Guide to Drives

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Guide to Drives

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Guide to Drives

Contents Description PageIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312

Load Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322

Centrifugal Load Applications . . . . . . . . . . . . . . . . . . . . . . . 328

Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

Basic Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

Tables and Formulas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367Maintenance of Industrial Control Equipment . . . . . . . . . . 381

Introduction The illustrations, definitions and equations presented in this sectionare for educational purposes only.

This reference material is provided to assist the reader in

understanding certain basic electrical and physical relationships

commonly associated with rotating machinery and adjustable speed

drive technology.

Since each machine or process has unique control parameters, no

individual formula can take into consideration all the requirements toaccurately apply a specific product or predict its performance.

Each drive application must be carefully examined by the user.
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Basics Introduction

A variable speed drive is an electronic device that controls the speed,

torque, horsepower and direction of an AC or DC motor. Allen-

Bradley manufactures variable speed drives to meet wide variety of

applications.

Adjustable frequency AC drives serve processing needs andnumerous general industrial applications such as fans, pumps and

conveyors in a variety of working environments. DC industrial drives

control material handling and processing equipment in the forest

products, mining, metals, printing, and other industries. System

engineered AC/DC drives are custom designed for highly specialized

applications. High performance motion control drives meet the needs

of special purpose, high volume production and assembly machines.

The following information provides the basics required to evaluate

AC or DC drives application needs.

DC Drive Control System

Any DC drive control system generally contains the following:

operating controls, drive controller and DC motor (see Figure 1).

The operator controls allow the operator to start, stop, change

direction and speed of the controller by simply turning potentiometers

or other operator devices. These controls may be an integral part of

the controller or may be remotely mounted. Programmable

controllers are being used more frequently in this area to achieve

greater flexibility in process or machine control.

The drive controller converts a constant potential AC voltage to an

adjustable DC voltage which is then applied to a DC motor armature.

Regulation characteristics of the controller allow the motor to run at

the desired speed set by a reference input. Additional circuits can help

protect the controller, motor and driven machine from line voltage

transients, overloads and various circuit faults.


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Figure 1DC Drive Control System

The DC motor converts the adjustable voltage DC from the drive

controller to rotating mechanical energy. Motor shaft rotation and

direction are proportional to the magnitude and polarity of adjustable

voltage applied to the motor. Normally, the motor shaft is coupled to a

gear reducer or other transmission device which is then coupled to the

driven machine (see Gear Reducer Selection in the Basic Mechanics

section for more information).

The DC motor in a typical drive control system can be of the shunt

wound or permanent magnet type. In adjustable speed DC drive

applications, the motor armature is connected to an adjustable voltage

supply. The motor field (if not of the permanent magnet type) is

connected to either a fixed or adjustable voltage supply.

The tachometer-generator (feedback device) shown in Figure 1converts actual speed to an electrical signal that is summed with the

desired reference signal. The output of the summing junction

provides an error signal to the controller and a speed correction is

made.

Constant Torque Applications

The following paragraphs discuss DC drives in regard to major

categories of applications. The term drive refers to an electronic

regulator, armature and field supply. These supplies could be in a self-

contained unit or packaged separately.

Armature voltage controlled DC drives are constant-torque drives.

They are capable of providing rated current at any speed between zero

and the base (rated) speed of the motor. These drives use a fixed field

supply and give motor characteristics shown in Figure 2.

As seen in Figure 2, the motor output horsepower is directly

proportional to speed (50% horsepower at 50% speed, etc.) However,

rated torque is available at any speed (constant torque).


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Figure 2Constant Torque Curve

Constant Horsepower Applications

Armature and Field Controlled DC Drives

Certain applications require constant horsepower over a specified

speed range. As seen in Figure 3, an armature voltage controlled DC

drive has constant torque characteristics. Two items should be noted

here.

A drive required to deliver constant horsepower over a 2:1 speedrange would need special motor and control devices. It should also be

noted that at half speed, an armature controlled DC drive only

develops 50% of its rated torque and horsepower.

Figure 3

Constant Torque and Horsepower Curves


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Variable Torque Applications

Field Controlled DC Drives

(Operation Above Motor Base Speed)

One characteristic of a shunt-wound DC motor is that a reduction in

rated field current, at a given armature voltage, will result in an

increase in speed. However, this also results in a lower torque outputper unit of armature current (see Figure 3).

This concept can also be seen in Figure 4. Armature current is a

function of motor load. As the demand on the motor increases, so

does the motors demand for more current. In order to keep the motor

within its rated current range, the motor load must inherently decrease

above base speed with a resultant decrease in motor torque output.

Figure 4

Motor Speed and Load Characteristics

A simple method of reducing rated field current is to insert a resistor

in series with the field current source. This may be useful for

achieving an ideal motor speed for the application.


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A more sophisticated method of reducing rated field current is by use

of a solid state variable current field regulator. This method provides

coordinated automatic armature and field current control for an

extended speed range in constant HP applications. The motor is

armature voltage controlled for constant torque-variable HP operation

up to base speed. Then the motor is transferred to field current control

for constant HP - variable torque operation up to maximum speed(shown in Figure 3).

AC Adjustable Frequency Drives

Much of the power that is consumed today by AC motors goes into

the operation of fans and pumps. With these type of devices however,

actual demand is often less than the design capacity of the system.

Direct variable speed control of the fan or pump provides an attractive

means of energy savings and cost efficiency.

AC Drive Characteristics

AC adjustable frequency drives convert 3 phase 60 Hz input power to

an adjustable frequency and voltage source for controlling the speed

of AC squirrel cage induction motors.

The frequency of the applied power to an AC motor determines the

motor speed and is based on the following equation:

Equation 1:

Where:

N = speed (RPM)f = frequency (Hz)

P = number of poles

The number of poles is considered a constant since this design

characteristic is already manufactured into the motor.

The AC adjustable frequency drive controls the frequency (f) and

voltage applied to the motor. The speed (N) of the motor is then

proportional to this applied frequency. Control frequency is adjusted

by means of a potentiometer or external signal depending on the

application.

To maintain constant motor torque, the drive controller automatically

maintains the voltage and frequency output at a constant relationship

for any motor speed. This is called the volts per hertz ratio (V/Hz).

AC Drive System

An AC adjustable frequency drive typically consists of three basic

parts: operator controls, drive controller (referred to as an inverter)

and an AC motor. Figure 5 shows an AC adjustable frequency drive

system.

N120f

P------------=


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Figure 5AC Adjustable Frequency Drive System

The operator controls allow the operator to start, stop, change

direction and speed of the controller by simply turning potentiometers

or other operator devices. These controls may be an integral part of

the controller or may be remotely mounted. Programmable

controllers are often used for this function.

The drive controller converts fixed voltage AC to an AC adjustable

frequency and voltage source. It consists of a control unit and a power

conversion unit.

The control unit oversees the operation of the drive and provides

valuable system diagnostic information. The power conversion unit

performs several functions. It rectifies the incoming fixed AC voltage

(changes AC to DC). The resultant DC voltage is then filtered through

an LC low pass filter to obtain a DC voltage bus. The power

conversion (inverter) unit then produces an AC current and voltage

output having the desired frequency.

The AC motor converts the adjustable frequency AC from the drivecontroller to rotating mechanical energy.

AC Adjustable Frequency Drive

Types

The most common types of AC adjustable frequency drives used are:

variable voltage input (VVI) and pulse width modulated (PWM). The

following paragraphs offer a brief description of each type.

Variable Voltage Input (VVI)

This type of drive rectifies AC input power and delivers variablevoltage DC to a section of the power conversion unit called the

inverter section. The inverter section then inverts the variable voltage

DC to variable voltage and frequency AC. This inverter section is

built with power transistors or thyristors (SCRs) depending on

horsepower requirements.

Figure 6 shows a block diagram of the power conversion unit in a

variable voltage inverter.

PowerConversionUnitControlUnitOperatorControls

3 PH AC Line

ACMotor

Drive Controller


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Figure 6Power Conversion Unit (VVI)

An SCR bridge rectifier converts the 3 phase input power to variable-

voltage DC which is then the input to the inverter section. The

inverter section generates variable voltage, variable frequency AC

power to control motor speed. Because a large filter capacitor

provides a voltage supply to the inverter, output voltage is not affectedby the nature of the load.

The output voltage from a VVI drive is frequently called a six step

waveform and is shown in Figure 7.

Figure 7VVI Output Waveforms


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Pulse Width Modulated (PWM)

Figure 8 shows a block diagram of the power conversion unit in a

PWM drive. In this type of drive, a diode bridge rectifier provides the

intermediate DC circuit voltage. In the intermediate DC circuit, the

DC voltage is filtered in an LC low-pass output frequency and voltage

is controlled electronically by pulse-width-modulating techniques.

Essentially, these techniques require switching the inverter power

devices (transistors or SCRs) on and off many times in order to

generate the AC variable voltage and frequency.

Figure 8Power Conversion Unit (PWM)

This switching scheme requires a more complex regulator than the

VVI. However, with the use of a microprocessor, these complex

regulator functions are effectively handled. The output voltage from a

PWM drive is shown in Figure 9.

Figure 9

PWM Output Waveforms


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Drives

As factory automation increases, many applications require position

control as well as speed and direction control. Parts being processed

in an automated manufacturing line may need machining or sorting.

Precise control of part location (accomplished by a motion control

drive) directly affects the quality of the product.

A positioning drive receives a signal from some type of position

controller: numerical controller (NC), programmable controller (PC)

or computer numerical controller (CNC). This position controller tells

the motion control drive at what direction, speed and time to move a

part from point A to point B.

One category of motion control drives is the servo controller.

Servo Controllers

Servo controllers offer extremely fast response and precise control of

acceleration/deceleration, speed and torque. Originally designed for

aviation control applications, servos can accelerate from standstill to

100 RPM in several milliseconds.

Many servos are built with three major system loops: position loop,

velocity loop and current loop (see Figure 10). A typical servo system

is comprised of a position controller, encoder, or resolver (feedback

device), servo controller (servo amplifier) and a servo motor.


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Figure 10Servo Controller System

Speed feedback (velocity) is usually supplied by a tachometer-

generator (tach) with an encoder or similar sensor giving the position

feedback. The velocity loop (contained in the servo controller) sums

the velocity feedback and position error signals and generates avelocity error signal. The current loop (contained in the servo

controller), sums the current feedback and current command signals

and controls current limit.

In some designs, the position feedback (encoder) is connected to the

system controller, and in others, it is fed to the drive controller. Most

conventional DC servos require the position loop to be closed

externally. Therefore, a tachometer-generator (tach) used for speed

feedback is the only feedback device needed by the DC servo

controller.

Most servo controllers have power transistors that produce a pulse

width modulated (PWM) DC output. This design offers faster

response with better utilization and protection of the DC servo motor

compared to SCR servo controller designs. However, these SCR units

are still used for the larger HP applications.

Many new drive controllers include the functions of the system

controller plus several servo drive controller in one unit. These are

usually termed multi-axis units.


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Load Characteristics Introduction

The process of selecting an adjustable speed AC or DC drive is one

where load is of primary consideration. It is important to understand

the speed and torque characteristics as well as horsepower

requirements of the type of load to be considered. AC drive

characteristics are somewhat different than DC drives. The demands

and economics of a particular application should be matched to the

drive capabilities. After this matching process is completed, the

decision regarding the type of adjustable speed drive can be made.

When considering load characteristics, the following should be

evaluated:

What type of load is associated with the application?

Does the load have a shock component?

What is the size of the load?

Are heavy inertial loads involved?

What are the motor considerations?

Over what speed range are heavy loads encountered?

Motor loads are classified into three main groups depending on how

their torque and horsepower varies with operating speed. The

following paragraphs deal with the various DC and AC motor load

type usually found in process, manufacturing or machining

applications.


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Motor Load Types

Constant Torque Load

This type of load is the one most frequently encountered. In this

group, the torque demanded by the load is constant throughout the

speed range. The load requires the same amount of torque at low

speeds as at high speeds. Loads of this type are essentially frictionloads. In other words, the constant torque characteristic is needed to

overcome friction. Figure 11 shows the constant torque and variable

horsepower demanded by the load.

Figure 11

Constant Torque Load

As seen in Figure 11, torque remains constant while horsepower is

directly proportional to speed. A look at the basic horsepower

equation also verifies this fact:

Equation 2

Where:

Torque = lb-ft.

Speed = RPM

5252 = a proportionality constant

Examples of this type of load are conveyors, extruders and surface

winders. Constant torque is also used when shock loads, overloads or

high inertia loads are encountered.

Constant Horsepower Load

In this type of load, the horsepower demanded by the load is constantwithin the speed range. The load requires high torque at low speeds.

From Equation 2, you can see that with the horsepower held constant,

the torque will decrease as the speed increases. Put another way, the

speed and torque are inversely proportional to each other. Figure 12

shows the constant horsepower and variable torque demanded by the

load.

TorqueDemanded

by theLoad(%)

HorsepowerDemanded

by theLoad(%)

Speed (%) Speed (%)

100 100

100100

HPTorque Speed

5252

---------------------------------------=


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Figure 12Constant Horsepower Load

Examples of this type of load are center-driven winders and machine

tool spindles. A specific example of this application would be a lathe

that requires slow speeds for rough cuts and high speeds for fine cuts

where little material is removed. Usually very high starting torques

are required for quick acceleration.

Variable Torque Load

With this type of load, the torque is directly proportional to some

mathematical power of speed, usually speed squared (Speed2).

Mathematically:

Equation 3

Horsepower is typically proportional to speed cubed (Speed3). Figure

13 shows the variable torque and variable horsepower demanded by

the load.

Figure 13Variable Torque Load

Examples of loads that exhibit variable load torque characteristics arecentrifugal fans, pumps and blowers. This type of load requires much

lower torque at low speeds than at high speeds.

100 100

100100 200

200

BASEBASE

TorqueDemanded

by the

Load(%)

Horse-powerDemanded

by the

Load(%)

TorqueCons ttan Speed( )2

100 100

100100

TorqueDemanded

by theLoad

(%)

HorsepowerDemanded

by theLoad

(%)

5050


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Table 4 summarizes load types, torque and horsepower characteristics.

Table 4: Load Types

HP and Torque Characteristics Application Examples

Constant HP, Torque var ies inversely with speed Metal -cutting tools operating over wide speed range. Some extruders, mix-

ers, special machines where operation at low speed may be continuous.

Constant Torque, HP varies as the speed General machinery, hoists, conveyors, printing press, etc. (represents 90%of applications)

Squared exponential, HP varies as square of the speed. Torque varies withspeed.

Positive displacement pumps, some mixers, some extruders

Cubed exponential. HP varies as cube of speed. Torque varies as square ofspeed.

All centrifugal pumps & some fans. (Note: Fan power may vary as the 5thpower of speed)

High Inertia Loads Are typically associated with machines using flywheels to supply most ofoperating energy, punch presses, etc.


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Other Functional Considerations

Shock Loads

Drives for crushers, separators, grinders, conveyors, winches, cranes

and vehicular systems often must manage loads which range from a

small fraction of the rated load to several hundred percent.

Under these considerations, a drive has two fundamental tasks: move

the load and protect the prime mover and driven equipment. If the

prime mover is an electric motor, as is the case with a large number of

industrial drives, shock loads can damage components such as

bearings, brushes and commutators, as well as components of the

drive control circuitry, by inducing signal irregularities and electrical

overloads in the power converter.

Size of Load

The size of the load determines the type of drive chosen. Adjustable

speed drives (AC, DC, fluid, traction, etc.) range from fractional to

multithousand horsepower. However, not all types of drives can bemanufactured in full range. Generally, power converter rectifier

technology is the limiting factor in what is practical or economical to

manufacture for any given type of electrical drive.

DC Motor Torque and HP

A DC adjustable speed drive is able to handle a variety of load

characteristics. Examples of load characteristics are: constant torque

loads, variable torque loads, constant horsepower loads or a

combination of both constant torque and constant horsepower.

Speed can be adjusted from 0 to 100% by controlling the armature

voltage from zero to rated nameplate voltage (assuming full rated

motor field). A DC motor can be selected to provide a constant torque

capability through nearly the entire controllable speed range.

Horsepower increases from zero to nameplate horsepower at base

speed (100% speed). Refer to Figure 14.

Figure 14

DC Motor Torque and HP Curves


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By weakening the motor field, speed can be further increased up to 3,

4, or even 5 times base speed. Over this speed range, the DC motor

now has a constant horsepower characteristic where torque decreases

and is inversely proportional to speed (Refer to Figure 14).

Many industrial processes require a combination of constant torque

and constant horsepower depending on specific requirements. Theseinclude rubber and plastics extruders where the torque requirements

at 25% speed may be double the torque requirements at 100%.

As seen in Figure 14, the DC motor allows a combination of constant

torque and horsepower, depending on the speed range used. Because

of this combination characteristic, a smaller HP motor and drive

rating would be required compared to a constant torque only drive. A

constant torque only drive has horsepower determined by the

maximum torque required (at any speed) and the top speed.

It should be noted that operation above base speed (in Field Control

Range) is not a standard feature on most DC Drives. This featurerequires a field supply like the Bulletin 1370-CHP Module or 1370-

RFS Module and a motor-mounted tach generator. With these field

supply modules, as speed increases (above base speed), torque

decreases. The selection of a drive with one of these modules should

be considered for applications requiring wide speed ranges with both

constant torque/constant horsepower load characteristics.


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AC Motor Torque

In an AC induction motor, torque results from the magnetic attraction

between the rotor and stator. In essence, the stator (stationary case)

has a rotating magnetic field at a frequency delivered by the inverter.

The rotor (rotating piece) is attracted to the stator producing a twist-

ing motion called torque. Figure 15 shows an AC induction motor

curve with the various torque ratings marked.

Figure 15AC Motor Torque Curve

Point A in Figure 15 is the torque produced at locked rotor when

rotor frequency is highest and inductive reactance is greatest

(breakaway torque). As the motor begins to accelerate, the torque

drops off, reaching a minimum value called pull-up torque. This isbetween 25 - 40% of synchronous speed. As acceleration continues,

rotor frequency and inductive reactance decrease. The rotor flux

moves more in-phase with stator flux and consequently, torque

increases. Maximum torque is developed where inductive reactance

becomes equal to the rotor resistance. Beyond the maximum torque

point, the inductive reactance continues to drop off along with the

current through the rotor. The torque capabilities of the motor

therefore also decrease.

Centrifugal Load Applications Introduction

Centrifugal fans and pumps are sized to meet the maximum flow rate

required by the system. However, system operating conditions

frequently require reducing the flow rate. Therefore, throttling

devices damper and valves are frequently installed to limit

pump and fan outputs. The throttling devices are effective but are not

energy efficient. Alternative means offer the ability to both vary the

flow and greatly reduce energy losses. The method: adjust the fan and

pump impeller speeds so the units deliver the required flow.


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Pump Energy Savings

Pumps are generally grouped into two categories, positive

displacement pumps and dynamic (centrifugal) pumps. The vast

majority of pumps used today are the dynamic or centrifugal type and

are the only type discussed in this article.

The graph in the box on pump terminology shows two independent

curves. One is the pump curve, which is solely a function of the pump

characteristics. The other is the system curve. This depends on the

size of pipe, the length of pipe, the number and location of elbows,

etc. The intersection of these two curves is called the natural

operating point, because the pump pressure matches the system

losses.

If the system is part of a process that requires adjustable flow rates,

then some method is needed to continuously alter the pump

characteristics or the system parameters. As mentioned, these include

valves for throttling, which change the system curve, or variablespeed control of the pump, which modifies the pump curve.

Figure 16 shows a throttling system with two operating conditions

one with the valve open and the other with the valve throttled or

partially closed. Closing the valve effectively increases the system

head that, in turn, decreases the flow.

Figure 16Typical Pump and System Curves for Pump With Throttling Valve for

Flow Control


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By comparison, the variable speed method changes the pump

characteristics when the pump impeller speed is changed, Figure 17.

Of these two, only the adjustable speed method uses considerably less

energy with reduced flow, thus offering significant energy savings.

For example, a particular pump with a 14 in. impeller operates at a

base speed of 1150 rpm in a system with a 63 ft head (no static head),and delivers 1200 gpm when the system is not throttled, Figure 18.

The process requires flow rates of 1200, 960, 720, and 480 gpm.

Figure 17

Typical Pump and System Curves for Pump Driven by Adjustable Speed Drivefor Flow Control

For a specific flow rate, the difference between points A and B,

Figure 18, gives a visual indication of possible energy savings. In

addition, changes in pump efficiency should be included in the

calculation to determine brake horsepower.


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Figure 18Pump and System Curves for 14 in. Impeller Operating at 1150 rpm. Points A

Indicate Operating Points for Throttled System and Points B are Operating

Points When Pump is Powered by Adjustable Speed Drive.

Table . lists the comparative brake horsepower values required by

throttling and adjustable speed methods for the four operating points.

Figure 19 graphically shows the power requirements and savings for

the various flow rates.

Table 5:

Comparison of Pump Brake Horsepower Requirements for Throttling

and Adjustable Speed Methods

Throttling Adjustable Speed

Flow

(gpm)

Head

(ft)

Pump

Efficiency Bhp

Head

(ft)

Pump

Efficiency Bhp

1200 (100%) 63 76.3% 25 63 76.3% 25

960 (80%) 69 73 23 40 75 13

720 (60%) 75 65 21 23 75 5.6

480 (40%) 81 54 18 10 75 1.6


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Figure 19Pump Power Requirements for Throttling and Adjustable Speed Methods, and

the Resultant Power Savings

This example does not include a static head. The magnitude of the

static head will affect the possible power savings. The less the static

head is in relation to the total head, the greater the power savings will

be achieved by using adjustable speed drives. For example, Figure 20

shows a pump curve with three system curves one with no statichead, and two with different amounts of static heads. For a given flow

rate, the difference between operating points A and B indicate

possible power savings with adjustable speed. Thus, the difference

between points A and B3 (no static head) is greater and offers

greater power savings than between A and B1, which has a 40 ft

static head.


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Figure 20Pump Curve and Three System Curves With Different Static Heads

How to Determine Curves

Pump curves are readily available from pump manufacturers.

However, system curves are more difficult to establish. One quick

method gives a fairly reliable approximation:

1. Determine the unthrottled (open) system flow rate (gpm) at the

location under consideration.

2. Measure the static head.

3. Plot these two points on a copy of the pump curve.

4. Connect these two points using approximately a square function

(Y = X2 or head = flow2).


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Duty Cycle and Energy Costs

Before the dollar savings can be calculated, it is first necessary to

establish the average duty cycle percent of time the pump delivers

the various flow rates. The horsepower requirements for each duty

cycle can then be weighted to give the average power requirements.

In this example, the average power requirement is 10.1 hp. This value

divided by the motor and drive efficiency and multiplied by the cost

of electricity, will give the monthly operating cost.

For example, assume a drive is 85% efficient, the pump operates for

400 hours per month, and electricity costs 7 cents per kWh.

The operating costs can be determined for each type of flow

regulation method to establish payback periods. Also, some

companies offer a computer analysis to give my cost and payback

comparisons.

Flow

(gpm)

Reqd. hp for Each

Flow Rate

Duty Cycle

(% of Time)

Weighted Power

Requirements (hp)

1200 25.0 10 2.5

970 13.0 40 5.2

720 5.6 40 2.2

480 1.6 10 0.2

100% 10.1

10.1hp0.85

-------------------0.746kW

hp------------------------ 400h

month---------------- $0.07

kWh--------------- $258

month----------------=


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Fan Energy Savings

The basic operation of centrifugal fans is similar to pump operation,

and energy savings are equally obtainable. However, the common

units are slightly different. Outlet pressure (static inches of water) is

used in place of head (feet of water) and flow is usually expressed in

cubic feet per minute (cfm).

Several different methods are used to throttle or regulate fan outputs.

The most common include outlet dampers and variable inlet vanes.

Outlet damper affect the system curve by increasing the resistance to

air flow, Figure 21, much the same as a valve throttles a pump output.

Figure 21Typical Fan Curve With Three System Curves for Various Settings of Outlet

Dampers

Figure 22, show that as the flow is decreased, the power requirement

is reduced only slightly. Variable inlet vanes direct the air flow as the

air enters the fan, and, in effect, modify the fan curve, Figure 25. With

these vanes, power requirements are significantly reduced as flow is

decreased, Figure 24.

As with pumps, adjustable speed drives offer the greatest energy

savings for fans. This adjustable flow method changes the fan curve,

Figure 25, and drastically reduces the power requirements, Figure 26,

even more than for inlet vanes.


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Figure 22Typical Fan Curve With Three System Curves for Various Settings of Outlet

Dampers

Figure 23

Typical System Curve With Fan Curves for Various Settings of Inlet Vanes


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Figure 24Power Requirements vs. Flow For Various Settings of Inlet Vanes

Figure 25

Typical System Curve and Various Fan Curves for Adjustable Speed Operation


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Figure 26Adjustable Speed Fan Drive Power Requirements vs Flow

Adjustable Speed Drives

Available in many different types, adjustable speed drives offer the

optimum method for matching pump and fan flow rates to system

requirements. One frequently specified type of drive is the electrical

adjustable speed type that uses an AC motor. Frequently termed an

inverter, this unit receives the fixed voltage and frequency plant power

typically 230 or 460 V, 60 Hz and converts this to adjustable

voltage and frequency to power the AC motor. The frequency applied

to AC motor determines the motor speed.

These AC motors are usually readily available standard units that can

be connected across the AC power line. This capability maintains

operation even if the drive controller (inverter) should fail.

Adjustable speed drives also offer an additional benefit increased

bearing and pump seal life. By maintaining only the pressure needed

in the pump to satisfy system requirements, the pump is not subjected

to any higher pressures than necessary. Therefore, the components

last longer.

The same benefits but to a lesser extent also apply to fansoperated by adjustable speed drives.

To obtain optimum efficiencies and reliability, many specifiers obtain

detailed information from the manufacturers on drive efficiency,

required maintenance, diagnostic capabilities within the drive, and

general operational features. Then, they make detailed analyses to

determine which system will give the best return on the investment.


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Technology and Formulas

Fans and Blowers

Where:

CFM = Cubic feet per minute

PSF = Pounds per square foot

PIW = Inches of water gauge

PSI = Pounds per square inch

Efficiency of fan = %/100

Pump

Head measurement of pressure, usually in feet of water. A 30 ft

head is the pressure equivalent to the pressure found at the base of a

column of water 30 ft high.

Static head pressure required to overcome an elevation change,

also expressed in feet of water.

Dynamic head (or friction head) pressure losses within the pipe

system due to flow. To get water to flow at a particular volume may

require overcoming a 10 ft static head plus a 1 ft dynamic head. Thedynamic head of a system usually increases proportional to the square

of the flow rate.

System head curve of the head required to satisfy both the static

head and the dynamic head for a range of flows in a given system.

Pump head pressure the pump produces at its outlet. Centrifugal

pump heads can vary depending on the flow through the pump and is

also determined by the impeller speed and diameter.

Pump curve characteristic curve of a pump showing the head-flow

relationship.

Operating point intersection of the pump curve and system curve.

Water horsepower energy output of the pump derived directly from

the outlet parameters.

HPCFM PSF

33 000, Efficiency of Fan-----------------------------------------------------------------------=

HPCFM PIW

6536 Efficiency of Fan----------------------------------------------------------------=

HPCFM PSI

229 Efficiency of Fan------------------------------------------------------------=


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Where:

Q = Flow rate (gpm)

H = Pressure head (feet of water)

S = Specific gravity (water is 1.0)

Brake horsepower horsepower required to operate the pump at a

specific point, and equals the water horsepower divided by the pump

efficiency.

Affinity laws A set of formulas used to evaluate the operation of a

centrifugal pump at any operating point based on the original pump

characteristics:

Where:

N = Pump speed (rpm)

Q = Flow (gpm)

H = Pressure head (feet of water)

P = Power (hp)

Motors Introduction

DC motors are used in a wide variety of industrial applications whenadjustable speed operation is required.

A DC motor provides quick and efficient stopping through dynamic

or regenerative braking. Additionally, the speed of a DC motor can be

smoothly controlled down to zero RPM and then be immediately

accelerated in the opposite direction. The DC motor can also respond

quickly to control signals due to their high torque capability.

DC Motor Types

Following are the four basic types of DC motors and their operating

characteristics. It should be noted that the performance curves usedhere to illustrate differences between the various types of motors are

those of motors connected to a pure DC power source (e.g. motor/

generator set). Always refer to the adjustable speed DC drive and

motor manufacturers specifications for speed and torque capabilities

under starting, continuous and overload conditions.

Water hpQHS3960-------------=

Q1Q2--------

N1N2-------

H1H2-------

N1N2-------

2

==

P1P2-------

N1N2-------

2

=


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Shunt-Wound

Shunt-wound motors have the armature connected in parallel across

the field winding. With constant armature voltage and field excitation,

the shunt-wound motor offers relatively flat speed-torque

characteristics. The shunt-wound motor offers simplified control for

reversing, especially for regenerative drives.

Compound-Wound

The compound-wound DC motor utilizes a field winding in series

with the armature in addition to the shunt field, to obtain a

compromise in performance between a series and a shunt wound type

motor. The compound-wound motor offers a combination of good

starting torque and speed stability.

Series-Wound

The series-wound motor has the armature connected in series with the

field. Although the series-wound motor offers high starting torque, it

has poor speed regulation. Series-wound motors are generally used onlow speed, very heavy loads.


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Permanent-Magnet

The permanent magnet motor has a conventional wound armature

with commutator and brushes, permanent-magnets replace the field

windings. This type of motor has excellent starting torque, with speed

regulation slightly less than that of the compound motor. Peak starting

torque is commonly limited to 150% of rated torque to avoid

demagnetizing the field poles.

EnclosuresThe basic protective enclosures for DC motors are: dripproof (DP),

dripproof fully-guarded (DPFG), splashproof (SP), force ventilated

from either a separate source or integrally mounted blower and motor

(FV), totally enclosed nonventilated (TENV), totally enclosed fan-

cooled (TEFC), and totally enclosed unit-cooled (TEUC). The totally

enclosed motor can be provided in explosionproof construction but is

limited in horsepower ratings available.

Ventilation

The system for ventilating motors depends on the type of motor

enclosure. The dripproof motor is ventilated by means of a shaft-mounted internal fan which draws air in the commutator bracket

openings, through the motor and out the back end bracket openings.

The same is true with the dripproof fully-guarded and splashproof

motors. When an integrally mounted blower and motor is supplied, it

is mounted to blow air into the commutator end bracket so that the air

flows from front to back. Since the internal fan is omitted from a

blower-ventilated or force-ventilated motor, it is possible to reverse

the air flow. In areas where the ambient temperature is too high or the

surrounding air is too dirty, fresh air can be supplied from an external

source through duct work that attaches directly to the motor end

bracket.

Forced-Ventilation provides constant cooling independent of

the motor shaft speed. It is used when motors must operate at full

torque for long periods at very low speeds (when a shaft-mounted

fan does not provide adequate cooling). When using a shaft-

mounted fan the air volume drops off as the speed is reduced. If

full torque (full current) is demanded at low speeds, the motor

may quickly overheat. As a rule of thumb, extra cooling is

required if full torque is demanded below 60% of the (DP)

motors rated base speed.


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Totally Enclosed Fan-Cooled motors are ventilated over the

frame by means of an external shaft-mounted fan with a shroud to

direct the air. The fan is located at the front end of the motor.

Since there is no interchange of inside and outside air, this type is

better suited for dirty environments. The internal shaft-mounted

fan is retained to circulate air within the motor, thus reventing

localized hot spots. TEFC motors are normally available inhorsepower ratings to 75 HP (1750 RPM).

Totally Enclosed Nonventilated motors have only an internalshaft-mounted fan to prevent hot spots within the motor. An

external fan is not supplied, making them suitable for applications

where a fan would become clogged and disabled. Totally

Enclosed motors dissipate heat through the motor frame, which

must be larger than a comparable dripproof motor to provide

adequate cooling. As a result, cooling ability becomes

independent of motor speed, making full torque available at very

low speeds. This type of cooling is suitable for small motors up to

approximately 7

1

/2 HP. Above 7

1

/2 HP, additional cooling isrequired.

Totally Enclosed Unit-Cooled motors have an internal air path

through the motor, a heat exchanger, fan and suitable duct work.

The external air path is through the heat exchanger, the fan and

then exhausted downward over the motor frame. The fans for the

internal and external air are driven from an integrally mounted

AC motor. This is an efficient method of ventilating a totally

enclosed motor. It allows the use of frames smaller than necessary

for fan-cooled ratings and provides constant cooling independent

of motor shaft speed.

Totally Enclosed Air-Over motor is a type of totally enclosed

fan-cooled motor which is also ventilated by air blowing over the

frame from another source. The air may be supplied by a

integrally mounted blower and motor or from a separate source.

An air-over DC motor has constant cooling dependent of shaft

speed. In general, air-over motors still carry the same rating as

fan-cooled motors in the same frame size.


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The following table is a comparison of the maximum continuous

loading and relative cost for different motor enclosures (normal Class

F temperature rise). It should be noted that special motors are

available for broader speed/torque ranges.

Insulation

DC motor insulation must havemechanicalas well asdielectric

strength. It must be able to withstand the thermal expansion and

contraction of the conducting parts which it is insulating and be

strong enough to withstand the normal handling necessary in the

assembly of the motor. It must also withstand the centrifugal and

electromagnetic forces on the conductors and possible mechanicalvibration. For long life, the insulation must be impervious to

moisture, oil, cleaning solvents, chemical fumes and dust of all kinds.

Great care is exercised in selecting the components of an insulation

system. The major classes of insulation are A, B, F, and H.

Class Ais the lowest grade, suitable for some household appliances,

but not normally found in industry.

Class B is general purpose. More demanding duty requiresClass For

Class H, heavy duty insulation capable of withstanding high ambient

and internal motor temperatures. Class Finsulation is presently the

industry standard.

Normal life expectancy of an insulation system is 10,000-15,000

hours, depending on temperature. Reducing the motors winding

temperature by 10C will double the insulation life, while increasingthe temperature by 10C cuts the expected life in half.

Type HP

% of Base Speed

Available atFull Rated Torque

Comparative

Price(Multiplier)

DPFG 1/8 to 400100% down to 80% 1

TENV 1/8 to 71/2

100% down to 5% 1.4 to 1.6

TEFC 11/2 to 75 100% down to 60% 1.4 to 1.6

TEUC 10 to 200 100% down to 60% 1.4 to 1.6

Blower Ventilated 3 to 200 100% down to 40% 1.1 to 1.4

Explosion-Proof 1/8 to 200100% down to 5% 2.2 to 2.4

Separately Ventilated 3 to 200 100% down to 5% 1.1

Splash Proof 3 to 200 100% down to 80% 1.1

Waterproof 3 to 200 100% down to 5% 1.5 to 1.7


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Motor Selection

When selecting a DC motor and associated equipment for an

application, the following points should be considered:

Environment

The surrounding environment that the motor is to be operating in is aprime concern. Conditions such as; ambient temperature, air supply,

the presence of gas, moisture or dust should all be considered when

choosing a motor.

Speed Range

The minimum and maximum speeds for the application will

determine the motor base speed.

Speed Variation

The allowable amount of speed variation should be considered. Does

it require constant speed at all torque values or will variation less than

2% be tolerated?

Torque Requirements

Determine the torque requirements at the various speeds.

Applications such as conveyors require constant torque, while others

such as centrifugal blowers, require torque to vary as the square of the

speed. Machine tools and winders are constant horsepower, with

torque decreasing as the speed increases. Thus, the speed-torque

relationship determines the most economical motor. Refer to the

section entitled,Load Characteristics for further information.

Reversing (Armature or Field)Reversing affects the power supply, control and motor. Motors with

series compound and series stabilizing windings should not be used if

full load torque is needed in both directions. The use of series fields in

these applications can cause a loss of approximately 7% torque in the

reverse direction due to opposing reaction of the series field.

Regeneration (Armature or Field)

The use of series compound or stabilizing windings with static

armature or field regenerative drives can cause a loss of

approximately 7% braking torque due to improper series field

interaction.

Duty Rating

Most DC motors carry one of three ratings:

Continuous Duty is applied to motors that will continuously

dissipate all the heat generated by internal motor losses without

exceeding rated temperature rise.

Definite Time, Intermittent Duty motor carry rated load for a

specified time without exceeding rated temperature rise.


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Indefinite Time, Intermittent Duty is usually associated with

some RMS load of a duty-cycle operation.

Peak Torque

The peak torque that a DC motor delivers is limited by that load at

which damaging commutation begins. Brush and commutator

damage depends on sparking severity and duration. Therefore, peaktorque depends on the duration and frequency of occurrence of the

overload. Peak torque is often limited by the maximum current that

the power supply can deliver.

Heating

The temperature of a DC motor is a function of ventilation and losses

in the motor. Losses such as core, shunt-field and brush-friction are

independent of the load and vary with speed and excitation. Losses in

the armature circuit are primarily dependent upon the load and the

current required to produce the desired torque. Operating self-

ventilated motors at reduced speeds may cause above normal

temperature rises. Derating or forced ventilation may be necessary toachieve the rated torque output at reduced speeds.

AC Motors

Introduction

Allen-Bradley AC adjustable frequency drives operate with various

types of standard 60 Hz motors. In some cases the existing motor or

motor normally sized for a given fixed speed application can be

directly applied to a drive. The user must understand the nature of the

application in terms of the speed range, load characteristics and driverequirements as they relate to the AC drive system. This allows proper

pairing of the motor and controller. It should be noted that the

performance curves used here to illustrate differences between

various types of motors are those of motors controlled by across the

line full voltage or other type starters. Always refer to the adjustable

frequency drive and motor manufacturers specifications for speed

and torque capabilities under starting, continuous and overload

conditions.

AC Motor Types

AC motors can be divided into two main types: induction and

synchronous.


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Induction Motors

The induction motor is the simplest and most rugged of all electric

motor. The three most popular types of AC induction motors are;

polyphase, wound-rotor and single-phase.

Polyphase

The polyphase motor is divided into four classifications according to

NEMA. The four classifications or designs are determined by the

locked rotor torque and current, breakdown torque, pull-up torque and

the percent slip. The speed-torque curve and characteristics of each

design are as follows:

Design A motors have a higher breakdown torque than Design

B motors and are normally designed for a specific use. The slip is

usually 5% or less.

Design B motors are a general purpose type motor andaccount for the largest share of induction motors sold. The slip ofa Design B motor is approximately 3-5% or less.


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Design C motors have a high starting torque with a normalstarting current and low slip. The Design C motor is usually used

where breakaway loads are high at starting, but are normally run

at rated full load, and are not subject to high overload demands

after running speed has been reached. The slip of the Design C

motor is 5% or less.

Design D motors have high slip, high starting torque, lowstarting current and low full load speed. Because of the high

amount of slip, the speed can drop if fluctuation loads are

encountered. The slip of this type motor is approximately 5 to

13%.

Wound-Rotor Motors

The wound-rotor motor allows controllable speed and torque over the

conventional induction motor. Wound-rotor motors are generally

started with a secondary resistance in the rotor. As the resistance is

reduced, the motor will come up to speed. Thus the motor can

develop substantial torque while limiting the locked rotor current. The

secondary resistance can be designed for continuous service to

dissipate heat produced by continuous operation at reduced speed,

frequent acceleration or acceleration with a large inertia load.

External resistance gives the motor a characteristic that results in a

large drop in RPM for a small change in load. Reduced speed is

provided down to approximately 50% rated speed with low efficiency.


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Single-Phase Motors

Single-phase motors are most commonly found in the fractional

horsepower range with some integral sizes available. The most

common single-phase motors are listed below.

Shaded-Pole motors have a low starting torque and are

available only in fractional horsepower sizes. The slip of ashaded-pole motor is 10% or more at rated load.

Split-Phase motors have low or moderate starting torque andare limited in size to about 1/3 horsepower.

Capacitor-Start motors produce greater locked rotor and

accelerating torque than the split-phase motor and are available in

sizes ranging from fractional to 10 horsepower.

Split-Capacitor motors are similar to the capacitor-start motor

but produce a higher power factor ratio.

Synchronous Motors

Synchronous motors operate at synchronism with the line frequency

and are inherently constant-speed motors without sophisticated

electronic control. The two most common types of synchronous

motors are nonexcited and DC-excited. When applied to applications,

the synchronous motor, typically, provides up to 140% of rated

torque. When controlled by an adjustable frequency controller,

provisions for volts per hertz adjustments should be provided for

setting optimum performance.

Nonexcited MotorsThese motors use a self-starting circuit and require no external

excitation. Reluctance, hysteresis and permanent-magnet design

motors are the three main types of nonexcited motors available.

Reluctance designs have horsepower ratings that range fromsubfractional to about 30 HP. The subfractional motors have low

torque, while the integral motors have moderate torque.

Hysteresis designs are made in the subfractional horsepower

ratings and are primarily used a timing and servomotors.

Hysteresis motors are more costly than the reluctance type and

are used when precise constant speed is a requirement.

Permanent-Magnet motors are becoming increasingly popularin the fractional and lower integral horsepower ranges of1/4 to 5

HP. The permanent-magnet motor has relatively high efficiency

and power factor.


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DC-Excited Motors

These motors require direct-current supplied through slip rings for

excitation. Because DC-excited motors have inherent low starting

torque and require a DC power source, a starting system providing

full motor protection is needed. The starting system must apply the

DC field excitation at the proper time, remove field excitation at rotor

pull-out, and protect the windings against thermal damage under out-of-step conditions.

Enclosure

The totally enclosed nonventilated enclosure (described in theDC

Motorsection) is the most common type of enclosure found on AC

motors. Additionally, many of the remaining enclosures normally

used for DC motors can also be used for AC motors. Described below

are the: pipe ventilated, weather-protected, totally enclosed water-air-

cooled, totally enclosed air-to-air-cooled and totally enclosed water-

cooled enclosures. It should be noted that many of the enclosuresmentioned can be found on DC as well as AC motors.

Ventilation

As described for DC motors, the system for ventilation depends on

the motor enclosure. The Pipe-Ventilated motor is available in an

open or totally enclosed type of enclosure and is used in very dirty

environments. Ventilating air (supplied by the customer) enters and

exits the motor through inlet and outlet ducts or pipes. The air is then

circulated by means either integral or external to the motor. The pipe-

ventilated motor is the most economical totally enclosed type ofenclosure.

The Pipe-Ventilated Weather-Protected motor uses and open type

enclosure for ventilation. The motor is constructed to minimize the

entrance of rain, snow and airborne particles to the electrical parts of

the motor. External air is circulated through the motor for cooling.

The Pipe-Ventilated Totally Enclosed Air-to-Air-Cooled and Totally

Enclosed Water Air-Cooled enclosures are normally used on high

horsepower motors that generate large amounts of heat. A heat

exchanger is used for both types to remove the heat generated by the

motor. An AC motor driven blower circulates air through thewindings and heat exchanger tubes. The heat in the heat exchanger is

removed by either an external air system (air-to-air) or water provided

by the user (water-air-cooled).

The Pipe-Ventilated Totally Enclosed Water-Cooled Motor is cooled

by circulating water. The water or water conductors come in direct

contact with the motor parts.


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AC Motor Selection

When selecting an AC motor and associated equipment for an

application, the following points should be considered:

Environment

The environment in which the motor operates is a prime concern.Conditions such as: ambient temperature, air supply, the presence of

gas, moisture or dust should all be considered when choosing a motor.

Speed Range

The minimum and maximum speeds for the application will

determine the motor base speed.

Speed Variation

The allowable amount of speed variation should be considered. Does

it require constant speed at all torque values or will variations be

tolerated?

Torque Requirements

The starting torque and running torque should both be considered

when selecting a motor. Starting torque requirements can vary from a

small percentage of the full load to a value several times full-load

torque. The starting torque varies because of a change in load

conditions or mechanical nature of the machine. The motor torque

supplied to the driven machine must be more than that required from

start to full speed. The greater the excess torque, the more rapid the

acceleration.

AccelerationThe necessary acceleration time should be considered. Acceleration

time is directly proportional to the total inertia and inversely

proportional to the torque.

Deceleration

The necessary deceleration time should be considered. Dynamic

braking or external mechanical braking may be required to achieve

stopping times.

Duty Cycle

Selecting the proper motor depends on whether the load is steady,varies, follows a repetitive cycle of variation or has pulsating torques.

The duty cycle which is defined as a fixed repetitive load pattern over

a given period of time is expressed as the ratio of on-time to the cycle

period. When the operating cycle is such that the motor operates at

idle or a reduced load for more than 25% of the time, the duty cycle

becomes a factor in selecting the proper motor.


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Heating

The temperature of an AC motor is a function of ventilation and

losses in the motor. Losses such as operating self-ventilated motors at

reduced speeds may cause above normal temperature rises. Derating

or forced ventilation may be necessary to achieve the rated torque

output at reduced speeds.

Basic Mechanics Introduction

In order to apply AC or DC drives, certain mechanical parameters

must be taken into consideration. The following section explains what

these parameters are and how to calculate or measure them.

Torque

Torque is the action of a force producing or tending to produce

rotation. Unlike work (which only occurs during movement) torque

may exist even though no movement or rotation occurs.

Torque consists of a force (lb) acting upon a length of a lever arm (ft).

The product of these two factors produces the term lb-ft which is the

unit of measurement for torque. Mathematically, it is expressed as:

Equation 4

Figure 27Calculating Torque

Note: The term ft-lb. is the unit of measurement for work.

Because most power transmission is based upon rotating elements,torque is important as a measurement of the effort required to produce

work.

Torque (lb-ft.) Force (lbs.) Distance (ft.)=

Guide to Drives

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