Speed Control of DC Motor

Speed Control of DC Motor

Speed control means intentional change of the drive speed to a value required for performing the specific work process. Speed control is a different concept from speed regulation where there is natural change in speed due change in load on the shaft. Speed control is either done manually by the operator or by means of some automatic control device.

One of the important features of dc motor is that its speed can be controlled with relative ease.

We know that the expression of speed control dc motor is given as,

Therefore speed (N ) of 3 types of dc motor – SERIES, SHUNT AND COMPOUND can be

controlled by changing the quantities on RHS of the expression. So speed can be varied by

changing

(i) terminal voltage of the armature V ,

(ii) external resistance in armature circuit R and

(iii) flux per pole φ .

The first two cases involve change that affects armature circuit and the third one involves change

in magnetic field. Therefore speed control of dc motor is classified as

1) armature control methods and

2) field control methods.

Speed Control of DC Series Motor

Speed control of dc series motor can be done either by armature control or by field control.

Armature Control of DC Series Motor

Speed adjustment of dc series motor by armature control may be done by any one of the methods

that follow,

1. Armature resistance control method: This is the most common method employed. Here the

controlling resistance is connected directly in series with the supply to the motor The power loss

in the control resistance of dc series motor can be neglected because this control method is

utilized for a large portion of time for reducing the speed under light load condition. This method

of speed control is most economical for constant torque. This method of speed control is

employed for dc series motor driving cranes, hoists, trains etc.

2. Shunted armature control: The combination of a rheostat shunting the armature and a rheostat

in series with the armature is involved in this method of speed control. The voltage applied to the

armature is varies by varying series rheostat R 1. The exciting current can be varied by varying

the armature shunting resistance R2. This method of speed control is not economical due to

considerable power losses in speed controlling resistances. Here speed control is obtained over

wide range but below normal speed.

3. Armature terminal voltage control: The speed control of dc series motor can be accomplished by supplying the power to the motor from a separate variable voltage supply. This method involves high cost so it rarely used.

Field control of DC Series Motor

The speed of dc motor can be controlled by this method by any one of the following ways –

1. Field diverter method: This method uses a diverter .Here the field flux can be reduced by shunting

a portion of motor current around the series field. Lesser the diverter resistance less is the field

current, less flux therefore more speed. This method gives speed above normal and the method is

used in electric drives in which speed should rise sharply as soon as load is decreased.

2. Tapped Field control: This is another method of increasing the speed by reducing the flux and it is

done by lowering number of turns of field winding through which current flows. In this method a

number of tapings from field winding are brought outside . This method is employed in electric

traction.

Speed Control of DC Shunt Motor

Speed of dc shunt motor is controlled by the factors stated below

Field control of DC Shunt Motor

By this method speed control is obtained by any one of the following means –

1. Field rheostat control of DC Shunt Motor: In this method , speed variation is accomplished by

means of a variable resistance inserted in series with the shunt field . An increase in controlling

resistances reduces the field current with a reduction in flux and an increase in speed. This

method of speed control is independent of load on the motor. Power wasted in controlling

resistance is very less as field current is a small value. This method of speed control is also used

in DC compound motor.

Limitations of this method of speed control:

A. Creeping speeds cannot be obtained.

B. Top speeds only obtained at reduced torque

C. The speed is maximum at minimum value of flux, which is governed by the demagnetizing

effect of armature reaction on the field.

2. Field voltage control: This method requires a variable voltage supply for the field circuit

which is separated from the main power supply to which the armature is connected. Such a

variable supply can be obtained by an electronic rectifier.

Armature control of DC Shunt Motor

Speed control by this method involves two ways . These are :

1. Armature resistance control : In this method armature circuit is provided with a variable

resistance. Field is directly connected across the supply so flux is not changed due to variation of

series resistance. This is applied for dc shunt motor. This method is used in printing press,

cranes, hoists where speeds lower than rated is used for a short period only.

2. Armature voltage control: This method of speed control needs a variable source of voltage

separated from the source supplying the field current. This method avoids disadvantages of poor

speed regulation and low efficiency of armature-resistance control methods. The basic adjustable

armature voltage control method of speed d control is accomplished by means of an adjustable

voltage generator is called Ward Leonard system.

This method involves using a motor –generator (M-G) set. This method is best suited for steel

rolling mills, paper machines, elevators, mine hoists, etc.

Advantages of this method –

A. Very fine speed control over whole range in both directions

B. Uniform acceleration is obtained

C. Good speed regulation

Disadvantages –

A. Costly arrangement is needed , floor space required is more

B. Low efficiency at light loads

AC Motor Speed Control

AC Induction Motor Speed Control

►So what can we do to control the speed of an AC induction motor?

Change the number of poles (in discrete increments - inefficient & rarely done)�Change the frequency of the AC signal�Change the slip�Change AC Frequency

►Variable speed AC Motor adjustable speed drives are known as inverters,

variable frequency drives (VFD) , or adjustable speed drives (ASD).�►Common ways to vary AC frequency:

Six-step inverter�Pulse-Width-Modulation�Vector Flux�

Six-step Inverter

►AC rectified to DC, then switched to imitate a sine wavealso called a Variable Voltage Inverter or VVI

Pulse-Width-Modulation

►DC voltage (rectified AC) rapidly switched to match "area under curve" also called Pulse-Density-Modulation

Changing Rotor Slip

►Important to match the motor to the load ensure that a change in motor power gives a desired change in load speed

►Load should have a substantial inertial components inertial torque can "carry" the load through brief periods when motor torque cannot

►Best used with motors designed for high slip

Variable Series Resistance

►Additional series resistance reduces voltage across main windings

Variable Voltage Transformer

►More efficient than previous method, no power wasted in the series resistance

Tapped Winding

►Commonly used with 3-speed fan motors (like the one in AC Motor Lab)

STARTER

Necessity of starter

At starting ,the speed of motor is zero so that the back e.m.f. In the armature is zero.

Armature resistance is so low, if it is connected to power supply directly ; huge current will pass thru armature.

The huge current may damage the machine, major heat, very high speed in case of DC series motor.

Ia = V/Ra

Function of starter

Start and stop the motor.

Limit inrush current where necessary.

Permit automatic control when required

Protect motor and other connected equipments from over voltage, no voltage, under voltage, single phasing etc.

Motor Starter Features.

Rated by current (amperes) or power (horsepower)

Remote ON/OFF control

Motor overload protection

Starting and stopping (electrical life)

Plugging and jogging (rapid making and breaking current)

Type of starter

For DC Motor

Two point starter for DC series motor

Three point starter for shunt motor

Four point starter for compound motor

For AC Motor

DOL Starter

Star-Delta

Auto-transformer

Variable Frequency drive

Wiring Diagram of DOL Starte

r:

Motor Starting Characteristics on DOL Starter:

Available starting current: 100%.

Peak starting current: 6 to 8 Full Load Current.

Peak starting torque: 100%

Advantages of DOL Starter:

Most Economical and Cheapest Starter

Simple to establish, operate and maintain

Simple Control Circuitry

Easy to understand and trouble‐shoot.

It provides 100% torque at the time of starting.

Only one set of cable is required from starter to motor.

Motor is connected in delta at motor terminals.

Disadvantages of DOL Starter:

It does not reduce the starting current of the motor.

High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).

Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.

Voltage Dip: There is a big voltage dip in the electrical installation

High starting Torque: Unnecessary high starting torque, even when not required by the load.

Suitability

DOL is Suitable for:

Small water pumps, compressors, fans and conveyor belts.

Motor rating up to 5.5KW

DOL is not suitable for:

The peak starting current would result in a serious voltage drop on the supply system

Motor rating above 5.5KW

Star delta starter

Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges.

To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed.

Star-Delta Connection in transformer

Star-Delta Connection in motor

Star delta power circuit

Star delta control circuit

Motor Starting Characteristics of Star-Delta Starter:

Available starting current: 33% Full Load Current.

Peak starting current: 1.3 to 2.6 Full Load Current.

Peak starting torque: 33% Full Load Torque.

Advantages of Star-Delta starter:

The operation of the star-delta method is simple and rugged

It is relatively cheap compared to other reduced voltage methods.

Good Torque/Current Performance.

It draws 2 times starting current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter:

Low Starting Torque, only 33% starting torque

Break In Supply – Possible Transients

Six Terminal Motor Required (Delta Connected).

It requires 2 set of cables from starter to motor.

The delta of motor is formed in starter and not on motor terminals.

Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-delta starter.

Low Starting Torque: reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of the current at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque .

Reversing of Direction

Variable frequency drive (VFD)

A variable-frequency drive (VFD), also termed adjustable-frequency drive, variable speed drive, AC drive, micro drive or inverter drive is a type of adjustable-speed drive used in electro-mechanical drive systems.

It controls AC motor speed and torque by varying motor input frequency and voltage.

Principle of VFD

Variable frequency Drive, principle of the same is by regulating the frequency, we can regulate the speed of drive

Example - n=120f/p

Where,

n - speed

f - frequency

p – nos. of poles

VFD flow diagram

VFD 3-phase power conversion

Application Application

Cane Carrier Drive,

Feeder Table Drive,

Milling Plant Drive,

Raw juice Pumps,

Sulphited Juice Pumps,

Molasses Pumps,

Injection Pumps,

Cooling Tower Pumps and

Centrifugals for a Sugar Curing and Chemical Dosing pump.

Auto transformer

Circuit and function

An auto-transformer starter makes it possible to start squirrel-cage induction motors with reduced starting current, as the voltage across the motor is reduced during starting.

In contrast to the star-delta connection, only three motor leads and terminals are required. On starting, the motor is connected to the tappings of the auto-transformer.

Starting Characteristics

Motor terminal voltage less than line voltage (by transformer ratio).

Motor current exceeds line current (by inverse of transformer ratio).

Starting torque is reduced by the square of the terminal voltage.

Applications:

Where complete acceleration at reduced amperes is needed.

Where line ampere reduction requirements are severe, and load torque is not minimal.

Difference between DOL/Star delta /Autotransformer

Sr.

DOL Starter

Star delta starter

Auto transformer starter

1 Used up to 5 HP

Used 5 HP to 20HP

Used above 20 HP

2 Does not decrease the starting current

Decreases the starting current by 1/3 times

Decreases the starting current as required

3 It is cheap

It is costly It is more costly

4 It connects directly the motor with supply for starting as well as

It connects the motor first in star at the time of starting in delta for running

It connects the motor according to the taping taken out from the auto transformer

for running

KV Series

Super-small Programmable Logic Controllers

General Specifications(R and T in model names indicate relay output and transistor output)

Model KV-10RWKV-10T2W

KV-16RWKV-16T2W

KV-24RWKV-24T2W

KV-40RWKV-40T2W

KV-80RWKV-80TW

Programming language

Ladder diagram and expanded ladder diagram

Number of instructions

Basic: 16, application: 34, arithmetic:26, interrupt: 4

Execution time(basic I/O instructions)

1.0 µs min., 1.92 µs average 1.4 µs min., 3.12 µs average

Avg. number of steps 500 steps/program 3000 steps/program

Input (Maximum extendable number of inputs)

6 inputs(70 max.)

10 inputs(74 max.)

16 inputs(80 max.)

24 inputs(72 max.)

48 inputs(80 max.)

Output (Maximum extendable number of inputs)

4 outputs(68 max.)

6 outpus(70 max.)

8 outputs(72 max.)

16 outputs(80 max.)

32 outputs(80 max.)

Maximum extendable number of I/Os

74 80 88 88 128

Internal utility relays(with latching function)

160 800

Special utility relays 160 160

Data memory(16 bits)

1,000 words 2,000 words

Temporary memory (16 bits)

32 words 32 words

Timer/counter

A total of 64 timers, up, and up-down counters are

provided: 0.1-s timer (0 to 6553.5 s)0.01-s timer (0 to 655.35

s)1-ms timer (0 to 65.535 s)1 analog timer (0 to 24.9

s, 0 to 2.49 s, or 0 to 0.249 s)

A total of 120 timers, up, and up-down counters are provided:

0.1-s timer (0 to 6553.5 s)0.01-s timer (0 to 655.35 s)1-ms timer (0 to 65.535 s)

2 analog timers (two of 0 to 24.9 s, 0 to 2.49 s, or 0 to 0.249 s)

High-speed counter 2 auto-reset up-counters (max. input response frequency: 10 kHz)

High-speed counter comparator

4 4

Direct clock pulse

2 channels, 2 kHz max.(output from 0500), 1.5 kHz max. (output from

0501)

2 channels, 2 kHz max.(output from 0500), 1.5 kHz max. (output from

0501)

2 channels, 20 kHz max. (output from

0500), 1.5 kHz max.

(output from 0501)

Memory backup

Program memory: EEPROM, programs retained for 10 years min., rewritable 50,000 times min.

Data memory: data retained for 2 months min. by capacitors (at 25°C)

Supply voltage 24 VDC +10% to -20%

Maximum current consumption

KV-10RW: 75 mA

KV-10T2W: 65 mA

KV-16RW: 105 mA

KV-16T2W: 70 mA

KV-24RW: 130 mA

KV-24T2W: 75 mA

KV-40RW: 220 mA

KV-40T2W: 115 mA

KV-80RW: 400 mA

KV-80TW: 300 mA

Ambient temperature 0 to 50°C

Withstand voltage 1500 VAC applied between power terminal and I/O terminal, and between terminals and housing (1 min.)

Weight

KV-10RW: approx. 130

gKV-10T2W: approx. 120

g



g



g



g


gKV-80TW:

approx. 500 g

input/Output Specifications

Type Basic unit

Model KV-10RW

KV-16RW

KV-24RW

KV-40RW

KV-80RW

KV-10T2W

KV-16T2W

KV-24T2W

KV-40T2W

KV-80T2W

No. of inputs

6 10 16 24 48 6 10 16 24 48

No. of outputs

4 6 8 16 32 4 6 8 16 32

Type of output

Relay Relay Relay Relay Relay BJT MOS-FET

MOS-FET

MOS-FET

BJT

Type Expansion unit

Model KV-8ERW

KV-8ET2W

KV-8EXW

KV-8EYRW

KV-8EYT2W

KV-16EXW

KV-16EYRW

KV-16EYTW

No. of inputs 4 4 8 - - 16 - -

No. of outputs 4 4 - 8 8 - 16 16

Type of output

Relay BJT - Relay BJT - Relay BJT

Power Supply Unit SpecificationsModel KV-U6W

Operation system Switching type

Power supply voltage 100 to 240 VAC (50/60 Hz) ±10%

Output voltage 24 VDC ±10% (Ripple: 240 m Vp-p max.)

Output current 0.8 A

Weight (brackets not included) Approx. 210 g

Analog I/O Specifications (Input: KV-AD4, Output: KV-DA4)Type Voltage Current

Analog I/O range -10 to +10 V 4 to 20 mA

http://europe.keyence.com/products/program/kv/kv_spec2.html#top

Input impedance 1 M ohm 300 ohm

Output impedance

0.5 ohm min. -

Number of I/O 4 channels of inputs or outputs 4 channels of inputs or outputs

Resolution 5 mV (1/4000) 4 µA (1/4000)

Speed Control of DC Motor

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