40093384 Single Phasing

ABSTRACT

In India there are so many industries in different fields. For example steel sector, Oil

sector, Irrigation etc. All industries have many drives and equipments like conveyor

belts, pumps, Mills etc.

All the drives of industries use electrical motors. Most of the electrical motors are

designed for three phase, 50Hz (in India) supply. These three phase motors are less

expensive than starting of DC motors.

Starting of AC 3-phase induction motors is less expensive than starting of DC motors

as they require simple D.O.L or Star/delta starters. D.O.L or Star/delta starters

generally have only over load protection. Three phase induction motors are very

sensitive and get damaged, when they are subjected to Single-phasing.

For three phase induction motor, it is necessary that all the three phases of supply

should present. While it is on load when any one of the fuse goes out, or missing, the

motor will continue to run with two phases only, but it will start drawing a huge

current for the same load. This high current may run the motor unless switched of

immediately.

A single phasing preventer avoids such a mishap with this circuit, the motor will not

run unless all the three phases are present.

In this context we need to design a preventer which prevents these mishaps and

protects the costly motor under such conditions. The single phase preventer is very

less expensive and protects reliably the motor which is very costly.

SINGLE PHASING PREVENTERPage 1

INDUCTION MOTOR

Figure1.1: Three-phase induction motors

An induction motor or asynchronous motor is a type of alternating current motor

where power is supplied to the rotor by means of electromagnetic induction.

An electric motor turns because of magnetic force exerted between a stationary

electromagnet called the stator and a rotating electromagnet called the rotor.

Different types of electric motors are distinguished by how electric current is supplied

to the moving rotor.

In a DC motor and a slip-ring AC motor, current is provided to the rotor directly

through sliding electrical contacts called commutators and slip rings. In an induction

motor, by contrast, the current is induced in the rotor without contacts by the magnetic

field of the stator, through electromagnetic induction.

An induction motor is sometimes called a rotating transformer because the stator

(stationary part) is essentially the primary side of the transformer and the rotor

(rotating part) is the secondary side.

Unlike the normal transformer which changes the current by using time varying flux,

induction motors use rotating magnetic fields to transform the voltage. The current in

the primary side creates an electromagnetic field which interacts with the

electromagnetic field of the secondary side to produce a resultant torque, thereby

transforming the electrical energy into mechanical energy.

Induction motors are widely used, especially poly phase induction motors, which are

frequently used in industrial drives.


Induction motors are now the preferred choice for industrial motors due to their

rugged construction, absence of brushes (which are required in most DC motors) and

thanks to modern power electronicsthe ability to control the speed of the motor.

History of Induction Motor

The induction motor was first realized by Galileo Ferraris in 1885 in Italy. In 1888,

Ferraris published his research in a paper to the Royal Academy of Sciences in Turin

(later, in the same year, Nikola Tesla gained U.S. Patent 381,968) where he exposed

the theoretical foundations for understanding the way the motor operates. The

induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky about a

year later.

Principle of operation and comparison to synchronous

motors

A 3-phase power supply provides a rotating magnetic field in an induction motor. The

basic difference between an induction motor and a synchronous AC motor is that in

the latter a current is supplied into the rotor (usually DC) which in turn creates a

(circular uniform) magnetic field around the rotor.

The rotating magnetic field of the stator will impose an electromagnetic torque on the

still magnetic field of the rotor causing it to move (about a shaft) and rotation of the

rotor is produced. It is called synchronous because at steady state the speed of the

rotor is the same as the speed of the rotating magnetic field in the stator.

Fig 1.2: A 3-phase power supply provides a rotating magnetic field in an induction `

motor.


By way of contrast, the induction motor does not have any direct supply onto the

rotor; instead, a secondary current is induced in the rotor. To achieve this, stator

windings are arranged around the rotor so that when energised with a polyphase

supply they create a rotating magnetic field pattern which sweeps past the rotor.

This changing magnetic field pattern induces current in the rotor conductors. These

currents interact with the rotating magnetic field created by the stator and in effect

causes a rotational motion on the rotor.

However, for these currents to be induced, the speed of the physical rotor must be less

than the speed of the rotating magnetic field in the stator or else the magnetic field

will not be moving relative to the rotor conductors and no currents will be induced. If

by some chance this happens, the rotor typically slows slightly until a current is re-

induced and then the rotor continues as before.

This difference between the speed of the rotor and speed of the rotating magnetic field

in the stator is called slip. It is unit less and is the ratio between the relative speed of

the magnetic field as seen by the rotor (the slip speed) to the speed of the rotating

stator field. Due to this, an induction motor is sometimes referred to as an

asynchronous machine.

Construction

The stator consists of wound 'poles' that carry the supply current to induce a magnetic

field that penetrates the rotor. In a very simple motor, there would be a single

projecting piece of the stator (a salient pole) for each pole, with windings around it; in

fact, to optimize the distribution of the magnetic field, the windings are distributed in

many slots located around the stator, but the magnetic field still has the same number

of north-south alternations. The number of 'poles' can vary between motor types but

the poles are always in pairs (i.e. 2, 4, 6, etc.).

Induction motors are most commonly built to run on single-phase or three-phase

power, but two-phase motors also exist. In theory, two-phase and more than three

phase induction motors are possible; many single-phase motors having two windings

and requiring a capacitor can actually be viewed as two-phase motors, since the

capacitor generates a second power phase 90 degrees from the single-phase supply

and feeds it to a separate motor winding.


Single-phase power is more widely available in residential buildings, but cannot

produce a rotating field in the motor (the field merely oscillates back and forth), so

single-phase induction motors must incorporate some kind of starting mechanism to

produce a rotating field.

They would, using the simplified analogy of salient poles, have one salient pole per

pole number; a four-pole motor would have four salient poles.

Three-phase motors have three salient poles per pole number. This allows the motor

to produce a rotating field, allowing the motor to start with no extra equipment and

run more efficiently than a similar single-phase motor.

Types of rotor in induction motors

i. Squirrel-cage rotor

The most common rotor is a squirrel-cage rotor. It is made up of bars of either

solid copper (most common) or aluminum that span the length of the rotor, and

those solid copper or aluminium strips can be shorted or connected by a ring or

sometimes not, i.e. the rotor can be closed or semi-closed type.

Fig 1.3: Diagram Of Squirrel Cage Rotor


The rotor bars in squirrel-cage induction motors are not straight, but have some skew

to reduce noise and harmonics.

ii. Slip ring rotor

Fig 1.4: Slip Ring Induction Motor

A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are

connected to slip rings. When these slip rings are shorted, the rotor behaves

similarly to a squirrel-cage rotor; they can also be connected to resistors to

produce a high-resistance rotor circuit, which can be beneficial in starting.

iii. Solid core rotor

Fig 1.5: Solid Core Induction Motor

A rotor can be made from a solid mild steel. The induced current causes the rotation.


Figure 1.6: Abnormalities in Induction Motors


Overview

Before discussing single-phasing, lets take a look at some of the ways that electric

motors fail. Historically, the causes of motor failure can be attributed to:

1. Overloads 30%

2. Contaminants 19%

3. Single-phasing 14%

4. Bearing Failure 13%

5. Old Age 10%

6. Rotor Failure 5%

7. Miscellaneous 9%

From the above data, it can be seen that 44% of motor failure problems are related to

HEAT. Allowing a motor to reach and operate at a temperature 10C above its

maximum temperature rating will reduce the motors expected life by 50%. Operating

at 10C above this, the motors life will be reduced again by 50%.

This reduction of the expected life of the motor repeats itself for every 10C. This is

sometimes referred to as the half life rule. The term, temperature rise, means that

the heat produced in the motor windings (copper losses), friction of the bearings, rotor

and stator losses (core losses), will continue to increase until the heat dissipation

equals the heat being generated. For example, a continuous duty, 40C rise motor will

stabilize its temperature at 40C above ambient (surrounding) temperature.

Standard motors are designed so the temperature rise produced within the motor,

when delivering its rated horsepower, and added to the industry standard 40C

ambient temperature rating, will not exceed the safe winding insulation temperature

limit. The term, Service Factor for an electric motor, is defined as: a multiplier

which, when applied to the rated horsepower, indicates a permissible horsepower

loading which may be carried under the conditions specified for the Service Factor of

the motor.Conditions include such things as operating the motor rated voltage and

rated frequency.


Single-Phasing

Figure 1.7: Diagram of a WYE/DELTA transformation with one primary phase open. The motor is protected by two overload devices. Note that one phase to the motor is carrying two times that of the other two phases. Without an overload device in the phase that is carrying two times the current in the other two phases, the motor will burn out.

The term single-phasing, means one of the phases is open. A single-phasing

condition subjects an electric motor to the worst possible case of voltage unbalance. If

a three-phase motor is running when the single phase condition occurs, it will

attempt to deliver its full horse power enough to drive the load. The motor will

continue to try to drive the load until the motor burns out or until the properly sized

overload elements and/or properly sized dual-element, time-delay fuses take the

motor off the line.

For lightly loaded three-phase motors, say 70% of normal full-load amperes, the

phase current will increase by the square root of three (3) under secondary single-

phase conditions. This will result in a current draw of approximately 20%more than

the name plate full load current. If the overloads are sized at 125% of the motor

nameplate, circulating currents can still damage the motor. That is why it is

recommended that motor overload protection be based upon the actual running

current of the motor under its given loading, rather than the nameplate current rating.


Two motor overload protective devices cannot assure protectionagainst the effects of primary single phasing. the middle linecurrent increase to 230% is not sensed.

Figure 1.8: Concept of single phasing


How to Calculate Voltage Unbalance and the Expected Rise in Heat

Figure 1.9

a) Step 1 Add together the three voltage readings as(248 + 236 + 230) = 714 volts.

b) Step 2 Find the average voltage.(714/3) = 238 volts.

c) Step 3Subtract the average voltage from one of the voltages that will indicate the greatest voltage difference.In this example: (248 238) = 10 volts.

d) Step 4100 x ( greatest voltage /difference average voltage)= 100 x (10238)= 4.2 percent voltage unbalance.

e) Step 5Find the expected temperature rise in the phase winding with the highest current by taking2 x (percent voltage unbalance)2 i.e. (2 x (4.2)2) = 35.28% temperature rise.

Therefore, for a motor rated with a 60C rise, the unbalanced voltage condition in the above example will result in a temperature rise in the phase winding with the highest current of 60C x 135.28% = 81.17C.


Fig 1.10: Motor during Normal Operation

Fig 1.11: Motor during Single Phasing


Hazards of Single Phasing for a Three-Phase Motor

When one phase of a secondary opens, the current to a motor in the two remaining

phases theoretically increase to 1.73 (173%) times the normal current draw of the

motor. The increase can be as much as 2 times (200%) because of power factor

changes. Where the motor has a high inertia load, the current can approach locked

rotor valves under single-phased conditions. Figures: 1.8 & 1.9 illustrate the 173%

current increase. Three properly sized time-delay, dual-element fuses, and/or three

properly sized overload devices will sense and respond to this over current.

Figure 1.12: (Delta-Connected Motor)Diagram showing the increase in current in the remaining two

phases after single-phasing


Figure 1.13: (WYE-Connected Motor) Diagram showing the increase in current in the remaining two

phases after single phasing


DESCRIPTION OF SINGLE PHASING PREVENTER

1. Suitable for any HP motors for complete protection against single phasing

unbalance supply.

2. Ensures correct phase sequence.

3. Automatic SWITCH OFF at dangerously LOW/HIGH voltage.

4. Built in time delay to bypass momentary transients.

5. Fail safe feature keeps the relay off against an open circuit in the control unit.

6. Voltage sensing & Current sensing.

Figure 1.14: Connection Diagram using single phase preventer


The SINGLE PHASING PREVENTER consists of the following blocks

1. POWER CIRCUIT

Basically it consists of step down Transformer, 4 Diodes, Shunt Resistance, Zener

Diode and Filter capacitor. During the positive half cycle of Secondary voltage Vi,

the diodes D2 and D3 are forward biased and conduct the current through load

resistance.

Whereas D1 and D4 are reverse biased and are in off state. It may be observed

that D2, R1 and D3 are in series. During the Negative half cycle of secondary

voltage Vi the current will appears diodes D1, D4 are forward biased and D2, D3

are reversed biased. Therefore the forward biased diode conducts the current

through load resistance. The most important result is that the polarity across the

load resistance R1 is same i.e. current flowing through R1 is same direction.

In this circuit the Zener diode reverse biased p-n junction and operates only in

break down region. Sometimes called as voltage regulator diode because it

maintains a fairly constant output voltage during reverse biased operation.

2. COMPARATOR

An op-amp used as a COMPARATOR. A fixed reference voltage Vref of 1V is

applied to the (pin-3) (-) input and the other varying signal Vin is applied to the

(pin-2) (+) input. Because of this arrangement the circuit is called the non-

inverting Comparator.

When Vin is less than Vref , the output voltage Vo is at +Vsat because the voltage at

the (-) input is higher than that at the (+) input. On the other hand, when Vin is

greater than Vref, the (+) input becomes positive with respect to the (-) input and

Vo goes to +Vsat.

Thus, Vo changes from one saturation level to another. The Comparator is a type

of analog-to-digital converter. At any given time the Vo wave form shows whether

Vin is greater or less than V ref.


3. TRIPPING CIRCUIT

It consists of one n-p-n transistor, diode and relay. The comparator output (pin-6)

is connected to base of BD 115 and collector is connected to voltage source. Relay

and diode are connected in between emitter (BD 115) and ground (-ve) supply.

Transistor BD 115 is used as emitter follower.

In a three phase supply the voltage is 120 degrees apart from each other. Thus the

addition of three phases gives zero voltage. If anyone of the phases goes off

voltage present at the summing point equals half the line voltage.

In this circuit the three phases (R,Y, B) are connected to the line neutral, which in

turn is connected to the ground of the circuit. When all three phases are present,

voltage at point D is zero. When phase goes out, voltage at point D goes up to

about half the line voltage. This voltage is divided by 150K and 50K resistors. The

voltage at point B is about 8V when 50K potentiometer is properly adjusted.

The voltage at point 6 is operating condition, so relay will operates when any one

of the phases goes out. This Relay when used in the control circuit of the three

phase motor, or with a circuit breaker will switch the power off on operation.

Figure 1.15: BLOCK DIAGRAM OF SINGLE PHASING PREVENTER


POWER CKT

COMPARATORCKT

TRIPP- ING CKT

STARTER

MOTOR

Figure 1.16: CIRCUIT DIAGRAM OF SINGLE PHASING PREVENTER


Xmer P

N D2 D1 D3 D4 D1 TO D4 IN 4007

12.1ZENER

1000mfd 25V

R

Y

B

150k

150k

150k

150k D B

BY 127 10K

1K

1K

+

_

2 4

3 7

6

B

C

E

3.8K 50K

32mfd15v

IN 4148

Relay

+12V

LM741

N

Figure 1.17: SINGLE LINE DIAGRAM FOR SINGLE PHASING PREVENTER


R

B

FUSE

OFF

OLR

SPP

ON HOLDING

CONTACTOR

COIL

LINK FUSE

Future Scope of Single Phasing Preventer

In this age of exploding population, the demand for power has increased manifold,

add to that the depleting natural resources of energy. The majority of equipments used

are outdated in this regard. What we want is the Optimum use of energy i.e nothing

should go wasted. The devices & equipments used should be designed to avail a free

flow of energy.

The three-phase induction motors come to use in everyday life, as well as in

industries. The phenomenon of Single Phasing causes a haphazard, the whole of the

system may blow away in seconds, the huge capital invested is gone,we need to

protect our devices & system from any such mishappenings, so the concept of Single

Phasing Preventer comes into the picture. It is not long before its utility was being

questioned, but now it has proved it worth.

Today all the major industries and Distribution Systems of the world are using the

Single Phasing Preventer, its scope is limited, but more efforts being put in its R&D

by the leading economies of the world, including ours would certainly pave its way

into many Nave sectors which uptill now have not been explored in this case, these

areas include its use along with the Thermal Relays in industries, along with

irrigational pumps in farms, tube wells and many more.


CONCLUSION

The rule of electricity in modern technology is that of an extremely versatile

intermediately. The chief advantage of this energy is that it can be transmitted,

controlled and utilized with relative simplicity, reliability and efficiency.

The primary objective of presenting this project named DESIGN AND

FABRICATION OF SINGLE PHASE PREVENTER is to protect the 3-phase

induction motor against single phasing.

This is designed as per our above mentioned requirements. This project states clearly

how it is functioning. In addition to that we discussed about the faults and remedies of

the circuit also.

Every care has been taken to design this project and we expect that this project is very

useful for avoiding single phasing problem for A.C. 3-phase induction motors and

saves the equipment from being damaged.


REFERENCES

i. www.msbte.com

ii. www.ieeexplore.ieee.org

iii. www.areva-td.com

iv. www.power.indiabizclub.com

v. www.protonelectronic.com

vi. www.itee.uq.edu.au

vii. www.progress-energy.com


INDUCTION MOTOR

40093384 Single Phasing

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Transcript of 40093384 Single Phasing