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 equipment’s 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.

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INDUCTION MOTOR 

Figure1.1: Three-phase induction motors

An induct induction ion motor or async asynchro hrono nous us motor motor is a type type of  alternating current motor  wher wheree powe powerr is supp suppli lied ed to the the rotor    by by mean meanss of  electromagnetic induction. An electric motor  turns because of magnetic force exerted between a stationary electromagnet calle calledd the stator  and and a rota rotati ting ng elec electro troma magn gnet et calle calledd the 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. 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. induction . An induction induction motor is sometimes sometimes called a rotating rotating transforme transformerr because 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 transformer  which changes the current by using time varying flux, induction motors use rotating magnetic fields to transform the voltage. The current in the the prim primar aryy side side crea create tess an electroma electromagneti gneticc field whic whichh inte intera ract ctss with with the the electromagnetic field of the secondary side to produce a resultant torque, thereby transforming the electrical energy into mechanical mechanical energy. Induction motors are widely used, used, especially especially  poly phase induction motors, which are frequently used in industrial drives. SINGLE PHASING PREVENTER Page 2

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 electronics—the 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.

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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 reinduced 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 PHASING PREVENTER Page 4

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

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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.

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Figure 1.6: Abnormalities in Induction Motors

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Overview Before discussing single-phasing, let’s 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 10°C above its maximum temperature rating will reduce the motor’s expected life by 50%. Operating at 10°C above this, the motor’s life will be reduced again by 50%. This reduction of the expected life of the motor repeats itself for every 10°C. 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, 40°C rise motor will stabilize its temperature at 40°C 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 40°C 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.

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Single-Phasing

Two motor overload protective devices cannot assure protection against the effects of primary single phasing. the middle line current increase to 230% is not sensed .

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.

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Figure 1.8: Concept of single phasing

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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 3

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

d) Step 4

100 x ( greatest voltage /difference average voltage) = 100 x (10÷238) = 4.2 percent voltage unbalance.

e) Step 5

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

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

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Fig 1.10: Motor during Normal Operation

Fig 1.11: Motor during Single Phasing

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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

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

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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

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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 the diodes

D2

and

D3

Vi ,

are forward biased and conduct the current through load

resistance. Whereas that

D1

and

D4

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

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

Vin

Vref  of

1V is

is applied to the

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

Vref , the (+) input

Vin

is

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 .

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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.

STARTER 

POWER  CKT

COMPA RATOR  CKT

TRIPPING CKT

MOTOR 

Figure 1.15: BLOCK DIAGRAM OF SINGLE PHASING PREVENTER  SINGLE PHASING PREVENTER Page 17

P mer 

X N

D1

D2

D3

D4

D1 TO D4 IN 4007 +12V

1000mfd 25V

12.1 ZENER

R

150k

10K BY 127

Y

150k

D

150k

B

1K

1K

B 150k

3  _  + 2

7 6

LM 741 4

C 50K 

3.8K B 32mfd 15v

E

IN 4148 Relay

N

Figure 1.16: CIRCUIT DIAGRAM OF SINGLE PHASING PREVENTER 

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R

FUSE

OFF

OLR

SPP

HOLDING

ON

COIL

CONTACTOR

LINK FUSE

B

Figure 1.17: SINGLE LINE DIAGRAM FOR SINGLE PHASING PREVENTER 

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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 Naïve 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.

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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 FABRICATION OF SINGLE PHASE PREVENTER”

“DESIGN AND

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.

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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

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