1. INTRODUCTION

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. The starting of

three phase motors are less expensive than starting of DC motors. 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 are

present. While it is on load if any one of the fuse goes out, or goes 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 off immediately.

Failure of any of the phases makes the appliance prone to erratic functioning and

may even lead to failure. Hence it is of paramount importance to monitor the availability

of the three-phase supply and switch off the appliance in the event of failure of one or

two phases. The power to the appliance should resume with the availability of all phases

of the supply with certain time delay in order to avoid surges and momentary

fluctuations. 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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2. WHAT IS SINGLE-PHASING?

Loads using three-phase power sources are subject to loss of one of the three

phases from the power distribution system. This condition is known as "single-phasing."

The loss of a single phase on a three-phase line may be due to a downed line or a blown

pole top fuse on the utility system. Loss of a single phase may also result from a single-

phase overload condition causing one fuse to blow, or an equipment failure within the

end-user's facility. About 14% of the motor failures are due to single phasing.

The loss of one phase, of a three-phase line causes serious problems for induction

motors. The motor windings overheat due primarily to the flow of negative-sequence

current, a condition that exists anytime there is a phase voltage imbalance. The loss of a

phase also inhibits the motor's ability to operate at its rated horsepower. If single-phasing

occurs when a motor is rotating, the torque produced by the remaining two positively

rotating fields continues to rotate the motor and develop the torque demanded by the

load. The negatively rotating field, the field associated with the lost phase, produces

currents in inductive loads resulting in voltages in the faulted leg of the three-phase

supply. These voltages may be nearly equal to the phase voltage that was lost. Therefore,

detecting a single-phasing condition by measuring the voltages at the motor terminals is

usually unproductive.

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2.1 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.

Fig.2.1 Diagram of a WYE/DELTA transformation with one primary phase open

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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.

3.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

The increase in current is in only phase of the motor in case of delta connected load and

in star connected load the increase in current occurs in two phases.

3.1 EFFECT OF SINGLE PHASING ON PHASE CURRENT

Fig.3.1Effect of Single Phasing on Phase Current

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3.2 EFFECT OF SINGLE PHASING ON STAR CONNECTED LOAD

Fig.3.2Effect of Single Phasing on Star connected Load

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4. CIRCUIT DIAGRAM OF SINGLE PHASING PREVENTER

Fig.4.1circuit diagram of single phasing preventer

For 3 phase induction motor, lit is necessary that all the 3 phases of supply are

present while it is on load. When any of the fuses goes out or a phase is 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 ruin the motor, unless

switched off immediately. A single phase preventer circuit avoids such a mishap.

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With the circuit motor will not run,unless all 3 phases are present. In a 3 phase

supply, the voltages are 1200 apart from each other. Thus the addition of 3 phases

gives zero voltages. If any one of the phase goes the, voltage present at the

summing point equals half the line voltage. In the circuit 3 phases(R, Y, B),

connected to line neutral with in turn connected to the ground of the circuit. When

all 3 phases are present, voltage at point d is zero. So, potential at pin 3 of IC 741

is also zero, but voltage at pin 2 is nearly 4V. Hence IC 741 is used as comparator

and the voltage at pin 6 is zero. Hence the relay cannot operate. When a phase

goes out, voltages at point D goes up to about half the line voltage .This voltages

divide by 150 k and 50k resistors. The voltage at pin 3 is about 8V when 50k

potentiometer is properly adjusted. The voltage at pin 6 is about 12V .So, the

relay would operate when any of the phases goes out. This relay used in control

circuit if 3 phase motor, or with a circuit breaker, would switch the power

OFF/ON operation.

4.1 COMPONENTS

The following are the components required to develop the single phasing preventer

circuit:-

Resistor 150 kΩ ,0.5W 4

10KΩ 1

3.8 KΩ 1

1KΩ 2

Transistor(NPN) SL 100 1

Relay 12 V ,200 ohm

IC 741 8 pin

Diode BY 127

Preset button 50 K (variable resistor)

1N4148 1,looks like Zener diode near relay

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Capacitor 32uF ,25V

4.2CIRCUIT DESCRIPTION

4.2.1 IC 741:

The LM741 series are general purpose operational amplifiers which feature improved

performance over industry standards like the LM709. They are direct, plug-in

replacements for the 709C, LM201, MC1439 and 748 in most applications. The

amplifiers offer many features which make their application nearly foolproof: overload

protection on the input and output, no latch-up when the common mode range is

exceeded, as well as freedom from oscillations

Absolute Maximum Ratings:

The maximum ratings of the IC are specified for parameters like supply voltage, input

and differential input voltages, storage and operating temperature ranges, soldering pin

temperatures, and output short circuit duration. The manufacturers advise not to exceed

these maximum ratings even under the worst operating conditions which is given in table

4.1

Sr.no

.

Operating ratings LM 741

1 Supply Voltage ±22V

2 Power Dissipation 500 mW

3 Differential Input Voltage ±30V

4 Input Voltage ±15V

5 Output Short Circuit Duration Continuous

6 Operating Temperature Range −55°C to +125°C

7 Storage Temperature Range −65°C to +150°C

8 Junction Temperature 150°C

Table no.4.1 absolute maximum ratings

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Pin Diagram:

The 741 IC is developed using the planar epitaxial process. The IC is made

ideal for use as integrator, summing amplifier, voltage follower and other

basic applications.

The 741 IC is available in the market as 8-pin metal can, 10-pin flat pack,

8 or 14 pin DIP. The pin configuration for these packages are shown below:

Fig.4.2 pin diagram of LM 741

Features of 741 IC:

1. Short circuit and overload protection provided.

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2. In theory, the dc output voltage will be zero if both the inputs of the 741 IC are

connected to the ground. But in practice, a small dc output may appear due to minor

internal unbalances. It is usually unnoticed in normal applications. But for critical

conditions, the output voltage can be set precisely to zero by connecting a 10K

potentiometer between terminals marked “offset-null”.

3. Low power consumption.

4. Large common mode rejection ratio (CMRR) and differential voltage ranges.

5. No external frequency compensation is required. It also does not need any external

compensation for phase component. This simplifies the circuit design and minimizes the

number of components used.

6. No latch-up problem.

4.2.2 Transistor SL 100 (NPN):

SL100 is a general purpose, medium power NPN transistor. It is mostly used as switch in

common emitter configuration. The transistor terminals require a fixed DC voltage to

operate in the desired region of its characteristic curves. This is known as the biasing. For

switching applications, SL100 is biased in such a way that it remains fully on if there is a

signal at its base. In the absence of base signal, it gets turned off completely.

The emitter leg of SL100 is indicated by a protruding edge in the transistor case. The

base is nearest to the emitter while collector lies at other extreme of the casing.

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Fig 4.3 Transistor SL 100 (NPN)

4.2.3Diode BY 127:

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Fig.4.4 Diode BY 127

Features of BY 127:

1. Low forward voltage drop

2. High current capability

3. High reliability

4. High surge current capability

4.2.4 1N4148 zener diode :

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Full switching diode chip selection from several manufacturers including 1n4148

switching diodes,. Simply choose from the switching diode technical attributes and

results will quickly be narrowed in order to match your specific switching diode

application needs.

Applications for Switching Diodes:

The switching diode is the most basic function of almost every electronic application.

Switching diodes are also used in high-speed rectifying applications, such as in

radiofrequency receivers. Applications also include high-speed switching, general-

purpose switching and reverse polarity protection in the consumer, automotive, and

telecommunication industry.

Ratings:

Maximum Reverse Recovery Time (1 ns, 4 ns, 50 ns..), Maximum Peak Current (4 mA, 2

A, 4 A, …) and Power Dissipation (from 80 mW to 1 kW) for 1n4148 switching diode or

any other type of switching diode.

4.2.5 Relay:

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Fig.4.5 SUN HOLD RAS -1210 relay

The relay (SUN HOLD RAS 1210) would operate when any of the phases goes out. This

relay used in control circuit if 3 phase motor, or with a circuit breaker, would switch the

power OFF/ON operation.

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5. ADVANTAGE

The main advantage of this protector circuit is that it protects three-phase appliances from

failure of any of the phases by disconnecting the power supply through the contactor and

automatically restores the three-phase supply to the appliance (with reasonable time

delay) when all the phases are available.

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6. CONCLUSION

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 are present. While it is on load if any one of the fuse goes out, or

goes 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 off immediately. Failure of any of the phases makes the appliance prone to

erratic functioning and may even lead to failure. A single phasing preventer avoids such a

mishap with this circuit, the motor will not run unless all the three phases are present. The

single phase preventer is very less expensive and protects reliably the motor which is

very costly.

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