Transcript of Single Phasing
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
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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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.
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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
Principle of operation and comparison to synchronous
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 `
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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 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
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.
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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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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
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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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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
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How to Calculate Voltage Unbalance and the Expected Rise in Heat
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 (10÷238)= 4.2 percent voltage unbalance.
e) Step 5Find 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
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
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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
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.
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-
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.
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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.
Figure 1.15: BLOCK DIAGRAM OF SINGLE PHASING PREVENTER
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TRIPP- ING CKT
D2 D1 D3 D4 D1 TO D4 IN 4007
R Y B
150k 150k 150k 150k D
BY 127 10K 1K 1K + _ 2
3.8K 50K 32mfd15v
Relay +12V LM741
Figure 1.16: CIRCUIT DIAGRAM OF SINGLE PHASING PREVENTER
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Figure 1.17: SINGLE LINE DIAGRAM FOR SINGLE PHASING PREVENTER
Future Scope of Single Phasing Preventer
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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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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.
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