Synchronous Motor - Construction and Working

A synchronous motor is a type of AC motor whose rotor rotates at the same speed as the rotating
magnetic field. The stator’s magnetic field revolves at a speed that depends on the supply
frequency known as synchronous speed. Hence the name synchronous motor. The rotor of the
synchronous motor is synchronized with the frequency of the supplied current.

Synchronous motor and induction motor are the most widely used types of AC motor.
Construction of a synchronous motor is similar to an alternator (AC generator). A
same synchronous machine can be used as a synchronous motor or as an alternator. Synchronous
motors are available in a wide range, generally rated between 150kW to 15MW with speeds
ranging from 150 to 1800 rpm.

Construction of Synchronous Motor

The construction of a synchronous motor (with salient pole rotor) is as shown in the figure. Just
like any other motor, it consists of a stator and a rotor. The stator core is constructed with thin
silicon lamination and insulated by a surface coating, to minimize the eddy current and hysteresis
losses. The stator has axial slots inside, in which three phase stator winding is placed. The stator
is wound with a three phase winding for a specific number of poles equal to the rotor poles.

The rotor in synchronous motors is mostly of salient pole type. DC supply is given to the rotor
winding via slip-rings. The direct current excites the rotor winding and creates electromagnetic
poles. In some cases permanent magnets can also be used. The figure above illustrates
the construction of a synchronous motor very briefly.

Working of Synchronous Motor

The stator is wound for the similar number of poles as that of rotor, and fed with three phase AC
supply. The 3 phase AC supply produces rotating magnetic field in stator. The rotor winding is

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fed with DC supply which magnetizes the rotor. Consider a two pole synchronous machine as
shown in figure below.

 Now, the stator poles are revolving with synchronous speed (lets say clockwise). If the
rotor position is such that, N pole of the rotor is near the N pole of the stator (as shown in
first schematic of above figure), then the poles of the stator and rotor will repel each
other, and the torque produced will be anticlockwise.

 The stator poles are rotating with synchronous speed, and they rotate around very fast and
interchange their position. But at this very soon, rotor can not rotate with the same angle
(due to inertia), and the next position will be likely the second schematic in above figure.
In this case, poles of the stator will attract the poles of rotor, and the torque produced will
be clockwise.

 Hence, the rotor will undergo to a rapidly reversing torque, and the motor will not start.

But, if the rotor is rotated up to the synchronous speed of the stator by means of an external force
(in the direction of revolving field of the stator), and the rotor field is excited near the
synchronous speed, the poles of stator will keep attracting the opposite poles of the rotor (as the
rotor is also, now, rotating with it and the position of the poles will be similar throughout the
cycle). Now, the rotor will undergo unidirectional torque. The opposite poles of the stator and
rotor will get locked with each other, and the rotor will rotate at the synchronous speed.

Characteristic Features of a Synchronous Motor

 Synchronous motor will run either at synchronous speed or will not run at all.
 The only way to change its speed is to change its supply frequency. (As Ns = 120f / P)
 Synchronous motors are not self starting. They need some external force to bring them
near to the synchronous speed.
 They can operate under any power factor, lagging as well as leading. Hence, synchronous
motors can be used for power factor improvement.

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Application of Synchronous Motor

 As synchronous motor is capable of operating under either leading and lagging power
factor, it can be used for power factor improvement. A synchronous motor under no-load
with leading power factor is connected in power system where static capacitors can not
be used.
 It is used where high power at low speed is required. Such as rolling mills, chippers,
mixers, pumps, pumps, compressor etc.

Starting Methods of Synchronous Motor

A device that converts electrical energy into mechanical energy running at synchronous speed is
called Synchronous Motor. Its speed is constant irrespective of load. It is a doubly excited
machine because its field winding is excited from a separate dc source. But the synchronous
motor is not self-starting. The average synchronous motor torque is zero at the start. For a net
average torque, it is necessary to rotate the rotor at a speed very near to synchronous speed.

Why Synchronous motor not self starting

Synchronous motors have lots of advantages but being not self-starting unlike 3 phase induction
motors, is a major disadvantage. In synchronous motors, the stator has 3 phase windings and is
excited by 3 phase supply whereas the rotor is excited by DC supply. The 3 phase windings
provide rotating flux whereas the DC supply provides constant flux.
The torque produced on the rotor is a pulsating one and not uni-directional. Considering the
frequency to be 50 Hz, from the above relation we can see that the 3 phase rotating flux rotates
about 3000 revolutions in 1 min or 50 revolutions in 1 sec. At a particular instant rotor and stator
poles might be of the same polarity (N-N or S-S) causing a repulsive force on the rotor and the
very next second it will be N-S causing attractive force. But due to the inertia of the rotor, it is
unable to rotate in any direction due to attractive or repulsive force and remain in standstill
condition. Due to this, the motor cannot start on its own. The rotor of the synchronous motor has
to be brought to synchronous speed by using external means.
The different methods used to start a synchronous motor are:
Below are the techniques used for starting a synchronous motor:

1. Using Pony Motors

2. Using Small D.C. Machine
3. Using Damper Winding
4. As a Slip Ring Induction Motor ( Synchronous Induction Motor )

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1. Using Pony Motors:

By using the small pony motors like a small induction motor, we can start the synchronous
motor. This small induction motor is coupled to the rotor of the synchronous motor. The function
of this induction motor is to bring the rotor of the synchronous motor to the synchronous speed.
Once the rotor attains the synchronous speed the pony motor is dis-coupled from the rotor.
The synchronous motor continues to rotate at synchronous speed, by supplying d.c. excitation to
the rotor through the slip-rings. One should remember that the motor used as the pony motor
must have less number of poles than the synchronous motor used.

2. Using Small D.C. Machine:

It is similar to above method with a slight difference between the two. A DC machine is
coupled to the synchronous motor. The DC machine works like a DC motor initially and brings
the synchronous motor to synchronous speed. Once it achieves the synchronous speed, the DC
machine works like a DC generator and supplies DC to the rotor of the synchronous motor. This
method offers easy starting and better efficiency than the earlier method.

3. Using Damper Winding:

When a 3-phase supply is given to the synchronous motor it fails to start. In order to make it
start copper bars circuited at both ends ( similar to the squirrel cage rotor of an induction motor )
are placed on the rotor, these bars or winding are known as 'Damper Winding'.

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 Now when the supply is given the field winding setups a rotating magnetic field. Due to the
damper winding used, the rotor starts rotating as an induction motor i.e., less than the
synchronous speed at starting. Once d.c. excitation is given to the field winding and the motor is
then pulled into synchronism.

The damper winding is used to start the motor and hence can be used for starting purposes only.
Because once the rotor rotates at synchronous speed the relative motion between the damper
winding and rotating magnetic will be equal, and hence induced emf and current will be zero.
The damper winding will be out of the circuit.
4. As a Slip Ring Induction Motor ( Synchronous Induction Motor ) :
In this method, an external rheostat is connected to the rotor through slip-rings. Here, ends of
the damper winding are brought of the motor and connected either in star or delta. The rheostat is
connected in series with the rotor. At starting high resistance is connected with the rotor to limit
the current drawn by the motor. As the motor starts as a slip ring induction motor at starting, it
draws large currents.
When the motor picks up its speed, resistance is gradually cut off from the rotor circuit. As
the speed reaches near to synchronous speed, d.c. excitation is given to the rotor and it is pulled
into synchronism.

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 The above figure shows the rheostat connected with the rotor circuit through slip-rings. From
the figure as the dc supply is given current 'I' flows through the positive terminal, then it divides
as 'I/2' through each phase at star point.
From these methods, damper winding is the most common method of starting a synchronous
motor.
Hunting or Surging or Phase Swinging
Basically hunting is observed in synchronous machines when there is sudden removal or
application of load. Rotor tries to hunt or reach the new equilibrium position . The oscillations of
the rotor about its new equilibrium position is called hunting. Hunting is the variation in speed
around the target speed.

Causes of hunting:

1. Sudden changes of load.

2. Fault occurring in the system which the generator supplies.
3. Sudden changes in the field current.
4. Cyclic variations of the load torque.
Effects of hunting:
1. It can lead to loss of synchronism.
2. Large mechanical stress may develop in the rotor shaft.
3. Machine losses increase and the temperature of the Machine rises.

Reduction of Hunting:

Hunting is reduced by following techniques

1. Damper windings.
2. Use of Fly wheels.

1. Damper windings.

Synchronous motors have their pole-shoes slotted for placing copper bars. The copper bars are
placed in these slots and short-circuited at both ends by heavy copper rings (like squirrel cage

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rotor of induction motors). This arrangement is known as damper winding in synchronous
motor.
Role of Damper Winding in Synchronous Motor
The damper winding in synchronous motor performs two functions:
 provides starting torque and
 prevents hunting in the synchronous motor
When the rotor is rotating at synchronous speed, then the relative velocity between the RMF
(rotating magnetic field of the stator) and the rotor is zero. Hence induced EMF in the damper
winding is zero.
Thus, under normal running conditions, damper winding in synchronous machine does not carry
any current.
Hunting in Synchronous Motor

The stator and the rotor poles of running synchronous motor are magnetically locked and, hence,
both run with the same synchronous speed. But centerlines of the two poles do not coincide with
each other. The rotor slips back behind the stator poles by a small angle δ.
This angle is known as the load angle or torque angle. This backward shift of the rotor is
essential for developing motor torque.
As the load on the motor is increased, the backward shift of rotor poles increases by a larger
angle, but rotor poles still continue to run synchronously. The value of load angle δ depends on
the load carried by the motor. This load angle also controls the stator current.
Greater will be the value of δ; higher will be the value of stator armature current. It is so because
the motor needs more input power to carry the increased load. If too much load is put on a
synchronous motor, the rotor will be pulled out of synchronism, and it will stop.
The damper winding in synchronous motor plays a very important role in hunting. When the
rotor oscillates, the relative motion between RMF and the rotor becomes nonzero. Hence an
EMF is induced proportional to relative motion in damper winding. This induced EMF is in such
a direction that it will try to oppose the cause of it (Lenz’s law). Here the cause is relative motion
due to hunting. Hence the hunting reduces quickly due to damper winding.
The time taken by the rotor to reach its final equilibrium position after hunting is known as
‘settling time’. It should be as short as possible. The use of damper winding in synchronous
motor reduces its settling time considerably.

 Uses of flywheels

A flywheel is an external arrangement connected between the motor and prime mover of that
motor. Usually, a high weighted flywheel is used for this purpose. Due to its great inertia, it
stabilizes the relative motion of the rotor due to oscillations, and it controls the rotation without
any variation in speed.

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Synchronous Motor on Busbar

Over Excitation: E Cos𝜹 > Q; Q is negative

The machine delivers the reactive power and operates at lagging power factor. The effect of
armature reaction is demagnetization.
Under Excitation: E Cos𝜹 > Q; Q is positive
The machine absorbs the reactive power and operates at leading power factor. The effect of
armature reaction is magnetization.
Normal Excitation: E Cos𝜹 = Q; Q is zero
The machine neither delivers nor absorbs the reactive power and operates at unity power factor.
The effect of armature reaction is cross magnetization.

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 Effect of change in Excitation

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Applications of Synchronous Motors:
(i) Synchronous motors were mainly used in constant speed applications.
(ii) Overexcited synchronous motors can be used to improve the power factor of a plant while
carrying their rated loads.
(iii)Synchronous condenser behave like a variable inductor or a variable capacitor, it is used in
power transmission system to regulate line voltage.

SYNCHRONOUS CONDENSER

• A synchronous motor takes a leading current when over-excited and, therefore, behaves
as a capacitor.

• An over-excited synchronous motor running on no-load in known as synchronous

condenser.

• When such a machine is connected in parallel with induction motors or other devices that
operate at low lagging power factor, the leading KVAR supplied by the synchronous
condenser partly neutralizes the lagging reactive KVAR of the loads. Consequently, the
power factor of the system is improved.

Like capacitor bank, we can use an overexcited synchronous motor to improve the poor power
factor of a power system. The main advantage of using synchronous motor is that the
improvement of power factor is smooth. When a synchronous motor runs with over-excitation, it
draws leading current from the source. We use this property of a synchronous motor for the
purpose.
The synchronous condenser is the more advanced technique of improving power factor than a
static capacitor bank, but power factor improvement by synchronous condenser below 500
kVAR is not economical than that by a static capacitor bank. For major power network we use

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synchronous condensers for the purpose, but for comparatively lower rated systems we usually
employ capacitor bank.

The advantages of a synchronous condenser are that we can control the power factor of system
smoothly without stepping as per requirement. In case of a static capacitor bank, these fine
adjustments of power factor cannot be possible rather a capacitor bank improves the power factor
stepwise.
The short circuit withstand-limit of the armature winding of a synchronous motor is high.
Although, synchronous condenser system has some disadvantages. The system is not silent since
the synchronous motor has to rotate continuously.

S. No Particular Synchronous motor Induction motor

1 Speed Remains constant (i.e., Ns) Decreases with load.

from no-load to full-load.
2 Power factor Can be made to operate from Operates at lagging power factor.
lagging to leading power
factor.
3 Excitation Requires d.c. excitation at the No excitation for the rotor.
rotor.
4 Economy Economical for speeds below Economical for speeds above 600
300 r.p.m. r.p.m.

5 Self-starting No self-starting torque. Self-starting

Auxiliary means have to be
provided for starting.

6 Construction Complicated Simple

7 Starting More less
torque

Types of Torque in a Synchronous Motor

In order to select a synchronous motor for a particular application, the following torques are
considered −
Pull-in Torque
As a synchronous motor is started as an induction motor and runs at a speed 2% to 5% below the
synchronous speed. Then, the DC excitation voltage is applied and the rotor pulls into step with

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the rotating magnetic field of the stator which is revolving at synchronous speed. Therefore, the
maximum torque at rated voltage and frequency under which a synchronous motor will pull a
connected load into synchronism when the DC excitation is applied to the motor, is known
as pull-in torque.

Pull-out Torque
There is a limit to the mechanical load that can be applied to a synchronous motor. With the
increase in the load, the torque angle (δ) also increases so that a stage is reached when the rotor
is pulled out of synchronism and the motor comes to the rest. Therefore, the maximum value of
load torque which a synchronous motor can develop at rated voltage and frequency without
losing synchronism is called as pull-out torque or breakdown torque.

Locked Rotor Torque

The locked rotor torque is the minimum value of the torque developed by a synchronous motor
with the rotor locked (i.e. stationary rotor) at any angular rotor position and rated voltage and
frequency is applied to the motor. The locked rotor torque is provided by the armature windings
of the motor.

Running Torque
The torque developed by a synchronous motor under running conditions is known as running
torque. The running torque is determined by the power rating and the speed of the motor.

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Synchronous Motor V Curves

The graphs plotted between armature current (Ia) and field current (If) for different constant loads
are known as the V curves of the synchronous motor.
The power factor of a synchronous motor can be controlled by changing the field excitation, i.e.,
by variation of field current (If). Also, the armature current (Ia) changes with the change in the
excitation or field current (If).
Now, let us assume that the synchronous motor is operating at no-load. If the field current (If) is
increased from a small value, the armature current (Ia) decreases until Ia becomes minimum. The
power factor of the motor corresponding to this minimum armature current is unity. Up to the
point of minimum armature current, the motor was operating at lagging power factor. If a graph
is plotted between armature current (Ia) and field current (If) at noload, the lowest curve in
Figure-1 is obtained. In order to obtain a family of curves as shown in Figure-1, the above
procedure is to be repeated for various increased loads.

Because the shape of the curves plotted between armature current (I a) and field current (If)
resembles the letter "V", thus these curves are known as V curves of a synchronous motor.
The point corresponding to unity power factor is the point at which the armature current is
minimum. The curve connecting the unity power factor points (or lowest points) of all V curves
for various loads is called the unity power factor compounding curve. Similarly, the
compounding curves for 0.85 power factor lag and 0.85 power factor lead are shown by dotted
curves in Figure-1.
Therefore, the compounding curves may be defined as the loci of constant power factor points on
the V curves of a synchronous motor. The compounding curves give the information about the
manner in which the field current (If) of the motor should be varied to maintain constant power
factor under changing loads.
Hence, the V curves are useful in adjusting the field current of the synchronous motor. From the
V curves shown in Figure-1, it is clear that decreasing the field current below that for minimum
armature current results in lagging power factor. Similarly, increasing the field current beyond
the level of minimum armature current results in leading power factor. Therefore, by changing

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the field excitation of a synchronous motor, the reactive power supplied to or consumed from the
power system can be controlled.

Inverted V Curves of Synchronous Motor

Inverted V curves of a synchronous motor are defined as the graphs plotted between power
factor and field current (If) of the motor.
A family of inverted V curves of a synchronous motor obtained by plotting the power factor
versus field current is shown in Figure-2.

The peak point on each of these curves indicates unity power factor. From the curves, it can be
seen that the field current (If) for unity power factor at full-load is greater than the field current
(If) for unity power factor at no-load.
The inverted V curves also show that if the synchronous motor at full-load is operating at unity
power factor then removal of the mechanical load from the shaft of the motor causes the motor to
operate at a leading power factor.

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