SYNCHRONOUS MOTOR: Construction - Types -Principle of operation -Starting of Synchronous

Motor -V curve – Inverted V curve – Operating at different power factors - Hunting - Damper Windings
- Applications.

Synchronous Motors: Applications, Starting Methods & Working Principle

Electrical motors are an electro-mechanical device that converts electrical energy to
mechanical energy. Based on the type of input we have classified it into single phase and 3
phase motors.
The most common type of 3 phase motors are synchronous motors and induction

motors. When three-phase electric conductors are placed in certain geometrical positions (i.e.

in a certain angle from one another) – an electrical field is generated. The rotating magnetic

field rotates at a certain speed known as the synchronous speed.

If an electromagnet is present in this rotating magnetic field, the electromagnet is

magnetically locked with this rotating magnetic field and rotates with the same speed of
rotating field.
This is where the term synchronous motor comes from, as the speed of the rotor of
the motor is the same as the rotating magnetic field.
It is a fixed speed motor because it has only one speed, which is synchronous speed.
This speed is synchronised with the supply frequency. The synchronous speed is given by:

Where:
 N= The Synchronous Speed (in RPM – i.e. Rotations Per Minute)
 f = The Supply Frequency (in Hz)
 p = The number of Poles
 Construction of Synchronous Motor

The construction is almost similar to that of a 3 phase induction motor, except the fact

that DC supply is fed to the rotor. In this motor an three phase supplied is applied to the stator

and DC supply is supplied to the rotor.

Main Features of Synchronous Motors

1. Synchronous motors are inherently not self starting. They require some external
means to bring their speed close to synchronous speed to before they are
synchronized.
2. The speed of operation of is in synchronism with the supply frequency and hence for
constant supply frequency they behave as constant speed motor irrespective of load
condition
3. This motor has the unique characteristics of operating under any electrical power
factor. This makes it being used in electrical power factor improvement.

Principle of Operation Synchronous Motor

Synchronous motors are a doubly excited machine, i.e., two electrical inputs are
provided to it. Its stator winding which consists of a three-phase supply, and DC to the
rotor winding.
The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating
magnetic flux. The rotor carrying DC supply also produces a constant flux. Considering
the 50 Hz power frequency, 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 instant 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 that attractive or repulsive forces, and the rotor remains in standstill condition.
Hence a synchronous motor is not self-starting.
So some mechanical means is used initially which rotates the rotor in the same
direction as the magnetic field to speed very close to synchronous speed. On achieving
synchronous speed, magnetic locking occurs, and the synchronous motor continues to rotate
even after removal of external mechanical means.

METHODS OF STARTING OF SYNCHRONOUS MOTOR

1. Motor starting with an external prime Mover:
Synchronous motors are mechanically coupled with another motor. It could be
either 3 phase induction motor or DC shunt motor. Here, DC excitation is not supplied
initially. It rotates at speed very close to its synchronous speed, and then the DC
excitation is supplied. After some time when magnetic locking takes place supply to
the external motor is cut off.
2. Damper winding :
In this case, the synchronous motor is of salient pole type, additional winding
is placed in rotor pole face. Initially, when the rotor is not rotating, the relative speed
between damper winding and rotating air gap flux is large and an emf is induced in it
which produces the required starting torque. As speed approaches synchronous speed,
emf and torque are reduced and finally when magnetic locking takes place; torque
also reduces to zero. Hence in this case synchronous motor first runs as three phase
induction motor using additional winding and finally it is synchronized with the
frequency.
Application of Synchronous Motors
1. Synchronous motor having no load connected to its shaft is used for power
factorimprovement. Owing to its characteristics to behave at any electrical power
factor, it is used in power system in situations where static capacitors are expensive.
2. Synchronous motor finds application where operating speed is less (around 500 rpm)
and high power is required. For power requirement from 35 kW to 2500 KW, the size,
weight and cost of the corresponding three phase induction motor is very high. Hence
these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills
etc.
 V Curves and Inverted V Curves
The performance characteristics of a synchronous motor are obtained by v-curves and
inverted v-curves. Synchronous machines have parabolic type characteristics (the graph
drawn is in the shape of parabolic).
If the excitation is varied from low (under-excitation) to high (over-excitation) value,
then the current Ia also changes i.e., becomes minimum at unity PF and then again increases.
But at starting lagging current becomes unity and then becomes leading in nature. V-
curves and inverted V-curves of a synchronous motor are used to analyze efficiency on no-
load and on-load conditions.

V-Curves of Synchronous Motor :

If the armature current Ia is plotted against excitation or field current for various load
conditions, we obtain a set of curves known as 'V-Curves' clue to their shape similar to
english letter V.
In the below figure V-Curve of a synchronous motor shows how armature current
Ia changes with excitation for the same input, at no-load, half full-load, and full-load.

From V-Curves it is observed that the armature current has large values both for low and
high values of excitation (though it is lagging for low excitation and leading for higher
excitation). In between, it has a minimum value corresponding to the unity power factor
(normal excitation).

Inverted V-Curves of Synchronous Motor :

 If the power factor is plotted against excitation for various load conditions,
we obtain a set of curves known as Inverted V-Curves.

The inverted V-Curves of synchronous motor shows how the power factor varies with
excitation. From inverted V-curves, it is observed that the power factor is lagging when the
motor is under excited and leading when it is over-excited. In between, the power factor is
unity

Content Synchronous Motor

 Definition The electromagnetic motor which converts electrical
01 energy into mechanical work at constant speed is called
as a ‘Synchronous Motor’.

02 Motor Speed Synchronous motor runs at the synchronous speed.

The synchronous motor works on the principle of the

03 Principle
‘Magnetic Locking’.

04 Starting Position It is a non-self starting motor.

It consists of two main parts- the stator and rotor.

Connected
05 Supply
Stator connects with the three-phase AC supply and
(AC and DC)
Rotor connects with the DC supply respectively.

Excitation The synchronous motor requires the DC excitation

06 System (DC system (or prime mover) to start the motor (i.e. for the
source for rotor) rotor rotating).

The construction of the synchronous motor is very

07 Construction
difficult.

For Synchronous motor, relative motion is not required

08 Relative Motion
in between the stator and rotor.

Slip
09 (magnetic field vs Zero slip (S=0) occurs in the synchronous motor.
rotor speed)

Motor Speed
10 It runs from 150 to 1800 rpm. synchronous of speed.
(in r.p.m)

It has unity or lagging or leading power factor.

11 Power Factor
Mostly it works on the unity power factor.

Efficiency
The synchronous motor has more power efficiency due
12 (Ratio of power
to the unity or leading power factor.
output & input)

It is used for power factor correction, constant speed

13 Uses load service, voltage regulation of transmission line,
etc.

14 Cost This motor is costlier than the induction motor.

15 Maintainance Maximum maintenance is required.

Fan, blowing fan, a dryer is an example of the

16 Example
synchronous motor.
 OPERATING AT DIFFERENT POWER FACTORS
The power factor of a synchronous motor is changed with a change in the excitation.
When the excitation of the motor is increased, the power factor changes from lagging to unity
and then to a leading power factor.
This property of the motor is utilized to improve the power factor of the leads, having a low
lagging power factor.
Normally, when the motor is utilized in this way to improve the factor, the
synchronous motor is run without any mechanical load. The excitation is adjusted in such a
manner that it works at a leading power factor. The synchronous motor is then referred to a
synchronous condenser.
In factories the majority of the motors used are induction motors, the full load power
factor of these motor may be around 0.8 lagging. The power factor of these motors may
operate at full load and many of them operate at light loads. This results in the power factor
of the installation reading as low as 0.6 lagging. A synchronous condenser may be connected
in parallel with the induction motors. This will improve the power factor of the installation.

The term HUNTINGis a study about three phase synchronous motor operations.
The word hunting is used because after the sudden application of load the rotor has to
search or ‘hunt’ for its new equilibrium position.
That phenomenon is referred to as hunting in a synchronous motor.
what is the condition of equilibrium in synchronous motor.
A steady state operation of synchronous motor is a condition of equilibrium in which the
electromagnetic torque is equal and opposite to load torque. In steady state, rotor runs at
synchronous speed thereby maintaining a constant value of torque angle (δ). If there is a
sudden change in load torque, the equilibrium is disturbed and there is resulting torque which
changes the speed of the motor.

What is Hunting?
Unloaded synchronous machine has zero degree load angle. On increasing the shaft
load gradually load angle will increase. Let us consider that load P1 is applied suddenly to
unloaded machine shaft so machine will slow down momentarily.

Also load angle (δ) increases from zero degree and becomes δ 1. During the first swing
electrical power developed is equal to mechanical load P 1. Equilibrium is not established so
rotor swings further. Load angle exceeds δ 1 and becomes δ2. Now electrical power generated
is greater than the previous one. Rotor attains synchronous speed. But it does not stay in
synchronous speed and it will continue to increase beyond synchronous speed. As a result of
rotor acceleration above synchronous speed the load angle decreases. So once again no
equilibrium is attained. Thus rotor swings or oscillates about new equilibrium position. This
phenomenon is known as hunting or phase swinging. Hunting occurs not only in synchronous
motors but also in synchronous generators upon abrupt change in load.
Causes of Hunting in Synchronous Motor
1. Sudden change in load.
2. Sudden change in field current.
3. A load containing harmonic torque.
4. Fault in supply system.
Effects of Hunting in Synchronous Motor
1. It may lead to loss of synchronism.
2. Produces mechanical stresses in the rotor shaft.
 3. Increases machine losses and cause temperature rise.
4. Cause greater surges in current and power flow.
5. It increases possibility of resonance.
Reduction of Hunting in Synchronous Motor
Two techniques should be used to reduce hunting. These are –

 Use of Damper Winding: It consists of low electrical resistance copper / aluminum

brush embedded in slots of pole faces in salient pole machine. Damper winding
damps out hunting by producing torque opposite to slip of rotor. The magnitude of
damping torque is proportional to the slip speed.
 Use of Flywheels: The prime mover is provided with a large and heavy flywheel. This
increases the inertia of prime mover and helps in maintaining the rotor speed constant.
 Designing synchronous machine with suitable synchronizing power coefficients.

ADDITIONAL DETAILS

Why Synchronous Motor is not self-starting?

Consider the rotating magnetic field is equivalent to the physical rotation of two stator
poles N1 and S1.
Consider an instant when two poles are at such a position where the stator magnetic axis
is vertical, along A-B as shown in the below figure(a). At this instant, rotor poles are
arbitrarily positioned as shown in the below figure.
At this instant, the rotor is stationary and unlike poles will try to attract each other. Due
to this rotor will be subjected to an instantaneous torque in the anti-clockwise direction as in
figure(a).

(a) Action of Synchronous Motor (b) Action of Synchronous Motor

 Now stator poles are rotating very fast i.e. at a speed Ns r.p.m. Due to inertia, before
rotor hardly rotates in the direction of anticlockwise torque, to which it is subjected, the stator
poles change their positions. Consider an instant half a period latter where stator poles are
exactly reversed but due to inertia rotor is unable to rotate from its initial position. This is
shown in figure(b).
At this instant, due to the unlike poles trying to attract each other, the rotor will be
subjected to torque in the clockwise direction. This will tend to rotate the rotor in the
direction rotating magnetic field. But before this happens, stator poles again change their
positions reversing the direction of the torque exerted on the rotor.

Starting a Synchronous Motor Using an Induction Motor

We need to bring the rotor of the synchronous motor to synchronous speed before we
switch on the motor. For that reason, we directly couple a small induction motor (pony
motor) with the synchronous motor. Note here, that the number of poles of the induction
motor should be less than the synchronous motor else it will never be able to achieve the
synchronous speed of the synchronous motor. This is because an induction motor always has
a speed less than the synchronous speed and for it to become equal to the synchronous speed
of the synchronous motor, its own speed has to be increased. After the rotor of the
synchronous motor is brought to the synchronous speed, we switch on the DC supply to the
rotor. After that, we simply de-couple the induction motor from the synchronous motor shaft.
Starting a Synchronous Motor Using a DC 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.
Starting a Synchronous Motor Using Damper Windings
In this method, the motor is first started as an induction motor and then starts running as a
synchronous motor after achieving synchronous speed. For this, damper windings are used.
Damper windings are additional windings consisting of copper bars placed in the slots in the
pole faces. The ends of the copper bars are short-circuited. These windings behave as the
rotor of an induction motor. When 3 phase power is supplied to the motor, the motor starts
running as an induction motor at a speed below synchronous speed. After some time DC
supply is given to the rotor. The motor gets pulled into synchronism after some instant and
starts running as a synchronous motor. When the motor reaches synchronous speed, there is
no induced emf in the damper windings anymore and hence they don’t have any effect now on
the working of the motor. This is the most commonly used technique for starting synchronous
motors.
Starting a Synchronous Motor Using Slip Ring Induction Motor
Here we connect one external rheostat in series with the rotor. The motor is first started as a
slip ring induction motor. The resistance is gradually cut-off as the motor gains speed. When
it achieves near synchronous speed, DC excitation is given to the rotor, and it is pulled into
synchronism. Then it starts rotating as a synchronous motor.

Functions of Damper Windings:

 Damper windings helps the synchronous motor to start on its own (self starting
machine) by providing starting torque
 By providing damper windings in the rotor of synchronous motor “Hunting of
machine”can be suppressed.When there is change in load, excitation or change in
other conditions of the systems rotor of the synchronous motor will oscillate to and
fro about an equilibrium position. At times these oscillations becomes more violent
and resulting in loss of synchronism of the motor and comes to halt