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Synchronous Motors

As we know that, a D.C generator can be run as D.C motor. In the like manner, a 3 phase alternator may operate as a motor by connecting its armature winding to a 3 phase supply. It is then called a 3 phase synchronous motor. As the name implies, a synchronous motor runs at synchronous speed (Ns = 120 f/P) i.e in synchronism with the revolving field produced by the 3 phase supply. The speed of rotation is, therefore, tied to the frequency of the source. Since the frequency is fixed, the motor speed remains constant (synchronous speed) at all loads provided the load on the motor does not exceed the limiting value, the motor simply comes to rest and the average torque developed by it is zero. For this reason, a Synchronous motor is not inherently self-starting. Therefore, in order to start a synchronous motor, it is brought up almost to its synchronous speed by some auxiliary means before it is synchronized to the supply.

Construction Of Synchronous Motor

A synchronous motor is a machine that operates at synchronous speed and converts electrical energy into mechanical energy. It is fundamentally an alternator operated as a motor. Like an alternator, a synchronous motor has the following two parts:

  1. Stator: Which houses a 3 phase armature winding in the slots of the stator core and receives power from a 3 phase supply.
  2. Rotor: That has a set of salient poles excited by direct current to form alternate N and S poles. The exciting coils are connected in series to two slip rings and direct current is fed in to the winding from an external exciter mounted on the rotor shaft.

The stator is wound for the same number of poles as the rotor poles. As in the case of an induction motor, the number of poles determines the synchronous speed of the motor.

Synchronous Speed, Ns = 120f/P

Where f = frequency of supply in Hz 

P = Number of poles 

An important drawback of a synchronous motor is that it is not self starting and auxiliary means have to be used for starting it.

Operating Principle

The fact that a synchronous motor has no starting torque can be easily explained.

  1. Consider a 3 phase synchronous motor having two rotor poles NR and SR. Then the stator will also be wound for two poles Ns and Ss. The motor has direct voltage applied to the rotor winding and a 3 phase voltage applied to the stator at synchronous speed . The direct (zero frequency) current sets up a two pole field field which is stationary so long as the rotor is not turning. Thus, we have a situation in which there exists a pair of revolving armature poles (Ns – Ss) and a pair of stationary rotor poles (NR – SR).  synchronous-motor-operating-principle
  2. Suppose at any instant, the stator poles are at positions A and B as shown above. It is clear that poles NS and NR repel each other and so do the poles SS and SR. Therefore, the rotor tends to move in the anticlockwise direction. After a period of half cycle, the polarities of the stator poles are reversed but the polarities of the rotor poles remain the same. Now Ss and NR attract each other and so do NS and SR. Therefore, the rotor tends to move in the clockwise direction. Since the stator poles change their polarities rapidly, they tend to pull the rotor first in one direction and then after a period of half cycle in the other. Due to high inertia of the rotor, the motor fails to start.

Hence, a synchronous motor has no self starting torque i.e. a synchronous motor cannot start by itself.

How to get continuous unidirectional torque? If the rotor poles are rotated by some external means at such a speed that they interchange their positions along with the stator poles, then the rotor will experience a continuous unidirectional torque. This can be understood from the following discussion:

  1. Suppose the stator field is rotating in the clockwise direction and the rotor is also rotated clockwise by some external means at such a speed that the rotor interchange their positions along with the stator poles.
  2. Suppose at any instant, the stator and rotor poles are in the position as shown below.
  1. It is clear that torque on the rotor will be clockwise. After a period of half cycle, the stator poles reverse their polarities and at the same time rotor poles also interchange their positions.
  2. The result is that again the torque on the rotor is clockwise, Hence, a continuous unidirectional torque acts on the rotor and moves it in the clockwise direction. Under this condition, poles on the rotor always face poles of opposite polarity on the stator and a strong magnetic attraction is set up between them. This mutual attraction locks the rotor and stator together and the rotor is virtually pulled in to step of revolving flux. synchronous-motor-unidirectional-torque
  3. If now the external prime mover driving the rotor is removed, the rotor will continue to rotate at a synchronous speed in the clockwise direction because the rotor poles are magnetically locked up with the stator poles. It is due to this magnetic interlocking between stator and rotor poles that a synchronous motor runs at the speed of revolving flux.

Making Synchronous Motor Self Starting

A synchronous motor cannot start by itself. In order to make the motor self starting, a squirrel cage winding (damper winding) is provided on the rotor. The damper winding consists of copper bars embedded in the poles faces of the salient poles of the rotor. The bars are short circuited at the ends to form in effect a partial squirrel cage winding. The damper winding serves to start the motor.

  1. To start with 3 phase supply is given to the stator winding while the rotor field winding is left un-energized. The rotating stator field induces currents in the damper or squirrel cage winding and the motor starts as an induction motor.
  2. As the motor approaches the synchronous speed, the rotor is excited with direct current. Now the resulting poles on the rotor face poles of opposite polarity on the stator and a strong magnetic attraction is set up between them. The rotor poles lock in with the poles of rotating flux. Consequently, the rotor revolves at the speed as the stator field i.e. synchronous speed.
  3. Because the bars pf squirrel cage portion of the rotor now rotate at the same speed as the rotating stator field, these bars do not cut any flux and, therefore, have no induced currents in them. Hence squirrel cage portion of the rotor is, in effect, removed from the operation of the motor.

It may be emphasized here that due to magnetic interlocking between the stator and rotor poles, a synchronous motor can only run at synchronous speed. At any other speed, this magnetic interlocking ceases and the average torque becomes zero. Consequently, the motor comes to a halt with a severe disturbance on the line.

Equivalent Circuit Of 3 Phase Synchronous Motor

Unlike the induction motor, the synchronous motor is connected to two electrical systems, a D.C source at the rotor terminals and an A.C system at the stator terminals.

  1. Under normal conditions of synchronous motor operation, no voltage is induced in the rotor by the stator field because the rotor winding is rotating at the same speed as the stator field. Only the impressed direct current is present in the rotor winding and ohmic resistance of this winding is the only opposition to it. synchronous motor equivalent circuit for 3 phase
  2. In the stator winding, two effects are to be considered, the effect of stator field on the stator winding and the effect of the rotor field cutting the stator conductors at synchronous speed.
  3. The effect of stator field on the stator conductors is accounted for by including an inductive reactance in the armature winding. This is called synchronous reactance Xs. A resistance Ra must be considered to be in series with this reactance to account for the copper losses in the stator or armature windings. This resistance combines with synchronous reactance and gives the synchronous impedance of the machine.
  4. The second effect is that a voltage is generated in the stator winding by the synchronously revolving field of the rotor. This generated e.m.f depends upon rotor speed and rotor flux Φ per pole. Since rotor speed is constant, the value of e.m.f depends upon the rotor flux per pole i.e. exciting rotor current If.

Synchronous Motor On Load

In D.C motors and induction motors, an addition of load causes the motor speed to decrease. The decrease in speed reduces the counter e.m.f enough so that additional current is drawn from the source to carry the increased load at a reduced speed. This action cannot take place in a synchronous motor because it runs at a constant speed at all loads.

What happens when we apply mechanical load to a synchronous motor? The rotor polls fall slightly behind the stator poles, while continuing to run at synchronous speed. The angular displacement between stator and rotor poles (called torque angle δ) causes the phase of back e.m.f Eb to change w.r.t supply voltage V. This increases the net e.m.f Er in the stator winding. Consequently, stator current Ia ( =Er/Zs) increases to carry the load.

The following points may be noted in synchronous motor operation:

  1. A synchronous motor runs at synchronous speed at all loads. It meets the increased load not by a decrease in speed but by the relative shift between stator and rotor poles i.e. by the adjustment of torque angle δ.
  2. If the load on the motor increases, the torque angle δ also increases but the motor continues to run at synchronous speed. The increase in torque angle δ causes a greater phase shift of back e.m.f Eb w.r.t supply voltage V. This increases the net voltage Er in the stator winding. Consequently, armature current ( = Er/Zs) increases to meet the load demand.
  3. If the load on the motor decreases, the torque angle δ also decreases. This causes a smaller phase shift of Eb w.r.t V. Consequently, the net voltage Er in the stator winding decreases and so does the armature current Ia ( = Er/Zs).

Pull Out Torque

There is a limit to the mechanical load that can be applied to a synchronous motor. As the load increases, 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 a standstill. This load torque at which the motor pulls out of synchronism is called pull out or breakdown torque. Its value varies from 1.5 to 3.5 times the full load torque.

Power Factor Of  Synchronous Motor

In an induction motor, only one winding i.e. stator winding produces the necessary flux in the machine,. The stator winding must draw reactive power from the supply to set up the flux. Consequently, induction motor must operate at lagging power factor.

But in a synchronous motor, there are two possible sources of excitation, alternating current in the stator or direct current in the rotor. The required flux may be produced either by stator or rotor or both.

  1.  If the rotor exciting current is of such magnitude that it produces all the required flux, then no magnetizing current or reactive power is needed in the stator. As a result, the motor will operate at unity power factor.
  2. If the rotor exciting current is less i.e. motor is under excited, the deficit in flux is made up by the stator. Consequently, the motor draws reactive power to provide for the remaining flux. Hence motor will operate at a lagging power factor.
  3. If the rotor exciting current is greater i.e. motor is over excited, the excess flux must be counter balanced in the stator. Now the stator, instead of absorbing reactive power, actually delivers reactive power to the 3 phase line. The motor then behaves like a source of reactive power, as if it were a capacitor. In other words, the motor operates at a leading power factor.

To sum up, “a synchronous motor absorbs reactive power when it is under excited and delivers reactive power to source when it is over excited.

Synchronous Capacitor

“An over excited synchronous motor running on no load is known as synchronous capacitor.”

We have seen that a synchronous motor takes a leading current when over excited and therefore, behaves as a capacitor. When such a machine is connected in parallel with induction motors or other devices that operate at lagging power factor, the leading kVAR supplied by the synchronous capacitor partly neutralizes the lagging reactive kVAR of the loads. Consequently, power factor of the system is improved.

Synchronous capacitors i.e. unloaded over excited synchronous motors are installed in power systems solely for power factor improvement. They are economical in large sizes than the static capacitors.

Applications Of Synchronous Motors

  1. Synchronous motors are particularly attractive foe low speeds because the power factor can always be adjusted to unity and efficiency is high.
  2. Over excited synchronous motor can be used to improve the power factor of a plant while carrying their rated loads.
  3. They are used to improve the voltage regulation of transmission lines.
  4. High power electronic converters generating very low frequencies enable us to run synchronous motors at ultra low speeds.Thus huge motors in the 10 MW range drive crushes, rotary kilns and variable speed ball mills.

Comparison Of Synchronous And Induction Motors

ParticularSynchronous MotorInduction Motor
SpeedRemains constant from no load to full loadDecreases with load
Power factorCan be made to operate from lagging to leading power factorOperates at lagging power factor
ExcitationRequires D.C excitation at the rotorNo excitation for the rotor
EconomyEconomical for speeds below 300 r.p.mEconomical for speeds above 600 r.p.m
Self startingNo self starting torque. Auxiliary means have to provided for startingSelf starting
ConstructionComplicatedSimple
Starting torqueMoreLess