synchronous motors

Synchronous Motor.

It is evident that a d.c. machine can work as a d.c. motor as well as a d.c. generator. If the machine is supplied with d.c., then it will work as a d.c. motor. On the other hand, if the machine is supplied with mechanical power, then it will work as a d.c. generator. It means that there is no practical difference in the construction of a d.c. motor and a d.c. generator. Similarly, a synchronous motor is an alternator which can run as a motor when supplied with A.C. If two alternators are connected in parallel and the prime mover of one of them is stopped, then it will start to work as a synchronous motor. Now it can supply the mechanical power to the load connected to it The construction of a synchronous motor is the same as that of an alternator, but there is a difference in their speeds. A high speed 2-pole motor is always of a salient pole type, whereas, the alternator may be of salient pole type or without salient pole type. A synchronous motor is not a self-starting motor, because the rotor does not gain the synchronous speed as the rotating magnetic field of the stator does. Let a stator to have a rotating magnetic field whose speed is equal to the synchronous speed, and the rotor to have definite polarities since it is supplied with D.C.

When N-S poles of the sine or are in front of the N-S poles of the rotor, then the stator field applies a drag on the rotor, in the direction of its field rotation, as shown in Fig. 17.1(a) and (b). But, almost at the same moment, the polarities of the stator field are changed from N-S to S-N, as shown if Fig. 17.1 (c). Thus the rotor is attracted back, because of opposite polarities of the stator and rotor. In this way, x the rotor, remains under attraction and repulsion 2o alternately but it does not rotate. Of course, it seems to be vibrating to and fro. Now, if the rotor is rotated at a synchronous speed by some external means, then it will set into continuous rotation at the synchronous speed. In case, the rotor speed is reduced below the synchronous speed or a difference has developed between the stator and rotor speeds, then the motor will stop and its torque will become zero.

Different Position Of Startor And Rotor Polarities

Construction and Working of Synchronous Motors.

(a) Construction- The construction of a synchronous motor is similar to that of an alternator. It has a stator, a rotor and an exciter. The stator is wound with 3-phase winding, which produces a rotating magnetic field when supplied with 3-phase A.C. The number of stator poles is kept in accordance with the synchronous speed: P=120*f /N The exciter is a d.c. shunt or compound generator, which is coupled to the shaft of the motor. It generates 110 to 250 volts D.C., which is supplied to the rotor winding through two slip rings mounted on the rotor shaft.

Connections Of Synchronous Motor

(b) Working Principle- When the rotor, is supplied with 3-phase A.C., it produces a rotating magnetic field having a synchronous speed. The rotor is excited by D.C. supply. The rotating field acts on the rotor and tries to drag it. Rotor takes a little time to move, but by the same time the stator polarities are changed, thus the rotor is attracted back and it does not move. If by any external means, the rotor is brought to the synchronous speed, then the flux of both the rotor and stator will become magnetically locked, and the rotor will continue to rotate at the synchronous speed. See Fig. 17.2. In this way, a synchronous motor runs at a constant speed for a constant supply frequency. Fig. 17.3 shows the magnetic interlocking of the rotor and stator poles. If any difference between the rotor speed and synchronous speed is raised, then the, average torque will become zero and the motor will stop.

Magnetic Interlocking Of The Stator And Rotor Pole

Effect of Load Variation on Synchronous Motor. A synchronous motor provides a constant speed to the load, its speed remains constant at load and at no-load conditions. Fig. 17.4. shows the curve of e.m.f. induced in a conductor of the stator due to magnetic poles N-S of the rotor. If the flux disturbance is negligible, then the e.m.f. induced in the conductor 'a' will be maximum while the conductor is in the centre of a pole. When the conductor 'a' will be in midway position between the two poles, then the e.m.f. induced in it will be zero, as shown by the curve 'e'

Induced e.m.f. In The Rotor Conductor

When a load is, coupled to the rotor shaft, then the rotor speed will reduce slightly for a moment, but it will immediately be compensated by itself and will regain its previous value. In this condition, 'e' represents the e.m.f. induced and its magnitude will become equal to 'y'. Thus, the curve of induced e.m.f. at load will lag behind the curve of induced e.m.f. at no-load by an angle a, as shown in Fig. Hence, the power available at the rotor pulley will be some-what lesser than P due to frictional and rotational core losses. The difference of P and P will be equal to the armature copper loss. In this way, the speed of a synchronous motor remains the same at load and no-load, but up to some extent. If the motor is over-loaded beyond the limit, then its speed will reduce, the torque will become zero and the motor will stop.

Damper Winding.

In addition to the field excitation winding, one more winding is wound on the rotor of a synchronous motor, which is known as damper winding. This winding is a short-circuited cage of copper conductors made on the rotor, exactly similar to the squirrel cage winding (Fig. 17.7). However, the damper winding may be wound in the same manner as it is wound on a slip-ring induction rotor. In the later case, the winding is connected to an external rheostat through three slip-rings. In this way, the rotor will have five slip rings-two for field excitation and 3 for damper winding. On starting a synchronous motor, a rotating magnetic field is produced in the stator. This field induces an e.m.f. in the cage winding, which sets up a flow of current in it. As a result, a torque is produced and the rotor begins to rotate like an induction rotor. When the rotor gains the synchronous speed, the induced e.m.f. in the damper winding falls to zero, since it is not cutting the magnetic flux any more.

Squirrel Cage Type Damper Winding

The purpose of the damper winding is to eliminate the hunting defect. If a motor is working at a normal load and suddenly the load is increased, then the torque will reduce and it will reduce the motor speed. The reduction in speed will cause an increase in the phase angle between the induced back e.m.f. and the terminal voltage of the stator winding. Thus, the winding will draw more current from the a.c. source and will produce a small magnetic angular drift in the stator with respect to rotor poles. This small drift will produce a large change in the energy component of the current. As a result, the rotor speed will start to oscillate to and for with respect to its mean speed. "The oscillating defect of a synchronous motor is called the hunting effect or phase swinging'. This effect produces a sound in the motor. As the rotor starts to oscillate, due to hunting effect, the speed of the rotor is changed. The change in speed results in an e.m.f. to be induced in the damper winding, due to which a torque is developed in the rotor. This torque opposes the change in rotor speed and brings the rotor back to its normal speed. In this way, the hunting effect is eliminated and the motor continues to run at its normal speed, irrespective of load fluctuations.

 Advantages, Disadvantages and Uses of Synchronous Motor

(A) Advantages

(1) It is capable to operate at different power factors.

(2) Its speed remains almost constant even at varying loads.

 (3) It is least affected by the effective voltage variations.

(B) Disadvantages

(1) It has a very low starting torque.

(2) Its speed can not be changed.

 (3) It requires A.C. and D.C. both.

 (4) It has hunting defect.

(C) Uses

 (1) It is used to operate a motor-generator set at a constant speed.

(2) It is used in frequency changing equipment’s to operate the prime mover.

(3) It is used to increase the power factor and to regulate the terminal voltage of transmission lines. (4) It is used to operate compressors, pumps, line-shaft etc.

 Methods of Starting Synchronous Motor. A Synchronous motor is not a self-starting motor, therefore it requires a prime mover to rotate its rotor. Following two main methods are used for this purpose:

(1) Prime mover method

(2) Damper winding method

(1). Prime Mover Method- In this method the rotor of the synchronous motor is mechanically coupled to a small a.c. motor, which is called a pony motor. As the power circuit of the pony motor is switched on, it starts to rotate the rotor of the synchronous motor. The pony motor is usually a high-speed motor. When the synchronous motor gains the synchronous speed, the A.C. to the stator and the D.C. to the motor is applied, and the pony motor is switched off. If the d.c. supply is available, then a d.c. machine may be utilised as a motor to rotate the rotor of the Synchronous motor in place of a pony motor. When the synchronous motor gains the synchronous speed, the d.c. supply is switched off. Now, the d.c. machine is utilised to work as a d.c. generator, and its, output may be applied to the synchronous rotor for field excitation. Before switching on A.C. supply to the stator of the synchronous motor, the induced e.m.f. in the stator windings is synchronised with the bus-bar voltage. And then the A.C. is supplied to the stator while the pony motor is withdrawn. A synchronous motor may also be started with a diesel or petrol engine.

(2). Damper Winding Method- In this method the rotor of the synchronous motor is wound with a squirrel cage type or slip-ring induction type winding. The motor is started either as squirrel cage or slip-ring induction motor. When the rotor of the synchronous motor gains the synchronous speed, then the rotor gets magnetically locked with the stator field poles, and the motor runs as a synchronous motor. After it the same cage winding can be used to eliminate the hunting effect.

Comparison between synchronous and induction motors

Synchronous Motor	Induction Motor
Advantages: It improves the power factor. Its speed remains almost constant atremains load. Disadvantages: It is not a self-starting motor. It requires d.c. excitation. It has a high starting torque. It has a low starting The motor is costly.	It reduces the power factor. factor. Its speed reduces with an constant at varying load. It is a self-starting motor. increase in the load. Motor. It does not require d.c. excitation. It has hunting defect. torque. It does not have hunting defect. The motor is cheap.

Specialties of Synchronous Motor over Induction Motor

(1) Its power factor does not reduce below rated value.

(2) It has-low losses.

(3) It has a greater efficiency.

(4) Its speed remains almost constant even at higher loads.

(5) The power factor of a distribution bus-bar can be increased with its use. The main advantage of a synchronous motor lies with its ability of increasing the power factor of an electrical installation. In factories having a large number of induction motors. The power factor becomes low and the current starts to lag, and thus it causes a reduction in the efficiency of the motors. In order to improve the power factor, a synchronous motor is connected is parallel to the bus-bar, and it is run without load at full excitation. In this way, the power factor becomes unity and the efficiency of the motors is increased.

Construction and Working-Principle of Auto- Synchronous Motor. An ordinary synchronous motor has a constant speed. But, due to its low starting torque, it does not start at load. In order to make it self-starting, it is modified in the form of auto-synchronous motor or synchronous induction motor. These are of following two types:

1, Induction type auto-synchronous motor

2. Salient type auto-synchronous motor.

1.Induction Type Auto-synchronous Motor- It is also known as auto-synchronous induction motor. In the start, it works as an induction motor and after gaining its full speed it is converted into a synchronous motor. Its rotor has a slip-ring rotor type winding, which is connected to an external rheostat. The air gap in the magnetic circuit is larger than that of an induction motor. It has a less number of rotor slots having more length. Three slip-rings are mounted on the shaft of the rotor, which are connected to an external rheostat, as shown in Fig. 17.8. A change-over switch is connected in such a way that the three rotor windings from a series circuit, consisting two windings in patrolled and the third one in series,

Connection Of Auto-Synchronous Motor

Initially, the rotor windings are connected in star. Therefore, in the start the motor operates as an induction motor and produces sufficient torque. As the motor attains its full speed, the change-over switch is operated. It switches off A.C. supply, changes the rotor winding connections and supplies D.C. to the rotor winding for field excitation. And now the rotor works as a synchronous motor. The current distribution is shown in 

Rotor Winding Connection For D.C. Excitation

One winding carries I, current while the other two carries will not affect the motor, since the conductors of each winding are distributed over the whole periphery of the rotor. Rotor winding connection for d.c. excitation In low and medium output machines, the change-over switch is omitted and the rotor windings are directly connected to d.c. source. The winding consists of two parts, one part works as induction field while the other as exciter field as shown in In such machines, the exciter is also coupled mechanically to the rotor shaft, so that it generates D.C without any external prime mover.

Auto-Synchronous Motor With Exciter

 As the rotor speeds up due to the action of induction field of the rotor a voltage is induced in the exciter field winding. As the rotor attains the synchronous speed, the function of the induction field winding of the rotor is finished and the whole rotor winding works as exciter field winding. In low voltage machines, the rotor consists of a 3-phase winding which is connected to 3-phase supply through 3 slip-rings and a starting resistance or a starter. The stator consists of 3-phase windings and a single exciter field winding. Two phase windings are directly connected to rheostats, while the third one through an exciter field winding, as shown in When the rotor is supplied with 3-phase A.C., it starts to rotate like a slip-ring type rotor, but it does not generate exciter voltage. As the motor speeds up, an exciter voltage is build up in the exciter field winding wound on the stator. At synchronous speed, full excitation voltage will be available at the stator and the motor will continue to run in synchronization. 

 Synchronous Induction Motor

2. Salient type Synchronous Motor-The rotor of this type of motor has projected poles. A squirrel cage type of slip-rings type winding is wound on these poles. The rotor has three slip rings, out of which one Is meant for exciter field and the other two for induction lield. The motor is started with a small load and a small voltage applied at the stator. Due to large air gap between the rotor poles, the machine has a strong field system. It is capable to supply a wattless leading KVA without becoming unstable. Hence, it may be used as a phase advance.

Advantages of Synchronous Induction and Salient Pole Synchronous Motor

(A) Advantages of Synchronous Induction (auto synchronous) Motor

1. It has a high-starting torque.
2. Its magnetic circuit has small air gap.
3. It has a constant speed.
4. It is capable to improve the power factor of an electrical installation upto unity.
5. It is used to drive heavy loads of cement, rolling. cotton and paper mills etc.

 (B) Advantages of Salient Pole Synchronous Motor

1. It has a constant speed.
2. Its load torque is steady.
3. It can be started without any load.
4. Separate damper winding is used in it.
5. It is used as a phase advancer or power factor improver.

 Faults in Synchronous Motors

• Fault- A motor fails to start. Causes
• The motor is overloaded.
• The field exciter of the motor is not working.
• The supply voltage is low.

Fault- A motor gets heated up. Causes

• The motor is overloaded.
• The armature coils have short-circuit defect.
• The field poles are getting low D.C. voltage.
• The field windings have short-circuit defect.
• The supply voltage is low,
• The bearing have worn.