1. Synchronous motors are generally not self-starting because
A. the direction of rotation is not fixed
B. the direction of instantaneous torque reverses after half cycle
C. starters cannot be used on these machines
D. starting winding is not provided on the machines
Answer: B. the direction of instantaneous torque reverses after half cycle
Explanation:
- 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 unidirectional.
- 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.
- The direction of instantaneous torque on the rotor reverses after half cycle.
- 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.
Important Notes:
By using the following four methods we can start a synchronous motor.
Using Pony Motors:
- The pony motor drives the main motor up to near synchronous speed. The motor then needs to be synchronized to the supply. The DC excitation is then switched on, the rotor field and stator fields than "lock-in" and the pony motor is disconnected. The main motor then runs at synchronous speed.
Using Damper Winding:
- When the 3 phase supply is switched on, the motor runs up as a normal squirrel cage induction motor.
Using Small D.C machine coupled to it:
- A small DC machine is coupled with the synchronous motor. The DC machine is made to act as a DC motor so that the synchronous motor is started.
Why synchronous motor is not self starting ?
- At the start the rotor is stationary and take a positive cycle of AC supply, N pole of the stator (Ns) is opposite to N pole of the rotor (Nr)
- The repulsion on the rotor will move clockwise or counterclockwise
- Now Take negative half cycle N pole of stator will become S pole
- Now S pole of the stator (Ss) will attract N pole of the rotor (Nr)
- Because of attraction, during negative cycle and repulsion during the positive cycle, the resultant rotation is zero
- So resultant torque is zero
- So synchronous motor is not self-starting
2. In case one phase of a three-phase synchronous motor is short-circuited the motor will
A. not start
B. run at 2/3 of synchronous speed
C. run with excessive vibrations
D. take less than the rated load
Answer: A. not start
Explanation:
Single phasing in synchronous motor:
- Single phasing in a synchronous motor means that one of the three phases of the supply has been cut off due to any reason, and one phase fuse blown or removed.
- The motor, when already running, keeps on running as a single-phase motor making a characteristic noise.
- This is a single phasing condition because the current in both the remaining lines now is the same single current.
- In order to compensate for the lost current, the motor starts drawing more amount of current heating the motor even in some cases could burn the motor.
Important points:
- Single phasing in a three-phase machine means that one of the three phases of the supply has been cut off due to any reason, one phase fuse blown or removed, or disconnection somewhere in one phase.
- The motor, when already running, keeps on running as a single-phase motor making a characteristic noise. This is a single phasing condition because the current in both the remaining lines now is the same single current.
- In star connected stator, the load 1s taken by the remaining two phases so the motor now can take a load of 1/√3 times, (57.7%) its rating.
- Since now in single phasing operation, for the same current and voltage P = VI cosφ; while in 3-phase operation, P = √3VIcosφ.
- So if the motor is operating at a load near full load, the active phases will be overloaded.
- If the stator is delta connected which is usually the case, the current distribution in different phases is not the same. One of the windings gets overloaded even when the current in the active lines is normal.
- In a case when any of the three phases fails, in order to compensate the motor, starts drawing more amount of current heating the motor even in some cases could burn the motor.
- In 3-φ synchronous motor, if one of the winding is short circuit then the motor will run with excessive vibrations.
More clearly it depends upon the load:
- Less than 1/3rd of the rated load, it will continue to operate without any harm.
- Higher than 1/3rd of rated load, the motor will continue to operate, but will draw current more than its rated value. The motor temperature will start increasing.
- Higher load (near to the rated load), the motor speed will drop gradually to zero.
3. A pony motor is basically a
A. small induction motor
B. D.C. series motor
C. D.C. shunt motor
D. double winding A.C./D.C. motor
Answer: A. small induction motor
Explanation:
Pony Motor:
- A pony motor is basically a small induction motor.
- The pony motor drives the main motor up to near synchronous speed. The motor then needs to be synchronized to the supply.
- The DC excitation is then switched on, the rotor field and stator fields than "lock-in" and the pony motor is disconnected.
- The main motor then runs at synchronous speed.
4. A synchronous motor can develop synchronous torque
A. when under loaded
B. while over-excited
C. only at synchronous speed
D. below or above synchronous speed
Answer: C. only at synchronous speed
Explanation:
- Synchronous motors are inherently not able to self-start on an AC power source with the utility frequency of 50 or 60 Hz. his is because synchronous motors can develop a torque only when running at the synchronous speed.
- Synchronous motors are designed to run at synchronous speed, and they develop synchronous torque at this speed only
- When a synchronous motor has achieved synchronous speed and a load is applied, the rotor will momentarily lag the rotating stator field (but still run at synchronous speed)
- As the rotor briefly slows, the DC field in the rotor makes a torque angle with respect to the rotating stator field and the motor develops more torque at the shaft and accelerating the rotor
5. A synchronous motor can be started by
A. pony motor
B. D.C. compound motor
C. providing damper winding
D. any of the above
Answer: D. any of the above
Explanation:
By using the following four methods we can start a synchronous motor.
Using Pony Motors:
- The pony motor drives the main motor up to near synchronous speed. The motor then needs to be synchronized to the supply. The DC excitation is then switched on, the rotor field and stator fields than "lock-in" and the pony motor is disconnected. The main motor then runs at synchronous speed.
Using Damper Winding:
- When the 3 phase supply is switched on, the motor runs up as a normal squirrel cage induction motor.
Using Small D.C machine coupled to it:
- A small DC machine is coupled with the synchronous motor. The DC machine is made to act as a DC motor so that the synchronous motor is started.
6. A three-phase synchronous motor will have
A. no slip-rings
B. one slip-ring
C. two slip-rings
D. three slip-rings
Answer: C. two slip-rings
Explanation:
- A three-phase synchronous motor basically consists of a stator core with a three-phase winding (similar to an induction motor), a revolving DC field with an auxiliary or armature winding and two slip rings, brushes, and brush holders, and two end shields housing the bearings that support the rotor shaft.
- An armature winding consists of copper bars embedded in the cores of the poles. The copper bars of this special type of “squirrel-cage winding” are welded to end rings on each side of the rotor. The function of a slip ring is to transfer electrical signals from rotary to stationary components or systems
- both the stator winding and the core of a synchronous motor are similar to those of the three-phase, squirrel-cage induction motor, and the wound-rotor induction motor.
- The rotor of the synchronous motor has salient field poles. The field coils are connected in series for alternate polarity. The number of rotor field poles must equal the number of stator field poles. The field circuit leads are brought out to two slip rings mounted on the rotor shaft for brush-type motors.
- Carbon brushes mounted in brush holders make contact with the two slip rings. The terminals of the field circuit are brought out from the brush holders to a second terminal box mounted on the frame of the motor. A squirrel-cage, or armature, the winding is provided for starting because the synchronous motor is not self-starting without this feature.
Important Point:
- A synchronous motor is called a doubly excited machine because both its rotor and stator are excited.
- To achieve magnetic locking between stator and rotor, double excitation needed.
- The stator of 3 phase synchronous motor is excited with a 3-phase supply while rotor is with DC Supply.
- That's why 3 phase synchronous motor needs 3 phase AC supply as well as DC supply.
- In 3-phase synchronous machine, excitation is given on rotor. Excitation in the synchronous machine is in DC. As rotor is a rotating part, in order to excite it with DC, we must need slip rings
- The slip ring is connected to rotor supply which is low voltage DC supply. Hence slip ring is insulated for low voltage.
7. Under which of the following conditions hunting of synchronous motor is likely to occur ?
A. Periodic variation of load
B. Over-excitation
C. Over-loading for long periods
D. Small and constant load
Answer: A. Periodic variation of load
Explanation:
Hunting is the phenomenon that occurs in synchronous motors due to varying load or supply frequency.
Causes of hunting
- Periodic variation of load
- Sudden changes in load
- Faults occurring in the system when supplied by the generator
- Sudden change in the field current
- Cyclic variations of the load torque
Effects of hunting
- It can lead to loss of synchronism
- It can cause variations of the supply voltage producing undesirable lamp flicker
- The possibility of resonance condition increases. If the frequency of the torque component becomes equal to that of the transient oscillations of the synchronous machine, resonance may take place
- Large mechanical stresses may develop in the rotor shaft
- The machine losses increases and the temperature of the machine rises
Reduction of Hunting
- Use of damper windings
- Use of flywheels
8. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected
A. the motor stops
B. it runs as a reluctance motor at the same speed
C. it runs as a reluctance motor at a lower speed
D. none of the above
Answer: B. it runs as a reluctance motor at the same speed
Explanation:
- Total power in salient pole synchronous motor is equal to the sum of air gap power and reluctance power.
- This reluctance power is independent of excitation.
- So even if the field gets disconnected, the reluctance power is developed due to the variable reluctance offered by the salient poles which are used to run the synchronous motor at synchronous speed.
Important Notes:
- When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected the motor run as a variable reluctance motor.
- Reluctance torque is the torque generated because the motor is moving to a position where the reluctance seen by the armature flux is declining.
- Reluctance exists only when there is an unsymmetrical air gap.
- Therefore it exists only for Salient pole synchronous machines and does not exist for the cylindrical machine.
- Reluctance torque comes into play when any winding is disconnected i.e. filed winding or armature winding in the running conditions.
- Reluctance torque also produces reluctance power which makes the machine more stable.
- The machine will continue to run at synchronous speed due to reluctance torque (i.e. even when field winding or armature windings are disconnected).
- The rotor will remain in synchronism for a short time ie.. until the residual flux of the rotor decreases below a certain value.
- Variable reluctance motor behaves as similar to silent pole synchronous motor unexcited.
- And after the loss of residual flux, it will run as a reluctance motor due to the generation of reluctance torque.
9. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as
A. V(applied Voltage)
B. V3/4
C. V2
D. 1/V
Answer: A. V(applied voltage)
Explanation:
- Breakdown torque is the maximum torque of the motor, which is produced at full rated voltage and frequency.
- Torque T is directly proportional to P. As power varies as applied voltage V, a torque of a synchronous motor is also varying as applied voltage V.
10. The power developed by a synchronous motor will be maximum when the load angle is
A. zero
B. 45°
C. 90°
D. 120°
Answer: C. 90°
Explanation:
- The maximum power developed in the synchronous motor will depend on rotor excitation (Ef), supply voltage(V), and the maximum value of coupling angle (δ).
- The power developed depends on the excitation, voltage, and coupling angle. The maximum value of θ and hence δ is 90°. An increase in the excitation results in an increase of Pmax. Consequently, the load angle decreases for a given power developed. The overload capacity of the motor increases with an increase in excitation and the machine becomes more stable.
- For all values of V and Eb, this limiting value of δ is the same but maximum torque will be proportional to the maximum power developed.
- the power develop will be maximum when δ is 90°.