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Viva questions of ac motor part 2

Viva questions of ac motor part 2


Q: What are the basic differences between the single cage and double squirrel cage induction motor? 

Ans: In a double squirrel cage rotor, there are two cages one of high resistance other of low resistance. In the case of a single cage rotor, it has only one cage of low resistance winding. 

Q: What is the utility of providing double cases in the rotor? 

Ans: The motor has better starting torque. 

Q: How is the running efficiency of the single cage and double cage rotor motor? 

Ans: Double cage rotor motor has low efficiency than the single cage rotor motor. 

Q: Name the induction motor which has better starting and good efficiency? 

Ans: Slip ring Induction motor. 

Q: Why the slip ring induction motor has better starting torque and good efficiency? 

Ans: Because at the time of starting the external resistance is introduced in the rotor circuit which causes better starting torque and after that, the external resistance is removed and the motor works as the squirrel cage induction motor. 

Read: Viva questions of ac motor part 1

Q: What is a starter? 

Ans: the starter is a safety device that is used for the starting of the motor. 

Q: Name the protective devices used in the starter? 

Ans: NVRC, OLRC, Pushbutton station, etc. 

Q: What are the NVRC and OL.RC? 

Ans: These are no volt release coil and overload release coils; the no volt release coil protects against supply failure and overload release against the overloading. 

Q: What are the different types of OLRC? 


  1. Thermal or bimetallic relay.
  2. Magnetic relay.
  3. Dash pot type magnetic relays. 

Q: What do you understand by the thermal or bimetallic relay? 

Ans: It is made of two different metals. It has two strips of different temperature coefficient substances. As the temperature increases, the strip bends towards the material having less temperature coefficient. This principle is used in bimetallic or thermal relays. 

Read: Viva questions of ac motor part 3

Q: What is a magnetic relay and how it protects the motor? 

Ans: It works on the principle of magnetism. Whenever normal current is flowing, the coil will produce a magnetic flux but it will not attract the arm attached. Whenever current exceeds the limit, the flux produced will be contractor and protect the motor against overloading. 

Q: On what principle the dash pot type relay works?

 Ans: Magnetic effect of current. 

Q: What is the unique characteristic of the dash pot type relay? 

Ans: It is having an arrangement of time lag. 

Q: How the time lag is adjusted? 

Ans: By adjusting either the holes of different diameters attached with the moving or attracting system or by adjusting the distance of the soft iron rod and coil. 

Read: Viva questions of ac motor part 1

Q: What is the function of no volt release coil? What is the resistance of this coil? 

Ans: The no volt release coil protects the motor from supply failure. It has more resistance. It is wound with thin conductor having more number of turns and it is connected across the line.

Q: What type of starters is used for starting the three-phase motor?


  1. Direct on line starter (DOL starter).
  2. Star-delta starter.
  3. Autotransformer starter.
  4. Slip ring induction motor starter.

Q: Which type of starters is preferred for 1 HP, 3 HP, and 10 HP motors? 

Ans: One HP motor even is started without a starter but should be started with a DOL starter. 3HP with DOL starter, 10HP with star-delta starter. 

Q: How many types of contacts are there in a DOL starter? 

Ans: Fixed contacts, moving contacts, and auxiliary contacts. 

Read: Viva questions of ac motor part 3

Q: What is the function of auxiliary contacts? 

Ans: In case when the starter is on, the auxiliary contacts will be in parallel with the stare button and the supply will be continued to the no-volt release coil through these contacts. 

Q: What is the remote controlling? 

Ans: To control the motor from more than one place is known as remote controlling. 

Q: How are all the start and stop buttons connected in remote controlling? 

Ans: all start buttons are connected in parallel and stop buttons in series. 

Q: What is the phase sequence? 

Ans: The systematic arrangement of connections of all phases is known as phase sequence. Let us say in three-phase winding if the phase is starting from θº elect, phase II will start from (θ + 120)° elect. And phase III from (θ + 140)º elect. This arrangement of winding is known as phase sequence. 

Q: What will happen if the phases are not arranged in sequence? 

Ans: Motor will not run properly and operations will not be smooth. The motor will make abnormal sounds and may blow fuses even. 

Q: Why the D.O.L. starter is called so? 

Ans: In this starter, the same supply voltage is fed to the motor that is why it is called a direct on line starter. 

Read: Viva questions of ac motor part 1

Q: What is a star-delta starter? 

Ans: It is the single starter in which the motor is connected in star at the time of starting and in delta in running condition. These can be manual or automatic.

Q: When the motor should be put in delta after starting in star? 

Ans: As soon as the attain 75% of the synchronous speed the motor should be connected in delta.

Q: If the handle is moved slowly to the delta position is it possible and advisable? 

Ans: no, in the star delta it is not possible; there a mechanical lever protection is given so that it should not be delayed. If it is moved slowly the lever will not allow connecting the motor in the delta. Secondly, if the speed drops down because of time lag, the current will increase more than the predetermined value if it is now connected in delta. So that the time limit is there, one should be quick enough to connect the motor into the delta. 

Q: How the changeover is governed in the case of automatic star delta starter? 

Ans: A timer provided that governs the changeover time of delta. 

Q: What type of timers is used? 

Ans: Generally motorized or bimetallic. 

Read: Viva questions of ac motor part 3

Q: Why it is essential to start the motor more than 5 HP with a star-delta starter? 

Ans: To limit the starting current.

Q: How the star-delta starter reduces the starting current? 

Ans: By connecting the motor in the star at the time of starting, the voltage per phase is reduced, so the motor will take less current than if it would have been started in the delta. 

Q: How many times it reduces the starting current than if it would have been connected in the delta?

Ans: About three times. 

Q: What is an autotransformer starter? 

Ans: The starter in which autotransformers are used to reduce the impressed voltage is known as autotransformer starter. 

Q: How it differs from the star-delta starter? 

Ans: In case of star-delta starter, the starting voltage to phase winding is only 12/13 but in an autotransformer depending upon the requirement the voltage can be adjusted. In running condition both are giving full voltage to the motor. 

Q: In which connections the autotransformer windings are connected? 

Ans: In star. 

Q: Can you tell whether the transformer remains energized throughout the operation of the motor or not? 

Ans: No, only at the time of starting and de-energized during running position. 

Read: Viva questions of ac motor part 1

Q: The no-volt release coil remains energized throughout the operation or not?

 Ans: Yes, remain energized. 

Q: Can merely a D.O.L. starter be used for starting a slip ring induction motor? 

Ans: No, for that a slip ring induction motor starter is used. 

Q: What precautions are to be taken while starting a slip ring induction motor? 

Ans: The complete rotor resistance must be in rotor circuit and then only put the starter in the on position. 

Q: What is the general part of the slip ring induction motor starter? 

Ans: The parts are rotor resistance which is variable, brass studs, handle, OLRC, NVRC, overload tripping contact, two extra push buttons, stop button, metallic container, and connecting terminals. 

Q: What is the function of the start push button? 

Ans: The motor will not start unless the whole rotor resistance is not in the rotor circuit of the slip ring induction motor.

Read: Viva questions of ac motor part 3

Q: What is the function of the second push button, generally which is being used (pressed) while moving the handle of variable resistance ahead? 

Ans: It is connected in series with no volt release coil and when pressed the coil of the starter (DOL starter) remains energized otherwise the movement of the handle is pushed forward and the start button which is a NO contact will open and de-energize the no-volt release coil. 

Q: Which types of overload release coil are used in this starter? 

Ans: Nowadays where the DOL starter plunger is used for main line, thermal type overload relays are used. But in the old starter, magnetic overload relays were used. 

Q: Initially if the starter handle is in between ON and OFF position and you press the button, will the motor start? 

Ans: No, the NVRC circuit is open-circuited and the plunger will not operate. 

Q: Can you change the speed of the motor if so desired? 

Ans: In the case of slip ring induction motor the speed can be regulated up to some extent say 5%.

Q: What is the utility of this starter?

 Ans: It reduces the starting current with high starting torque. 

Read: Viva questions of ac motor part 1

Q: Why the starting torque is increased? 

Ans: Because the starting torque is directly proportional to the rotor circuit resistance. The rotor resistance is increased by increasing additional resistance in the rotor circuit. 

Q: Can’t you increase the rotor winding resistance permanently or use more resistance short-circuiting bars? 

Ans: No, it is not advisable because at the time of starting the torque will be more but in running condition, the rotor copper losses will be considerable but high which will decrease the efficiency. 

Q: How will you calculate the copper losses of this machine? 

Ans: The rotor circuit current IR, rotor circuit resistance (Rr + Rex) so the starting losses are IR2 (Rr + Rex), and running condition current =IR2 .Rr watts. 

Q: What are the methods of speed control? 

Ans: Pole changing, frequency changing, rheostat control, and cascade control. 

Q: If the frequency is increased will the speed increase? 

Ans: Yes, the speed is directly proportional to the frequency. 

Read: Viva questions of ac motor part 3

Q: How is the speed linked with the poles? 

Ans: By increasing the number of poles the speed will decrease and vice versa. 

Q: What do you mean by rheostat control? 

Ans: The speed is controlled by introducing a rheostat either in the rotor circuit or in the stator circuit. This way the speed is controlled below normal. 

Q: Why is rheostat control not commonly used? 

Ans: Because of more copper losses (I2R) 

Q: What is the condition for maximum efficiency? 

Ans: The copper losses should be equal to the iron losses. 

Read: Viva questions of ac motor part 1

Q: What is reversing switch? 

Ans: The switch which changes the d.o.r. of the motor by putting it into the other position is known as reversing switch. 

Q: How the d.o.r. is changed by reversing the switch? 

Ans: By putting it into the other position the phases are interchanged and thus the d.o.r. is change. 

Q: What is the speed regulation? 

Ans: It is the ratio of the increase in speed when the full load is thrown off, shown in the percentage of the no-load speed.

i.e. % Regulation = [(No-NL)/Ns] x 100

Q: How the speed of an induction motor is measured?

 Ans: By means of speedometer or tachometer. 

Read: Viva questions of ac motor part 3

Q: What are the methods of finding out the slip? 


  1. By means of measuring rotor speed.
  2. By stereoscopic effect. 

Q: How is the slip calculated by means of the tachometer? 

Ans: Measure the rotor speed and as we know the poles that means the synchronous speed of the motor can be calculated so substitute in the formula [(No-NL)/Ns] x 100 and calculated the slip.

Q: In a star-delta starter, how the no-volt release coil is connected?

 Ans: It is connected across the phases i.e. line in delta condition and across phase voltage in starting condition. 

Q: Where the overload release contact is located? 

Ans: Generally on left-hand side in Striplex and Bantex starters. But it is always connected in series with no-volt release coil. 

Q: How overloading is protected? 

Ans: In the case of overloading, the thermal overload relays carry more current. As a result, more heat is developed causing the bending stresses. The bending will operate the overload releasing bar i.e. contact to open and de-energizing the no-volt release coil, as a result the handle will come to off position and the motor will be protected. 

Read: Viva questions of ac motor part 1

Q: Can you adjust the overload current? 

Ans: Yes, it can be easily adjusted according to the overloading capacity or HP of the motor, 

Q: The overload tripping contact bar is a straight rod with three lever-type overload relays, can you tell the tripping contact will only open if all the three overload relays operate? 

Ans: No, the bar is placed in such a way that it will open if either of the three relays operates. It is a protection, and each individual overload relay is connected in series of each phase. So if overloading is caused in either of the phases it will operate and reenergize NVRC. 

Q: If the terminals of the starter are not known how will you find out? 

Ans: In this starter, there are nine terminals bought out, three for lines and six terminals for motor windings. Initially put the handle in star and see with the series testing lamp the three terminals which are short-circuited by thick strip i.c. star connections.

Now when three ends are known whether starting point or endpoints, the remains could be easily traced out. Let the first terminal is marked as A1, then the second is B1, and the third is C1. In delta connections, A1 is short-circuited with B2 or C2. Let it be B2, then B1, will be connected with C2 and C1 with A2.

Now check which terminal is a short circuit by the same strip and mark it B2. Similarly, trace C2 and A2. Now all L1, L2, and L3, A1, B1, C1, A2, B2, C2 are known. Connect the motor and start. 

Q: In automatic star-delta starters this facility of the handle is not there, so how will you identify the terminals? 

Ans: In that case, a contractor, generally the last one, is of star connections, so identifies the terminals and connected leads. Then see the second contactor and note the terminals which are to be short-circuited. When the star contactor opens, they will have dissimilar ends.

Remember that A1 is short-circuited with B2, B1 with C2, and C1 with A2 or A1 is with C2, C1 with B2, and B1 with A2. Identify the terminals and connect with respective terminals. 

Read: Viva questions of ac motor part 3

Q: Which part of the starter helps for connecting the automatic star delta starter in delta after starting? 

Ans: Timer. It can be adjusted after how many seconds of starting the motor is to be connected in delta. 

Q: A squirrel cage induction motor is made to have more air gap clearance. Will it draw more current or less current than the determined value? 

Ans: It will draw more current and the windings will be heated up. 

Q: A DOL starter while switched on and the start button is pressed, the plunger is energized but the motor is not running. What could be the possible fault? 

Ans: The outgoing leads are disconnected or broken or damaged. The contacts are not coming in contact. The short-circuiting strops of the plunger are damaged or broken. The input is not coming or open-circuited. Either of these or in conjunction may be the reason.

Read: Transformers viva questions part 1

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