Dear Readers, Welcome to D.C MOTOR Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of DC Motor. These DC Motor Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.
A. The operation of a D.C Motor based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. Basically, there is no constructional difference between a d.c. motor and a d.c generator. The same d.c. machine can be run as a generator or motor.
A. When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator. The induced e.m.f. acts in opposite direction to the applied voltage V (Lenz’s law) and in known as back or counter e.m.f Eb. It is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.
A. Speed regulation is the change in speed when the load on the motor is reduced from rated value to zero. It is expressed as a percentage of rated load speed.
A. Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.
A. Dc series is in the used in trains to get high starting torque while starting of the trains and the operating voltage is 1500v dc.
A. Speed control of a d.c shunt motor can be done using
1. Flux control method ()
2. Armature control method
3. Voltage control method
A. Speed control of a d.c series motor can be done using
1. flux control method
2. Armature-resistance control control method
A. For DC motors, Voltage equation is
Where V = Terminal voltage,
Eb = Back emf in Motor
Ia = Armature current
Ra = Aramture resistance.
At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra, where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased. The excessive current will damage commutator and brushes and eventually blow out the fuses.
A. We can reverse the direction of rotation of a D.C Motor by either reversing the field current or armature current. If both the currents are reversed the motor will run in original direction.
A. The shunt motor will achieve dangerously high speed and may destroy itself.
A. The direction of rotation of motor remains the same as current in armature and field.
A. 1. The motor will run but it would not carry same load as it would on d.c supply.
2. More sparking at the brushes
3. Eddy currents will be high and will cause overheating and may eventually burn on a.c supply
A. 1. DOL – direct online starter
2. Star delta starter
3. Auto transformer starter
4. Resistance starter
5. Series reactor starter
A. There are two principal types of d.c. motor starters viz., three-point starter and four-point starter. As we shall see, the two types of starters differ only in the manner in which the no-volt release coil is connected.
A. The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting. As the starting current is reduced, the voltage drops during the starting of motor in systems are reduced.
A. 1. Brake test
2. Swinburne’s test or No-load test
3. Regeneration test or Hopkinson’s test
4. Retardation or Running down test
A. Swinburne’s test is conducted at No-load condition to predetermine the efficiency at different loads and to measure the losses separately.