1. Why is the speed of DC shunt motor dependent on Back EMF?
(A) Because flux is proportional to the armature current
(B) Because armature drop is negligible
(C) Because Back EMF is equal to armature current
(D) Because flux is constant in DC shunt motor
Answer (D) Because flux is constant in DC shunt motor
Explanation:
In the case of DC motors Speed ∝ Back emf(Eb)/flux(φ).
The field winding is connected across the armature. The resistance offered by the field is constant so the current is constant. As current is constant flux also becomes constant b the cause flux is directly proportional to current.
Therefore the speed of the DC shunt motor depends on the back EMF.
2. What will happen when the field of a DC shunt motor gets opened while the motor is running?
(A) Continue to run at the same speed
(B) Speed of motor will be reduced
(C) The motor will attain dangerous high speed
(D) Armature current will be reduced
Answer (C) The motor will attain dangerous high speed
Explanation:
In a DC shunt motor, if we cut the supply for the field winding, the speed would dangerously increase in order to maintain the back emf of the motor.
In the case of DC motors Speed ∝ Back emf(Eb)/flux(φ).
So if flux drops to ZERO theoretically speed will tend to infinity and practically it will damage the windings of the motor.
3. Which of the following DC motor have the tendency of load instablity?
(A) Cumulative compound motor
(B) Shunt Motor
(C) Series motor
(D) Differentially compound motor
Answer (D) Differentially compound motor
Explanation:
In a differential compound motor, because of the opposition of the series field to the shunt field, the flux decreases as the load and armature current increase. As we know that the decrease in flux causes an increase in speed. However, because the speed is proportional to EB/φ, if both factors vary in the same proportion, the speed will remain constant. This action may occur in the differential compound motor.
If more turns are added to the series coil, it is possible to cause the motor to run faster as the Load is Increased.
With over compounding, a slight increase in speed is possible with an increase in load. This speed characteristic is achieved only with a loss in the rate at which torque increases with load.
Because the field decreases with a load increase, a differential compound motor has a tendency for load instability.
When starting a differential motor, it is recommended that the series field is shorted because the great starting current in this field may overbalance the shunt field and cause the motor to start in the opposite direction.
4. The reason for using starter while starting of DC motor is
(A) To restrict armature current as there is no back E.M.F at starting
(B) Motors are not self-starting
(C) Restrict starting torque
(D) None of the above
Answer (A) To restrict armature current as there is no back E.M.F at starting
Explanation:
At the time of starting a DC Motor, the Back EMF developed is zero as its speed is zero.
We know that general emf equation E = Eb + laRa. At starting Eb= 0 so la = E/Ra
So very high current flow (which is limited by resistance only but resistance itself is very low usually 1-2 ohms) through the armature of the machine which can destroy it completely.
So to protect the machine from such large current, starters which can be simple resistors or the sophisticated 3 point or 4 point starters may be employed.