MCQs on Electrical Machines

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01․ The armature voltage control is considered suitable for a DC motor driven at constant

Armature voltage control is best suited for constant torque system because the output delivered by the motor decreases with a decrease in applied voltage and a corresponding decrease in speed occurs. In this system shaft torque remains constant over a given speed range but shaft power varies with the speed.

02․ The main disadvantage(s) of the ward-Leonard system is/are
high initial cost.
high maintenance cost.
poor overall efficiency.
all of above.

As two machines (one motor and one generator) are required, so arrangement is costly and maintenance is also costly. The overall efficiency of the system is low due losses of two machines.

03․ Ward Leonard control provides speed control
in both direction.
in one direction only.
below rated speed.
above rated speed.

By Ward Leonard speed control can be obtained in both above and below the rated speed. Actually the direction of rotation of motor armature can be reversed by reversing the direction of exciting current of generator with help of shunt regulator.

04․ In case of DC Shunt Motor, the speed will____ if the voltage increases, neglecting armature saturation.
remain the same.
be unpredictable.

In case of DC shunt motor, the flux per pole is always constant if applied voltage is constant. In no load condition of DC shunt motor, flux will have maximum value and after increasing flux by inserting resistance causes the magnetic saturation of armature. If magnetic saturation is neglected, the speed remains unaffected due to variation of applied voltage or flux.

05․ Which of the following DC Motor requires 3 point starter?
Shunt motor.
Series motor.
Shunt motor as well as compound motors.
Shunt, compound and series motor.

The three point starter is used for DC shunt and compound motor. DC series motor can be started by only two point starter.

06․ To get the speed of DC motor more than normal without wastage of electrical energy _____________ is used.
rheostatic control.
Ward Leonard control.
any of above method.
none of above.

The basic adjustable voltage armature control method of speed control acommplished by means of an adjustable voltage by a motor generator set is called ward-Leonard system. This system consists simply working the motor with a constant excitation and applying a variable voltage to its armature to provide the required speed. Very fine speed control over the whole range from zero to normal speed in both direction can obtained without any wastage of electrical energy from ward-Leonard system.

07․ For starting a DC motor a starter is required because
it limits the starting current.
It starts the motor.
it limits the speed of the motor.
None of the above.

As the there is no back e.m.f. during starting the current at the time of starting is very high. For protection of motor against the flow of excessive current during starting period, it is necessary to insert high resistance in series with the armature circuit.

08․ When two DC series motors are connected in parallel, the resultant speed is
normal speed.
less than normal speed.
more than normal speed.

When two DC series motor are connected in parallel then the resultant speed is more than the normal speed. If the voltage across each motor is V and current through each motor in parallel connection is I/2. So Hence the resultant speed is more than normal speed.

09․ If the handle of a three point starter goes to back to OFF position, it may be due to
over load.
high filed circuit resistance.
zero or very low voltage supply.
all of these.

The starter handle goes to 'OFF' position because
  1. The supply voltage is either zero or lower than rated voltage.
  2. There are high field circuit resistance.
  3. The motor may be over loaded. For all those conditions the no-load release coil or over-load release coil releases the starting handle back to off position.

10․ If the no-voltage release in a DC motor starter fails to operate on failure of supply, on resumption of supply the motor will
develop very low starting torque.
start automatically without trouble.
get damaged.
not start automatically.

When the power supply fails or the field circuit becomes open, the starting lever of a starter should fall back to off position with its spring. On resumption of supply to the motor being connected directly across the line, the motor might be damaged by drawing heavy starting current and motor speed might become dangerously high.