MCQs on Electrical Machines

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01․ In a DC Motor 3 point starter is used to
provide protection against Low-Voltage.
provide overload protection.
limit the starting current to a safer value.
all of above.

Three point starter is used to limit the starting current and to provide voltage protection by 'NO VOLT RELEASE' coil and overload protection by 'HOLD-ON' coil.

02․ If starter is not used with large DC motor for starting, it will
damage the armature and commutator.
draw a starting current many times its rated full load current.
provide execessive voltage drop in line.
all of above.

Large DC motor without starter will draw high inrush current at the time of starting and may cause -
  • Heavy sparking at the commutator and even flash over.
  • Damage to armature winding either by heat developed in the windings or by mechanical forces set up by electromagnetic action.
  • Large voltage drop in the supply voltage.

03․ Direct-on-line (DOL) starters are not suitable for starting large DC motors, because
the motor may run away.
the starting torque becomes low.
the motor may not start.
the starting current will be enormously high.

Direct-on-line (DOL) starting are not used for large DC motor because the starting current will be enormously high which will cause large voltage drop in the supply mains and armature may get damaged due to excessive heating. It is used for starting of induction motor of ratings up to 3.75 KW.

04․ Which of the following DC Motor uses 4-point starter?
DC shunt motor with field weaking control.
DC series motor.
DC compound motor.
DC shunt motor with armature resistance.

The armature current of DC shunt motor . At the time of starting, Eb = 0. If the full supply voltage is applied to the motor it will draw a large current due to low armature resistance. A variable resistance should be connected in series with the armature resistance to limit the starting current. Hence a 4-point starter is used to start and control speed of DC shunt motor.

05․ DC motors are stopped by opening line switches and not by forcing the handle of the motor starter back to OFF position. This is due to occurance of
Heavy sparking at brushes.
Heavy sparking at the first stud of starting resistance steps.
Both of above.
None of these.

Generally to stop the motor, the starter handle should never be pulled back, this would result in burning the starter contacts. When there is heavy sparking at the first stud of starting resistance steps, the motor is to be stopped by opening the main line switches.

06․ High inrush of current during a starting of DC motor can efficiently be restricted to a safer value by increasing the _________________ resistance.
armature circuit
shunt field
both A and B
none of above

The DC motor draws high current at the starting instant due to absence of back emf. In order to prevent this high inrush of current at start, a resistance is connected is series with the armature. It increases the voltage drop in series, so current is reduced.

07․ The current drawn by a DC motor at the time of starting is
Very high.
Very low.

When the motor is at rest the speed of DC motor is zero, therefore back emf is zero and if a DC motor is connected directly to the supply mains a heavy current (about 10-12.5 times of rated current) will be drawn because from emf equation of armature circuit , armature current at starting is given as Ia = Vt / Ra and armature resistance Ra being very small, so current drawn is very high.

08․ In a seperately excited DC Motor if field current is increased, it will
Increase speed
Reduce armature heating.
Reduce starting torque.
Increase acceleration duration.

For a large field current, a higher value of flux will result in low operating speed and in higher motor torque for a particular value of starting current because motor torque is proportional to the product of flux per pole and armature current. Thus for given load torque, the motor will accelerate quickly and take less time to reach the lower operating speed from the starting instant. This will result in less heating of armature during starting.

09․ A DC series motor is running with a divertor connected across its field winding. If the divertor resistance is reduced then the speed of the motor will
remain unaltered.
become execessively high.

The diverter of series motor is connected in parallel with series field winding to reduce the current flowing through the field winding. This reduces the flux and increases the speed. If the resistance of diverter is reduced then the current flowing through the series field is increased and therefore flux is also increased and thereby the speed will be decreased.

10․ In which of the following speed control of DC motor the speed obtained is below than the rated speed?
Armature voltage control.
Armature circuit resistance control.
Field resistance control.
Any of above.

In armature resistance control, the field is directly connected across the supply. Hence the voltage drop across the armature drops as the current passes through the series resistance and the remaining voltage applied to the armature is lower than the line voltage. Thus the speed is reduced in direct proportion to this voltage drop at the armature terminals.