20EE211-Electric Drives and Control-Surprise Quiz 1

Which of the following cases require frequent starting and stopping of electric motor?

• Grinding mills

• Paper mills

• Lifts and hoists

• Air Conditioners

A 200V, Separately excited DC motor runs with a speed of 1500 rpm at no load current of 5A. When operated at full load, current is found to be 50A. Assuming constant flux operation, with armature resistance of 0.2Ω, calculate the full load speed.

• 1400 rpm

• 1450 rpm

• 1432 rpm

• 1478 rpm

For speed control of DC shunt motor using controlled rectifiers, armature voltage control gives speed ________

• above rated speed

• below rated speed

• equal to rated speed

• both below and above rated speed

If a portion of armature current is diverted through the diverter resistance, then the speed of DC series motor _________

• remains same

• increases

• decreases

• zero

Which of the following is the plugging method of braking?

• Reversal of field connections

• Reversal of armature connections

• Addition of equal and opposite field

• Removal of field circuit from current machine circuit

A 200 V, 1200 rpm, 100 A Separately excited DC motor has an armature resistance of 0.1Ω.  It is braked by plugging from initial speed of 1200 rpm. What is the external resistance to be placed in series with the armature circuit to limit the braking current to twice the full load?

• 7.33 Ω

• 1.5 Ω

• 1.85 Ω

• 3.24 Ω

Which of the following load normally needs starting torque more than the rated torque?

• Blowers

• Air compressors

• Centrifugal pumps

• Conveyors

Emf and torque produced in a DC machine are proportional to ________ and _________ respectively.

• armature speed and armature emf

• armature emf and armature speed

• armature current and armature emf

• armature speed and armature current

As there is no back EMF at the instant of starting in a DC motor, in order to prevent a heavy current from flowing through the armature circuit ____________

• a high value resistor is connected across the field winding

• a resistance is connected in series with armature

• a resistance is connected parallel to the armature

• armature is temporarily open circuited

If a starter is not used with a large DC motor, it will draw a starting current which __________

• is many times its full-load current

• will produce very low starting torque

• will produce an excessive line voltage drop

• will damage the commutator