DC Machines

By looking at which part of the motor,it can be easily confirmed that a particular motor is DC motor?

• Frame

• Commutator

• Shaft

• Stator

If a speed of a DC motor is increased, the back emf of the motor will

• increase

• remain the same

• decrease

• become zero

As the load is increased, the speed of the DC shunt motor will

• reduce slightly

• increase slightly

• increase proportionately

• remain unchanged

If the field of a DC shunt motor is opened

• it will continue to run at its rated speed

• the speed of the motor will become very high

• the motor will stop

• the speed of the motor will decrease

A 4 pole lap wound armature has 144 slots with two coil sides per slot, each coil having two turns. If the flux per pole is 20mWb and the armature rotates at 720 rpm. What is the induced voltage?

• 138.24V

• 34.96V

• 69.12V

• 276.48V

A DC machine has a 4 pole, wave armature with 46 slots and 16 conductors per slot. If the induced voltage in the armature is 480V at 1200rpm, determine the flux per pole.

• 48mWb

• 1.2mWb

• 0.94mWb

• 16.3mWb

A 230V DC shunt motor takes 32A at full load. The back emf on full load, if the resistance of motor armature and shunt field windings are 0.2 ohm and 115 ohms respectively, will be

• 210V

• 220V

• 215V

• 224V

In a motor, the armature resistance is 0.1ohm. When connected across 110V mains, the armature takes 20amp and its speed is 1200rpm. Determine its speed when the armature takes 50A from the same mains, with field increases 10%.

• 1061rpm

• 1023rpm

• 1171rpm

• 1251rpm

A shunt motor is running at 1200 rpm for a load which requires an armature current of 50A from 230V source. At no load, the armature current is 5A. If the effect of armature reaction has reduced the air gap flux by 2% from no load to full load, determine the no load speed. The armature resistance is 0.15 ohm.

• 1212rpm

• 1012rpm

• 1200rpm

• 1182rpm

The armature of a DC machine running at 1200rpm carries 45A in current. If the induced armature voltage is 130V, what is the torque developed by the armature?

• 46.5Nm

• 4.65Nm

• 45.6Nm

• 456Nm