Which of the following method of speed control of D.C. machine will offer minimum efficiency?

The condition for maximum efficiency for a D.C. generator is

A. Eddy current losses = stray losses

B. Hysteresis losses = eddy current losses

C. Copper losses = 0

D. Variable losses = constant losses

If a D.C. motor is connected across the A.C. supply it will

A. Run at normal speed

B. Not run

C. Run at lower speed

D. Burn due to heat produced in the field winding by eddy currents

The speed of a D.C. shunt motor can be increased by

A. Increasing the resistance in armature circuit

B. Increasing the resistance in field circuit

C. Reducing the resistance in the field circuit

D. Reducing the resistance in the armature circuit

The armature voltage control of D.C. motor provides

A. Constant torque drive

B. Constant voltage drive

C. Constant current drive

D. None of the above