DC Motor Drives Questions and Answers

Answer: C

Because the field carries the armature current, the flux rises with load and the torque rises roughly with the square of the current. That gives the very high starting torque which made the machine the traction motor of choice.

Answer: D

Correct answer: its speed rises dangerously as the flux collapses

In a series machine the field carries the armature current, so at no load that current is small and the flux with it. From

a vanishing flux means a runaway speed, and the machine can destroy itself by centrifugal force. This is why a series motor is always coupled directly to its load by gearing or a shaft, never by a belt that could break, and why it is never started uncoupled.

Answer: A

A motor-generator set supplies the drive motor, and controlling the generator field gives smooth reversible voltage over the whole range. It is smooth and four-quadrant but needs three machines, which is why converters have displaced it.

Answer: B

Three machines of comparable rating are needed where a converter of a fraction of the size would do, and the losses of each are cascaded. Its smoothness and four-quadrant capability were what justified it before power electronics existed.

Answer: C

The freewheeling path prevents the output voltage from reversing, so power can flow only from supply to machine. Motoring in one direction is all that is available, and braking must be arranged mechanically or with a separate resistor.

Answer: D

Advancing the firing angle beyond ninety degrees reverses the mean output voltage, so the machine can regenerate while its current keeps one sense. Reversing the torque as well requires either a field reversal or a second bridge.

Answer: A

Correct answer: four-quadrant operation

A dual converter is two fully controlled bridges connected back to back across the armature. One conducts for each direction of current, and each can be operated as a rectifier or as an inverter by taking the firing angle beyond 90 degrees. Between them they give both signs of voltage and both signs of current, that is motoring and braking in both directions of rotation - all four quadrants of the torque-speed plane.

Answer: B

Correct answer: \( (2V_m/\pi)\cos\alpha \)

Integrating the supply waveform over the conduction interval of a fully controlled bridge gives

The cosine is what makes the bridge so useful: at \( \alpha = 0 \) the output is the full \( 2V_m/\pi \), at 90 degrees it is zero, and beyond 90 degrees it is negative, so the same bridge inverts and returns power to the supply. The form with \( (1 + \cos\alpha) \) belongs to a half-controlled bridge, which cannot go negative.

Answer: C

Six pulses per cycle give a higher mean and a much smaller ripple than a single-phase bridge, which is why three-phase supply is used for any drive of appreciable size. The cosine dependence on firing angle is the same.

Answer: D

Correct answer: 90 degrees

Since \( V_{dc} \propto \cos\alpha \), the mean output passes through zero where the cosine does, at 90 degrees. Below that angle the bridge rectifies and delivers power to the machine; above it the mean voltage is negative and, with the current still flowing in the same direction through the thyristors, power flows back to the supply. Ninety degrees is therefore the boundary between rectification and inversion.