Induction Motor Drives Questions and Answers

Answer: C

Because the diode rectifier in the rotor circuit passes power in one direction only, slip power can leave the rotor but not enter it. A Scherbius arrangement with a bidirectional converter covers both sides of synchronous speed.

Answer: D

Correct answer: can pass slip power in both directions

A Kramer drive uses a diode rectifier in the rotor, so slip power can only flow outward and the machine runs below synchronous speed. A Scherbius drive uses a fully controlled converter, or a cycloconverter, which can also inject power into the rotor at slip frequency and so drive the machine above synchronous speed. Both act on the rotor circuit and both need a converter; the difference is the direction of flow.

Answer: A

The stiff dc link and the switching pattern together define the voltage applied, and the machine draws whatever current results. It is the arrangement used in almost every modern drive, generally with pulse width modulation.

Answer: B

A large link inductor makes the source stiff in current, and the machine voltage is whatever the machine and the imposed current produce. The arrangement is robust and inherently regenerative but is now largely confined to very large drives.

Answer: C

Correct answer: synthesise a nearly sinusoidal current in the machine

The inverter switches the dc link voltage on and off at a high carrier frequency, varying the width of the pulses so that their average follows a sinusoid. The winding inductance smooths what is left, and the current in the machine is close to sinusoidal with the harmonics pushed up around the carrier, where they are easily filtered and contribute little torque ripple. Varying the modulation depth and the fundamental frequency together is also how a PWM drive realises constant V/f.

Answer: D

Higher frequency means smaller current excursions between switchings but more switchings to pay for. The choice is a compromise, influenced also by audible noise and by the stresses the fast edges place on the winding insulation.

Answer: A

Fast switching edges travelling down a cable reflect at the machine's higher impedance and can nearly double the voltage there, stressing the insulation. Output filters, slower edges and inverter-duty windings are the answers.

Answer: B

Correct answer: flux-producing and torque-producing components

Field-oriented control transforms the stator current into a frame aligned with the rotor flux, giving \( i_d \) along the flux and \( i_q \) in quadrature with it. The two can then be commanded separately, exactly as field and armature current are in a separately excited dc machine, so torque responds without disturbing the flux. This decoupling is what gives a vector drive its fast torque response.

Answer: C

The transformation into flux and torque axes is meaningless without knowing where the flux is, and estimating it accurately at low speed is the hard part. Direct schemes measure it and indirect schemes compute it from the currents.

Answer: D

A switching table chosen from the errors and the flux sector replaces the coordinate transformations and current loops, giving very fast torque response. The penalties are a variable switching frequency and greater torque ripple.