DC Motor Drives Questions and Answers

Answer: A

Correct answer: electrical energy into mechanical energy

A motor converts electrical input into mechanical output at the shaft, less the losses. The same machine run as a generator does the reverse, and a dc machine will do either depending on whether it is supplied or driven - which is what makes regenerative braking possible. The commutator does convert alternating conductor currents to a unidirectional terminal current, but that is a detail of its construction rather than a description of what the machine is for.

Answer: B

Flux and armature current together give the torque, and flux and speed together give the back emf, so the field enters both of the machine's defining relations. Controlling it is what gives the drive its upper speed region.

Answer: C

Correct answer: across the armature terminals

A shunt field is connected in parallel with the armature, so it sees the terminal voltage and carries a small current through many fine turns. Because the flux is then nearly independent of load, the speed falls only slightly as load is applied - the shunt motor is the approximately constant-speed dc machine. A series field carries the full armature current, and a separately excited field has its own supply.

Answer: D

Correct answer: both a series and a shunt field winding

A compound machine carries a shunt field of many fine turns across the armature and a series field of few heavy turns carrying the load current. Connected cumulatively, the series field strengthens the flux as load rises, giving a large starting torque with the shunt field still preventing runaway at no load - the combination of series and shunt virtues that makes the compound motor suit presses, shears and rolling mills.

Answer: A

Correct answer: the resistive drop in the armature grows with current

From \( N \propto (V - I_aR_a)/\Phi \), a larger load current means a larger \( I_aR_a \) drop, less voltage left for the back emf, and so a slightly lower speed. The fall is small because \( R_a \) is small - typically a few per cent from no load to full load on a shunt machine. Armature reaction weakens the flux slightly and works the other way, which is why the drop is smaller still than the resistance alone would suggest.

Answer: B

Correct answer: a voltage proportional to the shaft speed

A tachogenerator is a small permanent magnet dc generator on the shaft, and since its emf is \( k\Phi N \) with the flux fixed, its output voltage is directly proportional to speed. It closes the outer speed loop of the drive. Its accuracy depends on low ripple and freedom from temperature drift, which is why precision drives now often use a digital encoder instead.

Answer: C

Correct answer: a shunt or a current transducer in the armature circuit

The inner loop of a dc drive controls armature current, so that current must be measured - by the voltage across a low-value shunt resistor, or by a Hall effect or current transformer device that also provides isolation. Since torque is proportional to current at constant flux, this signal is effectively a torque feedback, and clamping its reference is how the drive limits torque and protects the machine.

Answer: D

With the measured speed reading low or zero, the controller commands ever more current in an attempt to correct an error that is not there. Feedback failure detection is therefore part of the protection of any closed-loop drive.

Answer: A

Correct answer: a few tens of milliseconds

The armature time constant is \( L_a/R_a \), and for a typical machine it comes to something between five and fifty milliseconds. That is fast enough for the current loop to be tuned for a bandwidth of tens of hertz, and it is roughly two orders of magnitude quicker than the field time constant of about a second - which is exactly why fast torque control acts on the armature and never on the field.

Answer: B

Correct answer: its torque follows a single current directly

In a dc machine \( T = k\Phi I_a \), and with the flux held constant the torque is proportional to one measurable current, which a simple loop can regulate. Field and armature are physically separate and orthogonal, so controlling one does not disturb the other. An early ac drive had no such handle: only with field-oriented control, which resolves the stator current into flux and torque components, did the induction machine acquire the same property.