DC Motors Questions and Answers

Answer: B

Correct answer: the current the machine and supply will permit

At standstill there is no back emf, so the current would be limited only by the armature resistance and would be many times the rated value. What actually caps the starting torque is therefore the current the commutator, the windings and the supply can tolerate, imposed through the starter or the converter - not the flux, which is at its full value, and not the load.

Answer: C

The surplus torque accelerates the machine until the back emf has risen enough to reduce the current to what the friction alone requires. In a shunt machine that is a few per cent above the loaded speed; in a series machine it is catastrophic.

Answer: D

How quickly the machine settles to a new speed after a disturbance depends on how much inertia has to be moved and how strongly the torque responds to the speed error. The electrical and thermal time constants are separate quantities.

Answer: A

A stepper is cheap and needs no feedback but is limited in torque and can lose steps, whereas a dc or brushless machine with an encoder handles large loads smoothly. The choice turns on the size of the load and the smoothness wanted.

Answer: B

Correct answer: has no brushes to wear or spark

Removing the mechanical commutator removes the one part that wears in normal service, together with the carbon dust, the periodic inspection, the sparking and the radio interference, and the speed limit that commutation imposes. The commutation is done instead by an electronic inverter switched from rotor position - so the machine requires a controller rather than dispensing with one, and it runs from a dc supply, not an ac one.

Answer: C

Correct answer: low-cost appliances and automotive auxiliaries

Where the duty is intermittent, the life required is modest and the cost must be as low as possible - a window motor, a cooling fan, a power tool, a toy - the brushed machine is still unbeatable, since it runs from a battery with no controller at all. Traction, aerospace and large industrial drives have all moved to brushless machines, where the maintenance and life penalties of brushes cannot be accepted.

Answer: D

One measurable current commands the torque and one voltage commands the speed, which is a control problem simple enough for the electronics of the time. Vector control brought the same simplicity to the cage machine and ended the monopoly.

Answer: C

The key gives the series motor as the one that develops the least torque at the light load end of the range between no-load and full load. A series motor has its field winding in series with the armature, so the flux grows with the armature current and the torque varies as the square of the current (\(T \propto I_a^2\)), which makes the torque at light load very small, while a shunt motor has an almost constant flux and a torque that varies only in proportion to the current, and the compound motors lie between them.

The wording of the item is loose, so this follows the key, option C.

Answer: A

The field current is \(I_f = 400/200 = 2\) A, so the armature currents are 3.6 A at no load and 66.3 A at full load. The back emfs are

\[ E_{b0} = 400 - 3.6 \times 0.18 - 2 = 397.35\ \text{V} \]

\[ E_{b} = 400 - 66.3 \times 0.18 - 2 = 386.07\ \text{V} \]

Speed is proportional to the back emf divided by the flux, and the flux at full load is 97 per cent of that at no load.

\[ \begin{aligned} \frac{N_{fl}}{N_{nl}} \\ &= \frac{386.07}{397.35} \times \frac{1}{0.97} \\ &= 0.9716 \times 1.0309 = 1.00 \end{aligned} \]

The weakening of the field by armature reaction just cancels the extra voltage drop, so the speed is almost unchanged. The answer is 1, option A.

Answer: B

In a DC motor at full load, the armature has a voltage drop of 25 V across its resistance, and the voltage actually developed in the armature (back emf) is the terminal voltage minus that drop.

\[ E_a = V - I_aR_a = 240 - 25 = 215\ \text{V} \]

The 205 V and 255 V options use a wrong drop of 35 V or add the 25 V to the terminal voltage, which is what happens in a generator, not a motor, and 240 V ignores the drop. The answer is 215 V, option B.