DC Motor Drives Questions and Answers

Answer: A

Correct answer: True, provided the flux is held constant

Since \( T = k\Phi I_a \), holding the flux constant makes torque and armature current strictly proportional, so a current loop is a torque loop. This is exactly why dc drives are built with an inner current loop and why they gave such excellent torque control long before vector control brought the same capability to ac machines. The proviso matters: above base speed, where the field is being weakened, the proportionality no longer holds.

Answer: B

The field needs only a fraction of the power and rarely needs fast control, so a small single-phase bridge or a fixed supply suffices. Independent control of the two is what gives the dc drive its characteristic two regions.

Answer: C

Correct answer: motor and brake in both directions of rotation

The four quadrants are the four combinations of torque sign and speed sign: forward motoring, forward braking, reverse motoring and reverse braking. A four-quadrant drive can produce all of them, so it can accelerate and brake in either direction and reverse through zero speed under control. Hoists, lifts, mill drives and reversing servos all need it; a fan or pump generally does not.

Answer: D

Below it the voltage is being raised at full flux, and above it the flux is being reduced at full voltage. The point divides the constant-torque region from the constant-power one, which is why every dc drive is described in terms of it.

Answer: A

The first region carries it up to base speed at full flux, and the second doubles the speed by halving the flux at full voltage. Torque in the upper region is half its rated value, which the load must be able to accept.

Answer: B

Torque follows the product of flux and current, so reversing either factor reverses the torque while reversing both leaves it unchanged. Armature reversal is faster and is what a dual converter provides electronically.

Answer: C

Correct answer: a much larger inductance and time constant

The field winding has many thousands of turns on an iron core, so its inductance is large and its time constant \( L_f/R_f \) is of the order of a second, against a few milliseconds for the armature. Reversing the field therefore takes far longer than reversing the armature current, which is why fast reversing drives reverse the armature - through a dual converter or a four-quadrant chopper - and leave the field alone.

Answer: D

Correct answer: reverse the armature current as each coil passes the poles

For the torque on every conductor to act in the same direction, the current in a coil must reverse as it passes from under one pole to the next. The commutator does this mechanically, switching each coil at the right instant through the brushes, so the machine produces steady unidirectional torque from a direct supply. It is thus a rotary switch synchronised to the rotor, and its inevitable sliding contact is the machine’s chief weakness.

Answer: A

Correct answer: there is no back emf at standstill

From \( V = E + I_aR_a \) with \( E = 0 \) at rest, the current is limited only by the armature resistance, which is deliberately small - a fraction of an ohm. A machine that draws 20 A running might draw several hundred at switch-on. The remedy is to insert resistance and remove it as the back emf builds up, or, in a converter drive, to start at a low armature voltage and raise it.

Answer: B

Correct answer: inserting resistance and removing it as the machine speeds up

The starter puts resistance in series with the armature to limit the initial current, and the operator or an automatic relay cuts it out step by step as the machine accelerates and its back emf grows. The three-point starter also carries a no-volt release and an overload trip; the four-point starter separates the no-volt coil from the field circuit so that field weakening does not trip the starter. Both are superseded in modern drives by converter control of the armature voltage.