DC Machinery

Question 1

Armature current isAC and is converted to DC by the commutator.

True
False

Answer 1

True

Question 2

The primary function of excitation fora DC generator is to reduce back voltage.

True
False

Answer 2

False - the magnetic fields of all generators are excited with DC current in order to obtain higher flux density and voltage.

Question 3

In a DC generator it is necessary that the brushes be kept ahead of or in advance of the neutral load plane in order to obtain satisfactory commutation under load conditions.
True
False

Answer 3

True

Question 4

Commutators should be polished at regular intervals to remove the oxide coating which, if allowed to remain, would cause excessive arcing.
True
False

Answer 4

False - the oxide film serves as a conducting and lubricating film and helps to keep arcing to a minimum.

Question 5

The armature conductors of a motor, due to their rotation, cut lines of force and generate an ____, _______, in direction to the _________ EMF.

Answer 5

EMF
opposite
impressed

Question 6

Large motors are usually provided with some type of _______ ________ ______ which prevents excessive starting currents.

Answer 6

current limiting device

Question 7

To reverse the direction of rotation of a DC motor, the direction of the current in either the ________ or the _____ winding must be reversed; not both.

Answer 7

armature

field

Question 8

The mechanical power of a motor is expressed in terms of __________.

Answer 8

horsepower

Question 9

A motor at slow speed draws ____ current resulting in ____ torque, and a motor at high speed draws ____ current causing a ____ in torque.

Answer 9

more
high
less
drop

Question 10

What is armature reaction?

Answer 10

The effect that the magnetism produced by the current flowing in the armature has on the magnetism ( magnetic lines of force ) produced by the field coils.

Question 11

What is armature resistance?

Answer 11

The resistance of the wire used in the windings of the armature measured between the brushes of from positive to negative terminals.

Question 12

What is a shunt motor?

Answer 12

Nearly constant speed, and races when field circuit is broken.

Question 13

What is a series motor?

Answer 13

Speed decreases with added load, and motor races at no-load conditions.

Question 14

What is compound motor?

Answer 14

Large torque at low speeds and will not race at low loads

Question 15

A shunt-compound DC generator is supplying 30 amperes to a load at 125 volts. Resistance of line wires is 0.05 ohms each. Resistance of shunt-field circuit is 91.4 ohms. Resistance of armature is 0.1 ohm, and that of the series field is 0.40 ohms.
What is the terminal voltage of the generator?

Answer 15

Terminal voltage = 125 + (0.5 + O.5) 30
= 125 + 3
= 128 volts across terminals TT’

Question 16

A shunt-compound DC generator is supplying 30 amperes to a load at 125 volts. Resistance of line wires is 0.05 ohms each. Resistance of shunt-field circuit is 91.4 ohms. Resistance of armature is 0.1 ohm, and that of the series field is 0.40 ohms.
What is the shunt-field current?

Answer 16

Shunt-field current = 128/91.4 = 1.4 amps.

Question 17

A shunt-compound DC generator is supplying 30 amperes to a load at 125 volts. Resistance of line wires is 0.05 ohms each. Resistance of shunt-field circuit is 91.4 ohms. Resistance of armature is 0.1 ohm, and that of the series field is 0.40 ohms.
What is the current in the armature and series field?

Answer 17

Armature and series field current = 30 + 1.4 = 31.4 amps.

Question 18

A shunt-compound DC generator is supplying 30 amperes to a load at 125 volts. Resistance of line wires is 0.05 ohms each. Resistance of shunt-field circuit is 91.4 ohms. Resistance of armature is 0.1 ohm, and that of the series field is 0.40 ohms.
What is the EMF of the generator?

Answer 18

IR drop in armature and series field = 31.4 ( 0.1 + 0.4 ) = 15.7 volts.
EMF of generator = 128 + 15.7 = 143.7 volts

Question 19

A DC motor develops a net torque of 40 pound-feet at the shaft and runs at 900 rpm. What horsepower does it develop?
HP = 2 x π TN / 33,000 where;
T = torque in pound-feet
N = revolutions of the armature per minute

Answer 19

Solution: HP = 2 π TN / 33,000
= 2 x 3.1416 x 40 x 900 / 33,000
= 6.8

Question 20

The field coils of a DC generator furnish the magnetomotive force which sets up the flux in the magnetic circuit.

True
False

Answer 20

True