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4th Year Electrical - Machines > Direct Current (dc) Machines > Flashcards

Flashcards in Direct Current (dc) Machines Deck (16)
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1

The unit of flux density is the ________. It's symbol is ______.

Tesla; T

2

Once a magnetic circuit is in saturation, it is ______ (more/less) difficult to produce additional flux density.

More

3

A current of 3A in a 90-turn coil produces a magnetomotive force of ______ ampere-turns.

270

4

Name the five (5) components of the main magnetic circuit in a generator.

- Yoke
- Pole Core
- Pole Shoe
- Air Gap
- Armature Core (rotor)

5

In a DC generator, the brushes should commute between commutator segments when the coil is connected to those segments is travelling ___________ (parallel/perpendicular) to the field flux.

Parallel

6

An interpole must have the same polarity as the main pole _________ (ahead of/behind) it in the direction of rotation.

Ahead of

7

Commutating windings are placed in slots or holes in the main pole faces.

True or False?

False; Commutating windings are the interpoles.

8

What are the windings called that correct the problems of armature reaction in the polar zone?

Compensating windings

9

What are the two (2) methods used to help control armature reaction?

- Interpoles with commutating windings
- Compensating windings in the face of the pole shoe

10

The shunt winding is made of a few turns of fine wire.

True or false.

True

11

The armature winding carries the same current as the shunt field winding.

True or false?

False

12

Why is a resistor connected in series with the armature needed for starting a shunt motor?

At start, there is no CEMF to limit current; there is only armature resistance. Armature resistance is low and will allow high values of current, which may cause damage to the armature.

13

Why does the series motor have such high starting to torque?

The series winding is designed to carry the same high current as the amature. At start, high current produces intense magnetic fields around the series field winding and armature winding that results in high torque. As the motor speeds up, the CEMF produced tends to limit the current, and the torque decreases by the square of the current.

14

What happens to the speed of a series motor as the load is decreased?

The motor speed increases as load is decreased.

15

A 12 HP DC motor runs at 1100 r/min at full load. If the no load speed is 1210 r/min, what is the speed regulation?

10%

1210/1100

16

______________ is the process in which dc output is taken from an armature that has an ac voltage induced into it.

Commutation