MODULE ONE: DC MACHINES

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N3 ELECTRO-TECHNOLOGY

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MODULE ONE: DC MACHINES

Please watch this video before attempting the exercise questions.

CONSTRUCTION OF A 2 POLE DC MACHINE

Give two reasons why mica is used as an insulating material between the segments of the commutator in a DC Machine. *

4 points

Mica is a good insulating material and can withstand high temperatures.

Mica is a bad insulating material and can withstand high temperatures.

Why are the segments of a DC Machine wedge-shaped? *

4 points

The commutator segments are wedge shaped to prevent them from moving out of position due to the centrifugal force of the stator

The commutator segments are pear shaped to prevent them from moving out of position due to the centrifugal force of the armature.

The commutator segments are wedge shaped to prevent them from moving out of position due to the centrifugal force of the armature.

Name two types of brushes used in a DC Machine. *

3 points

Copper, Graphite or Copper-Graphite

Carbon, Graphite or Copper-Graphite

Two types of bearings are used in DC Machines, namely ball bearings and roller bearings. State two functions of these bearings.

4 points

To support the rotating armature.

To reduce friction.

To reduce speed.

To reduce flux.

Brushes are used to make electrical connections between the stationary circuit and the rotating commutator. Explain bedding of brushes.

4 points

To ensure the face of the rotor is shaped to the surface of the commutator.

To ensure the face of the brush is shaped to the surface of the commutator.

To ensure the face of the pole shoe is shaped to the surface of the commutator.

Clear selection

State Faraday’s Law of electromagnetic induction.

4 points

An EMF will be induced whenever there is a change in magnetic flux, the size of the induced EMF will depend on the rate at which the flux changes.

An EMF will be induced whenever there is a change in relative density, the size of the induced EMF will depend on the rate at which the flux changes.

Maybe

Clear selection

Lamination's are thin layers that are bound together. Why are the field poles and armature cores of DC machines laminated.

2 points

To reduce the friction and speed loss

To minimize the effects of eddy currents

Clear selection

The field system is the stationary part of the DC machine. State the function of the field coils

2 points

To reduce the air gap between the rotating part and the stationary part of the machine.

Produce the magnetic field when current runs through them.

Clear selection

The field system is the stationary part of the DC machine. State the function of the field pole.

4 points

Distributes the magnetic flux over the air gap between stator in rotor. Hold the field winding's in place.

Distributes the current over the air gap between stator in rotor.

Holds the rotor in place. Reduces the air gap.

False

Clear selection

There are two types of armature windings in a DC machine. State which winding is used for: High voltage, low current machines

2 points

Lap Wound

Wave wound

Clear selection

There are two types of armature windings in a DC machine. State which winding is used for: Low voltage, high current machines

2 points

Lap Wound

Wave wound

Clear selection

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