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Question

Answer 1

Step 1:

(3.1)

The open-circuit characteristics of a seperately-excited DC generator are given in a table.

Plot the open-circuit characteristics with the values in the table.

Graph:

Draw the coordinate plane.

Plot the points from the table.

Connect the points to a smooth curve.

The graph shows the open-circuit characteristics of a seperately-excited DC generator.

Step 2 :

(3.1.1)

Find the open-circuit voltage if the field circuit resistance is 8 ohms.

Draw a 8 ohms resistance line on open-circuit characteristics curve.

Consider the points (0, 0) and (60, 480) with a slope of 8.

Now draw a line connecting (0, 0) and (60, 480) on open-circuit characteristics curve.

Observe the graph:

The 8 ohm resistance line intersects the open-circuit characteristics curve at (100, 800).

Therefore the open-circuit voltage when the field circuit resistance 8 ohms is 800 V.

Comments

Continued....

Step 3 :

(3.1.2)

Find the critical resistance.

Draw a resistance line on open-circuit characteristics curve such that it is tangent line to the curve at origin.

Observe the graph:

The line resistance represents critical resistance line.

Critical resistance is the slope of the tangent line.

Slope of the tangent line is .

Therefore the critical resistance is 30 ohms.

Solution :

(3.1.1) The open-circuit voltage when the field circuit resistance 8 ohms is 800 V.

(3.1.2) The critical resistance is 30 ohms.

Answer 2

3.2)

Step 1:

The short-shunt compound generator supplies a load current of 80 A.

Shunt field resistance is 40 .

Armature resistance is 0.2 .

Field resistance is 0.5 .

Terminal voltage is 360 v.

Construct a circuit daigram of short-shunt compound generator with specifications.

EMF generated by the DC generator is , where .

Step 2:

Calculate .

Armature current .

EMF generated by the DC generator is .

.

EMF generated by the DC generator is .

Solution:

EMF generated by the DC generator is .

Answer 3

3.3)

Three methods to overcome the armature reation:

1. Brush shift:

A solution to the problem appears to shift the brushes along the direction of rotation in generator action and against the direction of rotation in motor action, this would result into a reduction in air gap flux.This will reduce the induced voltage in generator and would increase the speed in motor.

2.Special field poles called inter Pole methods:

The limitation of brush shift has led to the use of inter poles in almost all the medium and large sized DC machines. Inter poles are long but narrow poles placed in the inter polar axis.They have the polarity of succeeding pole(coming next in sequence of rotation) in generator action and proceeding (which has passed behind in rotation sequence) pole in motor action. The inter pole is designed to neutralize the armature reaction mmf in the inter polar axis.

The base of the inter poles is made wider to avoid saturation and to improve response.

3. Compensating Winding:

Compensating winding consists of conductors embedded in the pole face that run parallel to the shaft and carry an armature current in a direction opposite to the direction of current in the armature conductors under that pole arc. With complete compensation the main field is restored. This also reduces armature circuit’s inductor and improves system response.

Answer 4

3.4)

Purpose of a DC motor starter :

The primary issue is that DC motors require a large amount of starting current which must be limited to prevent the internal circuit of the armature winding of the DC motor. Hence DC motor starter is used to prevent the motor from getting damaged due to heavy current.