A) In a shunt wound DC motor you can increase the speed by increasing the resist

Question

A) In a shunt wound DC motor you can increase the speed by increasing the resistance of shunt resistor RF. What happens to the motor torque when you do this and why?

1)Torque increases. Increasing RF increases current in the field windings and increases magnetic flux. Because torque is proportional to magnetic flux, it increases.  

2)Torque decreases. Increasing RF reduces current in the field windings and reduces magnetic flux. Because torque is proportional to magnetic flux, it decreases.  

3)Torque increases. Increasing RF increases current in the field windings and increases magnetic flux. Because torque is proportional to magnetic flux, it increases.

4)Torque decreases. Increasing RF increases current in the field windings and reduces magnetic flux. Because torque is proportional to magnetic flux, it decreases.

5)Torque decreases. Increasing RF increases current in the field windings and increases magnetic flux. Because torque is proportional to magnetic flux, it decreases.

B)Refer to the diagram on “Speed Control of a Shunt Wound DC Motor”.

Suppose the no load voltage V = 90V, Ra + R1 = 5 ohms, Rsh + RF = 140 ohms, and the motor runs at a rated speed of 1500 RPM when IL = 4 amps. What is the armature current IA in amps when the motor runs at IL = 6 amps?

Note: Type a 1-decimal number, rounded off to the nearest 0.1

C) What is true about the speed of a series wound DC motor?

1) The speed decreases as the armature winding resistance decreases

2) The speed increases as the magnetic flux increases

3) The speed decreases as the torque increases

4) The speed decreases as the input voltage decreases

5) Items (3) and (4)  

Explanation / Answer

A-2

The speed of a dc motor is inversely proportional to the flux per pole. Thus by decreasing the flux, speed can be increased and vice versa.

To control the flux, a rheostat is added in series with the field winding. Adding more resistance in series with the field winding will increase the speed as it decreases the flux.

B) IL= If+Ia

If=90/140= 0.6428A

Ia=6-0.6428=5.3572A

C-5

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