Question

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

Suppose the no load voltage V = 90V, R_a + R₁ = 4 ohms, R_sh + R_F = 130 ohms, and the motor runs at a rated speed of 1000 RPM when I_L = 4 amps. What is the back EMF V_B in volts when the motor runs at I_L = 6 amps?

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

B) How do you know that the compound motor provides good speed control at the no-load condition?

1) The torque speed curve is flat near zero torque. This means that small changes in load do not appreciably change the speed.

2) The torque speed curve is flat near zero torque. This means that large changes in load do not appreciably change the speed.

3) The torque speed curve has a large slope near zero torque. This means that large changes in load lead to very small changes in speed.

4) The compound motor runs at slow speeds when heavily loaded.

5) Items (1) and (4)

C) You are designing a new emergency brake mechanism for an automobile featuring one of the following DC motors. Which one would you choose?

1) Permanent magnet motor

2) Brushless motor

3) Servomotor

4) Compound wound motor

5) Torque motor

(D) Suppose you are building a machine where the timing sequence of two drive axes requires that they operate at precisely the same speed. What type of motor would you select: AC or DC and why?

1) You select identical DC motors because controlling frequency input gives simple and precise speed control

2) You select identical AC motors because highly accurate and reliable AC speed controllers can be bought for very cheap prices

3) You select identical DC motors because controlling voltage input gives simple and precise speed control

4) You select identical AC motors because the variance of torque and speed specs on NEMA specified motors is very small

5) It doesn’t matter whether you select an AC or a DC motor. The cost of supplying accurate speed control for either type of motor is the same.

Answer 1

For question B, ANSWER is 5. From the characteristics of DC compund motors, we can conclude that speed is maximum when torque is zero. ie., torque- speed curve is flat for zero torque. this indicates that speed will not change for small slight changes in load. Also the motor runs slow when heavily loaded. speed of the motor decreases.

For question C, ANSWER is 4, COMPOUND motor is used. Beacause of the advantage of compound wound motor when applied for sudden loads, the motor automatically undergoes substantial drop in speed, when load is applied. this application implies for emergency braking system.

For question D, ANSWER is 3, For better speed control, precise same speed , for easy operation, and economical point of view it is always better to use DC motors used controlling voltage input gives better control of speed. By using armature voltage control method it can be acheived.