Stony Brook University

MEC 410Chapter 21 Homework Answer Key
Note: Correct answer is in Blue Font
(A) What is the difference between the locked rotor (starting torque) and the
breakdown torque?
1) Breakdown torque is the torque output of the motor at standstill, while
locked rotor torque is the maximum torque output of the motor.
2) Breakdown torque is the minimal point reached in the torque speed
curve soon after startup, while locked rotor torque is the maximum torque
output of the motor.
3) Locked rotor torque is the torque output of the motor at standstill, while
breakdown torque is the maximum torque output of the motor.
4) Breakdown torque is the point in the torque-speed curve where the
motor is in danger of failing, while locked rotor torque in the torque output
of the motor at standstill.
5) Items (3) and (4).
Refer to slide 7 for chapter 21
B)In the video demonstration of the split phase motor, how do the cooling fins
keep the motor from overheating?
1)The fins rotate around the stator windings at the speed of the motor,
creating a large air current.
2)The large air current created by the rotating fins channels away the hot
air located around the windings.
3) As the fan cools the air around the windings, the differential
temperature between the windings and ambient air increases. This allows
heat to flow away from the windings.
4) Items (1) and (2)
5) Items (1), (2) and (3)
Refer to the video found on: https://youtu.be/mOPIYQnsJe8
C) Why was a universal motor used in the 3-speed fan in our video demo?
1)A universal motor operates at m