Chapter 18 - Motor Protection

Question 1

List 4 common motor faults?

Answer 1

1. Short Circuit.
2. Overloading.
3. Stalling.
4. Single Phasing.

Question 2

Explain what happens when a motor Short Circuits?

Answer 2

• A simple Short Circuit in the motor winding’s or control circuitry will cause extremely High Fault Currents.

Question 3

Explain what happens when a motor is Overloading?

Answer 3

• If the Motor is loaded above its Torque Rating, it will draw current above its maximum Current rating,.
• This draw of a high current could cause overheating damage to the motor winding’s and Insulation.

Question 4

Explain what happens when a motor is Stalling?

Answer 4

• Stalling occurs when the rotor fails to turn due to severe overloading or mechanical failure

Question 5

Explain what happens when a motor is single phasing?

Answer 5

• Single Phasing occurs when there is a open circuit in one supply line to a Three phase Motor.
• As a result, the only voltage supplied to the motor is the p.d between the 2 remaining supplies.
• Thus, the motor is effectively receiving a single voltage and operates like a single phase motor.

Question 6

Explain in detail the symptoms exhibited by a single phasing motor?

Answer 6

IF RUNNING:- the motor will continue to run but current in the remaining lines will approximately double. the high current will eventually cause the motor to overheat. the motor will also vibrate and run uneven, but will remain at nearly the same speed, so it may not be immediately obvious that a fault has occurred. IF STOPPED:- the motor will not start.

Question 7

Explain how Motors are Protected from each type of fault.

Answer 7

1. Short Circuits in motors are protected by an over current device
2. Overloading is protected by the MTPU if the Over Current Protection fails.
3. Stalling again is protected by the Over Current Protection Device but if it fails the MTPU is there has a backup.
4. If the Motor is Single Phasing the MTPU will operate as temperature rises above normal.

Question 8

Explain the purpose of the Motor Protection Thermal Unit?

Answer 8

• The main purpose of the MTPU is to protect the motor from overheating
• This is done by disconnecting supplies if temperature exceeds the safe limit (typically 140 Degrees C)
• Thermistors (temperature sensitive resistors) are embedded in each stator phase which are monitored by the MPTU

Question 9

test

Answer 9

check