Quiz sheet for motor protection

A microtherm is a device that

A) is in direct contact with the windings and used a snap action bimetallic disk to detect motor temperature and open circuit motor current on small motors.

B) tiny device embedded in the windings of a motor that directly detects the temperature of the motor and increases resistance in the control circuit supplying the coil of a starter.

C) detects motor temperature by measuring the current in a thermal overload and isolating the coil in the event of an overload.

D) is an electronic device that measures current in a motor using hall effect of the magnetic field around the conductors supplying the motor.


An isolator used in the supply circuit of a motor must be capable of breaking 

A) the full load current of the motor

B) seven times the full load current

C) the locked rotor current

D) four times the full load current.


Clause in the wiring rules states that protection against restarting or reversal of a motor must be provided where 

A) unexpected restarting of a motor might cause danger.

B) there are no gates or guards

C) the load would be damaged if the motor was reversed

D) there is no sign warning people of the motor starting without notice


Clause requires motor to have over temperature protection even if they can be seen at all times, if the rating of the motor is greater than

A) 2.25 kW

B) 480 watts

C) 240 Watts

D) No over temperature protection is required if they are attended at all times.


If a motor cannot be seen from where it is operated, what device is required to allow maintenance to be performed on the motor with safety?

A) An isolator 

B) A mirror 

C) A danger tag 

D) Warning sign 


Motors require overload protection because

A) they draw high current when overloaded and this overheats the motor

B) when they are overloaded they can snap a shaft.

C) the wiring rules tell me so.

D) they can be stolen if they don't 


Repetitive starting of squirrel cage induction motors such as frequently happens on a crane.

A) can cause motors to overheat because start currents are very high compared to run currents

B) has no significant impact on a motor

C) causes the cage to overheat due to eddy currents

D) weakens the shaft and causes it to work harden.


The device connected in series with the motor that uses full motor current to heat a bi-metal strip that will open a normally closed switch if current is too high for too long is called a 

A) thermal overload

B) microtherm

C) thermistor

D) dashpot


The device illustrated above is called a

A) microtherm

B) thermal overload

C) dashpot

D) thermistor


The item in the image above is called:

A) a Thermal overload

B) a Microtherm

C) a Thermistor 

D) an electronic overload


The most common over temperature protection method used to protect induction motor is a

A) Thermal overload

B) Circuit breaker 

C) Thermistor

D) Contactor


The reason we can't protect a 20A motor from overload using a 20A circuit breaker is because

A) Circuit breakers have a let through current that is 1.45 time the rating of the breaker and this current would most likely damage the motor before it tripped.

B) Circuit breakers are designed for protecting circuits, not motors.

C) The wiring rule prohibit it.

D) Circuit breakers have a magnetic trip mechanism that would trip every time the motor started, due to the high start currents. Motors have a very low resistance and this is similar to a short circuit.


The type of motor protection most commonly used is a

A) Thermal overload

B) Thermistor

C) Mocrotherm

D) Dashpot 


The type of overload illustrated above is called 

A) a dashpot relay

B) thermal overload

C) microtherm

D) thermistor


These little suckers are sometimes embedded in the windings of a motor and connected in series with the coil of the starter. They are called

A) thermistors

B) microtherms

C) overloads

D) despots 


This motor start fuse is marked 63M100 this means 

A) The fuse provides no protection for the motor but can tollerate a short term high current that is similar to the short circuit protection that would be provided by a 100A fuse but will protect the cable from a sustained over-current as though it were a 63 A fuse.

B) The fuse is designed to protect a 63 amp motor but will blow if subjected to 100 amps

C) The fuse is designed to protect the cable that is rated at 63 Amps but will not blow if the current is not more than 100A because fuses have a fusing current of 1.6 time the rating of the fuse.

D) This fuse is a 100A fuse but it can also be used to protect a motor if the motor is rated at 63A 


What happened to this motor?

A) It was connected in delta and lost a phase from the supply

B) It was connected in star and lost phase from the supply

C) it was overloaded

D) it had an open circuit in some of the windings


What happened to this motor?

A) It was connected in star and lost a supply line

B) It was connected in delta and lost a supply line

C) it was overloaded

D) it was short circuited


When a low resistance cage squirrel cage induction motor is plug braked (put into reverse before it has stopped turning).

A) the torque is not much higher than DOL starting torque and could be lower.

B) the torque can some be high enough to snap the shaft.

C) the torque is equal to breakdown torque

D) the torque will be negative. 


When the voltage applied to a squirrel cage induction motor is below the rated voltage 

A) more current is drawn for a given load because more slip is required to maintain torque

B) ohms law states that it will draw less current and not generate enough torque to run the load

C) the motor will not be able to start because the torque is too low

D) the motor will accelerate faster because slip is higher. 


Last modified: Friday, 28 May 2021, 5:36 PM