EPC 13 - Motor Starting, Connection & Protection

Question 1

Name THREE different types of three phase current limiting motor starters.

Answer 1

Primary resistance
Star-delta 
Autotransformer 
Electronic 
Secondary resistance

Question 2

What are THREE of the most likely causes of overheating of a typical three phase squirrel cage induction motor on load?

Answer 2

Overloading.
Frequent starting.
Blocked ventilation passages.

Question 3

Name the protection device that is used to protect a 400V/415V three phase squirrel cage induction motor from overheating due to excessively high line currents?

Answer 3

Overload relay.

Question 4

What type of load requires a fuse with a high rupturing ‘slow-blow’ capability?

Answer 4

Electric motors with a high inrush starting current.

Question 5

What type of protection is provided by ALL magnetically operated three phase motor starters?

Answer 5

Under voltage protection.

Question 6

How many line cables are required between an electromagnetic autotransformer starter and the associated squirrel cage induction motor?

Answer 6

Three line cables. An earth is usually required.

Question 7

What type of three phase motor starter causes the motor to draw the highest starting current?

Answer 7

A direct-on-line (DOL) starter.

Question 8

How can the direction of rotation of a three phase induction motor be reversed?

Answer 8

By reversing any two of the three incoming line leads.

Question 9

What determines the thermal overload setting of a three phase D.O.L. connected motor?

Answer 9

The Full Load Current (current rating on the name plate) of the motor.