Motor Starters

Question 1

List the two basic parts of a magnetic motor starter

Answer 1

contactor, overload device

Question 2

In what basic part of a starter are the operating coil and power circuit contacts located?

Answer 2

contactor

Question 3

Where does an overload device sense current?

Answer 3

in the power circuit

Question 4

Which set of contacts is designed to carry full load motor currents?

Answer 4

power circuit contacts

Question 5

What is the purpose of a holding circuit

Answer 5

keep operating coil energized

Question 6

For proper operation, the holding contacts should be connected in _____ with the start button

Answer 6

parallel

Question 7

What are the two basic factors you must consider when selecting a magnetic motor starter?

Answer 7

motor rating, how it is used

Question 8

What are the two basic styles of magnetic motor starters?

Answer 8

NEMA, IEC

Question 9

A squirrel-cage motor that is started and then stopped under normal running currents has a start selected according to which IEC use category?

Answer 9

AC-3

Question 10

List four reasons a three-phase motor could single phase

Answer 10

loose connection, break in one of the supply conductors/motor windings, blown fuse, burnt out contact in the contactor

Question 11

At what percentage of its rated load does a three-phase motor stall if the motor is operating is single phasing?

Answer 11

35%

Question 12

How much more current does a three-phase motor that is single phasing draw on its two remaining supplying conductors?

Answer 12

173%

Question 13

What is the most important factor affecting the life of a motor?

Answer 13

the amount of heat the motor is subjected to

Question 14

List seven factors that affect the heating of a motor

Answer 14

the magnitude of the load current, length of time of the overload, frequency fo starts, time taken for the motor to pick up speed, ambient temperature, cooling ability of motor, the duty cycle of the motor

Question 15

What are the two sections of an overload relay?

Answer 15

one that senses motor current, one that responds to the current

Question 16

What may active an overload device?

Answer 16

a response to heat or magnetic flux due to increasing motor current

Question 17

How do bimetallic overload devices work?

Answer 17

they rely on the bending of a bimetallic strip to open a NC contact in series with the operating coil

Question 18

How do thermal overload relays work?

Answer 18

heat causes an NC contact to open the control circuit and de-energize the contact operating coil

Question 19

Can melting alloy overload devices be of the automatic reset type?

Answer 19

no, they must be manually reset

Question 20

What type of overload relay is used when a variety of conditions are critical to a motors long-term survival?

Answer 20

fully adjustable or programmable microprocessor controlled electronic overload relay

Question 21

Electronic overload relays sense the magnetic flux of a motor current by using a ____ _____

Answer 21

current transformer

Question 22

Does a magnetic overload device require heat to operate?

Answer 22

no