Motor Starters Flashcards
List the two basic parts of a magnetic motor starter
contactor, overload device
In what basic part of a starter are the operating coil and power circuit contacts located?
contactor
Where does an overload device sense current?
in the power circuit
Which set of contacts is designed to carry full load motor currents?
power circuit contacts
What is the purpose of a holding circuit
keep operating coil energized
For proper operation, the holding contacts should be connected in _____ with the start button
parallel
What are the two basic factors you must consider when selecting a magnetic motor starter?
motor rating, how it is used
What are the two basic styles of magnetic motor starters?
NEMA, IEC
A squirrel-cage motor that is started and then stopped under normal running currents has a start selected according to which IEC use category?
AC-3
List four reasons a three-phase motor could single phase
loose connection, break in one of the supply conductors/motor windings, blown fuse, burnt out contact in the contactor
At what percentage of its rated load does a three-phase motor stall if the motor is operating is single phasing?
35%
How much more current does a three-phase motor that is single phasing draw on its two remaining supplying conductors?
173%
What is the most important factor affecting the life of a motor?
the amount of heat the motor is subjected to
List seven factors that affect the heating of a motor
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
What are the two sections of an overload relay?
one that senses motor current, one that responds to the current
What may active an overload device?
a response to heat or magnetic flux due to increasing motor current
How do bimetallic overload devices work?
they rely on the bending of a bimetallic strip to open a NC contact in series with the operating coil
How do thermal overload relays work?
heat causes an NC contact to open the control circuit and de-energize the contact operating coil
Can melting alloy overload devices be of the automatic reset type?
no, they must be manually reset
What type of overload relay is used when a variety of conditions are critical to a motors long-term survival?
fully adjustable or programmable microprocessor controlled electronic overload relay
Electronic overload relays sense the magnetic flux of a motor current by using a ____ _____
current transformer
Does a magnetic overload device require heat to operate?
no
