Unit 19 Motor controls
what is not a typical voltage used for relays contactors and motor starters?
12V
What is the difference between a contactor and a starter
A Starter has over current protection built into its framework. (starters are contactors with over current protection devices).
Can a contactor always be converted to a starter
A contactor cannot always be converted to a starter, but in some cases it can. The starter differs from the contactor because of the over current protection
Can a motor be started immediately after an overload?
A motor should not be restarted immediately after an overload condition because they may need time to cool. When possible, determine the reason for the overload condition before restarting the motor.
Why is it not a good idea to file or sand the contactor contacts?
Contacts may be cleaned but this should be done as a temporary measure only. Filing or sanding exposes base metal under the silver plating and speeds deterioration. Replacing the contacts are the only recommended repair
Why is it not a good idea to use circuit protection devices to protect large motors from overload conditions?
Fuses protect circuits, not motors. With 2 motors, on the same circuit ... 1 motor can be operating in an overloaded condition, without overloading the conductor = fuse will not open the circuit
The difference between inherent and external overload protection
Inherent - motor overload protection is provided by sensing devices within the motor. External - motor overload protection is applied to the current passing device : the relay, the contactor, or the starter.
The two amperages that influence the choice for replacing a motor starter are
LRA (Locked Rotor Amperage) and FLA (Full Load Amperage)
what causes an overload protection device to function
Overcurrent
What are the contact surfaces or relays, contactors and starters made of?
Silver Cadmium Oxide
Under what conditions are motors allowed to operate with slightly higher than design loads
The motor can operate above the FLA and within the service factor (SLA) without harm
What components can be changed on a contactor and a starter for rebuilding purposes
The springs, the coil, and the overload protection device (heaters and switches)
magnetic overload devices are very accurate and not affected by ...... .......
ambient temperature
starters are .......... with motor ........... protection built into the framework of the contactor
contactors overload
external overload protection is applied to the ....... ......... device
current passing
contacts on relays, contactors, and starters should never be ........... or ...........
filed sanded
large motors should be protected from .......... conditions
overload
Two types of relays are
pilot duty and throw away
What is the purpose of overload protection at the motor
protect the motor from over current and damage
contactors are large ........ that may be ................
relays rebuilt
The recommended repair for a defective relay is to
replace it
inherent motor overload protection is provided by ....... devices within the motor
sensing
relay, contactor, motor starter are three types of motor ........ and ......... devices
starting stopping
the relay is used for ........... circuits and motor ........
switching starting
bimetal and solder pot devices are ............ operated
thermally