13 - Contactors ------ Flashcards
What is a contactor? How does it work?
A contactor is a heavy-duty type of electromagnetic relay. It allows sensitive, low current sensors and controls to control large motors with higher currents. They are larger than relays, and can often be rebuilt or have parts replaced.
A motor control device sends an electrical signal, which energizes the electromagnetic coil inside the contactor. The energized magnet attracts the steel armature that holds the contactor’s contacts, which closes the large contact points that safely carry the large current needed for the motor.
How do you identify a contactors current/voltage?
The Rated Coil Voltage is the voltage needed to power the contactor. Look for the contactor label, often found on the top and usually near the screws on the front of the contactor.
What are the qualities that should be copied when choosing a replacement contactor?
Rated Coil Voltage, hz, number of poles, Contact Rating Per Pole, size, and number of contacts
How to wire a contactor
High voltage/high current wires are connected to the top and bottom contacts. Low voltage control wires are connected to the sides, where they run through the coil.
How to conduct examination/PM on a contactor
- Inspect electrical terminations and contacts for pitting (physical deterioration due to extreme voltage and heat).
- Examine coil for any signs of deterioration or burns.
- Check electrical resistance across contacts and coil.
- Check for proper operation.
How to measure resistance of contactor, what is the desired resistance
Measure across the coil, a 10% deviation from 10 ohms is considered potential issue territory
What is pitting? How to check for pitting, what causes it, and how to fix it
Clean contacts using contact cleaner and abrasive materials to remove buildup. If damage is really bad, replacement might be the solution.
How do you determine that a contactor has failed
- Coil is receiving the correct voltage, and the contacts are not pulling in (coil is faulty)
- Control coil has no continuity (coil has burned up and is OL)
- Resistance across the contacts when the coil is closed is too high (pitting, obstruction, bad contact)
- When coil is energized and contact is closed, no current is carried across the contacts. Could also occur with not all contacts.
What is the electrical symbol for a contactor?
NO/NC switch symbols next to a coil symbol
What are some of the signs that a contactor is bad?
Scorched or melted regions, pitting, corrosion, and wear. Unusual buzzing or humming noises (may indicate arcing or faulty contacts). Overheating (overload, defective connections, problem with control circuit).
