Overload relays protect a motor by sensing the current going to the motor. Many of these use small heaters, often bi-metallic elements that bend when warmed by current to the motor. When current is too high for too long, heaters open the relay contacts carrying current to the coil of the contactor.Jan 6, 2014
What causes a thermal overload?
Causes may include a large change in load (e.g., a scrap grinder is fed too much at a time), misalignment, a broken drive gear, or improper motor drive settings. Power problems (e.g., low voltage or low power factor) also may cause an overload condition.Jan 25, 2016
What is the purpose of the thermal overload unit?
The function of a thermal overload relay, used in motor starter circuits is to prevent the motor from drawing excessive current which is harmful to motor insulation. It is connected either directly to motor lines or indirectly through current transformers.
How do you set thermal overload?
If the 125% value is not built into the relay, you must set it at the motor's nameplate current + 25%. For example, assume you want to protect a motor with 60A of full-load current, and you have an overload relay that can be set from 50A to 100A. If the device already factors in the 125%, you must set it at 60A.Mar 7, 2019
How do you test an overload heater?
- Measure the normal motor running current (i motor).
- Turn off the motor and let it cool for about 10 minutes.
- Calculate the following ratio: i (motor) / i (overload min FLA). ...
- Set the overload to its minimum FLA and turn on the motor.
- Wait for the overload to trip.
How do you know if a overload relay is bad?
Give the compressor a shake if the start relay isn't plugged in. If you hear rattling on the inside of the start relay, it's time to replace it. You may have a problem with the actual compressor if it's not rattling and in good condition.
How do you troubleshoot a thermal overload relay?
- Remove overload. or adjust the. ...
- Is there a mechanical overload on the motor?
- Is the proper voltage available at the motor?
- Check for loose connections or single phasing at the motor. ...
- Adjust the relay or. ...
- Tighten the connections or. ...
- Test the actual trip point of the relay and replace if necessary.
What are motor heaters?
Motor Heaters (MH series heaters) are designed to control moisture build-up on the electric motor windings. Condensation build-up in the motor can result in premature failure of the electric motor. ... Flexible motor heaters provide heat to the electrical windings preventing moisture to collect in the motor.
What are motor overloads?
Motor overload occurs when a motor is under excessive load. The primary symptoms that accompany a motor overload are excessive current draw, insufficient torque and overheating. Excessive motor heat is a major cause of motor failure.