The thermal overload relay is designed to protect the motor or other load from damage in the event of a short circuit, or being over-loaded and overheating. The simplest overload relay is activated by heat caused from high current flowing through the overload and over a bimetallic strip.
How does a contactor overload work?
When current is too high for too long, heaters open the relay contacts carrying current to the coil of the contactor. When the contacts open, the contactor coil de-energizes, which results in an interruption of the main power to the motor.
Do contactors have overload protection?
Contactors are used to control power in a variety of applications. When applied in motor-control applications, contactors can only start and stop motors. Contactors cannot sense when the motor is being loaded beyond its rated conditions. They provide no overload protection.
How do you set a thermal overload on a motor?
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.
What is the thermal overload relay function for in a contactor?
Thermal overload relays are economic electromechanical protection devices for the main circuit. They offer reliable protection for motors in the event of overload or phase failure. The thermal overload relay can make up a compact starting solution together with contactors.
How does thermal overload protection work?
A thermal overload relay works on the heat produced by the excessive overload current. The heat produced by the overload current is utilized to trip the motor circuit. These are mostly used for protection of low-voltage squirrel cage induction motors or DC motors of lower output rating.
What does thermal overload protection mean?
The thermal protection is present to turn the motor off when excessive heat is generated within the motor circuitry. This safety feature halts the temperature climb before it can burn up the motor. Thermal protectors generally reset themselves once the motor cools down to a safe operating temperature.
What is the overload protection do on the contactor?
Overload relays protect the motor, motor branch circuit, and motor branch circuit components from excessive heat from the overload condition. Overload relays are part of the motor starter (assembly of contactor plus overload relay). They protect the motor by monitoring the current flowing in the circuit.
What is motor overload protection?
Overload protection is installed in the main control circuit or power line of motor to protect from damage due to mechanical malfunction overload conditions when it is running. The effect of an overload is an excessive rise in temperature in the motor windings due to current higher than full load current.
What is thermal overload?
Thermal Overload Relays Information. … When excessive current flows through the motor circuit, the relay opens due to increased motor temperature, relay temperature, or sensed overload current, depending on the relay type.
How do you calculate thermal overload protection on a motor?
The overload protection is sized per the motor nameplate current rating, not the motor full load current (FLC) rating. Thus, 60A×1.25=75A. Overload protection shall not exceed 75A, so you need to use a 70A dual-element fuse [240.6(A) and 430.32(A)(1)].
How do you set thermal overload?
Thermal Overload Relay setting = 100% x Full Load Current (Line).
- Thermal Overload Relay setting = 100% x Full Load Current (Line).
- Thermal Overload Relay setting = 100%x6 = 6 Amp.
How do you test a thermal overload on a motor?
Overload Protection Test:
- 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.
Why thermal overload relay is used in motor protection?
Thermal overload relays are installed in the main circuit when electromechanical protection is adequate. This protects the motor in case of phase failure or overload. The thermal overload relay does not directly shut down the load. Instead, it triggers one or several auxiliary contacts which then disconnect the motor.
What is the principle of thermal relay?
The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over current of the system, it bends and makes normally open contacts.
How do you protect an overloaded motor?
Motor overload protection is necessary to protect the motor and to help ensure the motor performs as expected. Continuous duty motors are protected against overload by a separate overload device sized between 115% and 125% of the motor nameplate full-load current, FLA.