Overload Relay

Overload relay is a device that prevents an electric motor from drawing too much current, overheating, and literally burning out. Overload relay is necessary to prevent burn out and to ensure maximum operating life.

What is an overload?

The term literally means that too much load has been placed on the motor. A motor is designed to run at a certain speed, called its synchronous speed. If the load on the motor increases, the motor draws more current to continue running at its synchronous speed.

It is quite possible to put so much load on a motor that it will draw more and more current without being able to reach synchronous speed. If this happens for long enough period of this, the motor can melt its insulation and burn out. This condition is called overload.

In fact, the motor could stop turning under a large enough load (called a lock rotor). This is another example of an overload condition. Even though the motor shaft is unable to turn, the motor continues to draw current, attempting to reach its synchronous speed.

Overload may be caused by low line voltage, excessive voltage unbalance, or by an open line in a polyphase system, which results in single-phase operation. Under any condition of overload, motor draws excessive current that causes overheating. Since motor winding insulation deteriorates when subjected to overheating, there are established limits on motor operating temperature. To protect a motor from overheating, overload relays are employed in a motor control to limit the amount of current drawn.

Although the overload current is not enough to blow the fuses or trip circuit breakers, it can produce sufficient heat to burn up the motor. The operation of overload relay should be such that the motor is allowed to carry harmless overloads but is quickly removed from the line when an overload has persisted too long.

What is difference between overload protection and overcurrent protection?

The overload relay dose not provide short circuit protection. This is the function of overcurrent protective equipment like fuses and circuit breakers. An overload protection device is required that does not open the circuit while motor is starting, but opens the circuit if the motor gets overloaded and the fuses do not blow.

Fuses are not designed to provide overload protection. Their basic function is to protect against short circuits (overcurrent). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishing between this temporary and harmless inrush current and damaging overload. Such fuses, chosen on the basic of motor full-load current, would blow every time the motor is started. On the other hand, if a fuse were chosen large enough to pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occur later.

The idea overload protection for motor is an element with current –sensing properties very similar to the heating curve of the motor (in figure) which would open the line to motor when full load current is exceeded.
An overload relay consists of:
- A current sensing unit (heater).
- A mechanism to breaker the circuit, either directly or indirectly.

Overload relay has a time delay to allow temporary overload without breaking circuit. It has a trip capability to open the control circuit if overload current continue over period of time. Overload relay have some means of resetting the circuit once the overload is removed.

Type of overload relay

1. Bimetallic Overload Relay
2. Melting Alloy Overload Relay
3. Solid State Overload Relay