A motor overload relay is an electro-mechanical relay that is operated by heat developed in the relay.When the level of current in a circuit reaches a preset value, the increased temperature opens a set of contacts.
The increased temperature opens the contacts through a bimetallic strip or by melting an alloy that activates a mechanism that opens the contacts depend on our setting for overload relay.This time i explain detail about common types of overload relay for motor control.
Types Of Overload Relay
1) Thermal overload relays
- Melting alloy
These are probably the most popular type of overload protection.The motor current passes through a small heater winding and under overload conditions.
The heat causes a special solder to melt allowing a ratchet wheel to spin thus opening the control circuit contacts.It must be reset by hand operation and Heater coil and solder pot in one unit non-tamperable.
- Bi-metallic strip
This design uses a bimetal strip associated with a current-carrying heater coil.When an overload occurs, the heat causes the bimetal to deflect and actuate a tripping mechanism which opens a set of contacts in the control circuit interrupting power to the coil and opening the power contacts.
Most relays are adjustable over a range from 85% to 115% of their value.They are available with ambient compensation. An ambient compensated devices’ trip point is not affected by ambient temperature and performs consistently at the same value of current.
2) Magnetic Overload Relays
A magnetic overload relay is an electro-mechanical relay operated by the current flow in a circuit.
When the level of current in the circuit reaches a preset value, the increased magnetic field opens a set of contacts.
Electromagnetic overload relays operate on the magnetic action of the load current flowing through a coil.
When the load current becomes too high, a plunger is pulled up into the coil interrupting the circuit.The tripping current is adjusted by altering the initial position of the plunger with respect to the coil.