OVERLOAD RELAYS
Overload relays are used to protect motors, and motor branch circuit conductors against extensive heating, it protects electric motors from overheating by the help of overload relay overheat can be detected overload relay can be connected in series and it could be placed among the motor and the contractor and thus when the overload trips motor restarting can be prevented. The relay interrupts the current when the heat sensor in the relay reaches a certain temperature
By using bimetal overload relay overload protection can be accomplished the bimetal strip is used as a trip lever when heat increases beyond a certain rate the strip will bend and it will trip the overload relay
OVERLOAD RELAY TYPES Overload relays are classified into two they are thermal overload relay and magnetic overload relay THERMAL OVERLOAD RELAY
Thermal overload relays are used with a line contactors for the protection of motors and its voltage ranges to 600VAC the thermal overload relay has three poles the current through the motor flow through the bimetals of the relay and these bimetals are indirectly heated due to the heat the bimetals tends to bend and it will cause the tripping of the relay and the auxiliary contact position will change These relay will protect itself in case of overloading until the short circuit protecting device activates
FEATURES AND BENEFITS OF THERMAL OVERLOAD RELAY • Protection of the motor • Starters can be created easily • It matches perfectly to ABB contactors • Current setting ranges are adjustable • Trip 10 range • Compensation of temperature up to 60 C • Availability of overload protection with phase loss sensitivity • Test and stop functions are available
Auxiliary contacts
The relays have two built-in auxiliary contacts both of them are separated physically separated and thus it can be used for two different circuits
MAGNETIC OVERLOAD RELAY
It is an electromechanical relay operated by the current flow in a circuit if the level of the current reaches a preset value the magnetic fields open a set of contacts magnetic overload relay has a coil connected in series with the motor circuit and a tripping armature or a plunger if the current exceeds certain level the contacts opens the overload relay
if the load current is very high a plunger is used to pull up into the coil interrupting the circuit By altering the initial position of the plunger with respect to the coil the tripping current can be adjusted
The difference between magnetic and thermal overload relays are the magnetic type is not responsive towards ambient temperature