Overload Relays are used to protect Electric Motor from failure due to high current drawn by motor.Electric motor has 33% of it's cost in total Air Compressor unit,so it is our duty to protect Electric Motor from any type of failure.Overload Relays are the only solution to protect motor from high current.
Overload Relays are generally of two types one is "Electro-magnetic" and another is "Thermal".
I have already given details about Thermal Overload Relay in past.In this post,you will find details about "Electro-magnetic Overload Relay" which is somewhat outdated from latest engineering,but still some old Air Compressor units are having these type of Relays.
The Electro-magnetic Overload Relays consist of a coil or solenoid carrying the line current (or a portion of it through current transformers) with an armature which when attracted sufficiently operates the release circuit or latch.
A time lag feature is in corporated to guard against unnecessary operation from temporary Overloads due to the normal operation of the machines which are being driven.Time lag is obtained by means of Dash pot (otherwise the action would be practically instantaneous) fitted to the Oil Circuit Breaker (OCB) controlling the stator circuits.
Construction and Adjustment of Dash pot.
The time lag consist of BODY CASTING,MOVABLE PLUNGER,OIL RETAINING SCREW CUP.The movable plunger comprises the core of the Overload release,to which is fitted a specially shaped brass disc moving in a restriction in the body casting,by turning the screw cap the position of the disc within the restriction is varied.
The time lag are dispatched dry and must filled with special time lag oil (SAE-60 grade) provided in separate containers in the Circuit Breaker.While fitting remove screw "A".It is not necessary to remove the bottom cup "B" and on no account should screw "C" be removed.
Adjustment of the Overload setting is made by slackening screw "D" and sliding the body casting up or down,until the top face coincides with the calibration marking required on the calibration tube,screw "D' is then tightened.The bottom rotatable cup "B" is now described reset themselves after operation within few second.
This type of Overload Relays are very simple in operation but due care must taken for accurate operation.All the marks "A","B","C"&"D" are embossed on respective spaces,you have to identify first before working on "Electro-magnetic Overload Relay".