Mostly Air Compressors are driven by electric motor.This is the cheap option available comparing with other options such as diesel generator etc.
The operating cost of any air compressor can be reduced by using electric motor as a driving media.This is true when your electric motor runs very smoothly.But think,if your electric motor failed to start due to burning of motor winding or failure of bearings etc.
What will be the cost of repairing or replacement? This cost may be near about 33% of your Air Compressor cost.Do you believe this? For small engineering units this costs much more in the form of breakdown and production losses.To keep Air Compressor running cost at lower side,we have to protect our electric motor from failure.Most frequently motor failed due to excessive current drawn by motor.
To protect electric motor from excessive current a "Overload Protection Relay" is placed in electric circuit.
Excessive current may be due to either mechanical overload on the motor or due to electrical system,due to unbalance supply voltage or single phasing or defective starter or defective in the motor itself.In either case it is essential that the supply should be disconnected before any damage is done to the motor.An overload device thus usually operates by releasing the latching-in device by disconnecting the supply to no-volt coil or in contactor starters by operating the operating coil circuit.
Overload Relays are generally of two types.
02)Thermal Electro-magnetic overload relays are old type which is not in use recent days that's why I am giving details about Thermal Overload relay.
These are mostly used in Air Compressor now a days.
Thermal Overload relay:-
This may be bi-metal strips or solder pot elements and in either case as the action is due to their heating up,a time element is always present.The action of bi-metal strips over load release depends on the movement resulting from the different rates of expansion of the two metals forming the combined strip when heated.
The bi-metal strip may be directly heated by the current or indirectly heated by a coil of resistance wire which carries the current.Thermal Overload relays are usually of the hand reset type,the reset feature is combined with the relay.
How to set Motor Overload Protection Relay.For closer protection of the motor,particularly against single phasing condition, the bi-metal overload relay is connected in the phase circuit.hence relay should be set for 0.6 times the actual line current drawn by the motor,as measured by an ammeter.In the absence of an ammeter the following procedure may be followed.
01)Start the motor with overload relay setting at 0.6 times the rated motor current for star-delta starter and for rest of the starter at motor full load current stamped on the name plate.
02)After the motor reaches the pick load condition,reduce the relay setting gradually till the relay trips.
03)Set the relay slightly higher than the tripping value.
04)To check the setting is correct or not,allow relay to cool down,then restart the motor and make sure,relay does not trip during starting.If trips,increase the relay setting slightly and check again.
05)To check the tripping,increase the unloading air pressure setting of air compressor by 1 kilogram per centimeter square from maximum rated air pressure,when the current increases than the normal peak load current,the relay should trip.(Allow air compressor to run for more than 5 minute from peak pressure condition).If relay don't trip,slightly reduce the setting.