Air compressor are driven by electric motor and these motors are started by different types of starters according to the requirement.These starters may be fully automatic with push button to operate or totally manual type,as per the design of the manufacturer and the choice of the customer.Here I will write about automatically operated and manually operated starters which are most important part of electrical engineering and air compressor.
01)Automatic Type Starters:-
This consist of contactors with built in coils and plunger.Pressing the starters button energises the operating coil which closes the contactor.In order that the contactor may remain closed when the "Start " push button is released,retaining contactors are required.
These are closed by the contactor lever itself and thus maintain the operating coil circuit,once it has been made and the contactors closed,irrespective of the position of the "Start" push button.Depression of the "Stop" push button or failure of supply to the operating coil immediately comes the contactor to open.
By energising,the operating coil from the same circuit as supplies the motor such an arrangement is equivalent to a no volt release.(Example-Automatic Direct-On-line (DOL) Starter and Automatic Star-Delta Starters are of these type)
02)Manual Operated Starters:-
For safety,all manual operating motor starting switch or circuit breaker return to the "Off" position in the event of failure of the incoming supply,and for this purpose an "Under voltage" or "No volt" release is fitted.The under voltage release forms an inherent part of the starting switch in all electro-magnetically operated starters,but is incorporated to the starting switch.
With drum type AC Starters having a manually operated starting handle,the starting switch is fitted with a spring which biases the switch to the "Off"position,but is retained in the "On" position by a mechanical latch fitted in the starter is a shunt-wound electro-magnet or solenoid excited by the supply voltage.
In the event of supply failure,the plunger or armature of the solenoid or electro-magnet is released and is arranged to knock off the hold-on latch and so return the switch to the "Off" position.
Mostly Star-Delta Starters are used in Air Compressor Technology.