N2 Electrical Trade Theory|The Easy Way! 51 3.4.3 No-volt release coil


Te starting arm is held in the on position by the magnetic attraction of the no-volt release coil.


If a drop in the supply voltage is large enough, it could cause a motor to stall (stop). Tis could result in excessive current being drawn when the supply is restored and is also a huge safety risk.


Fig. 3.18 Face plate starter for a series motor


Te aforementioned can be overcome by making use of a no-volt coil. In the event of a power failure, or if the power supply to the motor is switched off, the no-volt coil will de-energise. Te starter arm will be released and the return spring returns it to the off position.


Te motor will not restart unless the starter is deliberately operated. 3.4.4 Overload relay


Te overload relay (OLR) is a current sensitive relay which has a set of normally open and normally closed contacts. It is set so that it attracts its armature (soſt iron piece) when the current of the motor exceeds a predetermined value.


In the case of a shunt motor starter the movement of the armature short-circuits the no-volt coil and it is de-energised.


In the case of a series motor starter the movement of the armature opens the circuit of the no-volt coil and it is de-energised.


In this way the motor is automatically switched off whenever the load current exceeds a predetermined value.


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