Why a 220v supplied coil of magnetic contactor burnt without core?

Question

Why a 220v supplied coil of magnetic contactor burnt without core?

Answer 1

The coil of an AC contactor is designed to operate with its laminated metal core in place. The core actually increases the reactance of the coil and hence the impedance which keeps the current in the coil within its working limit. Removing the core reduces the reactance and hence the impedance of the coil and hence the current increases by a factor of around 5. So 5 times the normal current flows through the coil and causes rapid burn out.

With a DC coil the core has no effect on the current as the current depends on the resistance of the coil only there is no reactance in a DC circuit.

It is a common practise for electricians to place a screwdriver in the coil of a solenoid valve to check if it is energising with a DC solenoid no problem but with an AC solenoid if it i soperated without the valve body a similar problem occurs the impedance is reduced and the current increased causing it to burn out quite quickly.

This phenomenon can be used when electrical fault finding on hydraulic and pneumatic AC solenoid valves. If the coil has burnt out it usually indicates the valve has a mechanical problem and the metal core has not moved causing excess current in the coil so the whole valve should be changed not just the coil.    

Answer 2

A magnetic contactor coil normally operates with a core. Without it with 220volt supplied, not enough impedance can oppose the increased current flow.

Answer 3

Without the core, the impedance due to the inductance of the coil is not sufficient to limit the current to a safe value.