Misapplied voltage to an AC coil

[Banjoguy]

I had incorrectly ordered a relay with wrong volage. Realization came after it was installed on an upgraded project. Instead of 120VAC coil for a Safety Relay, it was 240VAC. I was informed it got burned up when 120VAC was applied. It was on a fused 120VAC circuit as design intended. The circuit was fused with 1A fast blow.

Since this weird issue happened, I thought 120VAC would make the 240VAC coil to underperformed. But to burn made no sense. Anyone care to comment?

 

[ptonsparky]

It is an inductive load. Current will go up as voltage goes down. Just like any motor with the wrong applied voltage, the smoke gets out.

 

[Banjoguy]

Ah, makes sense. Forgot all about good old college days learned circuit theory stuff. Amount of current passes up to limit of fuse would perhaps be enough to smoke this thing.

 

[LarryFine]

If I'm not mistaken, the low voltage also may fail to pull in the armature, which affects coil current.

 

[retirede]

If I'm not mistaken, the low voltage also may fail to pull in the armature, which affects coil current.

Correct.

 

[winnie]

If I'm not mistaken, the low voltage also may fail to pull in the armature, which affects coil current.

This.

A contactor coil is not a constant power load like a motor. Reducing the voltage to the coil will reduce its operating current, as long as the coil is in the same state.

But for most AC coils, the inductance increases dramatically when it 'pulls in', reducing the current flow. If the voltage is too low to 'pull in' the coil, then the low impedance high current state could be maintained for an extended period of time.

Jon

 

[ptonsparky]

This.

A contactor coil is not a constant power load like a motor. Reducing the voltage to the coil will reduce its operating current, as long as the coil is in the same state.

But for most AC coils, the inductance increases dramatically when it 'pulls in', reducing the current flow. If the voltage is too low to 'pull in' the coil, then the low impedance high current state could be maintained for an extended period of time.

Jon

Similar to Locked Rotor Current in a motor. Y/N?

 

[winnie]

Similar to Locked Rotor Current in a motor. Y/N?

Similar in the sense that at locked rotor the motor draws vastly more current.

Different in the sense that the contactor is very binary, with vastly different inductance in one state vs the other. The motor has a smooth transition between locked rotor and spinning under load. IMHO the analogy between motor and contactor coil works reasonably if you only compare locked rotor to spinning with no load at synchronism.

-Jon