Degauss electromagnet to remove magnetic remanence

[nodee]

Electromagnet - Remove magnetic remanence

Hi,

I have circuit that uses a electromagnet (holds about 50kg) to hold a door closed, the circuit is powered by 24V. The electromagnet is energised via a high side driver using a p-channel MOSFET connected to one terminal of the electromagnet and the other terminal to 0V.

The problem I have is that the door is not released once the electromagnet is powered down, instead some magnetic remanence holds the door closed with about 15kg of force.

I've looked online to find a solution and the suggestion of a LC tank (non-polarised capacitor in parallel with electromagnet) that creates a exponentially decaying oscillation when power is removed that will degauss the electromagnet. If I power up then down with the door open (no plate on the face of the electromagnet) I can see the waveform on my scope as described, If I have the door closed (plate on the face of the magenet) however the effect is gone.

Any suggestions at what is happening or a possible solution?

Thanks,
nodee

 

[mtwieg]

When the plate is close to the electromagnet, the inductance of the electromagnet will increase a great deal, because the reluctance of the magnetic path is greatly decreased. This means that it will take a much larger amount of energy to establish a given magnetic field in it, which is what you need to do to degauss it. You should still see some oscillating waveform, though it may be severely damped and at a lower frequency when the plate is present. But even if you see the oscillation, the actual induced field and current is probably quite small due to the high reluctance.

Basically, try using a much bigger capacitor. The energy it stores should approach the energy stored by the electromagnet in the closed state. Maybe 1/10th of that, at least. The required capacitor may turn out to be infeasibly large. If that's the case then degaussing can be achieved by applying the waveform with PWM and a full bridge driver.

 

[chuckey]

Try fitting an air gap (a bit of sticky tape!) to the pole piece. If the door stick with 15 KgF then there is no point in using 50 KgF
Frank

 

[KlausST]

Hi,

Is it possible to reverse polarize with about 25% of the usual voltage to compensate for the remanence.

****

The other way is to connect a capacitor in parallel to the electromagnet. Then you must switch off immediately, not slowly.
The magnet and the capacitor form a resonant. After switching off the magnet a part of the stored energy moves to the capacitor and back to the magnet with reversed polarity .... compensating remanence.

This is almost the same as with forced demagnetising... a sinewave with decreasing amplitude...

Klaus