Remanent magnetism in a solenoid 1

[felixc]

Hi,
a friend asked me if there can be a way to reduce remanent magnetism in a solenoid in his machine tool. The coil is driven directly by a switch. He says that the solenoid still has some magnetism left after he turns the switch off. My feeling is that a zero-crossing relay may do a better job by cutting the supply when no voltage is present, and perhaps let the current fade in a more gracious way that would let the magnetism go away. Will it reduce the remanent magnetism? Or is there a better solution?
Felix

 

[BrianG]

Does the solenoid "stick" on due to the remanence, or is it just slow to release?

 

[Skogsgurra]

If it is a DC solenoid (in spite of you talking about zero crossing) the best way is to find a suitable capacitor (voltage rating!) and connect it in parallel to the coil.

The coil/capacito will then ring when switched off and the ringing takes the flux down to zero through several excursions in positive and negative directions with decreasing amplitude until the core is completely demagnetized.

It will take rather big capacitors to do this and the inrush current can be quite high. You should use an oscilloscope to find out the right capacitor value. You have it when you can see at least five consecutive, decreasing oscillations with a period in the milliseconds range.

Usually not worth the effort, and remember: it only works with DC.

 

[felixc]

The solenoid uses 12VAC, at less than an ampere.
It is used to hold metal parts in place. It is slow to release the light parts after the switch is turned off.

 

[BrianG]

Hi Felix. If the solenoid is not releasing the metal parts after machining then the problem may not be remanence in the solenoid core but in the metal parts themselves.

Obviously running the solenoid from a.c. helps, but as you have already deduced, it is possible to interrupt the current at any random point on the phase such that the solenoid core could become magnetised in one direction, and could equally apply to the metal parts - hence your question about the zero-crossing detector. To get around this random magnetisation you could consider two solutions: the obvious one, a smoothly reducing a.c. field to zero, so that both solenoid and metal part are de-gaussed, or b)why not make remanence work for you?

For the alternative idea, first stop the current on a specfic known phase with a triac control circuit, then give a small pulse of the opposite polarity. If the remanence is in the metal part the oppposite field now produced in the solenoid should repel the metal part away.

 

[felixc]

Hi Brian. Thanks for the tip, I'll give it a try!
Felix