DC through contactor

I have an application that requires switching the polarity of 120VDC (full wave rectified from 120VAC, no filter cap) to demagnetize an electromagnet. The electromagnet will be switched on and off for normal "holding" operation, and the plan is to momentarily reverse the polarity of the power supplying the magnet to demagnetize it, in order to facilitate release. These operational steps will be controlled via a PLC.

I'm planning to use a pair of miniature contactors wired with a mechanical interlock for the polarity swap. I realize that the contactors are not rated to break DC; I will be controlling AC power ahead of the bridge rectifier independently.

Anyone see an issue with using a pair of these contactors to swap polarity?

https://www.automationdirect.com/ad...tactors_(4-Pole)/16_Amp_(AC3)/CWC016-00-40L03


They will only carry the DC; they will not switch under load. The voltage is well within their ratings, and there should be no arcing wear on the contacts.


Thoughts?





-rpoet

Bad Idea

After the AC power is removed, it will take a few seconds for the DC current to decay through the bridge rectifier. Opening the DC contactor before the current reaches zero will result in severe arcing on the contacts as they are not designed to switch DC, especially a highly inductive DC load. I realize you can sequence the switching with the PLC but what happens when there is a power failure or a fuse blows? The AC and DC contactors will open at the same time and probably result in welding the contacts of the DC contactor. Why not do it right the first time and use a DC rated contactor? You can still sequence the switching to minimize the wear and tear on the DC contacts.

Dealing with lifting magnets is a lot more complicated then you think
Check with these guys I know they can get you fixed up

https://www.eriez.com/NA/EN/eriez.htm

Using the wrong contactors will give you no end to the problems AC rated contactors will not work for DC. I used Word Lenard in the past but I understand they were bought out by somebody but I just can't remember who.

Download information for a controller from Ohio Magnetics to get an idea of what's needed to control an electromagnet.
See schematic for Hubbell controller here.

I don't have direct experience with lifting magnets so can only come at this this from a theoretical standpoint.

Maybe it is a European thing but you can use AC contactors for switching DC voltage, check the spec sheet for this Eaton/Moeller contactor:
ftp://ftp.moeller.net/DOCUMENTATION...76968-DILMP20(190V50HZ_220V60HZ)-20130328.pdf

The issue with using AC contactors to break DC circuits is the voltage that can be produced as the magnetic circuit collapses. I have often seen AC selector switches that are rated for 415V AC but only 60V DC, the work around is to series the contacts up to the voltage you require. There isn't anything magical about switches rated for DC loads, they just tend to have a larger gap at the contacts to allow for the high voltages produced.

You do need to be aware of the electromagnetic interference that a large spark can create.

This gives some good information on switching inductive loads:
http://www.artisancontrols.com/UtilityImages/ProductSupportPdf/Switching Inductive Loads.pdf

The problem with DC switching vs. AC is that AC naturally commutates through 0v, extinguishing the arc. With DC, the arc has to be extinguished purely through air-gap.

I have several customers who use contactors from these guys: http://www.gigavac.com/ I believe they are Argon-filled, which apparently has a higher insulation value than atmosphere.

We have used DC drives to control electromagnets in the past. By setting the current limit, we couldcontrol the power of the magnet. I wonder if you could just use a regenerative drive to reverse the polarity? You didn't mention the current, but if it's under 15A or so, maybe one of these: https://www.kbelectronics.com/data_sheets/kbrg.pdf You may want to give their applications guys a call.

I doubt you will be able to demagnetize the electromagnet by simply reversing the current through the magnet. This will probably just magnetize it in the opposite polarity. Electromagnet controllers I have seen reverse the current several times, reducing the current on each reversal until the current is very small. This is usually done by adding resistance to the circuit.

Vic said:

I doubt you will be able to demagnetize the electromagnet by simply reversing the current through the magnet. This will probably just magnetize it in the opposite polarity. Electromagnet controllers I have seen reverse the current several times, reducing the current on each reversal until the current is very small. This is usually done by adding resistance to the circuit.

Click to expand...

My research said the same thing, though oddly enough, it appears that's just what this machine had. One contactor connected rectified DC to the magnet, and a second, reverse-wired contactor driven through a one-shot timing relay. When you turned off the first contactor, it was supposed to fire the timing relay on the falling edge and trigger the "demag" contactor. Something in that morass of 45-year old contactors, timing relays, aux contacts, and unlabeled wires went wrong.



I told them it was going to be the most cost effective to simply replace the power supply to the magnet, rather than try to troubleshoot the existing. The magnet in question is actually a magnetic chuck for a surface grinder. They didn't like my quote; they're going to buy a "manual" non-electro magnetic chuck. Oh, well...



Thanks for the help, everybody!





-rpoet

That's the way a mag chuck works. It reverses polarity for a short time, enough to jumble the alignment of the magnetic domains. With the manual magnet they are going to have to buy a demagnetizer because when the part comes off the chuck it will retain enough magnetism to be a problem.
My first life was a toolmaker and there are thousands of those old walker chucks all over the world.
You may find that a couple of diodes for a few bucks will get them running again.