Electomagnetic discharge 2

[xcracer]

I'm having issues with arcing while controlling an electromagnet.

I realize arcing across a relays contacts is normally counteracted by using a snubber. But, there's another aspect to my problem which I think will require more than just a snubber.

After removing charge from the magnet, I pull the magnet away from its workpiece, which causes the electromagnet to work as a motor and generate energy.

Any suggestions, comments, etc?

Any help is appreciated, as I'm quite new to the electrical world (trained as electro-mechanical, which was really just mechanical & plc programming).

Thanks

 

[MJR2]

What you might want is a drop controller. Perhaps like one of these.

http://www.ecandm.net/products.aspx?category=LiftingMagnetControllers

Mike

 

[VEBill]

10A, even inductive, is well within the practical limits for solid state switching with a nice big fat FET (or several in parallel if required). If designed correctly, it would last 'forever'.

 

[Skogsgurra]

That is something I would like to test next time, MJR2. Do you have any experience with them? Safety and such things?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MJR2]

Drop controllers are used with lift magnets of all sizes. Particulary in rather automated systems. This particular brand I have no experieince with. Hubbell also has some.

Some controls actually drive the magnet negative to discharge/drop faster.

Safe? Never had what I thought of as safety issues.

They sure beat just pulling the powered lead off. Once is enough of that.

Mike

 

[Skogsgurra]

Soo right! Thanks

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[xcracer]

Thanks for the info.

Part of the reason I was hoping to stay away from a full blown magnet controller was due to the cost, and also the requirement. I'm only running 168 Watts (7Amps) @24Vdc. Most of the magnet controllers I've glanced at are rated for much higher.

I think I'll go the route of a DC contactor and look into using a reversed current for a short pulse.

Cheers

 

[MJR2]

If you don't have a drain for the current the voltage spike will eventually cause a winding failure in your magnet. And eventually can be a rather short amount of cycles. We have them integral on 40 watt lift magnets.

Mike

 

[biff44]

If I understand the problem, you have an electromagnet that is powered by 24V DC. With the power supply off/unattached/etc, if you move a magnet away from the coil, you get arcing somewhere in the system.

Sounds like you have reinvented the magneto!

Just put a large wattage zener diode across the coil terminals. You put I would pick perhaps 50 volts. The zener will clamp any reverse voltage greater than 50 volts, and any forward voltage greater than 0.7 V. Problem solved.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[Skogsgurra]

Problem with the zener is that the inductive kick is negative. So, to use the 50 V, you have to connect cathode to the negative terminal and anode to the positive terminal. Then, the zener will conduct heavily when you apply power. Not good at all. You could just as well use a diode, with the delay issues mentioned before. Or a diode and a resistor.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

For relays, zener diode paired with a standard diode back to back is what Tyco recommends... see Figure 3 on page 2 here:

http://relays.te.com/appnotes/app_pdfs/13c3264.pdf

It provides the best compromise when we want to get the fastest reduction of current possible (fastest dropout) while clamping voltage to a specified value.

The requirement to get fast reduction of currrent is important for relays with time-sensitive outputs. Also some have suggested slow operation of the relays can damage the output contacts if I understood correctly.

I don't think any of the concerns for fast deenergization apply to your application (??) so seems like diode with series resistor would do just as well.

There was also a lot of good discussion by contributors in this thread:
thread248-291225


=====================================
(2B)+(2B)' ?

 

[biff44]

"Problem with the zener is that the inductive kick is negative. So, to use the 50 V, you have to connect cathode to the negative terminal and anode to the positive terminal. Then, the zener will conduct heavily when you apply power. Not good at all. You could just as well use a diode, with the delay issues mentioned before. Or a diode and a resistor. "

No, you connect the zener so the cathode is on the + terminal of the power supply, and the anode is on the - terminal of the power supply. That way it does not conduct when the power supply is turned on.

When a inductive voltage kickback happens, it is thus limited to Either +50 Volts, or - 1 Volt.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[Skogsgurra]

The misunderstanding is that the kick-back is negative. Not positive. So your zener will act as a standard diode. Make a measurement to convince yourself.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

Gunnar is correct.

Connection using standard diode is shown in figure 2 of link above (does not rely on any reverse breakdown). Upon interruption the coil produces a voltage which (when seen by the rest of the circuit) will tend to keep the current flowing in the same direction as it had been.

The circuit using a zener is in figure 3 linked above, and provides best compromise of clamping the reverse voltage and rapid current reduction...but again rapid current reduction may not be important to your application.



=====================================
(2B)+(2B)' ?

 

[VEBill]

...coil .... tend to keep the current flowing in the same direction as it had been.

Perfectly correct (of course).

 

[xcracer]

ElectricPete - thanks for your input and the link to the other thread on the forum. After reading through the other thread, I've come to the conclusion that a diode & zener would be the best option, as I do need rapid current reduction.

As I've never specified diodes before, what are the major characteristics to take into consideration?
(The parts will be purchased from Allied Electronics, and their online catalog shows ALL parameters, so it doesn't give me a good idea of what are the main characteristics to consider.)

As for spec's (*these have changed*) (I had all 4 magnets in parallel, but re-wired so each magnet has it's own relay contacts)
Electromagnet with 8.4ohm resistance @ 24v, therefore 2.85 Amp.
What size diode and zener would I need to take up the spike produced?

Note - it was also determined that the motion of the workpiece being moved away from the magnet was not causing the issue, as I initially suspected.

My apologies if these questions seem overly simple; I am outside my scope of study and have very much appreciated the input from everyone.

Cheers