Electro Magnet Power Supply

I am looking into using electro magnets to grip and move parts in a system we are currently quoting to a customer. The cost of the magnets is not that high but the power supplies seem quite expensive.

They claim to have a feature that reverses the current when releasing to have the part come off the magnet more quickly. That sounds great but a 50watt powersupply with this feature is over $900. A little cheaper w/ a toggle switch but this one will take a PLC output to turn the magnet on and off.

Is this somethign I could do myself with a reversing contactor?

Thanks

I may be wrong but I do not think they are actually reversing polarity, since the magnetism is developed by having current flow. What is more likely is they are removing the current to the magnet and providing a "drain" to remove residuals.

You have to be careful with electromagnets, if saturated they may become permanent magnets.

I guess you could reverse polarity but let's say 24vdc powers with turnon at 5vdc and magnetic field increasing in strength with max stregth provided at 24vdc. In that case you could switch it from 24vdc to maybe -2vdc; which should quickly reverse the current flow but not have enough power to create a magnetic field. I do not have any electromagnets to play with or I would test this.

They said it may work with a standard supply just switched on and off but I would have to test it. The claim to fame for their power supply is that is should release the part almost immediately.

If it takes to long with a standard supply I guess I can always fall back to their supply.

Reversing the polarity depends on how the magnet is wound, but I dont think that what the "special" power supply does... Typically you would pick the part with high voltage, then when the part is secured with the magnet, lower the voltage (maybe by 60%). In many cases this is done with a timer or maybe a prox switch on the magnet. This would be more effective by using a PWM power supply such as used for a small dc motor.

$900.00 for 50watt supply sounds a bit high. My guess it's a modified PWM motor control with a sensor for the magnet. Consider the idea that a good PWM motor controller will cost about $300.00-400.00, then you have to modify it, and spend some time also, all risking poor results. Re-think your options and consider the $900.00 unit.

I have been doing this for years now using a standard 24 VDC power supply through 2 opto couplers. The opto couplers are controlled by PLC outputs. The first opto turns on and holds the part for the move. Then the second opto comes on for a couple of milliseconds in the reverse direction. there is also a resistor in series with the magnet in the reverse direction to drop the voltage to about to 1/3 of what it is in the forward direction. the size of the resistor depends on the resistance of the of the magnet.

Joe

Joe sounds like he's got it all figured out.

My experiments with using electromagnets to transfer a rubber coated steel belt strip (tire industry) led me to conclude that:

Yes, you do need to reverse the polarity through the magnetic coils to precisely release the product, and you want the current of that reverse polarity circuit to be adjustable to match your application, but it will be much less than the normal holding DC current.

If you just turn off the power, there will be a residual magnetic field that will fade away on its own over the course of a few seconds in my case, and the duration of that fading magnetism was quite erratic. That might be okay for some applications where the magnet and product are physically separated when that happens. In my case, the product still stayed in contact so I needed the current reversal to make it work well.

Putting just a couple volts DC reversed through my coils caused the product to release immediately. I used Magpower supplies that took an analog input and had a built in pot to set the reverse current, so in the PLC, when I wanted to drop the load, I just set the output to zero, but the magpowr card actually would go from 90.0vdc @ 1/2 amp to -2.0 volts and very little current (I never measured the reverse current flow).

I am sure you could do this just like Joe did for a lot less than $900/supply.

JMHO
Paul

Link to Schematic

In this link, the poster was concerned about a lack of ground on the output side of the transformer. There's a schematic and a good discussion about how it works.

Joe Loescher said:

I have been doing this for years now using a standard 24 VDC power supply through 2 opto couplers. The opto couplers are controlled by PLC outputs. The first opto turns on and holds the part for the move. Then the second opto comes on for a couple of milliseconds in the reverse direction. there is also a resistor in series with the magnet in the reverse direction to drop the voltage to about to 1/3 of what it is in the forward direction. the size of the resistor depends on the resistance of the of the magnet.

Joe

Click to expand...

Joe Thanks. Would you be able to post a schematic to save me re-inventing the wheel?

Take Care
Norm Dziedzic

Hey Norm,

I'll have a look tomorrow when I'm back at work and see if I can dig up something for you

Joe