Resistance problem in magnetic chucks

[Ron Mercer]

Hi, trying to find answers. I have large electro magnets in our plant. They all use 480v dc to turn on and off. I want to use one of the controls from a particuler brand to turn on another brand magnet. It worked, once, but blew the diode fuses and crydom bridge. I replaced them and found the original magnet to have 52 ohms of resistance, but the other brand has only 1.5 ohms resistance. Both magnet types require a 50 amp circuit to operate, but I'm not sure what the magnets actually pull. I have no wiring sch for either unit. Is there a way to add say 50 ohms in series with the 1.5 ohm magnet and if so, how large would it have to be? By the way, these are electro perm chucks. They require less then a second of energy to turn on then the power can be disconnected until ready to de-mag. Thanks for the help
Ron

 

[bmcculla]

How hot do the magnets opperate? It might be that the inrush current when the magnet is cold is what blew your fuses. If its a long coil I guess it's possible it would be 1.5ohms cold and 50 ohms hot. If this turns out to be the problem you can rig up a PWM soft start circuit to limit the current until the thing has heated up a bit.

A PWM circuit is still probably the way to limit the current to 50Amps. adding a 50Ohm resistor would cause huge power dissipation in the resistor(I^2*R = 125000W).

Hope this helps
Brent

 

[RGBrainbow]

Hi,
driving the 1.5ohm-coil with AC might work too, because the coil's impedance lowers the current. (If alternating magnetic field is acceptable)
regards
joachim

 

[Klaus]

A PWM circuit is still probably the way to limit the current to 50Amps. adding a 50Ohm resistor would cause huge power dissipation in the resistor(I^2*R = 125000W).

Hope this helps
Brent

He still would get this huge power dissipation with his original magnet, since that measured 52 Ohm!
I suspect the measurement is incorrect, if one divides the current (50Amp) into the supply voltage a value of around 10 Ohm results.
A much more likely figure :wink: .
The difference between this and the new magnet coil would still blow the fuses though. Electro magnetic force is a product of Ampere-turns at the coil. Since the turns cannot be changed easily the only choice is to alter the current and hence the circuit supplying it.
This assumes that both magnets are of identical lifting capacity in the first place.

Soft start would not work in this case as the magnet only gets energised for a second, as stated above.
Klaus

 

[Klaus]

The difference between this and the new magnet coil would still blow the fuses though. Electro magnetic force is a product of Ampere-turns at the coil. Since the turns cannot be changed easily the only choice is to alter the current and hence the circuit supplying it.
This assumes that both magnets are of identical lifting capacity in the first place.

Klaus

Re- reading the original message, the info on the magnets state identical currents (50AMP), no other details. This means the magnet with the lower resistance requires less Volts, else the current gets exceeded.
Being a DC magnet, a series resistance or lower power supply are the options.
Adding a series resistance is a huge waste of energy so the lower supply voltage option remains (75V - 1.5x50)
Klaus

 

[Ron Mercer]

:shock: Thanks for all the info, I agree, numbers do not jive. I will try to get more info from manufacturers. Hard to get voltage readings in less then a second of on time. Once again, THANKS TO ALL.
Ron

 

[Phasor]

Hard to get voltage readings in less then a second of on time.

Try using an analog meter - half a second is usually enough to get a rough reading.