Magnetic Chuck Power Supply For the Blanchard 42" No. 18

laatsch55

laatsch55

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Had a lot of problems with the Neu-t-Rol chuck controller. So I built a +/- 117 VDC power supply , with the only de-mag option is to reverse polarity manually., What has me stumped is this little paragraph in the controller manual------------


Breaking the D.C. supply either ahead of the Neu-t-rol and the Magnetic
Chuck, while the Neu-t-rol is in the full "on" posit ion by other means
than the Neu-t-rol is dangerous to the Magnetic Chuck and should never
be attempted. This practice surges the Chuck with the possibility of
puncturing the insulation and causing grounded Chucks, which can also
break down the insulation on the Neu-t-rol drive motor and burn out an
excessive amount of pilot light.

Being the voltage and current will be coming right off the power supply with no intervening components I could see a surge. Not talkin a whole lot of either, 230 VDC at 2.97 amps...

Sooooo.....how do I control the surge?
 
laatsch55 said:
Had a lot of problems with the Neu-t-Rol chuck controller. So I built a +/- 117 VDC power supply , with the only de-mag option is to reverse polarity manually., What has me stumped is this little paragraph in the controller manual------------


Breaking the D.C. supply either ahead of the Neu-t-rol and the Magnetic
Chuck, while the Neu-t-rol is in the full "on" posit ion by other means
than the Neu-t-rol is dangerous to the Magnetic Chuck and should never
be attempted. This practice surges the Chuck with the possibility of
puncturing the insulation and causing grounded Chucks, which can also
break down the insulation on the Neu-t-rol drive motor and burn out an
excessive amount of pilot light.

Being the voltage and current will be coming right off the power supply with no intervening components I could see a surge. Not talkin a whole lot of either, 230 VDC at 2.97 amps...

Sooooo.....how do I control the surge?
Click to expand...
So Lee you have to ask yourself when I turn off a DC powered inductor, especially one of this size, what happens.

You get a very high voltage flyback when the current is interrupted suddenly. E=L*di/dt

When you switch off the PS connection to the chuck you need to provide somewhere for that current to go until the inductor dissipates that energy through its own series resistance. A big ass flyback diode across the chuck should be added. If you wish the current to decay faster, put a series resistor in line with that flyback diode.

What does your power supply consist of?
 
A 169 volt center tapped toroid, 800va, 2---15, 400 uf /200 volt Kemet caps, a 50 amp BR...
 
Absolutely Joe, with enough contact blocks to reverse polarity. Allen-Bradley 800T ,3 position , 2 maintained one spring return.
 
Here it is...kinda rough...
 

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laatsch55 said:
Here it is...kinda rough...
Click to expand...
Is the switch just on off Lee or does it reverse polarity on the chuck? The big, high voltage flyback diode will be on the chuck side of the switch with its cathode pointing to the positive voltage of your supply when switched on. If the switch reverses the polarity on the chuck then we will need to consider something different. I would use another 50A bridge for that flyback diode. + terminal of bridge to the positive voltage downstream of the switch and the - terminal of the bridge to the minus voltage downstream of the switch. The 2 AC terminals will be left unconnected.
 
The switch will trip a 30 amp open face relay, ( 2 of them), and it will switch both + and - so we can reverse polarity. I can add a BR downstream of each relay.. Using a relay because DC is DC... Yes, a revision before the drawing was done..
 
So it's a very short duration surge...in like milliseconds?
 
laatsch55 said:
So it's a very short duration surge...in like milliseconds?
Click to expand...
Yes but would be thousands of volts if not snubbed like this with a flyback diode. That chuck is no doubt, multiple millihenries of inductance.
 
There are 6 coils under the 42", totalling 77 ohms...
 
So, our trick is going to be to take one snubber offline as we switch polarity, but not too soon, to be able to still snub, right?
 
laatsch55 said:
So, our trick is going to be to take one snubber offline as we switch polarity, but not too soon, to be able to still snub, right?
Click to expand...
Why are you switching polarity to the coils Lee?

BTW, with 77 ohms you have plenty of internal resistance to absorb that energy...
 
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