Electromagnetic Chuck

[rabler]

I'm trying to think how you'd wind any real electromagnet to get 800 ohms. That's an awfully long length of very thin copper wire. You'd almost have to add some intentional resistance.

This is also one of those cases where I'd wonder if you're getting some high voltage noise that your meter is measuring, especially if it is a less expensive (not true RMS) meter.

 

[Reddinr]

If you can lock your meter into its lowest Ohm range it may read an inductive load (magnet coil) better. Sometimes a digital meter (if that is what you are using) can get confused with inductive loads depending on the DMM design.

 

[ddickey]

I have a Fluke so true RMS. I'll do some more checking.

 

[ddickey]

I checked again and it is almost 900ohms. So around 4.5mA. Here's a photo of the electronic. The resistor is 2Kohm and I think the cap was almost 400micro Farads.

 

[Reddinr]

Can you read the part number on the tube? It is possibly a dual rectifier fed by a center-tapped transformer in a "full bridge" configuration. Is there a model number / manufacturer available for the chuck?

I am having trouble figuring out how you got 4.5mA from what you have said so far. 400V, 2000 Ohms, 900 Ohms. Where did the 400V number come from? Was your coil measurement done with the coil disconnected from the power supply?

One thing that concerns me is that the DC voltage you are talking about is very high and quite dangerous in the wrong circumstances. One bad wire nick inside the chuck could lead to high voltage on the case of the chuck. In any case, be very certain that all parts of the chuck and the power supply enclosure are well-grounded to the building's ground system. The chuck is very old so bad or near-failure point insulation is a pretty good bet.

If my guess about the circuit is correct, the 2000 Ohms is in series with 900 Ohm chuck coil (sort of), so that is 2900 Ohms. I = V/R. I = 400V / 2900 Ohms ~= 138 mA. From a glance at the assembly, it looks like the rectifier feeds the 2K resistor which feeds the capacitor. That would make for smooth DC output from the capacitor to the chuck. The math above is not exact because the 2K resistor current will be pulsing at 2X the line frequency and the 900 Ohm chuck current will be much more like "pure" DC.

This all can be easily replaced but it is important that the voltage rating of the chuck be known and that the insulation of the chuck be tested. Possibly ask around your area to see if there is someone that can "hipot" the coil for you to check the insulation between the coil and the case of the chuck.

 

[wlburton]

My little 3x5 chuck on my Sanford works fine but I'm thinking about replacing the antiquated electronics that provide power to the chuck.
Anyone know of an affordable way to rectify 120Vac into 400Vdc?

Another option that does fit on the little Sanford is this chuck from Shars (with no electronics in the way): https://www.shars.com/4x7-strong-magnetic-pull-fine-pole-permanent-magnetic-chuck. If yours was not working that would be one way to go, but since it is working what you're trying to do probably makes more sense.

 

[ddickey]

From LathesUK
A small electrically-excited magnetic chuck was offered from the beginning of production, the necessary rectifier unit being housed within the base and suitable plugs, wiring and switches provided as part of the installation. The earliest version of the power supply used a Lafayette #4612, type 80, full-wave rectifier vacuum tube of only 1/8 amp output, the components being fitted to a metal plate inside the base. Later versions used a solid-state diode providing only half-wave rectification, although this was boosted by a matched capacitor. This much smaller circuit mounted directly on the switches, isolated by a fiber sheet, with the switch positions marked "On", "Off" and "Demagnetize". To replace the old, now-unobtainable components, a modern solid-state, full-wave bridge rectifier power supply has been designed and the schematic is available on the Internet.
I know @vtcnc has an SG48. Maybe he could put a meter on the output to the chuck?

 

[mksj]

Looks very dangerous with the broken down insulation. If it is possible to measure the transformer voltage output, the voltage drop of the tube rectifier and RC (resistor capacitor) can be modeled and the output voltage determined. I would consider the power supply you have a total toss, but one can determine output DC voltage.

PSUD2

 

[rwm]

If that really operates at 400 vdc I would be concerned. I would keep it as a display piece and get the permanent magnet chuck!
Robert