Fixing a Magnetic Drill Base?

I picked up a spares/repairs mag drill from you can guess where. It has two coils in the base (wired in parallel), only one works which is OK but not great so I'm contemplating whether it is worth trying to fix the other. Replacement part costs almost half the price of a brand new drill (+VAT+shipping) but that's the usual spares pricing right-to-repair rathole so we won't go there.

So, I am assuming it is a simple round coil in resin which could be dug out followed by winding a new coil on a former to be slid in, connected and then resin refilled. However my attempts to calculate required wire size don't add up - it's 110V and I get ~420 ohms resistance with my DVM, but playing with numbers from Wikipedia table of enamelled wire sizes suggests that the length I would need to match this won't handle the current or fit in the allocated space.

Where have I gone wrong? How should this be done?

p.s. additional info, coil is 50mm ID, 75mm OD, 25-30mm deep so cross section of ~ 25*12.5mm^2

There could easily be a poor connection or a damaged piece of wire in the circuit that is giving a higher resistance than expected. Since one coil had failed there is a possibility the second is on its way out as well. Since you are considering digging one out you will be able to find what the existing size is from the dead coil.

Martin C

PS Found this YouTube video

Repairing a Hougen mag drill electromagnet

Edited By Martin Connelly on 12/01/2022 15:36:24

Thanks - Clarification: the measured resistance is from the working coil, the non-working one is open circuit.

There isn't much circuitry inside, just a black box rectifier than turns input AC into DC and splits it to two connectors wired in parallel.

p.s. to your p.s. Yes, that video is what I had planned but I started with the desk calculations rather than pulling anything to bits. Interesting that his coil resistance is 1/3 of mine. I don't have the CNC lathe coil winder either!

Edited By Adrian R2 on 12/01/2022 15:44:16

I had a look at some data from new mag base drills (Hougen, quoting 115v for USA) and they had a system current 1 amp over the motor current. As indicator lights are likely low power this gives slightly less than 1/2A per coil so I would expect a reactance of of about 230 ohms and a DC resistance below this. If you can find similar data for the one you have it may help figure out if the resistance you have measured is a properly representative value for a good coil.

Martin C

Ok, so my measured resistance is supiciously high. No docs available, so options are:
1) Try a different DVM
2) Measure the current in operation for the good coil and see if that agrees
3) Dig out the coil and examine it
4) Just go with 31 gauge & cross fingers

Edited By Adrian R2 on 12/01/2022 16:41:25

If the coils are fed from a rectifier as Adrian said they will see a messy mix of DC and AC unless there's a BIG smoothing cap which I doubt.

As J H says, without smoothing capacitors and or a choke, the DC will have a ripple.

Half wave rectification will block every other half cycle whilst full wave rectification will give constant supply, but withe voltage constantly fluctuating between zero and peak, at supply frequency..

The ,inductance of the coil and hysteresis in the magnetic circuit will mean that the magnetic flux never drops to zero, so the base will stay in place as long as the forces imposed on it are not excessive..

With one coil U / S you will only have half the retaining force available, so returning the other coil to operation will be the way to go to ensure safe operation of the complete unit..

It is probably 110 Volt since it is intended for site operation from a transformer with two 55 volts windings centre tapped to earth.

Howard

Howard

Indeed - the rectifier is a black blob and I haven't examined it further - from measurements110VAC goes in and 110VDC comes out, but I don't have an oscilloscope so I don't know how clean it is. With two working coils you can use the thing upside down for drilling overhead beams (with safety strap for power outages) but I don't intend to attempt that.

I will investigate further at the weekend and report back if anything interesting results.