Any motor and clutch experts in the group?

Evening all, nice out, isn't it?
I'm hoping to pick some brains, I recently acquired an ancient and abused [1] Challenger lathe, mainly for the tooling and motor that came with it,
and I think I'd like to get it running to get back in the hang of machining while I find what I'm actually looking for (a Holbrook would be nice...) at a price I can afford!
Being a fan of skips, the dump etc. I came across an old Audi air-con pump complete with electromagnetic clutch - I believe that these are called on to transmit upwards of 5HP (certainly putting 12V at 4A up it locked it pretty firmly), it occured to me that I could use this as a drive clutch for the 3HP (eek! possibly a bit over-sized for a small lathe...) single-phase motor that came with the lathe, on either the motor output shaft or the countershaft-that-needs-to-be-built, can anyone see any glaringly obvious snags that I haven't?
I've had the motor connection box open, and have sussed out how to reverse it, should I want to, but would like the added safety of braking it - I've seen a few references to DC braking, wonder if anyone has had any practical experience of it? I assume that it's just a case of dumping Several Amps into the motor coil and letting back-emf/induced current in the rotor drag it to a halt?
Thanks in advance for any help!
Dave H.
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(The engineer formerly known as Homeless)

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The air con pump clutch is probably rated at 5hp @ 6000rpm. A lathe is 3hp @ 1500rpm so the torque is much higher. Personally I prefer the feel of a manual clutch.
You can get DC braking by shorting out all the motor windings so that it generates on the overun. It's not as good as injecting real DC but will help slow things down. I hope your chuck isn't screw mounted like mine. Stopping the headstock causes a torque reversal that unscrews the chuck so it falls off and chases you around the shop. All great fun but it sctratches the workpiece while it's doing it.
John
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"John" wrote...

Well, I've got a 3HP motor for a 5" lathe, a bit of slip in moments of stress would probably be a good thing... I can see a way to vary the current to control the frictional force, might just work to give a smooth take-up? I've got plenty of high-current low voltage transformers about the place, so DC braking's a real possibility from the sound of it, will have to investigate dumping a few amps through the windings - the start/capacitor winding's a lot higher DC resistance, so may have to use the main winding (about 1.5 ohms to DC), 12V at 8 amps? 24V at 16? I'm a little concerned that the thermal losses might cook the motor coils if I'm not careful, so some sort of timer might be required. My chuck does, indeed screw on, but before putting DC braking into action I'll add a spider on a drawbar (L/H threaded?) to retain the chuck in panic stops - I've seen a 12" chuck take a 1000RPM trip across the shop floor, very glad I wasn't in its path, like the wall was (temporarily)!
Cheers, Dave H.
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(The engineer formerly known as Homeless)

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