motor-generator won't put out

I have a 440 volt, 3 phase, induction motor, from a large marine
windlass, winch application. This motor has variable speed capability,
and the rotor is wound with 3 slip rings. When I first seen it, I
thought it would be easy to make into a genset, as it had lots of
windings on its rotor. I spun it up to around its rated RPM of about
1200, and experimented with putting 12 to 36 volts dc into the slip
rings, with resistors, in series. It put out 110 or 220 quite easily,
on each leg, with a draw of about 7 amps dc into the rotor. It powered
a lamp OK, but when I plugged in a handdrill ot skillsaw, it barely
worked, and the voltage dropped to about 25 volts ac. I suspect the
voltage output is very choppy looking, and not a nice sine wave. When I
take the rotor out and put 12 volts into the slip rings, the magnetism
patterns appear at about 5 spots around the surface, and are
staggered, with different ring combinations. The rotor is about 10
inches in diameter, 400 lbs, and about 30 slots one-half inch apart.
Is there anything I can do to furthut test this thing, short of a
rewind??
Reply to
magna7878
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Your problem is that a 'wound-rotor' induction motor is not meant for generation. The rotor winding connections are wrong.
If rated speed is 'about 1200' rpm, then the stator is a 6 pole winding. The wound rotor for such an induction motor would also be six poles. A generator rotor would have six poles, and the windings around the six poles would be arranged alternating so you would get N-S-N-S-N-S as you went around the rotor.
But a wound-rotor induction motor's rotor is not connected the same. The windings in such a machine are connected much like a six-pole three-phase stator would be, either delta or wye. And only the three tie points are brought out to the slip rings. Tracing around the rotor, the poles would be A, C', B, A', C, B', A, C', B, A', C, B', A, C', B, A', C, B'. That's 'six poles', but three-phase's worth of six-poles. Six 'A' poles, six 'B' poles, and six 'C' poles.
If you're really interested, you could separate the connections for groups of windings on the rotor and re-connect them to try and create just six N-S poles, alternating each one. Then bring each end of the string out to a separate slip-ring (the third slip-ring would be 'an installed spare'). Then you would have a true six-pole generator rotor. You'll have to see if you can find the connections, they may be underneath some 'banding' around the end. If all the connections are on one end of the rotor, reconnecting just takes some good solder/brazing skill and some quality insulation tape (not just that black polyvinyl stuff sold in Home Depot or RadioShack). Obviously, if you start cutting the banding off and poking around breaking connections, you may ruin this thing as far as any future life as a wound-rotor motor (unless you can put things back to right, do you have the equipment to do this?).
If the connections are on both ends (e.g. some coils tied together at ring-end, and others tied together at opposite end), then it won't be so easy to reconnect. But you still may be able to do it without completely rewinding.
Based on 1200 RPM, the rotor should be wound for six poles, and with 30 slots, that would be 30/18 = 1.66 slots per pole-phase-group. Not a nice even number. Are you sure you counted correctly? Still, it's possible with some overlapping of coils (goodness knows, just about every conceivable winding configuration has been tried at least once)
Anyway, if you reconnect it, you're still not going to have a great machine. The magnetic poles will probably be larger than they should be and generate a not-so-great waveform. And the DC current needed in the field winding of a typical generator has to be increased by quite a bit as load is added to the machine. Many machines I've worked with, the field current more than doubles between AFNL (amps, field, no-load) and AFFL (amps, field, full-load).
daestrom
Reply to
daestrom
Hi daestrom, Thanks for the information, the numbers I used are approximates. I will take your advice and see if I can determine how the slip ring wires are connected to the rotor wires. I've already spent so much time on this project, that a little more wont hurt. Supposing that it does work OK, is there an aftermarket field controller/regulator, generic type of device, that I can buy for controlling the field current, and maybe even the throttle.?
Reply to
magna7878

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