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Old 11-07-2011, 07:14 PM   #1 (permalink)
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effect of driving one coil in a 3 phase motor

I'm trying to understand the "theoretical" effect of driving one coil at a time in a three phase motor. I.e. you basically have one pole that is north on at a time.

Would it be coggier?

What would the effect on torque be precisely? Would it be disproportionately less torque? (does the average magnetic gap increase?)

Would it affect speed constants?

Affect on efficiency?

???

I'm just considering my options for a cheezy application that does not need as much motor as I have, where if there wasn't a huge efficiency penalty for just having low side drivers, I would be sorely tempted to do so: etek brushless on a honda 50, w 24 volts in batts.

Is it even doable? maybe there isn't an appropriate hall event even or coil pattern. I guess I would have to figure out a center tap to even consider it and hope it is a Wye.

Thanks for your consideration.

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Old 11-07-2011, 08:15 PM   #2 (permalink)
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If you're talking 3-phase AC, it won't spin.
When we say "the motor is single phasing" it is driving 2 of the 3 coils.
This is a very bad thing that will quickly burn the motor out.

Think this way:
Coil A pulls the North pole towards it, then releases it. With no Coil B pulling it to it, the pole gets pulled back to A the next sine wave. It goes no where.
When two are working, the motor bounces betwen coil A and B and just flutters.
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Old 11-07-2011, 08:16 PM   #3 (permalink)
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here is a basic schematic:


basically DC bus energizes one coil at a time with one polarity (or some permutation of that). Etek is a permanent magnet "brushless" motor fyi.
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Old 11-07-2011, 08:55 PM   #4 (permalink)
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A three phase motor will run just fine when single phased IF:
1) It's already up to speed AND
2) The load is less than 1/3 of nameplate power.

The whole idea behind three phase power is that the motor has three sets of electromagnets connected to three phase power. Three phase power has three different phases - Phase 3 lags behind Phase 2 which lags behind Phase 1.

Phase 1 energizes electromagnet 1 and pulls the rotor toward electromagnet 1. Then Phase 1 turns off and Phase 2 turns on. Electromagnet 2 is rotated 60 degrees from electromagnet 1, so the rotor is pulled toward electromagnet 2. Repeat for Phase 3. Then the phases reverse polarity for the next half revolution. The rotor is continuously chasing next electromagnet.

In practice, the three phases have a sinusoidal waveform and overlap. An electrical engineer can explain all this better and more accurately.
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Old 11-07-2011, 09:11 PM   #5 (permalink)
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Me referring to this motor as three phase is probably causing some confusion. But you hint at another consideration. If I were to take a "normal" three phase coil excitation pattern, then just lop off half of the waves, it would be a little "lumpy", there are three closely spaced pulses then a gap, would probably want to see if I can space the pulses evenly, not sure the geometry even allows for it yet.

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Old 11-07-2011, 09:23 PM   #6 (permalink)
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maybe "3-Phase Unipolar Converter" is a better term for this?
Technical Paper - Low Cost Multi-Phase Brushless DC Motors - PowerPulse.Net

hmm, "inductive energy stored in the windings at turn-off must be dissipated within the motor rather than being returned to the supply", wonder how big a penalty that is...
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Old 11-07-2011, 09:38 PM   #7 (permalink)
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Quote:
Originally Posted by JRMichler View Post
A three phase motor will run just fine when single phased IF:
1) It's already up to speed AND
2) The load is less than 1/3 of nameplate power.
DCB's questions about torque and speed constraints lead me to think variable vehicle drive eliminating the above.
While they will run it does bad things to the windings. If they stop and go into "shudder" the burn out quickly.
We normally install phase monitors to shutdown the motor starter to prevent burning the motor up.

Chopping half the phase would reduce the torque but not the speed.
Frequency controlls speed in a 3 phase motor.

Maybe I am lost, isn't the E-tek a DC motor?

Per your schematic it is a 3 pole DC motor.

Your best and cheapest bet would be to use a 555 chip the fire the IGBTs from 0 to 100 percent.
If it is low enough amperage it will work fine, perhaps sized for a bicycle or scooter. No room or weight allowance for "Paul and Sabrina's cheap controller" type situation.
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Old 11-07-2011, 10:04 PM   #8 (permalink)
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hmm, I will be "halving" the length of the current path too, so I may be putting more current into each leg for a given bus voltage, which might reclaim some torque. Dunno, gonna have to make a center tap and do some experiments with a battery and a scale and an ammeter

Edit, and see if I can find a combination of hall effect readings from the existing hall sensors that coincides with when the poles should be energized (120 degrees apart, leading pole alignment by 120 degrees)

I would still use something like an atmega to control it, easier for me to figure out/modify than a collection of 555s (easier for me to add a throttle too also)
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Old 11-08-2011, 12:20 AM   #9 (permalink)
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ok, thanks all for the input, I think I have my head around it.

I'm wrestling w/disassembly of the etek, and am thinking the problem becomes "why"?

Is a unipolar PM 3 phase hall effect controller with custom center tapped etek worth the effort over a used brushed etek? Doesn't seem likely.

Probably best to just sort out the high side. A hand full of 3 watt transformers and various regulators and some optos is a little bulky, but working is better than not, and it can have a negative gate shutoff.
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Old 11-08-2011, 06:01 AM   #10 (permalink)
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The brushless is more efficient and you don't have to replace brushes.

Now I get what you're up to.
Make 1/3 the controller !
If your motor pole timing is correct, it will work, except you would need to hand spin to get it started.
Not handy in a vehicle but would be fine for a table top / science fair / demo unit.

Personally those E-teks are too expensive to mess with and risk letting the magic smoke out.

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