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| Driving 3-phase motor from 2 amplifier phases |
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| Circlotron:
Got a 3-phase permanent magnet synchronous motor. One wire tied to ground and the other two are fed with sine waves 120 deg apart. The motor won’t rotate, it only jiggles back and forth a bit. If the windings from ground to each phase are fed equal sinusoidal voltages 120 deg apart then shouldn’t the third winding that goes between the driven points be getting the same voltage and the right phase displacement? |
| Circlotron:
Looks like it's more complicated than I thought... Putting it another way, say we had a 3-phase supply that is completely floating. If we ground one of the motor wires and look at the first ungrounded wire with a scope, what is the relative phase of the second ungrounded wire to the first, and to ground? |
| soldar:
--- Quote from: Circlotron on December 27, 2018, 03:45:29 am ---Got a 3-phase permanent magnet synchronous motor. One wire tied to ground and the other two are fed with sine waves 120 deg apart. The motor won’t rotate, it only jiggles back and forth a bit. If the windings from ground to each phase are fed equal sinusoidal voltages 120 deg apart then shouldn’t the third winding that goes between the driven points be getting the same voltage and the right phase displacement? --- End quote --- All this is extremely confused and confusing. You have power with two phases and neutral? Where did the third phase go? Can you post a clear schematic of the whole thing? |
| Circlotron:
Looks like the phase difference between the two amplifier outputs has to be 60 deg. That way at least there is equal voltages applied across each motor winding. Still won't run! |
| Circlotron:
Well, eventually got it to run! Maxed out at 120Hz, 30V peak, 2400rpm. (6 pole motor) The amplifier was *very* sensitive to volts/Hz. When the volts was right for the Hz the current would be lowest. Slightly off and the amp (2x LM3875) would go into current limiting and the motor would growl. What was really interesting though is the phase difference between the two amplifier outputs. In theory at least, it should have been 60 deg all the way, but in fact the optimum varied from 50 to 70 deg. Dialed it back and forth a degree at a time to find the smoothest running. Next step is to get a proper electronic speed control like used on drones etc. Not sure whether to go for sensor type or sensorless. The motor does have 3 hall effect position sensors, just I don't know which would work better. |
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