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I need occasionally drive watergate with gear box (e.g. http://www.servomech.com/main/screw-jacks.htm). Because location is far away from 220V I suppose powering from SLA 12V battery charged from solar panel. Battery capacity 7-20Ah. Required motor power is 50-100W (torque I think >0.2Nm). Because of outdoor usage I need IP66 or so. Price is also essential.

I did not find any 12V DC motors and seems that 3 phase AC motor (e.g. SIEMENS 1LA7063-6AB, 0.09kW,870 rev/min) produced in large quantities is the only option.

I can use power inverter 12V-DC/220V-AC and 3 phase frequency changer (e.g. Sinamics G110) to power motor even with trapezoidal profile. Expected efficiency 85%*95%. It could be assembled from stock products and it should work. I don't think it's worth to do it as home-made application.

I wonder if I can do it much simpler when I make simple inverter (IRS2153+current sensing IR2127+FETs+toroid transformer 2x12V/220V) and connect to 3 phase motor using Steinmetz method. Output of inverter is rectangular 220V voltage with adjustable frequency (easy to implement IRS2153 has option to force switching frequency from MCU). enter image description here

The questions are:

1) Steinmetz decrease motor power to 70% and starting torque to 50%. But I believe efficiency remains unchanged, i.e. in other words I can use stronger motor without power loss compared to regular 3-phase triangle connection. Is it true ?

2) Is it possible to regulate motor revolutions via changing of inverter output freqeuncy (i.e. apply trapezoidal profile) ? I'm uncertain how Steinmetz capacitor value (70uF/1kW) is related to voltage frequency (50Hz).

3) how motor copes with rectangular AC waveform? I think it may affect insulation bacause of voltage peaks.

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To actually answer the questions:

1) The efficiency is most certainly changed when you use a capacitor so generate a phase shift. The winding's magnetic field is going to be reduced. It will get it turning in the right direction, but not efficiently.

2) Yes, you can (and do) regulate a three phase induction motor's speed by controlling its frequency. They can run (with simple Volts/Hz control) down to nearly zero speed while maintaining decent torque, and up to beyond base speed as well. But, your capacitor phase shift isn't going to like variable speeds unless you use an absolutely huge value capacitor, which is going to be difficult to find since it has to be non-polar.

3) The most common early AC Variable Frequency Drives (VFD) used a totem pole of SCR's to generate a six-step waveform that went to the motor. These had little impact on the actual driven motors windings. The newer drives do need motors with a better rated insulation system since newer drives use a high frequency PWM waveform to generate the low frequency output to the motor. Older insulation systems don't like modern high frequency waveforms.


All that being said, you would be better off with a simple 12V DC motor. If you MUST go AC, and need variable speed, build at least a full three phase inverter, even if only using 6-step technology.

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  • \$\begingroup\$ Addressing 1) - sometimes there is a starting capacitor and a working capacitor. The starting capacitor gives higher torque until you reach your desired condition, then you switch to the working capacitor(optimized for the working point). This way you get the best of both worlds. Variable frequency is another thing to think about when considering capacitors and one phase, as it will change the impedance a lot (each working point (frequency) has an optimal capacitor). about 3) - well you had the mechanical Ward-Leonard group shivers :-) \$\endgroup\$ – WalyKu Aug 25 '15 at 12:31
  • \$\begingroup\$ @Kurtovic Start/Run caps are used in Single phase AC Motors, and the motor is designed with them in mind. A three phase motor, with balanced windings expecting a 120 degree phase shift between them, with an external capacitor is not anywhere near the same thing. \$\endgroup\$ – R Drast Aug 25 '15 at 12:42
  • \$\begingroup\$ I just wanted to mention that it is something that could be done too. I wouldn't agree that it isn't anywhere near the same thing. Except for the electromagnetic design of the motor to cope with the induced harmonics because of single phase use, the motor itself will be the same. The circuit is fully external to the motor, is what I wanted to say. (Positive attitude intended) \$\endgroup\$ – WalyKu Aug 25 '15 at 12:57
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    \$\begingroup\$ I meant the Steinmetz case, probably a misunderstanding then. \$\endgroup\$ – WalyKu Aug 25 '15 at 12:59

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