I have seen some boxes (with some capacitor circuits inside) which do single-phase power supply to 3-phase power supply conversion. The problem with them is that they cause the load motor to heat up, because the phase difference between the legs is 90 degrees instead of 120 degrees.

Is it possible to design a circuit that produces the third phase with exact 120 degree phase difference using only the original single phase?

Is it possible to use the available phase legs equally to produce the third phase, so that loading on a single phase is avoided?

How would this circuit look?

Note: The motors in question are 3.7 kW-5.6 kW induction motors.

  • \$\begingroup\$ how do you know that motor get heated just because not 120 degrees? \$\endgroup\$ Oct 8, 2012 at 3:51
  • \$\begingroup\$ @sandundhammika The installation Engineers told that the heat is because of not exact 120 degree. \$\endgroup\$
    – vivek
    Oct 8, 2012 at 4:01
  • \$\begingroup\$ oky do they check for 3rd harmonics? \$\endgroup\$ Oct 8, 2012 at 4:03
  • \$\begingroup\$ @sandundhammika No.They just install and go. Not doing any test on site. \$\endgroup\$
    – vivek
    Oct 8, 2012 at 4:06
  • \$\begingroup\$ the installation engineers are correct; if your phases aren't balanced you will end up generating more heat for a given torque output. \$\endgroup\$
    – akohlsmith
    Oct 9, 2012 at 13:42

3 Answers 3


I'm understanding this question to mean that you're trying to run a three-phase motor off a single-phase line. If you're trying to run the motor directly off the AC line, the phase angles involved will make it difficult to get the motor started, which is part of the reason three-phase exists in the first place. Single-phase motors usually have motor start caps for just that reason. That sounds like what you're describing.

The simple answer to your question is that to get three-phase AC from single-phase AC, you need to rectify the single-phase AC line into DC, then run the DC back through an inverter to get controlled three-phase AC. There are other electronic approaches, but they're less common in my (limited) experience. There are also mechanical approaches, which may be more convenient if you have the parts.

I'd suggest using a drive to operate your three-phase motor. Typical variable-frequency three-phase drives are exactly what I described above: a rectifier, followed by an inverter. I can't speak as to what's on the market in a given power class, but larger three-phase drives typically have terminals for the three-phase AC line input, the DC bus, and the three-phase motor output. If you have those terminals, you have two options.

One is to run single-phase AC through the three-phase input of the drive. If the voltages are correct, the drive should operate fine. The caveat is that you'll have to derate the drive somewhat. The input diodes are spec'd assuming that the drive's constant-power load will be distributed among three legs of the rectifier. If you distribute that same load over just two legs, those diodes will get hotter. The internal bus capacitors will also get hotter, because they'll see more ripple current without the third phase. Check with the drive manufacturer for the derating info.

If your drive has DC bus terminals, your other option is to skip it's internal rectifier and use an external one. Rectify the single-phase AC, then use that DC as the input to the drive. This will let you avoid derating the drive. My company makes something exactly for that purpose, though its power range may be larger than is cost-effective for your application. You'd have to price both options out to find out for sure. Read this for more details.

  • 2
    \$\begingroup\$ The "other approaches" link only lists electronic means. A single-phase motor turning a three-phase generator will also perform this feat and give you proper 120-degree phase differences, at the cost of mechanical noise and loss. It's called an M-G (motor-generator) set. \$\endgroup\$
    – akohlsmith
    Oct 9, 2012 at 13:44
  • \$\begingroup\$ Quite right! Editing to include. \$\endgroup\$ Oct 9, 2012 at 13:48
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    \$\begingroup\$ You may want to also add that almost any inverter can be used as a single-phase to three-phase converter if you derate the inverter to 67% of its nominal rating. The derating is required because your rectifier diodes on the front end will be sourcing significantly more current than they would if three phases were present on the input, and also your capacitor bank will have higher ripple current, which is the #1 thing that kills capacitors. So you derate a 100kW drive to 67kW, disable the "missing phase" fault and away you go. The motor will run great and the problem is solved. \$\endgroup\$
    – akohlsmith
    Oct 9, 2012 at 14:04

This is actually a very common and legitimate desire, for example to run quasi-industrial machine tools equipped with 3-phase motors in a home workshop.

Three approaches are generally available:

  • use capacitors to accomplish a phase shift to manufacture at least enough sense of a third phase to tell the motor in which direction you wish it to start. This is closely related to how single phase motors are often started with a shift capacitor which is then cut out by a centrifugal clutch. Some capacitors may remain in the circuit while running. Although low cost and simple, this is the least advantageous solution, and the uneven usage of the phases won't let you operate at designed efficiency. You can sometimes see these solutions listed in machinery catalogs as "static phase converters"

  • use a "rotary converter" which is sort of a rotational-inertia auto-transformer. Basically, you get another 3-phase motor of comparable or larger size, and spin it up with a start cap (or even, though not recommended given what can go wrong, a pull rope). You connect both motors in parallel, with the idler generating a third phase for the load motor from its rotational inertia (any 3-phase machine will readily operate as a motor or a generator, depending only on if it is mechanically rotating ahead or behind the electrical phase rotation). Some will further fine tune these with capacitors. Although this is a complicated setup it tends to work fairly well, and with a well sized idler gives advantages of 3-phase power such as quickly reversing during lathe threading operations.

  • use modern power semiconductors to rectify the available power source to DC, then PWM synthesize 3 perfectly phased sine waves. These are typically called variable (or sometimes vector) frequency drives, and have the additional advantage of enabling variation in the applied voltage and frequency, which is to say the speed of synchronous rotation, allowing the user to fill in the gaps between manual belt/gearing options or potentially eliminate the need for a transmission altogether. Prices for this technology are closing in on that of rotary converters, especially when you consider the cost of copper in a rotary converter idler motor.


The motor's current varies with the load, with the voltage, etc, so it's not something stable and that can be counted on. For this matter, from a two-phase (assuming 180deg) it's impossible to do a stable 3 phase conversion with only passive elements since you would be needing reactive devices to do the phase shift and all their reactances (impedances) depend on the current, therefore both the amplitudes and the phases will vary. I suspect your box-circuits just need recalculation for the proper power consumption, but they will only be able to supply a motor of a specific power. Still, even if you only have a 90deg phase-shift the motor should still be running, but with lower efficiency and stability, possibly heating if the loads are close to their maximum.

If, however, you have a quadrature two-phase (which I doubt), it's simple with just a transformer and proper windings (Scott-T transformer). The best option, though, would be a motor controller, but that can be expensive.

  • \$\begingroup\$ @vivek, you surely realize that the same is classified as illicit tampering by the utilities, please refrain from asking questions aiding and abetting shady activities. \$\endgroup\$ Oct 8, 2012 at 9:32
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    \$\begingroup\$ @VaibhavGarg, We recently had a discussion in relation to this. Here we discussed helping someone learn to crack a safe. The issue is that we can not reasonably enforce rules as to the use of advice. Any person can easily word something that has malicious intent to sneak by your radar and get answered, so you have to make the decision to answer what you are comfortably answering and if you feel someone is ignorant of regulations/laws inform them as best you can, but you cant force them. \$\endgroup\$
    – Kortuk
    Oct 8, 2012 at 10:51
  • \$\begingroup\$ For my part, this thing is done around here with more or less good results, because not everyone has 3ph. In order not to fall into the illicit category, the "tuning" has to be good enough and, somehow not surprisingly, it is done so because nobody wants to spend extra money on another motor or on the electric bill; still, it's not something encouraged. This is just an example for the saying "different worlds teach different things". What ohters decide to learn from it, it's up to them but one should remember that the messages are particular to each and everyone. \$\endgroup\$
    – Vlad
    Oct 8, 2012 at 11:29
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    \$\begingroup\$ @VaibhavGarg No, you have no such factual knowledge, you have only speculation as to the poster's intent. There are also many perfectly legitimate needs for wanting to solve this problem. \$\endgroup\$ Oct 9, 2012 at 14:25
  • 1
    \$\begingroup\$ @ChrisStratton, the details were there, but were edited out of the question, please see revision history \$\endgroup\$ Oct 12, 2012 at 6:38

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