I have a goal to build a circuit from scratch to simulate a 3 phase motor. I do not have any specific requirements, rather, I want to make sure that I understand what is wrong with my approach and current understanding. From what I remember in school, a capacitor can be used to induce a phase shift in an AC source, my question therefore follows, how can I design a circuit that shifts phases at intervals of 120 degrees? enter image description here

The crude drawing below attempts to portray my attempt at understanding this. 3 sets of electromagnets, same size and number of windings, are placed in this configuration and all three are connected to mains, with the addition of three capacitors set at different values to induce the desired phase shifts (We are also assuming all applicable safety circuits and practices, etc). From what I have read, it seems as if this is a gross oversimplification of a practical circuit, if any degree of accuracy can even be located. Please help me understand.

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    \$\begingroup\$ You are very crudely describing what is called a "static phase converter". It's generally a bad idea. Rotary phase converters using an idler motor are a better one, and VFD's which rectify the single phase input and electronically synthesize 3 phases are a good one. All of these are quite hazardous if you don't know what you are doing. The bigger problem though is that you said you need to "build a circuit from scratch to simulate a 3 phase motor" - "simulating" a motor doesn't make much sense here, you might as well do that with a computational model as with mains voltage. \$\endgroup\$ – Chris Stratton Aug 9 '17 at 0:46
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    \$\begingroup\$ Please read up en.wikipedia.org/wiki/Scott-T_transformer \$\endgroup\$ – Janka Aug 9 '17 at 0:52
  • \$\begingroup\$ consider a motor-generator and how energy is converted and impedance is low. then compare to the impedance of Caps \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 9 '17 at 0:56
  • \$\begingroup\$ @ChrisStratton, I poorly worded my intentions, I wanted to understand the practicality/feasibility of this approach, I haven't intended to attach a load to the circuit or conduct any simulations. \$\endgroup\$ – Alex Riphin Aug 9 '17 at 20:44

Theoretically you are on the right track but there are practical considerations. As the load varies, the phase shift would also vary and with three phases, the phase relationship will always be wrong for all loads but except one. Since three phase motors are used for high loads, the capacitors would have to be very large, and it is hard to conceive of a network that would accomplish the 120-degree phase shift that would be required between the three phases and provide it with both the high torque loads at start-up and the reduced torque while running or idling.

That having been said, your approach is actually commonly used in two-phase motors. If you research "split phase motors", you will find that two-phase motors are common in which the second phase's current phase shift is accomplished by capacitors, just as you propose.

  • \$\begingroup\$ Thank you, John. In clarification, my goal is not to run a motor, or attach any load for that matter, I simply want to get my feet wet with some calculations, and understand what some practical considerations should be, as you and others have stated with this approach. I will take a look at your recommendation of split phase motors. Thank you. \$\endgroup\$ – Alex Riphin Aug 9 '17 at 20:41

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