# Single phase motor starting from high leg voltage

Typically stinger voltage is said to be nonuseable, the 208V between a high leg and a bastard tap. High-leg-delta configuration (Wikipedia).

But since single phase motors require a phase offset to start anyway, what's to say you couldn't use that high leg instead of a cap start for a 120v single phase motor?

• Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
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Commented Sep 24, 2021 at 20:24
• "... what's to say you couldn't use that high leg instead of a cap start for a 120v single phase motor?" Voltage, for instance. The series capacitor will drop some voltage so the start winding probably won't even take 120 V. Commented Sep 25, 2021 at 11:05

In a 3-phase induction motor, the phase windings, placed 120° apart, are fed by voltages phase-shifted by 120° to generate a rotating magnetic field to drive the rotor.

Likewise, in a 2-phase induction motor the phase windings would need to be placed 90° apart and fed by voltages phase-shifted by 90°. In fact 2-phase electric power was generated at the turn of the 20th century at Niagara Falls, obviating the need for capacitors to run motors.

Single-phase motors are actually 2-phase motors, with 2 windings 90° apart, and the phase-shift of 90° being provided by a capacitor.

Your suggestion, to use a high-leg delta system to drive a single phase motor without a capacitor, is not practicable in the absence of voltages having a phase-shift of 90°.

Edited as follows:

There is absolutely no question vis a vis the phase shift of 90° between the line-to-neutral voltages (120 V) and the high leg voltage (208 V).

The only problem would be non-availabilty of the high leg voltage with split-phase consumers.

Availability of the high leg voltage would mean availability of the three-phase system and that would make use of a three-phase motor more valid.

• The highleg voltage is 90deg out of phase with the 120v. If the reference phase is B to A with B being the highleg and the center tap being across A and C, we would expect to see 120v at 120deg and 208v at 30deg Commented Sep 25, 2021 at 22:52
• Andrew Graham, To which terminals of the high-leg delta transformer would the motor be connected? A schematic would help understand your concept. Commented Sep 26, 2021 at 10:02
• See edit. Added schematic. Commented Sep 28, 2021 at 2:31
• Got it! My answer has been edited. Commented Sep 28, 2021 at 6:47
• For sure most consumers in NA would never run across a high-leg in the first place. The only feasible context would be a case where someone is on a highleg and happened to need to run a single phase motor. The only question left, some experimentation required, is whether there is an advantage to starting with a highleg instead of a start capacitor. Commented Sep 28, 2021 at 11:24