Understanding AC Motor Basics: Why a 120V Motor Can't be 3-Phase

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Discussion Overview

The discussion revolves around the feasibility and characteristics of 120V AC motors, specifically addressing the question of why a 120V motor cannot be three-phase. Participants explore the technical aspects of motor types, voltage standards, and power supply configurations.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that they have never encountered a 120V three-phase motor.
  • One participant suggests that while there is no physical reason a 3-phase motor can't operate at 120V, it would necessitate a 3-phase 120 VAC supply, which is not commonly available.
  • Another participant mentions that in the US, the lowest standard for 3-phase voltage is 208 VAC, which is 208 VAC line-to-line and 120 VAC line-to-neutral.
  • A later reply introduces the idea that a variable frequency drive (VFD) could drive a three-phase motor at 120 volts, but at reduced speed to maintain the frequency to voltage ratio.

Areas of Agreement / Disagreement

Participants express differing views on the existence and practicality of 120V three-phase motors, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

The discussion does not resolve the assumptions regarding the availability of 120V three-phase power sources and the implications of using VFDs with such motors.

Soleste23
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TL;DR
I have never seen a 120 volt three phase motor.
I have just started my electrician career and I am studying electric motors. why an 120v ac motor can not be 3 phase ? .
 
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Soleste23 said:
Summary: I have never seen a 120 volt three phase motor.

I have just started my electrician career and I am studying electric motors. why an 120v ac motor can not be 3 phase ? .
Are you aware of any 3 phase power sources that are 120 volts leg to leg?
 
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There's no physical reason that a 3-phase motor can't be 120 Volt, but it would require a 3-Phase 120 VAC supply (which is not a standard anywhere that I know of). In the US, the lowest common 3-Phase voltage is 208 VAC - that's 208 VAC L-L and 120 VAC L-N.
 
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1652988611774.png

https://www.plugsocketmuseum.nl/ThreePhaseTransformers.html
 
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Something to think about: A VFD driving a three phase motor would output 120 volts at about half speed in order to keep the frequency to voltage ratio correct.
 
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