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

• Soleste23
In summary, a 120 volt three phase motor is not commonly seen because there is no standard 3 phase power source that operates at 120 volts leg to leg. Although there is no physical limitation, a VFD would need to output 120 volts at half speed to maintain the correct frequency to voltage ratio.
Soleste23
TL;DR 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 ? .

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?

Soleste23 and russ_watters
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.

Soleste23 and russ_watters

https://www.plugsocketmuseum.nl/ThreePhaseTransformers.html

Soleste23
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.

Soleste23

## 1. Why can't a 120V motor be 3-phase?

A 120V motor cannot be 3-phase because it requires three separate power sources, each with a phase difference of 120 degrees, to operate. A 120V motor only has access to one power source, which is not enough to produce the necessary phase difference.

## 2. Can a 120V motor be converted to 3-phase?

No, a 120V motor cannot be converted to 3-phase. The motor is designed and built to operate with a specific voltage and phase, and altering this would require significant modifications that may not be feasible or safe.

## 3. What is the difference between a 120V motor and a 3-phase motor?

The main difference between a 120V motor and a 3-phase motor is the number of power sources they require. A 120V motor only needs one power source, while a 3-phase motor needs three separate power sources with a phase difference of 120 degrees between them.

## 4. Can a 3-phase motor be operated with a single phase?

No, a 3-phase motor cannot be operated with a single phase. It requires all three phases to function properly and efficiently. Attempting to run a 3-phase motor with a single phase can cause damage to the motor and potentially be dangerous.

## 5. Why are 3-phase motors more efficient than 120V motors?

3-phase motors are more efficient than 120V motors because they use three separate power sources, which allows for a smoother and more consistent power delivery. This results in less energy loss and better performance compared to a single-phase motor.

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