We created this page for multiple reasons – one, being to introduce our new line of digital phase converters from DPS and to answer a question we seem to get every week. How do I run this drill, lathe, milling machine, that has a three phase motor on it in my home shop?

 

John has been building rotary phase converters since the 1970’s and continues to do so to this day for our customers. Some of the key characteristics of rotary phase converters are as follows:

 

Voltage Imbalance: The generated third leg often differs in voltage from the input legs. For example, a system might show voltages of 252V, 244V, and 280V. Significant imbalance (e.g., >10%) can damage motors if they are loaded to full capacity. 

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Sizing Requirements: The idler motor typically needs to be larger than the load it powers (e.g., a 10 HP idler for a 7.5 HP load) to ensure sufficient starting current and better voltage balance. 

Efficiency: Efficiency can drop as low as 70%, especially if the converter is significantly oversized for the load.

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Load Sensitivity: Voltage balance fluctuates with changing load demands and utility voltage variations. They perform best when restricted to operating a single machine or a consistent set of loads. 

Do they work? Yes, John has had units in service for decades, only requiring capacitor replacement at some time.

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You have three choices – rotary, digital or use a VFD – we have VFD’s that operate on 120V or 240V single phase input that output 240V 3-phase.

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The thing to remember with ALL these choices is proper sizing and how the efficiency matters. You should only run one motor with these, and you have to choose the correct size for EACH motor – this keeps the efficiency in check.

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So, with a VFD you size it for the one motor you want to run – and that’s the only one you run on it. That's the rule you follow with digital or rotary phase converters also.

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How does a digital phase converter compare to a rotary?

 

Feature               Digital phase converter                         Rotary phase converter

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Power Quality     Utility grade <2% imbalance                 Good, but can vary up to 20%      

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Efficiency           97-98%                                                    Lower (idler motor draws power) 

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Noise Level        Quiet (cooling fans only)                         Louder (spinning motor) 

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Moving Parts      Minimal (only fans)                                  Yes (idler motor)  

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Best For              Sensitive electronics, CNC machines    General machinery, high horsepower 

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Cost                    Moderate                                                  Moderate 

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