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Balanced Voltage Claims – Truth or Fiction?

For years, many companies have used (and continue to use) the inexpensive method of “stacking” run capacitors in order to get a phase converter started. Those capacitors are not taken out of the circuit once the phase converter is running, which results in extremely high voltage and current imbalance. This condition causes current overloads that will burn out motors that are smaller than what the phase converter was sized for (ie. running a 2HP mill on a 40HP converter). Furthermore, this condition will ultimately cause the phase converter itself to burn out.

The justification for continuing to build phase converters this way is that the starting circuit is fail proof. But the consequences of the resulting high voltage and imbalance can be avoided by simply using new designs and updated technology.

With the proven quality and reliability of starting capacitors and contactors, new methods to create starting circuits using these components allow the rotary phase converter to have two stages: starting and running. This gives the best of both worlds – a robust starting circuit, and a balanced running circuit.

To attain the balanced voltage, the phase converter requires a high quality custom idler/generator (not a motor), and a very specific balancing of the capacitance across the windings of the idler. Placing all of the run capacitors across one winding will not produce a balanced voltage!

Outrageous claims of voltage balance of 2% or better. The only phase converter on the market that can produce this type of voltage balance, (actually 1% or better) is the Phase Perfect digital phase converter, which we will cover in another post. Please visit for more information about this product!

The only way a rotary phase converter might achieve such a voltage balance is if the load is constant (which is very rare), and the capacitance is “tuned” across each winding of the idler. Even then it would be very difficult. Switching in and out capacitors with contactors is a difficult thing to do properly. Switching them out once is fine, but not back in. The contactors just can not handle the “in theory” infinite current and the moment of contact.

It is so important to verify voltage claims! In the age of the internet, pretty much anyone can say anything…All in all, look for a (provable) 5% voltage balance, and that it can be achieved in a range of about 50-60% of the full load current of the converter.

There are several other marketing-style claims that are made by various companies. These advertising methods can be quite deceptive. The best defense is a good offense. We will continue to cover key features and benefits of phase converters, what to look for, what to avoid.

As always…please feel free to email if there is a specific topic you would like to see covered on this blog!

6 thoughts on “Balanced Voltage Claims – Truth or Fiction?

  • Walt Sweet

    May 10, 2019 at 12:09 am

    My idler is 230V, 7.5hp. It runs the lathe motor, which is 3hp. I tuned the capacitors, and during run, the voltage is within 3.5%: I think this is good. Continuous running is good, but if I have a lathe job that requires lots of starts and stops, the thermal overloads inside the lathe motor will trip, and I have to come back in 10 minutes. It takes about 2.5 seconds to bring the motor up to speed, and I think it would be about half that time under the utility’s 3 phase power. Again, continuous running is no problem, but I think the startups are causing the tripping. Does it make any sense to configure a special contactor to put extra capacitors online during startup only?

    • Sarah

      June 4, 2020 at 4:42 pm

      Hi Walt, I am sorry for the delayed response we are happy to help answer any of your questions. If this issue is still a concern please reach out to our technical support team at 888-743-6832 and one of our team members will be happy to look into it for you.

  • Brad DuBois

    November 24, 2019 at 8:48 pm

    Should the 3 phase voltages be balanced at idle with no loads connected,or with loads connected with no load on them.I am referring to grain augers and a grain leg.

    • Sarah

      May 28, 2020 at 2:09 pm

      Hi Brad! When the converter is not under a load the voltages will vary and the manufactured third leg will naturally be higher. But, once the load is applied all voltages should be within 5-7% of your single phase voltage that you have coming in. Side note, to make sure you are measuring the voltages correctly – make sure you measure them from the output lines and phase to phase. For example: T1-T2, T1-T3, T2-T3.

  • Paul

    March 7, 2020 at 11:24 pm

    Hi, I have a 1hp , 3 phase Bridgeport mill and am buying a 7.5 hp 3 phase lathe. If I size the converter for the lathe, can I use it to power the milling machine also? They will NEVER be run at the same time ! Thanks, Paul

    • Sarah

      May 26, 2020 at 2:57 pm

      Hi Paul, thank you for reaching out! That would be correct, since you are only running one at a time you would want to size based off of the largest load (7.5hp lathe). Based off of that and the type of loads you are running I would recommend our model AD15! It is capable of starting up to a 7.5hp load and will be good for running your 3hp mill as well. I would also verify the voltages off of both pieces of equipment and make sure they match with what you have coming in from your single phase side. If it is different you may need a step up/down transformer. If you have any questions feel free to reach out to our office at 888-743-6832!

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