Page 2 of 5 pages on selecting the right converter. Page one is here. 

      GWM Corporation Smith Electricsmith electric motor works rotary phase converter

     Here is a rotary converter manufacturer that gives us page after page of information.

     This is in contrast to what seems to be the trend today of just glossy photos and great promises, but no in depth technical explanations. 

     Likely because most people don’t want the technical stuff. 

     Eighty percent of the potential customers are probably not interested in the behind the scenes technicals. 

     I pulled some of the information out of Smith Electric site and have it quoted here. These are abridged quotes and the full site is here. Smith Electric 

      So the words in italics below are from the founder of

   GWM Corporation Smith Electric – Gary Werner

     My text is in blue.

Compared to any static phase converter, a rotary converter is versatile and powerful. Compared your local power company, a rotary phase converter is a finite compromise — but a viable option. This is stated to prevent unwarranted expectations of the equipment. Rotary phase converters have weaknesses and strengths which should be considered before a purchase is made.

Exactly what I have been telling people, you really need to look at all options to make the best decision.


     On the downside: A rotary converter in a standard, multi–motor installation will never balance each line’s power (current), as well as a utility-supplied, 3-wire, 3-transformer 3-phase system. The rotary converter is as good as an “open delta” 3-phase (2 transformer) system, however, and may in fact even be better under certain conditions.

     Rotary converters will not deliver the starting torque of a solid 3-phase line, and the rotary unit often must be greatly oversized relative to the largest motor operated to produce high starting torque.

That is exactly right about oversizing.

     Rotary converters will not maintain close voltage balance over a wide range of operation, and line-to-line 3-phase voltages will drop precipitously when heavy loads are introduced. If you have voltage-sensitive equipment such as computerized machine tools (CNC), best results can be obtained by using a separate converter for the CNC, and another one for general shop machines. A special CNC converter such as GWM’s Digi-Series typically adds five to seven percent to the machine’s cost.

A couple of times on his site he mentions that you should have two converters, one for cnc and one for the other machines, Or one for each cnc machine.

This same thought pattern seems to be brought up in the American Rotary information as well.

Below he is talking about an example he gave of three different motors.

The important point is the newer Epact motors and their extra demands on rotary converters.

Something I knew about but never ran into.

   Machine C will place very high starting and running demands on a rotary converter. If it were powered by 10 HP non-EPact motor (EPact motors are universal in the U.S. after November 1, 1997 except for special-purpose applications) fully loaded, within normal starting torque requirement, it would require a model 256 (20 HP) frame rotary unit. With the EPact motor it may require a 286 (30 HP) rotary. Why? Because in a phase-converter-generated power system, balanced currents and voltages are a function of rotor slip in the largest load motor, and a low-slip motor –that is, a motor that wants to run very near its synchronous rpm (1800 in this case) at the full-load — will place stresses on the converter that can only be overcome by a greater rotating magnetic field. If the larger rotary is not available, the motor does not attain its desired rpm and currents are severely unbalanced. Additional capacitors will not help, either.

The point at issue is this: for a converter to work properly on a heavily-loaded application using modern motors — especially “energy efficient motors” — mandated by government, you may find that the best way to lower overall costs on the installation is to change the motor on the machine to a special-purpose (non-EPact) or higher HP motor. This will not lower the machine’s electrical efficiency, since the capacitors in a rotary converter provide increased overall system efficiency.

One item he mentions, is putting a larger hp motor on the machine. He means the actual machine you are planning on running.

I can see in my experience that could work.

The seat of pants feel sometimes is that a larger motor would not struggle so much with the load, and you would be better off.

     Each model of standard rotary phase converter has a “minimum load operated” and “largest single motor” rating, as well as a total operated HP load. The minimum load applies because a standard rotary’s 3-phase voltages are dictated by its large capacitor bank. Two of the voltages may be quite high under light-load or unloaded conditions. As loads on the converter increase and the capacitor current is spread to the system, voltages decrease. On large multi-motor loads this is necessary so that good voltage under full-load is available. But high no-load voltages may “saturate” the magnetic fields of a small motor that does not require enough current by itself to bring the voltages down to reasonable level. The little motor may then overheat after 15 minutes or so when operated alone. To avoid this, you may either, (A) disconnect several motor run capacitors from the rotary’s capacitor bank if loads are not too great, or (B) idle enough 3-phase equipment to exceed the minimum when a small machine is operated. Electrical cost will not increase enough to notice. As a third option, a small-motor operation switch may be installed on the rotary converter to manually switch off some of the rotary’s capacitor bank under light loads.

This again is exactly what I have known about for years.

I did tests and found that the rotary would not balance right for different loads.

This was especially irritating to me because people criticize the transformer converter because it would only balance under full load.

They are right but at less then full load, the motor is drawing way less amps and never overheats.  

Below, Gary talks about voltage regulation.

As you read you can see that his company has had trouble with simple fixes and did design an elaborate machine to do this.

At this point you are getting into why someone designed the Phase Perfect. 

They were trying to update the converter to modern day electronics. Sort of like welders going to inverter types. 

   Some manufacturers of phase converters offer capacitor switching in an “automatic” mode or offer an “automatic voltage regulation” device (AVR) on certain models. Our knowledge and experience in this field mitigates vigorously against such a “solution” to high-voltage problems.

     Unfortunately, a reliable method of capacitor switching that does not damage operated equipment requires complicated technology and is fairly expensive to build. In 1988, GWM Corp. collaborated with Tom Lehman of Adaptive Design Company to build a computerized phase converter that would switch capacitors to adjust for varying load conditions. After a number of revisions and several thousand hours of work, the result was the Substation SS-1, rev. 7.0, which achieved stellar performance, but it was very complicated and expensive to manufacture.

     Since we wanted the SS-1 to have stand-alone capability as an alternative to a rotary converter on moderately-loaded multi-motor applications such as machine shops, we were forced to add some very elaborate and expensive surge protection devices, mainly to protect the Substation’s electronics from sudden load changes.

     As a complement to a rotary motor, the SS/Rotomax became a variable-capacitor rotary converter that would produce very clean power and well-regulated voltages under a wide load range.

     I want to draw a contrast here between the computerized voltage regulation device we developed–the Substation–and a mechanism that is making the rounds of more than one converter manufacturer under the guise of “automatic voltage regulation.” The device simply consists of a voltage sensing relay linked to a contactor that divides the rotary’s capacitor bank. At load, about half of the rotary’s capacitors are switched off. As the load increases, one of the voltages drops to a preset threshold and the contactor adds back the other half of the capacitor bank.

     We noticed the above devices popping up a few years ago when we began advertising our Substation. What troubles me is that this device is a voltage regulator in the same fashion that a guy standing over a car engine with a garden sprayer full of gasoline has fuel injection–the aim and regulation is terrible, and has the potential to cause a great harm. Here’s the problem: when capacitors are switched under load, bad things can happen, since capacitors will store a charge at full voltage, and the line–which, remember, we said changes from positive (+) to negative (-) 60 times a second–can have a different charge than the capacitors do when the device switches “on”. The sensor used on this “voltage regulator” has no way of relating to the line or capacitor charge, it just switches arbitrarily.

     And that’s another thing: our Substation varied power in 447 steps, not just two (off-on). And the SS-1 used a computer and triac switching to ensure that no reverse charges would resonate the transformers in your equipment or double the voltage going to your sensitive CNC electronics through random switching.

     I am presuming the existence of some type of bleeder resistor on the device. But, absent a microprocessor drive, there is no guarantee that the device won’t chatter– negating the bleeder effect–or continue working if the bleeder malfunctioned.

     Don’t make the mistake of using any kind of “voltage regulation” such as I’ve described here with a phase converter. It can be damaging to your equipment. I’m not criticizing the method without good reasons. My company–along with a few other major manufacturers of phase converters–has worked to minimize damage to our industry by hare-brained speculators and people who should know better. We all get tarred with a big brush when one of these ideas bears rotten fruit. Please don’t fall for it.

     And if you are thinking the “regulator” would work well on CNC–it doesn’t at all. Any capacitor switching device–our SS-1 included–could not anticipate a load change–it only responds after the load is initiated or removed. Best results are obtained by using an oversized converter designed for CNC and dedicating one converter to one machine. A steady power supply to your CNC gives it the freedom to run smoothly and do what it does best–make parts with no alarms and no downtime. This will make you the most money.

I believe that this could be old information and not applicable to some of the new stuff coming out today. 

He mentions developing his system back in 1986 which is ancient history in electronics.

Here he goes into what is called a High Resistance rotor.

I don’t know if and who are using that today.

The conclusion is he doesn’t recommend them.

     The popular rotary for the past 40 years uses what the industry calls a High Resistance (HR) rotor. In its original construction, the end, or “short” rings of an ordinary 3-phase motor’s rotor were drastically cut or alternately slotted end-for-end. This altered the rotor’s magnetic field, and would allow the rotary to then start and attain operating speed with only a few motor-run capacitors connected.

     The invention of this device was critical to the development of a workable multi-motor rotary phase converter, since at the time the only other starting means available involved starting the rotary motor mechanically with a “pony motor”–not the most user-friendly method–or using some single-phase motor-starting (electrolytic) capacitors and a single-phase motor starting relay to start the converter motor, in much the same fashion as the static converter back in figure 4.

    Operating multiple CNC machines on the same converter is an iffy proposition, although many of our customers have done so with some success.

     The best explanation is this: Some CNC will tolerate another motor starting and stopping during operation, some won’t. The best recommendation I have is–try it! If it doesn’t work, try advancing the power leads going to the machine that upsets regularly on the converter: specifically: move machine input lead No. 1 to terminal No. 2, lead No. 2 to terminal No. 3, and lead No. 3 to terminal No. 1. If that doesn’t help, do it again. The third time you do so, of course you will be back to the original wiring configuration. Doing this will sometimes take a critical control circuit off of T3 (the manufactured phase) to permit multiple operation. In this writer’s opinion use a separate converter for each CNC.
Unfortunately, the HR rotor has some problems. Some have been addressed recently by manufacturers, but many are simply the trade-off you get with the design. From a performance standpoint, the major benefits
The “Sales Gimmick” at our company is a superior product. Every company trying to fit their product into a market needs an identity–a distinguishing mark to set itself apart from the blur of similarity among its competitors.
We decided to go exclusively with phase converters in 1986. This field was ripe for a performance-oriented manufacturer not afraid of research & development. At that time our competition consisted of 5 or 6 major players, who were all buying from the same suppliers and essentially assembling the same equipment. Each had bragging rights to devices they patented 30 or 40 years ago. Motor technology has made major advances in the last 20 years, but the phase conversion industry remained stagnant. On top of this all the products were of thrift-store quality, and the rotarys were as noisy as can be imagined. As long as no one upset the status quo, however, change or improvement was not necessary. One company, for example, claimed quality as their driving force, but it was obvious that they had no idea what it was or how to sell it.

 Advantages ……..from the HR rotor are attained at low rotor rpm, that is, when the rotary motor is starting. When used in a normal 3-phase motor, a high resistance rotor is a high starting torque, low full-load rpm device–in other words, it is a high-slip rotor. Remember that this is just the opposite of what a high-performance rotary phase converter needs, to produce magnetic fields and voltages which stay ahead of operated load motors.

Summary of this is; it seems that these people take their business very seriously and they have been doing this since 1986.

If you like information you should go to their site and read through all the information they have.

It is very thorough. It appears that the Smith Electric Company doesn’t rely on the small consumer for business.

They must concentrate on large industrial sales. They have no prices on their site and you would have to call them to get a quote.


Let explore the only manufacturer that sells both the transformer converter and the rotary converter.

Ronk Electric