Common Phase Converter Issues and Misconceptions

The internet has become the principle source of product information and training for those shopping for a phase converter. Unfortunately, most folks come away from the experience more confused than when they began the research. The reason is that there are simply no industry wide standards that govern the sizing, performance and configuration of phase converters. Consequently every manufacturer is free to set their own standards which allow them to:

  • Rate the converter any way they choose

  • Use terminology that is NOT universally accepted or ambiguous in meaning

  • Take advantage of a buyers’ lack of technical knowledge

Truth be told, a well manufactured converter from most any manufacturer will run a 3-phase load. But from a buyer’s perspective, the single most important factor in assuring a successful selection is the supplier’s application knowledge on your type of load equipment. Anyone can build a phase converter but few companies combine the application experience and electrical system knowledge that is essential to a trouble-free installation. Phase converters are not commodities and treating them as if they are is a recipe for field trouble.

Kay Industries has been in the converter business for nearly 50 years. Our specialty is providing customers with the essential knowledge to make an informed decision. Toward that end, following are some of the issues that our customers tell us are the most confusing and why in the end, Kay Industries was the only company that explained things in a manner they could understand and MOST IMPORTANTLY, made good technical and economic sense!

Converter Horsepower rating – Does it really have to be larger than the load?

Many prospective converter purchasers report that they are advised to purchase converter with nameplate rating from 1.5 to 3 times larger than the load motor to be operated. Kay Industries flatly rejects that recommendation. Nowhere in electrical theory or application is that practice justified. The only way a motor would require a converter of twice its horsepower rating is if the converter nameplate did not honestly represent its true capability.

Common economic sense suggests that we should buy only what we need. If a load is 10HP, why should the required converter be rated 20HP? The answer is it doesn’t! No one would logically accept being told they had to buy two pounds of something in order to get one pound of the product. Yet that is exactly what the converter buying public is being told every day.

The only conclusion to be drawn from the above example is that if it takes a 20HP converter to run a 10HP load, then that converter is in reality a 10HP converter and not 20. In other words, the nameplate ratings of most converters are not a true measure of their actual capability. They are routinely misrepresented. This is the principal reason that converters CANNOT be compared on the basis of their nameplate rating.

Back to top


Heavy duty, light duty, hard start, easy start….what does it all mean?

Unless you are experienced in the concept of motor speed-torque characteristics, load curves, and starting inertia, it is virtually impossible for the average person to define their load in these terms. And it should not matter. These are jargon terms that sound meaningful and important but factually are so ambiguous as to be useless in determining the converter selection. More often than not, these descriptions are simply dropped into conversation to suggest a plausible reason for a converter selection. This is done with the full knowledge that the average consumer would not or could not factually challenge the terms. Kay Industries rejects the use of these descriptions.

While it is true that some loads are definitely harder to start than others, there are so many other factors that can have equal (or greater) effect on the converter’s ability to start and successfully run the load that it just adds confusion to bring these terms into the selection. Worst of all, when a converter does not do the job, it becomes easy for the manufacturer to assign the blame to “hard starting” rather than to acknowledge the true reason which was inadequate converter size.

Back to top



This is one of the most overused and misunderstood terms in all of converterdom. Most people have no idea of the true meaning of efficiency. It does not refer to how much motor capacity is “lost” when using a converter (you should get all of it).

As a concept, efficiency is a measure of the losses incurred in the operation of your machines. In other words it is the difference between the amount of energy that performs useful work and the amount of energy that utility charges you for.

This difference between the amount used and the amount billed represents the “losses” of your machines. In the case of the converter, two thirds of the energy passes directly through to the load without going through the converter. The result of this phenomenon is that phase converters are effectively 95% efficient. In other words, the converter costs very little to run.

Back to top


What is the significance of different operating voltages and how do they impact the selection of phase converters: 208-220-230-240-380-400-415-575

Single-phase is the predominant mode of distribution in most residential and light commercial areas in North America. The most common single-phase voltage is 120/240V, meaning the service consists of two 120V lines plus a neutral wire. Each of the 120V lines and neutral (commonly but INCORRECTLY referred to as 110V) are used for lighting and small appliances and tools. The two 120V lines without the neutral produce 240V (also commonly but INCORRECTLY called 220V) which is often used for small HP single-phase motor loads. This 240V supply is the single-phase input to the phase converter.

208 is normally a 3-phase voltage and usually includes a neutral. It is more commonly referred to as 120/208.

Operating Voltages 220-230-240 are effectively the same. In North America, residential and commercial single-phase voltages are actually 240V. Rarely if ever does someone actually have 220V. That term is a relic of the 1950’s as virtually all single-phase services are 120/240V. Ironically, despite being operated on 240V systems, motors and most other electrical apparatus built for the North American market are rated 230V but it is understood they will run on voltages plus or minus 10% of nameplate voltage rating.

European machines are built to an IEC standard which is built around 220V vs 240V and 380 or 400 vs 480V in North America. It is often necessary to employ a transformer to adjust the output voltage of a phase converter to be compatible with the requirements of offshore equipment built to another electrical standard.

In Canada, many motors built to run on 575 or 600V. Once again, a transformer may be required to match the converter output to the load voltage rating.

Back to top


What does the term “Clean Power” mean and can I get it from a phase converter?

Unquestionably one of the most ambiguous terms anyone could apply to electricity is this term as a description of its quality or “cleanliness.”

Even though the term has no real definition, it is actually intended to describe the amount and duration of voltage variations from the normal value of 240V (or whatever the service voltage supply). Only the electrical utility supplying the service can truly control that. With the exception of voltage changes as the load varies, a phase converter has very little control over the quality of the electrical power that serves a facility.

When the supply voltage exceeds the normal value it is commonly referred to as a voltage “spike” or transient voltage spike. Theses transient voltages can be caused by lightning, switches, circuit breakers tripping and similar events. When the voltage drops below 240V it is known as a “voltage sag.” Sags are much more common than spikes and are usually caused by starting large motors or faults on the system.

Another form of unwanted utility line conditions is harmonics. Harmonics very commonly are created by variable frequency drives. Harmonics can cause overheating of wires, can affect computers and electronic equipment. Usually they must be removed by using line reactors.

All of the above conditions occur for a variety of reasons and, more often than not, except for harmonics, originate in the utility system. There is very little a phase converter can do to eliminate these utility line conditions. Manufacturers that call for exceedingly tight restrictions on supply voltage range are not being realistic about what you can control.

Back to top


Is there such a thing as CNC rated converter?

Of course there is. If a company calls its converter CNC rated, who can argue? But in reality there really is no such thing beyond a marketing claim. True CNC (Computer Numerical Control) machines went out of production a generation ago. The original CNC machines used punched tape that instructed a machine tool to perform specific operations, usually with point to point directions.

The modern CNC machine uses microprocessors instead of punched tape to provide similar but far more sophisticated directions. But the CNC part of the machine has a very small electrical demand compared to the full electrical demand of the machine. That microprocessor is usually no bigger than the power supply in a notebook computer (less than a few hundred watts) and is virtually always single-phase. It rarely sees the output of a phase converter because it is connected only to the two converter lines that come directly from the utility supply.

The big power demand on these machines comes from the spindle motors and servo drives that are usually powered from variable frequency drives. A converter can easily operate a motor fed from a VFD as long as the converter’s output voltages are kept within a range acceptable to the VFD or other voltage sensing controls. Kay Industries was the first manufacturer to offer an accessory that automatically adjusted the converter output voltage to compensate for wide ranging connected loads. That made it suitable for manufacturing shops that had both large and small machines that needed to operate on a single converter. It also turned out to be the perfect accessory to run the VFD’s on CNC machinery. That accessory which was originally marketed under the name of “voltage sensing load range control” or VS for short was quickly copied and incorrectly assumed to mean “voltage stabilizer.” Consequently, the –VS accessory became synonymous with CNC service and the whole notion of CNC rated converters was born. However, if a CNC machine does not have a VFD, the –VS accessory is not necessary and a standard unmodified converter will operate that CNC equipment just fine.

Back to top


Frame sizes – What are they and why they are important to guarantee you are getting the most value

A rotary phase converter resembles a motor but it is actually an induction generator. It should be obvious that the converter is not a motor because it has no external shaft. Nonetheless, the induction generator portion of a rotary converter is commonly called a motor and in fact is governed by the National Electrical Manufacturers Association (NEMA) standard MG1 standard for Motors and Generators. The principal active materials in both motors and generators are copper wire and electrical steel.

One important similarity between motors and induction generators is that they are built on a series of standard frame sizes. These frame sizes define the physical dimensions of the generator. The larger the HP or KW rating of the motor or generator, the more iron and copper contained within. The only way to determine if two converters are identical in capacity is to compare their frame sizes. A manufacturer that does not reveal the converter frame size may be misrepresenting the converter’s starting horsepower rating.

Converters of different frame size simply cannot carry the same loads regardless of the rating stamped on the nameplate. As a simple rule, heavier is better. You should never pay more money for less iron and copper.

Back to top


The two supply lines are 180 degrees out of phase. How does adding one line make them all 120 degrees apart?”

If the premise of the question were true, the answer would be “it can’t.” But the first sentence is absolutely NOT true. The two utility supply lines represent a single voltage, usually 240V, which consists of two so-called hot lines that measure 120V to neutral and 240V relative to each other. That 240V represents a single voltage. It has no phase relationship because there is no other voltage to reference against. In an AC circuit, the polarity is constantly reversing means that each line switches from being positive to negative relative to the other. In this context, it is fair to say that each line is 180 degrees different in polarity than from the other. But relative polarity IS NOT a phase relationship. For a more detailed discussion of this topic, see the expanded version below.

Back to top


Expanded version "The two supply lines are 180 degrees out of phase....."

The single most common misstatement we hear about single-phase systems is that the 240V utility supply lines L1 & L2 are 180° out of phase. This assertion is absolutely false. It reflects a misunderstanding of the concept of phase relationship. Here’s why.

We start with the definition of voltage. Voltage is the electrical potential measured between two conductors. Note, it always takes two conductors or reference points to have a measurable voltage. If you place one probe of a voltmeter on one pole of a battery, the meter doesn’t budge. It is only when the other probe is placed on the opposite terminal that the meter registers voltage. A load connected to a single terminal would see no voltage and thus no current flow until connected across to the other terminal. The same principle allows birds to safely perch on a single bare overhead power line without getting electrocuted. L1 standing alone is not a voltage. L2 alone is not a voltage. A Neutral alone is not a voltage. However, L1-L2 or L1-N or L2-N is a measurable voltage.

In a single-phase alternating current system the magnitude of the voltage between the two lines follows a sine wave. Thus the voltage starts at zero, rises to a positive peak and then falls back to zero exactly half way into the cycle. It then goes to a negative peak and returns to zero to start the next cycle. In other words, the relative polarity between the conductors is switching positive to negative every half cycle or 180°. Most people mistakenly take this phenomenon to mean the voltage between these conductors is 180° out of phase. But the operative question to be asked here is “out of phase with what?” In reality this condition is nothing more than a continuous polarity change. It is not a phase relationship!!!

Most 240V single-phase systems consist of two “hot” lines and a neutral. Each hot line to neutral measures 120V and the voltage between the lines is 240V. For the reasons described above, while L1-N is measuring a positive voltage, L2-N is measuring the same magnitude of negative voltage. Note that we now are dealing with three wires to get two separate 120V voltages. We can see how technically the two 120V line to neutral voltages are 180 degrees out of phase with each other. But as soon as we drop the neutral and measure only between the two “hots” we are back to only two wires and thus a single 240V voltage. With only one voltage present there is no longer a phase difference.

In a 3-phase system there are three conductors (L1, L2, L3) and thus three separate voltage relationships, (L1-L2, L1-L3, L2-L3). With three separate voltages present, we can now compare their magnitude as a function of time. Phase difference is defined as the distance between the peaks or zero crossings between any two voltage waveforms and is usually measured in degrees. When these three voltages are displayed on the same time line it is apparent that each sinusoidal voltage reaches its peak value (or its zero crossing) at a point 120° before or after the previous voltage waveform.

The point of this discussion is: Do not confuse polarity with phasing. Two conductors define a single voltage and relative polarity. But it takes at least three conductors to have more than one voltage and you must have at least two voltages to have a phase difference. Two wires can be opposite polarity (180° apart) but they can’t be out of phase with each other.  However, a pair of conductors with a measurable alternating voltage can be out of phase with another pair of conductors because the distance between their peaks or zeros can be compared to each other.

Back to Common Phase Converter Issues and Misconceptions.