Phase converter NEMA Frame comparison

Standards applicable to (Canadian and USA products).

Bulletin 107A-FDC

(Note: For easier readability,  print this document  -  about 4 pages).

"I have just talked to four different phase converter manufacturers, and I am now more confused than ever."-  Sound familiar ?

One of the problems individuals face when pricing out converters, is that many phase converter manufacturers provide confusing or conflicting information, making it quite impossible to compare one manufacturer's converter with that of a different manufacturer.

The single most important item to consider when comparing one manufacturer's phase converter with that of any other manufacturer, is the converter FRAME size. The lack of a governing agency (such as NEMA) to set standards for phase converters, allows converter manufacturers to establish and state any standard for their converter - including standards for a) the largest motor allowable, and  b) the total output capacity of the converter.

While the methods to provide reduced locked rotor amperage and proper phase angle displacement in a converter differ between converter manufacturers, what is common to any make of converter, is the converter FRAME size output load capacity.   Any  3 phase generator or phase converter has a limitation as to the largest motor allowable on the system. The larger an electric motor is - the higher its starting current will be, and the more it will collapse the converter generated voltage. What is important to note, is that an electric motor that experiences a 10% drop in rated voltage will have an approximate 19 - 23% loss of starting torque. Hence, a Rotary converter that is sized too small for a particular motor will not allow the motor to develop the required load starting torque, nor reach its rated speed.

Since no governing body has established standards for phase converters, it is up to each phase converter manufacturer to determine if a particular FRAME will maintain adequate voltage during the motor start cycle (ensuring that the motor will be capable of starting the load). This determination by several different converter manufacturers will vary for the same converter FRAME size. Each of several converter manufacturers may suggest a different FRAME size for the same load.  Additionally, the determination by several different converter manufacturers for the total load capacity of a specific FRAME may also vary.

Some manufacturers will refer to a converter in terms of horsepower. One manufacturer may refer to their 256T FRAME as being a "15 HP" converter (- the reference directed at the largest motor they have determined that the 256T FRAME can operate), while another manufacturer may refer to the same 256T FRAME as being a "30 HP" converter (- the reference directed at their determination of the total load capacity of the 256T FRAME). References in terms of a HP rating for a converter are also not standardized, hence have little meaning. The individual seeking phase converter information, selection, and pricing, will only receive conflicting information on possibly every aspect of converter selection from each of these converter manufacturers.

For example, a PhaseMAXX - 10T is supplied in a 256T FRAME. The PhaseMAXX engineering staff has determined that this 256T FRAME will produce and maintain adequate voltage during the start cycle of a design "B", 1800 RPM, 10 HP motor where the torque demanded by the load is less than the motor locked rotor torque (A typical Class 1 application). A different converter manufacturer may decide that their 254T FRAME is sufficient for this same load.

Still as example, a PhaseMAXX - 10T  (256T FRAME)  has been determined by the PhaseMAXX engineering staff to be capable of a total load capacity of 30 HP (with provision for an 80% diversity factor), while another converter manufacturer has stated their total load capacity of the same 256T FRAME to be much higher at 40 HP.

Regardless of each individual manufacturer's determination, it will be unanimously agreed by all converter manufacturers, that a 10 HP electric motor connected to a 256T FRAME will produce higher starting torque than the same 10 HP motor connected to a 254T FRAME (for the same type of converter), and the 256T FRAME will provide a greater total output capacity than the 254T FRAME.

How then can an individual make sense of so much conflicting information?

Well, regardless of the self-standardized claims provided by several different converter manufacturers towards their converter's HP rating, KVA rating, Largest allowable motor rating, and Total output capability, etc., an individual seeking information needs only to decide which converter manufacturer has the best expertise in determining a certain FRAME size for a particular load. Then, the individual can request pricing for that specific FRAME size from each of several different converter manufacturers.

One quick indication of FRAME size is the converter's weight. Request the weight from the converter manufacturer for the converter they suggest. Compare this weight to other manufacturers suggested converters. The weight varies in relation to the converter FRAME, and provides an easy quick reference. 
Don't be misled. (Less weight - Less converter).

NEMA Frame sizes

Electric Motor
 NEMA Frame sizes

 Electric Motor
 HP Rating

184T 5 HP
213T 7.5 HP
215T 10 HP
254T 15 HP
256T 20 HP
284T 25 HP
286T 30 HP
324T 40 HP
326T 50 HP

The above chart illustrates how modern 4 pole (1800 RPM) design "B" electric motors are produced within certain FRAME sizes. Notice that the 20 HP motor FRAME size is a 256T which is not half of the numerical value of the 324T FRAME used for a 40 HP motor. However, the 256T FRAME has exactly half of the HP rating of the 324T FRAME.  The same applies for any FRAME used in the development and production of a phase converter.  A converter supplied in a 324T FRAME has twice the output load capability of a converter supplied in a 256T FRAME.

While NEMA standards exist for specific groups of electrical products,   a standard for phase converters has not been legislated. One of the stated objectives of the National Electric Manufacturers Association reads; "To promote the standardization of electrical apparatus and supplies". Until a governing agency such as NEMA establishes standards for the phase conversion industry, conflicting and inaccurate information from various converter manufacturers will run rampant. 

So shop around  -  but compare pricing for a specific converter FRAME size and compare that only (along with PhaseMAXX's included magnetic starter feature for S, D, T, and CNC of course).


Electram Rotary Equipment Ltd is part of the PhaseMAXX Power Products Group.