How To Speak Fluent IEC for Motors

As U. S. manufacturers step up export activities, PT designers should become familiar with IEC motor parameters. Here's a primer on the most common designations and how they relate to NEMA standards.

Fortunately, in the world of electric motors, there are basically only two languages: NEMA in North America, and IEC in most of rest of the world. Until now, we have pretty much co-existed with little need of interaction. That's all changing as the market continuous to become more global.

Just as the National Electrical Manufacturers Association goes by its acronym, "NEMA" and like NEMA, IEC establishes and publishes mechanical and electrical standards for motors. Many IEC standards have been somewhat nationalized. For example Germany has its VDE 0530 standard; Great Britain, its BS 2613. But they parallel IEC34-1 standards. And, in general, the same can be said with minor exceptions for standards worldwide. They are likely to be IEC clones or close derivatives.

More good news: Though NEMA and IEC standards use different terms, they are essentially analogous in ratings and, for most common applications, are largely interchangeable. If anything, NEMA standards tend to be more conservative----allowing more room for "design interpretation," as has been U.S. practice. Conversely, IEC standards tend to be more specific, more categorized ---some say more precise--- designed with less overload capacity.

In Europe and most of the rest of the world, the power grid furnishes 50-Hz power than rather than the 60-Hz power of North America. What is the effect on the performance when motors designed for 60-Hz are applied on 50-Hz supplies? More to the point, can you safely specify a 60-Hz design motor for use on a 50-Hz supply?

Three-phase motors. A 60-Hz, three-phase motor will operate satisfactorily (at the nameplated horsepower) on a 50-Hz supply if the voltage is reduced by the same ratio as the frequency. Therefore, a 230/460-V, 60-Hz motor operating on the high voltage connection with a 380-V, 50-Hz input will perform acceptbly at full nameplated horsepower, through shaft speed would be 5/6 nameplated speed.

For other 50-Hz voltages, a 230 voltages, a 230/460-V, 60-Hz, three phase motor may not be satisfactory without derating the 60-Hz nameplated horsepower by a factor of 0.80 to 0.85. This all relates to keeping motor heating in check on the 50-Hz load. Most manufacturers indicate in their literature or will advise on request whether a given motor is satisfactory for 50-Hz input and what horsepower rating.

So ask. It could save a world of worry.

The accompanying chart is a guide to effects of operating a 60-Hz, general-purpose, three-phase motor on a 50-Hz supply. Note that the biggest detriment is the heating.

Single-phase motors. For general purpose, single-phase motors, the answer to the 60-Hz/50-Hz application question is: Do not do it! The reason: Most single-phase motors require a speed-sensitive starting method. It must be sized differently to account for the lower operating speed of motors on 50-Hz supplies: 5/6 that of motors on 60-HZ supplies. For the specific applications, a motor manufacturer may be able to design a single-phase motor suitable for both 50-Hz and 60-Hz operation.

The bottom line. Some foreign users of motors are suspicious of U.S. motor manufacturer' assurances that certain 60-Hz motors will perform satisfactorily on 50-Hz supplies. When possible, choose a motor designed specially for the power supply.

Frame relationships

NEMA and IEC standards both use letter codes to indicate specific mechanical dimensions, plus number codes for general frame size. The letters can get especially tricky because, for example, a "D" in NEMA is really an "H" in IEC, while an "H" in NEMA is "K" in IEC.

Frame relationships are a bit easier; in only one case, 56 frame, do IEC and NEMA use the same nomenclature with different meanings. IEC's 56 frame is what we'd call a subfractional motor, while NEMA's 56 frame is our most common, covering about 1/4 to 1- 1/2 hp.

Table 1 provides a translation guide for most common mechanical parameters, with dimensions solely in millimeters to avoid to much clutter. (Moreover, as long as we are "speaking IEC," we should talk a little metric, too.) What you will notice is that, though the dimensions are certainly not identical, they are pretty close. The dimension least in synch is NEMA "N-W" (IEC "E")---- shaft shoulder-to-shaft end measurement. Most often, the NEMA measurement is much greater.

Common dictionaries tell you a watt is a unit of elerical power measurement based on amperes and volts (an input unit), while a horsepower is a power measurement based on mechanical work (an output unit)--- a horsepower equaling the power to lift 33,000lb 1ft in 1min. Seems like apples and oranges, but in electric motors they compare because watts are also used as output units: 1hp = 745.7 W = 0.7457kW

IEC uses kilowatts; NEMA, horsepower. And like NEMA, IEC assigns comparable power ratings to standard frame sizes.

IEC and NEMA kW/hp comparisons flows smoothly in smaller ratings, but in larger sizes they can vary enough to cause concern in some design applications. An example is IEC 115S/NEMA364T areas for 4-pole motors. (See Table 1 footnote.) Here, NEMA calls for 75 hp in the frame size in which IEC calls for 50 hp. Dropping to a NEMA 326T frame provides the 50 hp needed, if the dimensioning differences can be tolerated. If you need the 364T dimensions, be sure not to damage the drive train or load with the higher-power motor.

Here's where the IEC penchant for being specific really shows. "Open drip-proof" and "totally enclosed" are the descriptive words NEMA uses. However, IEC gives numbers---lots of them.

It all makes sense when you think about it. IEC refers to its enclosure designations as "degrees of protection," and gives ratings based on a two digit numbering scheme. The numbers follow the letters "IP." Think of them as standing for "ingress protection."

The first digit indicates how well-protected the motor is against entry of solid objects such as dust, wire, tools, or fingers. Here's what the first digit means:

0 - No protection.
1 - Protection against objects larger than 50mm (about 2 in) in diameter, like hands.
2 - Protection against objects larger than 12mm (about 1/2 in) in diameter, like fingers.
4 - Protection against objects larger than 1mm (about 0.04 in) in diameter, like small tools/wires.
5 - Complete protection, including dust-tightness.

The second digit signifies protection against water entry. Here are those ratings:

0 - No protection.
1 - Protected from water falling straight down.
2 - Protected from water falling as much as 15 deg from vertical.
3 - Protected from spraying water as much as 60 deg from the vertical.
4 - Protected from splashing water coming from any direction.
5 - Protected from water sprayed from a nozzle in any direction.
6 - Protected from heavy seas.
7 - Protected against immersion for a given time.
8 - Protected against immersion indefinitely.

For most industrial application, IP 22 relates to open drip-proof motors, IP44 or IP54 to totally enclosed, IP45 to weatherproof, and IP55 to washdown-duty motors.

Incidentally, if you are dealing with explosion-proof motors, take heart: The hazardous atmospheres defined by our national electrical code parallel those IEC "flame-proof" motors.

Again, IEC uses a letter and number code to designate how a motor is cooled. The code covers nearly every known cooling method, including those for very large liquid-cooled motors. It can extend all the way to a four-letter, four number code. For more common purposes, a few "short- code" designations should see us through:

IC 01 - The first digit means there is free exchange of coolant into and out of the motor. The second digit means the exchange takes place because of "self-circulation," or a fan mounted on the motor shaft. This is most likely a NEMA standard open motor, because of the internal-fan action.
IC 40 - The first digit means the frame surface is cooled; the second, that cooling by convection only with no fannig action. The motor is totally enclosed, non-ventilated.
IC 41 - The first digit again indicates frame-surface cooling, but the second shows fanning. You guessed it : totally enclosed, fan-cooled.
IC 48 - The frame is cooled, as the first digit tell us. But the second says the coolant and motor move relative to each other. Translation: a totally enclosed air-over motor. This relates to uses where the motor is in airstream of the fan or blower it drives, and is thus cooled by fan action.

Most NEMA-speakers refer to duty cycle in one of two terms: continuous or intermittent. IEC breaks it into eight ratings:
S1 - Continuous duty. The motor works at constant load for enough time to reach temperature equilibrium.
S2 - Short-time-duty. The motor works at constant load, but not long enough to reach temperature equilibrium, and rest periods are long enough for the motor to reach ambient temperature.
S3 - Intermittent periodic duty. Sequential, identical run and rest cycle with constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise.
S4 - Intermittent periodic duty with starting. Sequential, identical start, run and rest cycles with constant load. Temperature equilibrium is never reached, but starting current affects temperature rise.
S5 - Intermittent periodic duty with electric braking. Sequential, identical cycles of starting, running at constant load, electric braking, and rest. Temperature equilibrium is not reached.
S6 - Continuous operation with intermittent load. Sequential, identical cycles of running with constant load and running with no load. No rest period.
S7 - Continuous operation with electric braking. Sequential, identical cycles of starting, running at constant load, and electric braking. No rest period.
S8 - Continuous operation with periodic changes in load and speed. Sequential, identical duty cycles of start, run at constant load and given speed, then run at other constant loads and speeds. No rest.

Insulation designations

Happily, IEC and NEMA use the same classification system for winding insulation. It is based on the highest temperature the material can withstand continuously without degrading or reducing motor life. (However, NEMA has no Class E.) temperatures are :
Class A - 105° C (221° F).
Class E - 120° C (248° F).
Class B - 130° C (266° F).
Class F - 155° C (311° F).
Class H - 180° C (356° F).

Most industrial-duty motors use Class B or Class F insulation, depending on the application. Table 2 compares temperature rises, allowed under IEC and NEMA standards. IEC and NEMA 1.00 service factor ratings are nearly identical; NEMA 1.15 ratings, higher.

Torques of IEC Design N (think of it as "normal" torque) motors in general mirror those of NEMA Design B motors ---- the most common industrial type. Moreover, torques of IEC Design H (think of it as "high" torque) are nearly identical to those of NEMA Design C.

As table 3 shows, where NEMA's torque requirements differ, they are usually slightly higher, especially for 4-pole motors.

As with motor ratings, IEC ratings for motor starters and contactors tend to more numerous and application-oriented. NEMA controls are designed to fit broad range of needs. Therefore, by nature they will be over-designed for all but the highest ratings within a given range. IEC controls can be much more closely matched to the task at hand; there are roughly two-thirds more IEC ratings.

Because of greater rating variety and the flexibility this offers designers, IEC-style controls have already become common in this country, even among domestic control manufacturers.

That's a short course in IEC/NEMA comparisons. There is little question that IEC ratings in general follow a more logical, systematic, and descriptive path than those of NEMA. Its simply a matter of getting familiar with the way the rest of the world talks about electric motors.

by Ron Clarke, former Senior Vice President at LEESON Electric Corporation.