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Another question about motor life

In answer to an internet post, someone was told that “design life” is 30 years for large motor insulation. What’s the basis for this claim?

We know of none. Certainly some motors have exhibited life that long or longer; many others have not. But no recognized design practice, test procedure, or widely publicized experience supports any specific life span to be expected.

What’s a NEMA B speed-torque curve?

A 2016 paper from an IEEE conference included a graph of motor speed vs. torque for NEMA Design B motors, described as a “NEMA Motor Torque Curve.” I don’t see anything in my copy of NEMA standards that defines any motor speed-torque curve as such. What might this refer to?

There’s an unfortunately widespread publication of motor speed-torque curves identified as “NEMA standard.” But NEMA MG 1 standards do not define a curve shape for any motor, only the minimum values of torque at locked-rotor, pull-up, and breakdown, without indicating the speeds at which the latter two values can occur.

How long should grease last?

We understand that the useful life of grease in a ball bearing will be cut in half for every 10°C rise in temperature. But how can we tell how long the life would be without any temperature increase?

Bearing manufacturers have offered such statements as: “Several interactive factors influence grease life, the effects of which are extremely complex to calculate. . . .” According to another source, “greases contain oil and thickeners, both of which . . . influence life time . . . larger bearings [and] high speeds shorten grease life.” In other words, grease life can’t be precisely predicted, and half of an unknown quantity must also be unknown.

Torque versus voltage

Someone was asking on an internet forum about the practice of rating motors for 208/230 volts, and one response stated that “torque is proportional to the square of the voltage.”

I have seen that before, and I don’t believe it’s accurate. Comments?

No, it’s not quite accurate. Although the ratio does change somewhat with motor design and polarity, as well as throughout the speed range during acceleration, a-c motor torque varies more nearly as the 2.2 to 2.3 power of terminal voltage.

Example: at 80% voltage the torque will not reach 64% of the full-voltage value, but only about 60%. That can make the difference between a successful and an unsuccessful start.

What’s a ‘high resistance’ joint?

Everyone agrees that “high resistance” joints in wiring can be dangerous, with overheating being a cause of fires or accelerating corrosion. But how high is “high”? If joint resistance is measured, what’s the limit?

That’s a tough question. A soldered or brazed joint is normally of much lower resistance than a bolted joint. But much depends upon the contact area involved; the larger that is, the lower the overall resistance.

We’ve seen many publications concerning “effective” and “desirable” electrical contact, but hard numbers are difficult to come by. Joint resistance depends upon some combination of bolt size and tightening torque, contact area and shape, crimping or soldering variations, and corrosion, as well as the current involved.

One publication recommends a contact resistance of one “nanoohm” (a billionth of an ohm) as a maximum. Another cites a figure of 35 milliohms as a maximum. Still another calls for a range of 1000 to 5000 microhms (millionths of an ohm).

Overvoltage alarms and medium-voltage motors

On an internet forum, someone has asked about overvoltage alarm and trip settings for 4,160 volt motor control. The answer was that although NEMA design standards do specify a plus/minus 10 percent voltage variation, ““that doesn’t apply to medium voltage motors.”

I always assumed NEMA would apply regardless of motor size. Am I right about this?

Yes, sort of. Whether a “large” medium voltage motor, in Part 20, or a “medium” motor in Part 12, NEMA MG 1 specifies that “successful performance” is possible with that much voltage variation. The problem is that we’re not sure exactly what “successful” means in this context.

The status of IE5 motor efficiency

A motor manufacturer has begun advertising “NEMA IE5 motor efficient” products. Is this a new addition to NEMA standards?

Not that we know of. This category exists in international (IEC) standards (how widely available on the market we don’t know), but the IEC category of IE5 isn’t included in NEMA MG 1 as far as we know. Also, IEC standard ratings, in kilowatts, don’t all match NEMA standard ratings in horsepower.

The meaning of ‘ambient’

Does “ambient temperature” mean something different outdoors than indoors?

Yes. “Ambient” refers only to the temperature of the air surrounding some object. If that object is outdoors, it is also subject to heat produced by solar radiation, over and above the surrounding air temperature.

The temperature of the exposed object will be determined by the combined effect of air temperature and the radiation. The radiation effect will depend upon how closely the object is to a “black body” (which determines the radiation effect on temperature). Accordingly, the temperature rise of an outdoor object will depend to some extent upon its color.

A question about hipot motor test voltage

A recent internet post about testing a 2,300/4,000 volt motor winding stated that the test should apply an a-c voltage because “due to damage, d-c hipot is no longer done.” We know that the “final” overvoltage test is always a-c, but is d-c “in-process” testing no longer a legitimate practice?

MG 1 motor standards Part 3 has included this note: “A direct instead of an alternating voltage may be used for high-potential test.” Also, the “standard” (a-c) test voltage of “twice rated plus 1,000 volts” applies to the final test on an assembled machine.

Tests made at various stages of the winding process are up to the manufacturer. In any case, we don’t know of any ban on d-c testing. EA Edited by the EA staff