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Does NFPA 70E apply to d-c circuits?

Presumably the provisions of NFPA 70E are based on a-c voltages. In d-c circuits, we know that arcs behave differently, for instance. Are voltage levels in 70E applicable to d-c?

Voltages in the NFPA standard are meant to be either a-c RMS or d-c-the same numbers apply for either case. That was the intent of the standards-writing group, although the standard doesn't make that intention clear.

Hotter bearing may have reduced load rating

For a Class H insulated motor driving an exhaust fan in a drying kiln, we're told the ball bearings need to be "heat stabilized" for use at 180°C. Besides the right grease, do we need to be concerned about any other problems with such bearings?

Heat stabilization tends to reduce the hardness of ball and race material, so that the basic load rating of the bearing is reduced by 10% to 15% at 180°C. In direct-coupled service, that shouldn't be a problem.

Why not higher voltage for motor hipot test?

If we rewind a 460 volt motor, we give it a "new winding" hipot test at twice rated voltage plus 1,000 volts, which is 1,920 volts. But if the insulation system is supposed to be good for 600 volts, why isn't the test based on that, which would be 2,200 volts?

The standard practice is what NEMA MG 1 states in Part 12; "rated voltage" is the nameplate value, not the capability of the insulation system. A 4,000 volt winding, for example, may use a 5,000 volt insulation system but will be tested based on twice 4,000 plus 1,000, not twice 5,000plus 1,000.

Voltage drop limits vary around the world

For a 60 Hz motor to be used outside North America, do international standards allow for the same voltage drop as in the U.S.?

No. Most of the IEC standard voltage ratings aren 't the same anyway, but for those that are the same as in U. S. practice, here is a comparison:

The U.S. values (per ANSI C84.1) are for "Range A" service, usable for a "reasonable time." An ANSI "Range B" allows lower values, for which prompt corrective action is to be taken; even those values exceed the IEC limits. These relationships may not be final; the 7th edition ofIEC-60038 was still in draft form early this year.

What's lower limit for insulation resistance?

We have always used 500 megohms as a reasonable minimum for insulation resistance on form-wound Stators. We wonder why someone would insist on 5,000 as the lowest acceptable value. Is this coming out of some new standard, and if so, why so high?

Probably the answer is that a measured value that is quite high at ambient temperature will decrease rapidly when the winding is at normal operating temperature.

For example, a reading of 1,000 megohms at 500C may be only 20 megohms at 155°C. Even a small amount of surface contamination might cause trouble. There is no generally agreed standard beyond the general provisions of IEEE 43.

Does speed matter when regreasing bearings?

Some motors in a wet environment need greasing every week. The motor RPM varies a lot between 700 and 3,600. Should we be concerned about the actual speed when we put in new grease?

Opinions differ on whether to add grease while a motor is running, or only when it's stopped. The motor manufacturer's recommendation, tempered by your experience, has to be the guide. But if you do add grease during operation, actual RPM should not be a factor. The bearing assemblies were presumably designed to operate properly through the range.

Why cable rating doesn't match motor voltage

Why use 600 volt wire to connect a 230 volt motor? Since 300 volt wire exists, why not use that?

The lower-voltage material is normally intended for control and instrumentation use where application conditions are less severe. The stress of handling and installing power circuits, with generally larger and less flexible conductors, would tend to damage wire with insulation only thick enough for 230 volt service.

What's the meaning of loading factor"!

What's meant by "loading factor" for a motor?

Although commonly used, loading or load factor isn't defined by standards. It generally means the ratio between actual and rated output. For example, a 50 hp motor supplying 35 hp would exhibit a load factor of 35/50, or 70%.

Edited by the EA staff

Copyright Barks Publications Mar 2008
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