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You are invited to send questions to this department. Your identity will be kept confidential. but we ask that you include your name, affiliation, postal (not e-mail) address, and telephone number. If you'd like a pre-publication reply, enclose a stamped envelope and we'll try to respond promptly. We cannot provide personal consultation or supply winding data.

Is skimpy insulation a problem for motors?

It seems to us that more 2,300 and 4,000 volt motor windings of recent design are failing compared to what we used to see. We're hearing that insulation on formed coils today is thinner than it used to be, which leads to earlier breakdown. We don't make our own coils, and our suppliers stand behind their insulation systems, but this makes us wonder. Comments?

Insulation does occupy space otherwise available for conductors, so thinner materials that will do the same job have always been sought after. But thickness alone is hardly the only basis for judging insulation integrity.

Many years ago, the only available materials were paper, cotton, and various natural oils. Today's materials are much stronger and longer lasting at higher temperatures. We don't know what the rotating machine industry in general is doing, but we did find one pertinent comparison for 2,300 volt windings. Discounting allowances for varnish buildup alone, consider the table below.

The difference doesn't appear significant.

Any limits on motor surface temperature?

Most of our mechanics and troubleshooters seem to know better than to get their hands on the surfaces of motors running at service factor loads in some plant locations with ambient temperatures pushing 110 deg F. But one or two employees have raised safety concerns. Is there some OSHA regulation limiting motor surface temperature or requiring any sort of safety screening for motors?

We know of no pertinent regulations. Barriers of some sort might be prudent, but they're often impractical because of their effect on motor ventilation or process operation. Warning signs could be helpful, but the first step should be to alert everyone to the possible problem as part of a comprehensive safety program.

What's a Faraday shield all about?

Several of our customers have had problems with bearing currents in drive motors. Some have tried grounding brushes that don't always work, and many motors don't allow room to install them anyway. One user heard about a "Faraday-shielded" motor that fixes the problem. Is this some new product? Our distributor is unaware of it.

The Faraday shield is a grounded conducting sheet (typically thin copper) wrapped around the inside of the stator in the air gap. Laboratory studies several years ago established its effectiveness in preventing the capacitive voltage coupling that can cause the trouble you describe.

Unfortunately, as with many such theoretical developments, several practical problems limit its usefulness. First, such motors aren't generally available on the market.

Second, any highly conductive layer in the motor air gap can significantly increase stray load loss. Finally, a motor rewind necessitates removing the shield. Replacements--and the technique of replacement-are also not generally available to the service center.

Sizing contactors for capacitor circuits

We're installing a number of capacitor banks to deal with variations in plant power factor. Based on the rated capacitor current, how should contactors be sized to control a capacitor circuit?

A common rule of thumb is to apply a contactor sized to handle 1.5 times the rated capacitor current. This accounts for a 10% tolerance on that current plus an additional 35% margin in the circuit wiring.

How to specify motor inrush current

Please advise how we ought to give the inrush current limit at reduced voltage in a motor specification. We have used 650% for full-voltage starting. What should the percentage be at 85% voltage?

Power supply system behavior doesn't depend on percentages, only on amperes. Your 650% figure is a ratio of locked-rotor amperes to full-load amperes. The latter figure has no meaning at reduced voltage, so there can be no such ratio under that condition. Even at full voltage, you can't always know the full-load amperes, so specifying 650% of that unknown value is indefinite. Translate your percentages into actual ampere values, placing whatever limits on them that your power system imposes.

What causes vibration change with temperature?

In monitoring vibration on some large sleeve bearing machines, we've seen variation in the vibration pattern with changes in bearing temperature. So far there does not seem to be much correlation, and we're wondering what the relationship ought to be, and why. Do you have any information on this?

The probable explanation is a sensitivity to bearing support stiffness. That involves the oil film within the bearing, which is subject to change with oil viscosity and therefore with temperature. The relationship is likely to be unique to each machine. Some designs may not be affected, while others could be greatly influenced.