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PQ Tip: Shorter Leads, Longer Motor Life

Sep 1, 1998 12:00 PM, By John A. DeDad, Editor-in-Chief

You're seeing more premature motor failures. Is there a connection between these failures and the type of drive used? What effect has the length of the motor lead on the problem?

According to a Power Quality Testing Network (PQTN) application (No. 7) published by EPRI's Power Electronics Application Center (PEAC), three-phase motors can withstand 600VAC (rms) at 60 Hz, as required by industry standards such as IEEE 17-1974 and ANSI C50.21-1976. These standards don't apply to repetitive voltage transients. Thus, there's no way of predicting whether voltage pulses generated by PWM drives will cause insulation damage.

Yes, most motor manufacturers design motor winding insulation systems to withstand voltages greater than the IEEE and ANSI standards require. That's why hundreds of thousands adjustable speed drive-driven motors operate without premature failures.

But, some motors have insulation systems that can't tolerate the fast-voltage pulses of a PWM drive, especially those having long motor leads.

Here's what happens. According to PEAC, fast-changing PWM voltage pulses are interacting with the distributed inductance and capacitance of the motor leads. This can result in an amplified peak voltage as high as 1600V at the motor terminals. This peak voltage stresses and degrades the insulation around the stator winding of the motor.

Remember, the peak voltage magnitude at the motor terminals depends on the motor lead characteristics and the surge impedance of the motor. The smaller the motor and longer the leads, the greater the peak voltage.

Here's what to look for. Voltage pulses from the PWM drive damage the insulation in the stator windings over a period of days or even months. The amplitude of the voltage pulse is directly proportional to the length of cable between the drive and motor. In other words, the longer the cable length, the greater the peak overvoltage at the motor terminal.

Here's what you can do.

• Keep the motor lead length at 50 ft maximum.

  Industry experience shows that if done, you should have no problems.

• Observe cable length guidelines.

  SCR- and GTO-type switching devices allow the greatest motor lead lengths.

• Program modern IGBT drives to operate at different frequencies.

  This will allow a longer lead length (more than 50 ft).

• Consult your drive Owner's Manual

  for the maximum cable length.

• Install matching filters.

  If you can't reduce the lead length or switching frequency, a matched filter reduces the long motor lead effect by matching the terminating impedance of the motor to its lead impedance. Some manufacturers offer these filters as options to install at the motor terminals. You can purchase them separately and install them per the drive manufacturer's recommendations.

Here's what to do with new drive/motor installations.

• Address the durability of motor winding insulation during your initial specification process.

• Tell the drive manufacturer and system integrator the intended length of motor leads.

• Specify only a motor that complies with NEMA MG1, Part 31, Definite Purpose Inverter-Fed Motors. These motors are designed for use with PWM drives and can withstand the stress of 1600V peak with a 0.1m sec rise time.

• Don't fall for the claim "inverter duty" or "suitable for inverter power."

The benefits are worth your efforts: Reduced emergency maintenance and troubleshooting costs; reduced costly motor repairs and replacements.

EAC recommends you look into the following countermeasures: * Investigate installations of motors and drives for motor leads longer than 50 ft. * If motors fail because of stator insulation damage, then try to eliminate the long-lead effect. * Reduce the lead length to less than 50 ft, or reprogram the drive to operate at a lower switching frequency. * Install matching filters per your drive manufacturer's recommendations. * Use motors that comply with NEMA MG1, Part 31.

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© 2012 Penton Business Media, Inc.

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