The war between voltage spikes and motors

Knowing what to look for gives your motors the edge in surviving AC adjustable-speed inverter power.

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By Michael Clemente, Product Manager, Lincoln Electric Motor Division

PlantServices.com

Two significant events in the motor world are having a major effect on the way synchronous AC motors are designed, purchased and applied. One is driven by technology, the other by politics and economics.

Adjustable-speed technology

The insulated-gate bipolar transistor and the simplicity of the AC synchronous motor have made inverter drives the first choice for precise, adjustable speed and torque control. They are basically maintenance-free, efficient, and cost-effective compared to mechanical, electromagnetic, or DC motor drives. However, the AC motor insulation pays the price for this performance and convenience.

High-voltage spikes are the result of high switching frequencies (up to 20 kHz) and rapid rise times (dV/dt) that are characteristic of insulated-gate bipolar transistors. Further amplification from transmission line reflections and impedance mismatches cause a damaging corona effect. Together with thermal stresses from non-sinusoidal wave-form harmonics, they cause premature insulation failure in motors that lack suitable designs, material selections, and manufacturing methods.

Energy policy act impact

The motor efficiency mandates of the Energy Policy Act of 1992 (EPAct) do not specifically cover motors used in adjustable-speed applications. However, merely labeling a NEMA Design A or B motor as being suitable for inverter duty on adjustable-frequency, adjustable voltage power does not, by itself, exempt it. If it meets the other EPAct criteria (and most do) it must also meet the new efficiency standards.

The criteria for EPAct-covered motors as defined in NEMA Standards Publication MG 1-1987 include:

• single-speed, poly-phase T Frame,
• 1 through 200 hp,
• 3,600, 1,800, or 1,200 rpm,
• foot-mounted, squirrel-cage induction motors, NEMA Designs A and B,
• continuous rated, and
• operating on 230/460 volts, constant 60 Hz line power

Knowing how to recognize an inverter-duty motor that can deliver long life and high efficiency brings maximum value for your adjustable-speed motor dollar.

Corona concerns

The corona effect occurs when the electrical potential between two conductors reaches a level where the air around the conductors loses or gains electrons and becomes charged. When this happens, there may be a discharge through the air between the conductors if the applied voltage exceeds the dielectric strength of the insulation.

Corona effect was not a concern in low-voltage motors (600 volts or less) operating on utility-generated, 60 Hz, pure sine wave power. The special materials and steps taken to prevent corona insulation damage in medium and high-voltage (600 V+) and formed-coil motors were not necessary.

Because voltage spikes as high as 2,600+ volts occur in a 575 volt system, corona prevention is a major consideration for the survival of low-voltage inverter-duty motors.

Partial discharges

A single voltage spike is normally not enough to create a full catastrophic discharge within the windings of an inverter-duty motor. It can, however, lead to a partial discharge that can cause steady degradation of the organic material used in the magnet wire insulation. It can also affect the varnish separating the coils.

These partial discharges generate heat, radiation, and mechanical and chemical energies. When partial discharges cause sufficient accumulated insulation damage, the applied voltage (including spikes and reflections) causes rapid insulation failure.

Corona inception sites

The level at which these partial discharges occur is the <I>corona inception voltage<I>. When the applied voltage exceeds the corona inception voltage, air-containing voids in the insulating material become sites for partial discharges, insulation damage, and eventual failure. Voids include bubbles in the insulation, crevices between turns that did not fill with varnish, and cracks from mechanical damage.

Depending upon the nature of the site and the cumulative number of partial discharges, failure may occur in days or even hours.

Insulation failure locations

Partial discharges can occur anywhere within the insulation system. Ground insulation failures have been noted, as well as failures between adjacent phases (simultaneously at different potentials) and between coils in the same phase. However, turn-to-turn failures within the same coil are the most common.

The greatest potential difference lies between the first and last turn of a coil. Studies show that the voltage difference between randomly touching turns can reach 40 to 90 percent of the terminal voltage.

Through random coil winding, the first and last turn of a coil may touch. This creates an ideal site for insulation failure from partial discharges. Positioning the coils carefully within the slot nearly eliminates the chance of turn-to-turn failures.

What to do about it

Take two basic approaches to combat premature failure of inverter-duty motors. The external approach tries to prevent harmonics and over-voltage spikes from reaching the motor. The internal approach calls for motor materials and designs that withstand any voltage stresses the motor may encounter.

External solutions include:

• modifications to the drive controller,
• a variety of power conditioning accessories (filters, reactors, isolation transformers), and
• restrictions on the distance (cable length) between the drive and the motor to limit voltage build-ups as voltage waves reflect back and forth along the line.