How reflected waves damage ac motors

Nov. 1, 2000
To understand what reflected waves are, why they occur and how to prevent them, it’s important to have an understanding of ac drive operation and motor insulation systems.

Today’s sophisticated ac drives offer ease-of-use, higher performance, more features, smaller size, and lower cost-per-function than ever before. Most recently, the insulation systems within ac motors are being refined for long-term operation with ac drives. Along with these advances, new application techniques and standards can help adjustable- speed drive-and-motor users make informed choices to ensure proper drive-motor operation.

The majority of low-voltage ac drives on the market today are PWM , where incoming ac power is converted to a fixed dc voltage by the drive’s input rectifier, and then filtered to reduce ripple. The fixed dc power is then fed through an inverter section that changes it to variable frequency ac output power, which is then fed to the motor.

The rectifier outputs a waveform consisting of a series of rectangular pulses with fixed height and adjustable width. The adjustable width pattern produces near-sinusoidal current in the motor, and the overall pattern of positive versus negative pulses controls the frequency.

One cycle of the output waveform at a given output voltage can be made from many narrow pulses or a few wide pulses. To generate a waveform that contains more pulses, the transistors in the inverter must switch more often. The rate of switching is called the carrier frequency.

The evolution of power semiconductors

Earlier generations of adjustable-frequency drives were built with power bipolar transistors. These current-controlled devices had several drawbacks. They had relatively slow switching speeds, which resulted in high forward voltage drops, loud motor noise, and large switching and conduction losses. They also cost more and were more difficult to control than today’s insulated gate, bipolar transistor (IGBT) devices.

IGBT devices, on the other hand, are voltage-controlled devices. They are less expensive than power bipolar transistor devices and have smaller drive circuitry. There is less power lost during switching, which lets drive manufacturers use smaller heatsinks and fans.

New technologies, new considerations

IGBT-based drives, though, can cause problems in some applications. Faster voltage rise times and higher carrier frequencies can produce greater commonmode, conducted, and radiated noise. To eliminate these problems, engineers can install a common-mode choke at the drive output to reduce high-frequency current to ground. However, conducted ground currents can find their way into CNC, PLC, and computer grounds, creating conducted noise. Shielded insulated power leads on the motor can prevent this. Wireless drives and effective ground planes within the drive minimize radiated noise, or radio frequency interference (RFI).

Corona (partial discharge)

Whenever cable surge impedance does not match the motor winding surge impedance, a reflective voltage wave occurs which will result in voltage spikes, Figure 1. Although this happens regardless of PWM power semiconductor technology (IGBT, BJT, GTO, etc.) it is more severe with the fast switching IGBT-based drives. These voltage spikes occur on every generated pulse and will cause corona (partial discharge) to occur in the motor winding. The voltage required for corona to occur is referred to as the Corona Inception Voltage (CIV). If the reflective wave voltage spike is higher than the motor winding CIV, then corona will occur in the motor winding.

The motor insulation system should have a CIV rating above 1,600 V to meet the standard developed by NEMA (MG1 Part 31). The standard defines both the limit on peak voltage magnitude and how quickly the voltage can change. The acceptable gradient is a 0.1-msec rise from 10 to 90% of steady-state voltage with a 1,600-V maximum peak. For an inverterduty, 460-V motor, the CIV should be 1,600 V or greater.

Corona can cause both physical and chemical damage. It creates high-temperature hot spots on the motor and erodes the insulation between phases, turns, and coils within the motor. When this insulation fails, current can flow phase-tophase, turn-to-turn, or coil-to-coil, and the drive will typically trip, shutting off the motor. In cases where the windings are coated with resin, small bubbles in the resin, known as voids, provide the air necessary to begin the corona process.

In many cases, these failures are not severe enough to cause complete motor failure. Motors with insulation damage can sometimes run across the line without effect. However, today’s intelligent drives with sensitive electronic circuitry often detect small problems quickly and will not operate a motor with insulation damage.

There are a number of ways to raise a motor’s CIV (See box.). Shorter motor cables raise the frequency and contract the pulses over the CIV level. And special insulation systems are also available.

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Some of the steps engineers can take to avoid reflected wave phonomena are:

• Choose inverter-duty motors for adjustable- speed ac drive applications. These motors are rated to withstand temperature rise and feature a wide speed range.

At a minimum, adjustable-speed ac motors should meet NEMA MG1 Part standards. They should also have a minimum CIV rating of 1,600 V at rated operating temperature for 460-Vac and should have a higher voltage rating for 575 Vac.*

• Always follow the lead length recommendations provided by the drive manufacturer. Most have conducted extensive testing to understand both the dv/dt and the reflected wave voltage amplitudes created by their products. Where practical, locate the drive and motor as close together as possible.

A properly CIV-rated ac motor will operate under the effects of reflected waves even when the drive and motor are placed apart at extended distances, Figure 3.

• Use reactors and filters when the distance between the drive and the motor exceeds the manufacturers’ recommended lead length. Place an output reactor at the drive or a filter at the motor to reduce the peak voltages.

• Choose compatible drive-motor packages. Many manufacturers offer adjustable- speed drives and motors designed and tested for compatibility in various operating conditions. The selection data manufacturers provide often eliminates guesswork when choosing a drive and motor.

Components of a motor insulation system

There have been advances in materials, manufacturing processes, and in insulation systems for motors used with IGBT-based ac drives. A typical insulation system includes the magnet wire, slot insulation, motor coil head insulation, and insulating resin.

Magnet wire — The magnet wire is typically film coated with enamel. The wire material, its thickness, and concentricity of the coating all affect the quality of the insulation system.

Slot insulation — The motor’s slot insulation physically separates the motor’s components. Slot liners and top sticks increase phase-toground insulation and the mid sticks increase phase-to-phase insulation. They also add to the dielectric strength of the insulation system.

Coil head insulation — Additional insulation can be added to the coil head (that part of the winding that is outside of the slot). The manufacturer places phase paper between windings of different phases and adds insulation to the leads that emerge from those windings. This greatly increases the dielectric value of the insulation from phase-to-phase as well as from coil group to coil group.

Insulating resin —After the winding is assembled, it is coated and impregnated with an insulating resin. This resin should fill in as much space as possible between the wires and stator slots to create a solid unit. The resin will add to the dielectric capability of the insulation system, and by displacing the air around the conductor, will increase the voltage required for corona inception.

There are a number of assembly considerations that improve the insulation system’s performance. In winding design, for example, designers should keep conductors with a high difference in electrical potential as far apart as physically possible.

In addition, the resin application process can greatly affect insulation performance. Some resins are applied by a dipping process, after which the windings are baked to help the material flow throughout the entire winding. Other windings have the resin flowing over the stator before it is baked. Another method is called VPI or vacuum pressure impregnation. It forces resin into the windings to increase the thickness of the coating using heavy or filled resins to eliminate voids. These processes can be performed many times to improve the insulation system.

See associated figure

Other Rockwell Automation contributors to this article include: Gary Skibinski, Mike Melfi, Rhode Nelson, Jeff Ray, and George Weihrauch

* Rockwell Automation Test Method

Dave Petro is product marketing manager for ac general-purpose and high-performance drives, Rockwell Automation, and Sid Bell is materials engineer at Reliance’s Athens Motor Plant.

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