Solutions to motor insulation failures

>Many adjustable speed drives use insulated gate bipolar transistors (IGBTs). These power control devices offer many advantages and improve the control of ac motors. But, there are tradeoffs. Some IGBT characteristics, particularly their fast switching capability, combined with long lines between the drive controller and motor can shorten motor life.

The phenomenon

IGBTs let drives turn voltage on and off 18,000 to 20,000 times a second. To do this means the voltage rise time is short, usually less than a microsecond. These short rise times combined with long power lines between the drive and controller can produce voltage reflections, also called reflected waves that have high peak voltages. If the voltages are large enough, they will generate potentially destructive stresses in the motor insulation, Figure 1.

In simplistic terms, the shorter the voltage rise time and the longer the power lines, the higher the amplitude of the reflected wave. To quantify, under the “right” set of conditons, the voltage can reach two to three times the dc bus voltage in the drive. Thus, if a drive has a 575 Vdc bus, voltages at the motor may reach 1,700, enough to cause some motors to fail.

This problem occurs regardless of the type of switching device used by the drive, be it IGBT, gate turn off (GTO) or bipolar junction transistor (BJT). However, IGBTs are the most common and are usually the component of choice for the higher carrier frequencies (also called chopping or modulating frequencies).

Although, this phenomenon may affect only a small portion of installations, it is important to recognize the situation and consider solutions before installation.

Effects on motors

In addition to using insulated wire, most motors receive additional insulation by dipping the windings in a tank of insulating material to form another coating on the windings. This process adds insulation value to the magnet wire, compensating for nicks and thickness variations in the original wire insulation.

Some windings get a second dipping (with a curing process between the dippings) or an additional “drip” process that increases insulation material at the end turns. This addresses the problem of motor insulation failures that occur on the end turns of the winding, but not in the slots. Additional insulation measures assure that motor windings with different potentials are electrically insulated from each other.

The insulation process, however, usually leaves microscopic air voids in the coating. These holes can be insulation failure points when voltage peaks are impressed on the stator winding by a reflected wave. Frequently, 60 to 80% of the voltage can be distributed across the first turn of the motor winding, Figure 1.

At this time, conclusive data is not available to determine the exact cause of insulation failure; motor manufacturers are split on which of the following situations results from the reflected wave phenomenon:

• The electrical stress voltage exceeds the breakdown voltage of the air void, causing a partial discharge. Successive partial discharges slowly degrade the insulation.
• The voltage ionizes the surrounding air, leading to arcing across the windings (known as corona or corona discharge), causing immediate motor failure.
• The peak voltage is greater than the magnet wire insulation rating, causing dielectric stress and eventual insulation failure.

Smaller frame motors (1 hp and below) are at greater risk for two reasons:

• They are typically not as well constructed as larger motors. Larger motors tend to use more insulation. Also, motor manufacturers tend to use form winding rather than random winding techniques in large motor construction. Form winding reduces the voltage difference between adjacent windings.
• Smaller horsepower drives typically have faster rise times.

While no definitive research has been done on the effects of reflected waves on cable insulation, most cables are immune to this phenomenon because of their substantial insulation.

Why IGBT drives?

Reflected wave phenomenon gained greater attention with the introduction of 460-V and 575-V IGBT-based drives. Generally, 230-V applications are unaffected because the reflected wave amplitudes are low while the typical motor insulation is the same as a 460-V motor. Motor failures have been documented with BJT and GTO-based drives, but they generally occurred with motor cables of 600 ft and longer.

The rise time of a typical BJT (0.2 - 2 μsec) or GTO (2 - 4 μsec) enables longer motor cable lengths than today’s third generation IGBTs, which have rise times of 50 to 400 nanosec (0.05 to 0.5 μsec).

Even earlier generation IGBTs do not have the short rise times of their present day counterparts, which offer increased carrier frequency for reduced audiable noise and minimal power loss. As drive designers realize these advantages of the newer devices, use of IGBTs in drives increases. This, in turn, heightens concerns about motor protection.

Moreover, replacing older drives with newer ones that have the shorter rise times can cause reflected waves and motor failures.

The solutions

Maintaining performance requires examing three areas:

The motor. Some manufacturers offer “inverter duty” motors. However, many of these only improve the motor’s thermal capacity, not its ability to withstand voltage spikes. Therefore, to assure reliable operation on drives with fast-acting IGBTs, the preferred solution is to install a motor with insulation that can withstand the typical amplitudes of reflected waves.

Most standard motors are insulated to withstand a specific level of surge voltages, with higher levels available for longer rise times, Figure 2. For example, Vendor A offers motors that can withstand surges of 1,000 V if the rise time is 0.1 μsec, and 3,000 V for a 4 μsec rise time. By contrast, the preferred choice for IGBT drives is Vendor B with a 1,600-V (or higher) fast-rise, peak voltage capability.

A few motor manufacturers offer such 460-V motors with stator insulation systems that can withstand peak voltages of 1,600 V, meeting the latest NEMA specification MG1-paragraph 31.40.4.2 on voltage spikes for inverter motors.

Motor cable lead length. Because the amplitude of the reflected wave is proportional to the length of the motor cables, the most effective way to protect motors is to keep cable length short. Table 1 offers guidelines for cable lengths for 460-V drives based on drive type and motor insulation values.

Installation. If you can’t use motors insulated for high voltages, or shorter cable lengths, or you have a retrofit application where you can’t relocate the motor and drive, then the solution may require adding devices to attentuate the high voltage peaks. Such devices include line reactors and electrical filters.

Although incurring expense, this solution protects the motor, eliminating motor replacement costs and costly downtime.

Output reactors. Installing reactors between the drive output and the motor, Figure 3, reduces the rate of voltage rise (increases the voltage rise time). This limits the reflected wave amplitude and extends the allowable distance for motor cables. In most installations, using shorter cable lengths and placing the reactors at the drive output reduces the voltage peaks and cable stress, plus it will improve the quality of the voltage waveform seen by the motor.

Placing the reactor at the motor reduces the voltage peaks (and thus, allows greater lead lengths) and creates a better waveform to the motor. It does not reduce cable stress.

A reactor regardless of where it’s placed, however, will introduce a voltage drop that may reduce the ability of the motor to produce rated torque. The reactor will still see the same voltage stress from the reflected wave and must have sufficient insulation values to withstand these peaks.

These general rules for placement may not apply in all cases. Users should verify effectiveness with the drive manufacturer. Some drive manufacturers regularly recommend output reactors, and some require them, voiding the warranty if reactors are not installed. Some manufacturers have limited inductors built into the product.

Custom output filters. Another effective way to protect the motor is to install a specially designed filter at the drive output. Such a unit combines appropriate R-L-C values to form a dampened lowpass filter. Depending on the construction, it may also reduce electromagnetic interference (EMI). You can select a filter to match an existing installation, but this requires a study of the electrical system and can be the most expensive solution for the problem.

Terminator filters. The most cost-effective method for controlling voltage reflection is often a terminator filter designed to handle a range of applications, Figure 4. This keeps the amplitude of the reflected wave below potentially destructive levels. Installed at the motor terminals, the filter can cover a variety of installations. For larger horsepower installations, they are smaller and can cost less than 10% of the cost of a full output- filter or reactor.

Summary. For the newest IGBT drives, Table 2 gives the most probable solution for the various lengths of cables. These typical values are for a motor with 1,200-V insulation. This table does not represent all installations because of the variety of drive specifications relative to rise time.

Don’t avoid use of IGBT drives

It’s important to protect motors from voltage reflection. Most reflected wave concerns can be handled during installation. Certainly, the majority of installations use cables significantly shorter than the critical distances defined here. And the majority of motors controlled by IGBTbased drives do not suffer insulation degradation or failure due to this phenomenon. Additionally, many other, more prevalent reasons for motor failure have been and will continue to be identified.

These issues do not occur so often that you should avoid a new technology such as IGBTs. You must always weigh the benefits in control that IGBT drives offer versus the possible effects of voltage reflection in a small percentage of applications. Some manufacturers continue to offer BJT and IGBT drives to provide a lower performance solution where desirable.

Both drive and motor manufacturers are aware of these issues and are taking steps to minimize the impact.

Rhode Nelson is product manager and Gary Skibinski is principal engineer of the Standard Drives Business at Allen-Bradley Co. Inc., in Mequon, Wisc.