Many topologies have an advantage because they operate with a relatively small voltage step compared to older NPC topologies and have an input transformer that avoids any common-mode voltage.
by Mark Harshman (Siemens)
March 9, 2015

Motors and variable frequency drives (VFDs) have been used for upstream oil and gas artificial lift applications (with electric submersible pumps and injection compressors) for many years. VFDs provide energy savings, process control and improvement in older well development. When VFDs were first manufacture, operators discovered that their high harmonic content and voltage characteristics shortened the motor's life by damaging the insulation system. Motor companies designed inverter-grade motors that had increased insulation systems and larger housings to dissipate the additional heat. Eventually, VFD manufacturers found new topologies and methods to mitigate the harmonics and voltage transients so that their products were compatible with standard motors. This development led to the ability to retrofit an existing motor (designed for an across-the-line start) with a VFD. Originally, VFDs had only two or three output levels, and each one had voltage output characteristics that could severely damage the motor insulation system. The addition of output filters and many changes in control and internal VFD design helped these drives handle standard motors. Some types of VFDs were eliminated after providing particularly unfavorable performance with motors. Current-source inverters (CSI) were manufactured by most early VFD designers because the semiconductor devices offered good failure mode performance at a low cost. The output waveform was difficult for the motor to handle, and these topologies have been abandoned—with the exception of very high power ranges above 15 megawatts. They retain some popularity as a transformerless design, which has caused new problems with bearing currents. Operators face three major difficulties with motor insulation when a motor is controlled by a VFD:

  • The peak differential voltage on the windings with respect to ground, phase to phase and turn to turn
  • Voltage on all the windings with respect to ground, which is the common-mode voltage
  • The rate of change of voltage with respect to time (dV/dt)

A motor's windings are not intentionally grounded, but considerable capacitance exists from the winding to the stator frame through which alternating currents will flow. These currents are much larger when the motor is controlled by a VFD because of the high frequency components of the voltage. The conductors in the motor windings are insulated from each other and from the stator. The magnet wire has a heavy varnish overcoat and is usually also wrapped with insulating tape. The insulation in the slots is commonly known as groundwall insulation.

Destructive Effects of Voltage Stress

When voltage is present on the motor windings, an electric field is generated between all conductors, which are at different potentials. This electric field passes through the insulation materials, including air. Each material has a limit regarding the electric field it can withstand before breaking down. The electric field rarely becomes large enough to cause a breakdown of the insulation and create an arc. However, if the electric field becomes too large in the air, it can ionize the air and create ozone. Ozone is an extremely reactive gas that attacks all the organic molecules in the insulation and degrades them, eventually leading to failure. This phenomenon is called partial discharge. It may take days, months or years to destroy an insulation system. Low-voltage and medium-voltage motors typically have different coil constructions, even though the shape and location of the coils is similar. A typical stator coil orientation is shown in Figure 1.

Figure 1. A standard stator coil winding configuration<br />
<em>(Graphics courtesy of Siemens)</em>Figure 1. A standard stator coil winding configuration (Graphics courtesy of Siemens)

Because the voltage stress is less, low-voltage motors (below 1,000 volts alternating current) have random-wound coils with only varnish as the insulation. Random-wound means that no defined location exists for a given turn. This means that the start turn may lie in contact with the finish turn so that the entire coil voltage occurs between those turns. Medium-voltage motors, in contrast, have insulating tape wrapped over the varnish between the turns. Each turn has a defined location with respect to the other turns, so the start turn does not lie next to the finish turn.

Equation 1 and 2

These coils are much better at withstanding voltage than the random-wound, low-voltage motor coils. After the introduction of insulated gate bipolar transistor pulse width modulation (PWM) drives for low-voltage motors, an increase of low-voltage motor insulation failures occurred because of the unexpected harmful effects of the fast-rising pulses on the coil insulation.

Motor Insulation Design

If the motor designer does not know that the motor may be driven by a VFD, the insulation will be designed as follows:

  • The motor neutral will be assumed to lie near ground potential, which is the same voltage as the stator frame.
  • The terminal voltages will be assumed to be symmetrical with respect to ground with no common-mode voltage. This is the best situation for the groundwall insulation.
  • The phase-to-neutral voltage will be assumed to be sinusoidal with a maximum value of the source voltage plus 10 percent. The peak will be calculated using Equation 1. This will determine the groundwall insulation.
  • The phase-to-phase voltage will be assumed to be sinusoidal with a maximum value of the source voltage plus 10 percent. The peak will be calculated with Equation 2. This will determine the phase-to-phase insulation at which the coils of different phases that are in the same slot are touching.
  • The turns in the stator coils will each have the same voltage impressed on them that is the root mean square (rms) line-neutral voltage divided by the number of turns.
  • The dV/dt will be no more than the 50 to 60 hertz utility frequency that the sine wave creates.
Figure 2. Insulation areas of concernFigure 2. Insulation areas of concern

The listed concern areas are shown in Figure 2. Since the motor windings consist of coils embedded in slots in the stator, they have inductance as well as turn-to-turn capacitance and turn-to-ground capacitance. At high frequencies, they behave like transmission lines with wave propagation phenomenon. This means that, for fast-rising voltages, the applied voltage will not be uniformly distributed among the turns. The result is excessively high dV/dt and some turns being more stressed than others. VFDs do not produce sinusoidal voltages. Voltage source drives produce a series of pulses of different widths and amplitudes to approximate a sine wave. Power conversion circuits (more than a few kilowatts) cannot generate a smoothly changing voltage like the utility sine wave. The pulses have very high dV/dt values (1,000 volts/microsecond) on the leading edge compared to the utility sine wave. Because of the PWM process, the waveform has a higher peak value than the utility voltage. Also, for drives that have a thyristor bridge for a line-side converter, the output voltages have a displacement from ground that is referred to as common-mode voltage.

Types of VFDs

VFDs can be divided into voltage-fed and current-fed categories. Voltage-fed drives create a defined voltage at the output. The motor leakage inductance blocks the high-frequency components so only fundamental current flows. Current-fed drives create a defined current at the output. In these CSI drives and the line commutated inverter (LCI), a large capacitive (0.2 to 0.3 per unit) filter absorbs the high-frequency components of the current. The current-fed drive has low dV/dt and low peak voltage but can have a common-mode voltage issue, often necessitating a specially insulated motor. The voltage-fed PWM drives have different pulse patterns but generally fall into a few categories:

  • Neutral-point-clamped (NPC) VFDs have three levels of voltage at the output and five levels line-to-line. The voltage step is half the direct-current link voltage (about 3,300 volts for a 4,160-volt alternating-current drive)
  • In three-cell, multilevel VFDs, the cell is an NPC inverter. Here, the step is half the direct-current link voltage of a cell, or about 1,000 volts.
  • In multi-cell, multilevel VFDs, the voltage step is the value of the direct-current link voltage of a cell, about 900 to 1,000 volts.
 Figure 3. Relative stress values caused by VFD output levels Figure 3. Relative stress values caused by VFD output levels

The amplitude of the step and the rise time are the key factors in determining how a VFD waveform will affect the motor insulation after the waveform has propagated along the cable. The relative stress caused by the VFDs output levels is illustrated in Figure 3. As the number of levels increase, the voltage step decreases, resulting in lower dV/dt values and reduced stress on the motor insulation system.

Conclusion

The use of VFDs created some motor insulation issues because the motor insulation was designed for the benign sine waves of the utility. The insulation vulnerabilities occurred because of the presence of fast-rising voltage pulses in the VFD output and the use of power conversion circuits without a transformer, which creates common-mode voltage on the motor windings. Many topologies have an advantage because they operate with a relatively small voltage step compared to older NPC topologies and have an input transformer that avoids any common-mode voltage. When applying a VFD to a motor, operators should consider any potential risks. Most VFD manufacturers can evaluate a motor based on its topology. Manufacturers that produce drives and motors can ensure compatibility and often offer increased warranties because of this relationship.