The application of new generation Variable Frequency Drives, (VFD’s), utilizing Insulated Gate Bipolar Transistors, (IGBT’s), in the inverter section with motors connected by long leads has been a source for concern and expense. Motors run by variable frequency drives at some distance away can fail due to high voltage insulation breakdown caused by the fast switching time of the drive IGBT output. When left unmitigated, the high rate of change in voltage can result in spikes that add to any reflected voltage wave. This can add to the peak voltage in a way that produces a large voltage overshoot or spikes. These voltage spikes can and will damage motors and or cable insulation over time and may lead to premature motor failure. The problem gets worse as the length of the motor cables increase.
Drives and motors often need to be separated by distance. In some plants, motors can withstand the harsh surroundings. However, sensitive VFD electronics cannot tolerate such environments, forcing long distances between the motor control centers that house the drives and the motors that they control. Conveyors and presses often utilize single drives to operate multiple motors that are positioned along the length of the conveyor. The length of the conveyor often dictates the longest distance between a drive and a motor. Most manufacturers of VFD’s publish a maximum recommended distance between their equipment and the motor. The restriction of that maximum distance often makes application difficult, impractical, or unfeasible. Maximum tolerable distances vary by manufacturer but might be 100 to 250 feet. Many users of VFD’s have elected, or have been forced, to disregard the maximum recommended distance. These users are now replacing or rewinding motors after a 2-week, a 6-week, or a 6-month life span. In some cases, motor failure occurs even though the installation is within, but close to, the maximum recommended distance. Both the cost of these repairs and the downtimes that they demand are mounting quickly.
VFD’s generate the useful “fundamental” voltage and frequency via a modulation technique known as “Pulse Width Modulation (PWM)”. For a 480V system, the typical fundamental voltage ranges from 0 to 460V and the fundamental frequency varies from 0 to 60Hz. The inverter circuit “switches” rapidly, producing a carrier upon which is contained the useful fundamental voltage and frequency. This switching is quite like an AM or FM radio where the useful information, music or talk, is transmitted to the radio receiver at some assigned radio frequency. The carrier, or switching frequency used for IGBT-based VFD, generally ranges between 4 to 32 kHz with default settings of 6 or 8 kHz. Switching time is the time required for the IGBT inverter to transition from the “off” (high impedance) state to the “on” (low impedance) state and visa-versa. For the latest generation of IGBT’s, the switching time varies from 100 to 200 nanoseconds, (ns). Because these devices are used in circuits fed by approximately 650 V DC, for a 480V system, the rate of change of voltage with respect to time, (dV/dT), can exceed 7500 volts per microsecond, (V/s).
The relatively recent availability of high voltage, high current IGBT’s has led to the wide use of these devices as the main switching element in the D-C to A-C inverter section of 1-phase and 3-phase AC Pulse Width Modulated VFD’s. Virtually all the manufacturers of these types of power conversion circuits have developed, or are developing, product lines that utilize these relatively new devices. One of the main reasons for the widespread use of these devices is their extremely fast switching time. This switching time results in very low device transition losses and, therefore, results in highly efficient circuits. In addition, a fast switching time allows drive carrier frequencies to be increased above the audible range.
Reflective Wave Phenomenon
Voltage wave reflection is a function of the voltage rise time, (dV/dT), and of the length of the motor cables which behave as a transmission line. Because of the impedance mismatch at both ends of the cable, (cable-to-inverter and cable-to-motor), some portion of the waveform high frequency leading edge is reflected in the direction from which it arrived. As these reflected leading edges encounter other waveform leading edges, their values add, causing voltage overshoots. As the carrier frequency increases, there are more leading edges present that “collide” into one another simultaneously, causing higher and higher voltage overshoots. If the voltage waveform was perfectly periodic, it might be possible to “tune” the length of the wire. However, since the width of the pulses varies throughout the PWM waveform, it is not possible to find any “null” points along the lead length where the motor may be connected without the fear of damage.
Resonant Circuit Phenomenon
Another way to analyze the problem is with respect to system resonance. Because multiple conductor wire runs contain both distributed series inductance and distributed parallel capacitance, the conductors can be viewed as a resonant tank circuit. Knowledge of the Inductance, (L), and the Capacitance, (C), values of any circuit allows for the calculation of the circuit’s natural resonant frequency. As wire lengths grow, L and C will both increase, reducing the resonant frequency. In those applications where the physical length of conductors connecting the motor to the inverter exceeds 50 ft., L and C values combine to form a typical resonant frequency range between 2 to 5 MHz, depending on wire characteristics. If the length is longer than 250 ft., the resonant frequency will be lowered to the range of 500 kHz to 1.5 MHz. These self-resonant frequency ranges are at, or below, the high frequency components of the voltage waveform produced by the IGBT inverter. (A spectral analysis of the voltage waveform generated by inverters employing IGBT’s would reveal frequency components ranging in excess of 1 to 2 MHz). Furthermore, whenever the self-resonant frequency of the conductors approximates the frequency range of the IGBT voltage waveform, the conductors themselves go into resonance. The conductor resonance then creates a “Gain”, or an amplification of the voltage components at, or near, the conductor’s natural resonant frequency. This results in voltage spikes at the waveform transition points. These voltage spikes can readily reach levels in excess of 2 to 2.5 times the DC voltage feeding the inverter.
For a 480 V system, it is common to find voltage spikes at the motor terminals ranging between 1200 to 1550 V. (575/600V systems are even more vulnerable, as peak voltages are further amplified by the higher system voltage.) Also, recall that these voltage spikes can have a rise time, dV/dT, in excess of 7500 V/s. This can have an extremely detrimental effect on the motor windings and on the insulation system, often causing premature motor failure. Most motor manufacturers believe that the life of the motor will be greatly extended by limiting both the magnitude of the voltage spikes to levels below 1000V and the dV/dT at the motor terminals to levels less than 1000 V/s.
All manufacturers of motors and of other electromagnetic components, such as inductors, perform one or two dielectric tests on their equipment during the manufacturing stage to detect any defects in the insulation system components. For 600V class equipment, these tests consist of applying a relatively high voltage, 2500 to 3000V, for a short period of time. These types of tests stress the insulation system components and, if applied too many times or for too long a period, damage the insulation system. When long motor leads create a voltage overshoot, each spike acts like a little dielectric test. If enough of them occur, the insulation system will fail, and the motor will need to be repaired or replaced.
Seldom, if ever, do large motors fail due to insulation punch-through. This is because they are usually “perfect” wound, which means that the location of each turn of wire in the phase winding is precisely controlled. Therefore, the level of voltage from turn to adjacent turn is controlled. In smaller motors, however, the wire size is quite small, and the number of turns is large. Usually, these motors are “random” wound and do not lend themselves to control over the proximity of adjacent turns. Therefore, it is quite possible to have two turns of wire next to each other with a high voltage potential that is close to the maximum allowable limit of the insulation system. Even in the absence of an overshoot voltage, when a high dV/dT is applied, the insulation components may experience punch-through, causing motor failure. Normally, these types of failures occur within hours or weeks of start-up.
As the voltage associated with the high dV/dT increases, the likelihood of partial discharge, or “corona”, also increases. When corona is present, highly unstable ozone, O3, is generated. This very reactive by-product then attacks the organic compounds in the insulation system. Corona can easily develop whenever the dV/dT and the resulting voltage overshoot are not controlled. Even the larger motors, whose turn-to-turn voltage can be controlled with “perfect” winding techniques, are vulnerable to corona. Overall, this corona effect will lead to motor failure.
Applying a line reactor at the drive terminals has been attempted. Unfortunately, adding inductance merely reduces the resonant frequency of the total circuit. Because there are additional losses associated with the inductor, both in the copper and in the core, overall circuit dampening increases. This dampening may reduce the overshoot slightly, but it will also increase the duration of the overshoot voltage, applying additional stress on the motor windings. Applying a line reactor at the motor terminals has also been attempted. Since line reactors and motors share common construction materials, line reactors applied in front of motors simply become sacrificial lambs. They will eventually fail due to the same voltage-induced stresses.
RL reactors are unequaled in absorbing power line disturbances. They are built to withstand even the most severe power spikes. Construction of this filter does not include any capacitive components. They reduce nuisance tripping, reduce harmonic distortion, and minimize long lead effects. Some filters of this type have both common mode and differential mode inductances. This can be accomplished by separate common and differential mode inductors, or by an integrated single core construction. The filter contains wire-wound resistors. These type of filters are usually used for cable leads up to 300 ft. Dampened low pass filters (LC) are designed to reduce motor failures caused by IGBT-based drives connected by long leads of up to 1,000 feet. The sine wave filter reduces the effects of the reflected wave phenomenon (dV/dt), which can cause motor heating, insulation damage, and excessive audible noise. The reduction of high frequency VFD output currents extends the life of motors and transformers and eliminates the need for special motor cables. This filter also significantly reduces the common mode current available that can damage motor bearings and produce unwanted signals in control and other analog signals. Low-pass sine wave filters are designed to deliver conditioned power to motor loads driven by PWM drives with switching frequencies from 2kHz to 16kHz. This filter is suitable for VFD drives configured to Volts per Hz modes without DC braking only. Maximum output frequency is 80 Hz.
A tuned low-pass filter can be designed to remove all carrier frequency voltages. These application-specific, custom filters were originally designed to strip low frequency carrier energy from Bipolar and Darlington transistor-based drives to limit audible motor noise. While this approach removes all frequencies above the fundamental, and affords the ultimate in motor protection, it comes at a severe price. These filters are large, costly, and consume large amounts of power. In addition, they reduce the fundamental voltage due to high inductor insertion losses and force the motor to draw higher fundamental currents to produce rated horsepower. Finally, the specific tuning frequency of a carrier-stripping filter greatly restricts the ability to alter carrier frequencies after installation. This limits fine-tuning of the drive application.
Voltage Clippers and Snubbers
These energy-consuming devices must be applied at the motor terminals, which is difficult in most industrial and commercial applications. They require the addition of extra junction boxes or equipment enclosures as well as alterations and additions to the conduit scheme.