The evolution of power semiconductors has been so dramatic that today an insulated gate bi-polar transistor (IGBT) can be turned on in just 0. I micro-second. This results in the voltage rising from zero to peak in only one-tenth of a microsecond. Unfortunately, there are many motors in existence that do not have sufficient insulation to operate under these conditions.

HIGH PEAK VOLTAGES
High peak voltages can be experienced at the motor terminals especially when the distance between the inverter (drive) and the motor exceeds about 15 meters. This is typically caused by the voltage doubling phenomenon of a transmission line having unequal line and load impedance's. Motor terminal voltage can reach twice the DC bus voltage in long lead applications. When the characteristic load impedance is greater than the line impedance, then voltage (and current) is reflected from the load back toward the source (inverter). The absolute peak voltage is equal to the sum of the incident peak voltage traveling toward the motor plus the reflected peak voltage. If the load characteristic impedance is greater than the characteristic line impedance, then the highest peak voltage will be experienced at the load (motor) terminal. If the DC bus voltage is 850 volts, then motor terminal voltage could reach 1700 volts peak.

FAST VOLTAGE RISE TIMES
Fast voltage rise times of 1600 volts per microsecond can be typical as the motor lead length exceeds just a few hundred feet. Voltage rise time is referred to as dv/dt(change in voltage versus change in time). When the rise time is very fast the motor insulation system becomes stressed. Excessively high dv/dt can cause premature breakdown of standard motor insulation. Inverter duty motors typically have more phase-to-phase and slot insulation than standard duty motors (NEMA design B).

When motors fail due to insulation stress caused by high peak voltage and fast voltage rise times (high dv/dt) they have common symptoms. Most failures of these types occur in the first turn as either a phase-to-phase short or phase to stator short. The highest voltage is seen by the first turn of the winding and due to motor inductance and winding capacitance of the motor, the peak voltage and dv/dt decay rapidly as the voltage travels through the winding. Normally, the turn to turn voltage in a motor is quite low because there are many turns in the winding. However, when the dv/dt is very high the voltage gradient between turns and between phase windings can be excessively high, resulting in premature breakdown of the motor insulation system and ultimately motor failure. This problem is most prevalent on higher system voltages (480 & 600 volts) because the peak terminal voltage experienced often exceeds the insulation breakdown voltage rating of the motor.

STANDARD MOTOR CAPABILITIES
Standard Motor Capabilities established by the National Electrical Manufacturers Association (NEMA)and expressed in the MG- I standard (part 30), indicate that standard NEMA type B motors can withstand 1000 volts peak at a minimum rise time of 2 u-sec (microseconds). Therefore to protect standard NEMA Design B motors, one should limit peak voltage to 1KV and reduce the voltage rise to less than 500 volts per micro-second.

SOLUTIONS
There are several solutions available to solve this problem, each offering a different degree of protection at a different price.

1. Inverter Duty motors should be considered for all new IGBT drive installations. They offer increased winding slot insulation, increased first turn insulation, and increased phase- to-phase turn insulation. They are more expensive than standard design B motors but are the best motor for the job when it will be controlled by an IGBT variable frequency inverter. The NEMA Standard MG- I (part 3 1) indicates that inverter duty motors shall be designed to withstand 1600 volts peak and rise times of >0.1µsec. Nevertheless, it is wise to confirm the actual motor capability with the manufacturer.
2. Minimize Cable Length between the inverter and motor. Quite often this is somewhat uncontrollable, especially when the application is downhole pumping where the motor is required to be a great distance from the inverter. The longer the cable, the greater the capacitance of the cable, the lower the impedance of the cable and thus a greater mis-match will result between the characteristic line and load impedance's, resulting in higher peak voltage at the motor (load) terminals. Minimize this length whenever possible to avoid problems.
3. Tuned Inductor & Capacitor (LC) Filters are an effective means of taming the output voltage waveform and protecting the motor. An "LC" circuit can result in the best voltage waveform but at a relatively high cost and with some future considerations. Of course these filters are "low pass shunt type filters" tuned for some specific frequency, often in the range of 1 kHz to 2Khz. Because these filters have essentially zero impedance at there resonant frequency, it is very important that the inverter switching frequency not be set too low. The threat exists that someone may vary the carrier frequency (at a later date) without consideration for the existence of a low pass filter resulting in damage to the inverter or filter. One should be very careful when applying this type of filter on the output of an inverter with variable carrier frequency. LC filters for this purpose cost approximately 3-4 times the cost of a load reactor.
4. RC Snubber Networks can reduce the slope of the voltage waveform leading edge and reduce the peak voltage of the waveform but they have a minimal effect on the actual waveshape. They perform marginally when compared to the other solutions discussed herein. At an intermediate cost, they provide a marginal benefit. The cost of these network can be 2-3 times the cost of a load reactor.
5. Load Reactors are the most cost effective means of solving high dv/dt and peak voltage problems associated with IGBT inverters. Typical experience is that peak voltage is limited to I 000 volts or less (actual value varies based upon system voltage). Voltage rise time (dv/dt) is typically extended to several micro-seconds resulting in only about 75 - 200 volts per micro- second rise times. Usually the load reactor is all that is needed to adequately protect the motor from dv/dt and to allow full warranty of the motor in IGBT inverter applications. (Some motor manufacturers do not offer a warranty in IGBT applications if a load reactor is not installed).

Whether you install the load reactor at the inverter or at the motor, it will provide you with protection for your motor. It offers the best dv/dt reduction when it is placed at the inverter and this is usually the easiest place to add the reactor. Placement at the inverter also provides voltage stress protection for the motor cables. Of course there are some applications that may require the addition of the load reactor at the motor terminals. This will also provide very good protection of the motor because the IGBT protected reactor acts like the first turns of the motor. The motor is protected well in this case, however the motor cables are not protected from voltage stress.