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