Water and Energy International

The Variable Frequency Drive (VFD) technology has been well established with several options for the end user. The exponential increase in cost of Insulated Gate Bipolar Junction Transistor (IGBT) with increase of rated voltage, has forced industry to make Low Voltage motors up to 2000kW. It has also lead to introduction of new voltage level of 690V for VFD motors. Cheaper solutions for larger High Voltage motors have also been developed and available with many suppliers using multi-cell Low Voltage IGBT VFD technology. Other technologies with SGCT, IGCT and thyristor based LCI are also commonly available for high voltage VFD application motors.

In a power generating station the VFD finds application right from coal unloading area paddle feeder, stacker re-claimer, cable reeling drum operation, coal conveyors, coal feeders, compressors, oil pumps, ID fan, CEP and BFP etc. The other area covers wind generators and hydro project generators for pump storage option. This exponentially increasing demand for VFD in the industry had resulted in several motor winding failure cases experienced globally when existing motor is used with retrofit VFD. This happened due to non availability of standards covering all design requirements in motors, when they are used for VFD application. The practices adopted by different manufacturers for designing VFD motor varied at large. This has drawn attention for establishing the new design requirements for motors to make them suitable for VFD application.

Whereas the electrical system on the input side of IGBT based VFD is stress relieved, the motor connected on output side is subjected to a train of high voltage surges enough to puncture the turn to turn insulation of random wound Low Voltage Motors or form wound motors. The paper discusses the generation of train of spikes from IGBT output, and their amplification by refracted wave phenomena while travelling long distance. It also analyse the usefulness of filters on input or output side of VFD. The harmonics and associated heating of motors especially when run at low speed, the damage of bearing liner due to common mode voltage, the exclusions of critical speed of combined train of motor and load during variable speed operation has been elaborated in the paper. The paper list the issues needs to be addressed for VFD application and the design improvements required for Low Voltage as well as High Voltage VFD motor to ensure reliable and trouble free operation as well as long life.