Post by Prasad Ganmote

MV Induction Motors on VFDs: What the Datasheet Doesn't Tell You Most discussions stop at "use inverter-duty insulation." For medium-voltage induction motors, however, that's where the real engineering begins. A few areas deserve much more attention: 1. Reflected-wave overvoltage at MV dv/dt isn't just an LV cable-length issue. With long cable runs to remote compressor or pump stations—or even with certain multi-level converters—motor terminal overvoltage can still approach or exceed twice the DC-link voltage. This makes it essential to size the output filter based on the actual cable length, converter topology, and voltage rise time, rather than relying solely on generic vendor recommendations. 2. PDIV under PWM excitation Turn insulation qualified under sinusoidal voltage behaves differently under fast PWM pulses. The front-end coil turns experience disproportionate voltage stress due to non-uniform voltage distribution. For this reason, PDIV validation under pulse excitation, in accordance with IEC 60034-18-41/42, provides a far more representative assessment than power-frequency testing alone. 3. Rotor harmonic losses in retrofit projects Reusing an existing DOL rotor for VFD operation can be risky. PWM-induced MMF harmonics increase rotor surface and bar losses beyond what conventional sine-wave thermal models predict. Before approving rotor reuse, it is therefore worthwhile to reassess the thermal model using harmonic loss factors derived from the actual converter switching pattern. 4. Torsional resonance across the operating range Unlike DOL operation, VFD-driven compressor, pump, and mill trains continuously sweep through critical speeds during acceleration and normal operation. A torsional analysis supported by a Campbell diagram across the complete operating range, including transient starting conditions, can help identify potential resonance issues early in the design phase. 5. Shaft voltage isn't always predictable Common-mode voltage reaches the rotor through air-gap capacitance. Even small variations in rotor eccentricity can change shaft voltage behaviour between seemingly identical machines. Measuring shaft voltage during commissioning, as recommended in IEC 60034-25, provides valuable confirmation instead of assuming insulated bearings alone will eliminate the risk. The more I explore the MV VFD applications, the clearer it becomes that many reliability issues originate not from individual components, but from the interaction between the converter, motor, cable system, and driven equipment. Views are my own and do not necessarily represent those of my employer.