The Pulse Width Modulation voltage takes some time to rise from its minimum to its maximum value. This period is often called “rise time”. Due to the great rapidity of switching on the variable frequency drive stage, the growth of the voltage wavefront takes place too fast and, with the power electronics advance, these transition times tend to be more and more reduced.
Then the variable frequency drive fed motor is subjected to extremely high dV/dt rates, so that the first turn of the first coil of a single phase is submitted to a high voltage level. Therefore, a VFD can considerably increase the voltage stress within a motor coil, though owing to the inductive and capacitive characteristics of the windings, the pulses are damped on the subsequent coils.
The rise time has a direct influence on the insulation life, because the faster the pulse wavefront grows, the greater the dV/dt ratio over the first coil and the higher the levels of voltage between turns, causing the insulation system to wear more quickly away.
According to NEMA MG-1, Part 30 Motor Specification, Design B Motors (non-inverter duty) should be capable of handling rise time: 2 microseconds or slower (≥2 microseconds).
According to NEMA MG-1, Part 31 Motor Specification, Inverter Duty Motors should be capable of handling the following Rise Time: 0.1 microseconds o slower for < 600 volts; 1 microsecond or slower for > 600 volts.
MTE offers a full line of best-in-class products to protect motors and improve productivity. Our dV Sentry™ filters (for cable leads up to approximately 1,000 ft.) features the patented Triple Defense Core, is proven reduce common mode, protect against peak voltage, and reduce rise time – all in one unit.
Our SineWave Guardian™ filters (for cable leads up to approximately 15,000 ft.)deliver unequaled performance in cleaning the PWM waveforms generated by VFDs. It virtually eliminates high frequency content and voltage peaks, thereby reducing motor heating to provide extended motor life – and less downtime.