Different Medium Voltage VFD Manufacturers use various topologies in their design. The term topology refers to the switching scheme employed in the inverter section of the medium voltage VFD to, in effect, switch the DC bus into a synthesized variable frequency, variable voltage output. Great strides in these designs have been accomplished.

To begin with.. a two level topology is the most common DC bus switching in low voltage voltage source VFDs. (see figure to left) A typical 480 volt input VFD will have a bus voltage in the 650-700 volt range. At those levels, the switching devices, typically, IGBTs, can easily switch the entire bus level without issues.

Once you raise the input voltage to medium voltage VFD levels, typically 4160 volts, that becomes more of a concern.

The most common scheme used in a medium voltage VFD is three level topology. The Toshiba T300MVi uses five levels (See image on right) which makes it one of the most motor friendly VFDs in today’s market. Yaskawa’s new MV VFD is very motor friendly as well.

Peak voltages on two level topology 480V VFDs can produce peak voltages twice as much as the DC bus; roughly 1300-1400 volts. That level of voltage can be very stressful to the motor insulation on a 480V motor.

For the information of a potential MV VFD buyer, let’s consider a two level topology on a 4160V; there are a few manufacturers who use this topology even within the medium voltage VFD market. The DC bus on a 4160V two level topology will be roughly 5880V, hence we can generate peak voltages of over 11,000V on a 4160V VFD.

The majority of medium voltage motors are designed for and running across the line, which means the insulation was never designed to withstand those peak voltage levels. Motors have failed prematurely with two level MV VFD topology operating without output filters. Medium Voltage VFDs using this topology normally require such filters.

Toshiba, for instance, employs a different design. By switching the DC bus at five levels the T300MVi significantly reduces those peak voltages. The typical DC bus on the T300MVi is 1650V. This is a much more motor friendly design, and allows operation on existing motors without a filter. It also permits much longer cable lengths.
EMA services and sells the Toshiba T300MVi medium voltage VFD. Although we do not sell the GE Symphony MV drive, we are very familiar with this medium voltage VFD as well, and commonly provide field service repair on this drive and many others. 

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