VFDs work by chopping the incoming power to create a frequency that can be changed to adjust the speed of the motor.

Energy is lost every time chopping occurs and, currently, manufacturers believe this to be minimal hence the quoted 97% efficiency figure. However, this figure is at full speed but, in reality, the speed varies and the efficiency drops significantly as the speed drops, which is what happens in a real-world operational cycle and is quietly ignored by manufacturers.

It is similar to car manufacturers only quoting fuel usage at the optimal speed and not mentioning the real-world figures of an urban cycle.

To reduce the energy wasted and therefore be more efficient, you have to chop faster, transitioning from off to on or on to off, as quickly as possible. This is because, during the switching time when the transistor is neither on nor off, it is dissipating huge amounts of power, and this accounts for most of the energy losses.

GaN transistors are capable of running at frequencies of up to 20 MHz to provide ultrafast switching in 1-2 ns, as opposed to 20-50ns for Si and SiC transistors operating at 10 to 100 KHz. Thus, they are in this high energy loss region for almost no time at all and, consequently, they waste very little energy chopping the DC up into variable frequency AC to drive the motor at different speeds.

But this ultrafast switching or chopping of the DC bus generates considerable Radio Frequency Interference (RFI), that won’t pass EMC (ElectroMagnetic Compatibility) standards.  So, until now, all high power, GaN transistors implementations have had to be throttled back to reduce the RFI to acceptable levels, losing all the benefits of chopping faster.