There is a delicate balance between electric motors and the power electronics that control them. This is because semiconductors conduct electricity in a completely different way than wire wound inductors. Yet, they need to be designed to perform well together because, these days, almost all electric motor technology depends on the result of the two components operating together as a circuit.
The main difference is inrush current. In an AC motor, the current inrush and current carrying capability is unlimited until the winding burns out and catches fire. Putting electricity through a motor winding creates heat. Electrons must have some sort of friction to them. That’s what makes superconducting so attractive. Conductivity without heat. But it comes with a cost, superconducting only takes place at cryogenic temperatures. So you have to have a system large enough that the cost of keeping things at liquid hydrogen temperatures is beneficial.
For power semiconductors voltage is less the issue, we have mosfets for low voltage systems and IGBT for higher voltage systems. The limitation is that power semiconductors can usually handled double their rating and no more. Also, the rate of current inrush is a slope that cannot be exceeded. These are critically different behaviors from inductors and require users to make sure that they have a good match between the current requirement for a given application.
In both systems, however, any form of thermal management can make a huge difference in the size, weight and cost of an electric motor. The size and weight of electric motorcycle power converters and motors are amazingly small, and they are all liquid cooled. The liquid cooling adds some volume and cost, but this is far less than the size and weight increase required for ambient air cooling.
Fan cooled AC motors were notorious for failure with the early variable frequency systems. The motor was designed for constant 1800 RPM operation and the fan cooling, using ambient air over the motor housing was sufficient for normal operation. If significant operating time took place at lower speed, the fan was no longer able to move enough air to keep the motor cool, leading to overheating and eventual failure of the winding.
Mis-matching of variable frequency drives to high starting loads was also a common failure in early systems. High inrush current that are acceptable to an AC motor across the line would cause power electronic failures if they were applied strictly based on the horsepower of the load.
More next week.