OK, so Tesla won. The AC motor became much more popular because it turned without the need for complex and expensive speed controls. With mass production, the AC motor costs around $75. per horsepower and runs reliably with no maintenance requirements for years. Which is exactly the conversation you will get with any happy electric car owner.
30 years ago lifting payloads from a cargo boat required a variable speed motor and control system, and among the choices you might find the popular GE DC-300. It didn’t really matter that the DC system cost twice or more the cost for a comparably sized AC motor. The fact was that an AC motor was designed for constant speed operation and for applications requiring variable speed, DC brush motors and drives were required.
Advances in power semiconductor technology made possible the first AC inverters and many suppliers have worked to perfect the technology. The real leverage has been the declining cost of power devices and the improved processing capability that has allowed the AC motor to behave comparably to the DC motor at lower cost. DC motor costs have increased to the point that in all the applications that were exclusively DC like cranes and elevators, are now all AC.
In the migration of AC motor control technology, many features that were once considered options due to expense and now standard features. In particular the output of most AC drive electronics includes current transformers on all three phases. With a sensor in the circuit, the drive electronics can regulate current accurately and provide better control to the motor and load.
More importantly, the current sensing can be used to measure transients going to the motor. Which is precisely what motor protection relays do. The fact that there are 3 current transformers also means that current imbalances between the phases can be detected. Depending on the loop update rate of the processor, current regulation can be in 100’s of Hertz, which is generally fast enough to detect and prevent a problem before it becomes a disaster.
Electrical engineering treats an electric motor as a load on branch circuit. The only major concern is the current required and what type of circuit protection will be provided. The main requirement is limiting current, often through a circuit breaker or fuse, although generally fuses are not considered fast enough to protect the motor. Thermal overloads using melting alloys are common as well, with the same concern.
In the presence of a variable frequency drive with a processor that can measure current values in microseconds and calculate rate of current the options are different. The drive must internally regulate voltage and current in order to control the IGBTs which are used to output sine wave currents to the motor. So precise regulation of voltages and currents is a requirement of operating an ac motor with a drive.
So the nature of protecting an electric motor has changed along with everything else. Using an AC drive and solid state fusing provide the most reliable motor protection available in addition to varying the speed of the motor.