VFD and Motor Selection

There certainly are "inverter duty" AC motors, they are different in that they have features that help to mitigate some of the problems VFDs create for motors; cooling and voltage spikes. They have what is called "spike resistant" insulation on the magnet wire, because the fast rise time and steep wave front of the PWM output from the VFDs can create what are called "Standing Waves" in the conductors. These waves travel up and down the conductors and can be additive / cumulative (in laymans's terms) to the pint of reaching voltage levels far exceeding what was used on standard insulation. So what happens is a turn-to-turn short in the first turns of the motors. This is well documented. If you are running your motors at 230V or less however, most of this is unimportant because the spike level is a lot lower and typically not dangerous.

Also, turning down the speed also affects the motor's cooling system as well, so inverter duty motors have that in mind when they are designed.If you want a turn-down ratio of more than 5:1, consider using a motor with a separately powered cooling fan so that the fan runs at full speed regardless of the motor speed.

Can you use standard motors? Sure, but most likely they will fail sooner than you may like. So when considering the added cost for inverter duty motors, make sure to factor in the cost of unscheduled downtime into your analysis. You can also buy filters for the spike issues, something that is highly recommended if using a non-inverter duty motor, i.e. when it is a special design that is difficult to duplicate.

Many modern VFDs (Variable Frequency Drives) are more than capable of running standard three-phase motors without the need for more expensive VFD-rated motors (although care is needed to shield and filter the motor output and/or input (refer to VFD manual and datasheets)), specifically the motor winding insulation is better to handle voltage spikes that cheaper VFDs produce.

A properly installed modern VFD with a standard three phase motor is fine for most applications.

If in doubt, check with your VFD supplier (or data sheets) to ensure that your application and VFDs are suitable for use with standard motors before buying VFD-rated motors.

VFD is designed for use on standard motors. So, most applications and selection is mostly straightforward with a little knowhow and experience. If in doubt, refer to your supplier for guidance.

Occasionally, there will be selection considerations applying to certain applications or installations as outlined briefly in other posts e.g. high load at low speeds, long cable runs to motor (high capacitance), super synchronous operation above normal motor speed etc., so seek guidance on these. This normally ends with a standard VFD and motor and extra filters or chokes if needed. So called VFD motors are normally standard motors selected and nameplated for the application, or have an electric cooling fan in place of shaft mounted. Larger motors may use a standard winding in a larger frame to help cooling at lower speeds, for example.

I have seen only rare problems in many 1000s of applications. Notable problems have occurred with 'non-standard' motors such as: integral fan motors, submersible, high speed exhibiting problems / failure because of such reasons like lower cooling capacity, lower insulation levels, no interturn insulating paper for example. So, if you have special requirements or motors, check with VFD and motor supplier for compatibility to your needs to avoid problems later on.