We recently had a customer ask, would their 20-year-old pump motor work on a Variable Frequency Drive (VFD), and would a VFD manufacture guarantee that the VFD would not damage the motor. This can be a tricky conversation to have and too often people move forward without giving it the due diligence that it deserves. Without taking the right amount of time to understand the factors in a conversion such as this you could be walking down a road of headaches which is what we want to prevent. Here are some highlights of that discussion.
First, we need to understand heat and how that can affect a motor in operation. Standard AC induction motors can be operated with a VFD but are susceptible to premature failures associated with insulation breakdown due to voltage spikes generated by VFD operation and thermal damage due to inadequate cooling over the speed range. This has a major impact on motor performance and operating at higher than rated temperatures can severely damage the insulation and reduce the run life of the motor. You may be able to offset temperature rise with additional cooling or potentially you would need to consider a motor with a higher insulation class rating for desired performance.
If you find the need to convert to a true inverter duty motor that does offer improved performance in several areas. Inverter-duty motors are designed to withstand the voltage spikes associated VFD operation and operating at low speeds without overheating, delivering a wider constant torque speed range than a standard motor. While this adds cost to a conversion project this could be a necessary step to get the reliability out of the system that is desired.
A great reference that we recommend when looking a motor application is NEMA’s MG1 standard. This is the governing standard on motor & generators and provides performance standards for general purpose motors used with VFDs and detailed guidance on specifying invertor-duty motors.
In the end we were able to help our customer understand these factors and apply the right solution. By starting with understanding the requirements of the application and align your expectations based on the capabilities of both approaches you are setting yourself up for success.