In our October 2016 e-newsletter we mentioned that “…by limiting the amount of inrush current to motors, VFDs can also extend motor life.”
This inspired some conversation so to provide more information we looked at what Jerry Muehlbauer of Marathon Electric had to say on the subject. In a Q&A he address the subject as follows:
Q: Can a motor handle more starts/hour on VFD power?
A: Yes, provided that the VFD is properly tuned and its output frequency is steadily increased (or ‘ramped up’) from zero to the set point and the motor is not ‘line-started’, which causes extremely high (6-8 times rated) motor currents. When a motor is line-started the enormous heat generated in the motor windings limits the number of starts/hour (motor must cool down between starts). Line starting could be compared to starting your car while on jack stands: Put it in gear, start the engine, place a brick on the gas pedal, and then push it off the jack stands. This results in uncontrolled acceleration & huge mechanical stress on the transmission and engine. How many times can you do this before something breaks? Across the line (ACL) starting produces similar thermal and mechanical stresses in the motor and driven load. According to NEMA MG1, the life of a motor is basically determined by the number of ACL starts.
When the VFD ramps up the output frequency, inrush current is eliminated and mechanical stress is greatly reduced, permitting the motor to be repeatedly started without significantly shortening insulation life. Acceleration rates as short as 1/2 second (for small motors) or 1-2 seconds (for larger motors) are still long enough to avoid high inrush currents. In the automotive example, this is comparable to starting the engine, putting the transmission in gear, then driving away at the selected acceleration rate.