Sometimes, trying to figure out the right way to either start or operate your electric motor is confusing. Each situation is unique and requires its own specific solution. A VFD may work perfectly in a situation where a soft starter wouldn’t, and vice versa.
Writing at the Border States Electric website, Daniel Messina shares a great question from one of his customers: “Do I need to use an inverter duty motor with a VFD? I’ve read there can be problems if you use a continuous duty NEMA motor with a VFD.”
His answer? “A standard duty AC motor will work just fine although there are a few exceptions.”
Inverter duty motors are engineered with higher-quality winding insulation that make them more resistant to voltage spikes. There have been many articles, white papers and application notes written on this subject. The vast majority of the publications I have read make the case that if you don’t use an inverter duty motor with a VFD, dire consequences will befall you. Ironically, many of those same publications are also written by motor manufacturers.
Variable Frequency Drives (VFDs) do not pass a pure sine wave to a connected motor like an across-the-line starter does. The Pulse Width Modulation (PWM) wave form output from a VFD reflects medium frequency waves to a connected motor, especially over long wire lengths. These reflected waves create voltage spikes which can create additional heat in a non-inverter duty rated motor. This is why it’s a best practice to place a VFD as close to the driven motor as possible.
Messina concludes by saying that, in specific situations, a VFD may cause a motor to fail prematurely like we’ve written before about the “Dark Side of VFDs.” For Fair Use, please read his whole article.
In a related post from a different source, Shree Electronics presents a table that compares a soft starter to a VFD.
|Maintanence||Low maintenance required, medium skill required similar to a trend electrician||Low maintenance required, High skill required, similar to an engineer|
|Starting Current||3 times roughly of motor FLR||1.2 times roughly of motor FLR|
|Starting Time||10 to 100 seconds||Programmable from 10 to 9999 seconds|
|Energy Saving||Very small energy saving||Large energy savings|
|Power factor||Poor as per motor nameplate||0.9 Power factor|
|No of start/stop per hour||Maximum 6 start / hour||No limit, 20 or 30 start per hour|
|Motor Protections||General MPR dependent||High-class semiconductor and microprocessor biased protections for voltage, current, and thermal protection|
|Speed regulation||Not possible to vary the motor speed||Controls the motor speed from 0 rpm to rated rated, means 0 to 100% speed variation is possible|
|Control room interface||Hard wire only||Both hard wire and communication interface possible with PLC/DCS System|
Do you think this list is accurate? Let me know!