Inverter duty AC motor basics

I'm going to open a can of worms on this one. Keep in mind, this post on motors used with AC Drives is meant to provide a basic understanding and not meant to be over the top on technical terms.

When using an AC motor with a variable frequency drive, you actually have a few things to keep in mind. Just because the motor is namplated as "Inverter Duty" or "Inverter Rated" doesn't mean it will fit your application. These terms are used on motor nameplates and in catalogs with a very loose meaning. The only real industry standard is Nema MG1 part 31. It addresses voltage insulation issues.

• Voltage issues and Nema MG1 part 31. It states that a motor can withstand voltage peaks of 3.1 times the motors rated contiguous voltage, usually 600v ac, with a rise time of .1 microsecond or more. Basically, in English, this means the motor can withstand the pulse spikes created by the drive up to 1800 volts. If the motor can't handle these the windings can fail and short and voila! you have a dead motor. Ask the motor manufacturer what the inverter rating is on the windings. Many have motors that are good to 1800 volts. Some even offer 2000 volt insulation. (motor lead length is also an issue here to be covered in a later bullet)
• Thermal issues. This issue is not very well regulated and your on your own. Not only can voltage spikes kill a motor, but heat does too. Motors are rated at a base rpm (1200, 1800, 3600 etc). The fan on a TEFC (totally enclosed fan cooled) motor is designed to give optimum cooling at that speed. When you start to slow the motor down, the fan slows down and the motor can get hot. Find out what the turndown ration will be for the application. This means if you are going to use the drive to run a 1800 rpm motor at 900 rpm this is a 2:1 ratio. Make sure your motor is rated for the application's turndown! Otherwise you cook the motor, bearings and insulation and viola! you have a dead motor. Motor manufacturer's usually have different motors to address turndown ratios needed. Ratios from 4:1 all the way to infinite:1 (rated to run at zero rpm continuous) are not uncommon. Motor manufacturers can do this by oversizing the frame, adding a constant speed blower to the motor or even by increasing the iron content in the frame to help dissipate heat.
• When looking at what motor to use with your AC drive, also factor in "how far away will the motor be from the AC drive?" The higher the voltage, the more important this is. For 460 and 575 electrical systems this is crucial. When you have long leads to the motor you can build up dvdt spikes and the spikes can exceed the voltage rating on the motor. Be sure to consult both the AC drive manufacturer and motor manufacturer for the recommended maximum lead length. In some cases, it will be necessary to use a load reactor or filter to clean these spikes up to avoid a viola! dead motor and downtime in the plant. I've seen everything from 50 feet maximum to others publishing 1000' with their drive and motor. Most drives have made improvements on cleaning up the dvdt spikes but always ask about this!

This post won't make you an expert, but hopefully it will give you some ammo to make a better decision next time you're looking for an AC motor and AC drive combo.

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