Richard R. Schaefer is the General Product Manager for AC and DC variable speed motors at Baldor Electric Company, a member of the ABB Group. During the live Q&A portion of the on-demand webinar "The Next-Generation Motor Efficiency Performance: Beyond NEMA Premium," Richard along with Steve Evon, Baldor Engineering Manager took several questions related to hybrid permanent magnet motor technology.
PS: You mentioned that hybrid permanent magnet (PM) motor technology is fairly new. Are these hybrid motors available today?
RS: Yes, they are. Several motor manufacturers are making these motors today. They are available in different ratings, but yes, the motors are available today. This technology really goes back to the late '70s, so these motors, at least in smaller horsepowers, have been around for decades.
PS: Will the magnet strength of PM motors deteriorate over time?
RS: That's a good question, and one that we get asked a lot. The answer is no – other than a very abnormal condition, the rotor should never demagnetize, and the magnet should never demagnetize. Once the rotors have been magnetized, they will stay magnetized and inherently those magnets will not want to demagnetize over time.
Really there are two considerations in terms of possibility for demagnetizing the rotor. One is excessively high temperatures, and hybrid motors are designed such that they should never reach temperatures high enough to cause a de-mag condition. The other consideration is if the motor is being used on a drive, in which case high instantaneous overcurrent could potentially de-mag the motors. For Baldor hybrid motors, we set the drive to actually protect the motor and set the instantaneous overcurrent level of the drive so the drive is actually acting to protect the motor.
PS: Are there any overspeed limits related to hybrid motor technologies?
SE: The overspeeds are basically the same as an induction machine, and certain PM motors can run at well over 10,000 rpm, so there wouldn't be any more limitations on speed. The only caution would be, as with any PM machine, if you get too wide of a constant power speed range, you'll have a back EMF voltage on a fault. This is a concern with the drive: they can be above rated voltage, so you have to be careful if you're operating these on a drive greater than probably 2:1 or 3:1 constant horsepower range.
RS: Keep in mind that we recommend these motors be applied in centrifugal load applications. As you overspeed a motor in a centrifugal load application, you're going up the load curve exponentially, so consequently just a little bit of additional speed has a lot of additional torque that's required. In centrifugal load applications, we say the base speed of the motor is the top speed of the motor unless a user takes the additional overspeed torque requirement into consideration.
PS: How does the cost of a hybrid motor compare to the cost of a standard type induction motor?
RS: Cost can vary between motor manufacturer to motor manufacturer. The way our motors are priced, the hybrid motor, which we called eXtreme Efficient or IE4/IE5, is a 20% premium above the premium efficient motor, above the IE3 level. The payback depends on the hours of operation of the motor and on the cost of power, but typically it's going to be a six-month to a two-year payback with that 20% premium. However, because the full load amps are reduced when using a drive, you can downsize the drive; in most cases, the lower drive price offsets the incremental motor price, so you end up with a premium efficient system at an equivalent or near equivalent price.
PS: Since hybrid motors are synchronous motors with PM rotors, when you run these with adjustable frequency drives, do these motors require a feedback device like other PM motors?
SE: No. In the testing that we've done, any drive will work running in "scalar" or simple volta/herts mode without any feedback device. So in this case, the induction cage really aids in the ability to run these motors on a drive, and that's one of the huge advantage of them is that you get PM performance without a feedback device.