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Permanent Magnet v Induction

Posted: Wed, 25 Nov 2015, 11:14
by T1 Terry
Not sure if this has already been posted here but I thought it may be of interest http://drivesncontrols.com/news/fullsto ... otors.html
Clearly funded by one side of the industry so liable to be some what bias but interesting all the same

T1 Terry

Permanent Magnet v Induction

Posted: Thu, 26 Nov 2015, 01:47
by acmotor
"The motors were tested in three phases over a variety of speeds and torque values on the same variable-frequency drive to compare their efficiencies"
So one VFD was programmed for each motor and each motor type IM/PM in turn. Considerable differences in control algorithm. There may be variability in the VFD's skill at servicing each motor type. That alone would account for some variations in the test. Not exactly apples with apples.
Surely cost, weight, size and longevity would be of more concern when choosing motor type ?
One observation I would make is that PM motors have high loss when coasting, unlike IMs who can be quite inert when spinning in coast mode in an EV. This is related to the inability to turn the magnets off so iron losses persist and the motor drags more when coasting.
Other than that, it is perhaps more down to individual motor design than just an IM/PM who is best debate.

Permanent Magnet v Induction

Posted: Thu, 26 Nov 2015, 02:16
by T1 Terry
Glad to hear some feed back on the article. I was extremely sceptical when I read that it was the copper promotions people who were paying for the trials so great to hear that there are other down sides to permanent magnet motors in an EV application. Can they be set up for regen the like induction motors can? In the applications I'll be look at, much heavier vehicles than the average EV, regen is the gold at the end of the rainbow, recharged batteries rather than smoked brake linings will be a real selling point now the price of fuel has gone through the floor.

T1 Terry

Permanent Magnet v Induction

Posted: Thu, 26 Nov 2015, 03:01
by acmotor
Both IM and PM regen fine. It is just that a IM has control over the rotor magnetic field strength so that in the hands of a clever VFD it can optimise itself for a greater range of load conditions and frequencies.
Overall efficiency may be more a function of the VFD than the motor.