Basic Motor Question (PMM vs Induction)

[dforesi]

Hello everyone,

While I have a background in mechanical engineering, I am not motor specialist, and I know several of you are. I was reading an article (credibility TBD) recently and two items stuck out at me that I thought were incorrect.

The paper is discussing the efficiency differences between standard AC induction motors and Permanent Magnet Motors. Keep in mind, the application here is steady state operation. The following claims were made:

(when discussing the switch from AC induction to PMM) - Efficiencies are gained by the magnets working to spin the rotor, as opposed to a standard induction motor that requires additional electricity to induce the magnetic field into the rotor.
- Is this even remotely correct? I thought the advantage of PMM was better starting torque with the draw back of poor high speed efficiency due to back EMF

Additionally:
PMM motor design is much more energy efficient when compared to the standard induction motor, achieving efficiency ratings of 90% while the average AC Induction motor will have efficiency ratings between 30% and 70%. - This seems completely incorrect based on my understanding. We are talking about 2000-3500rpm operation so I assume back EMF on PMM wont be a huge difference, but I thought AC induction and PMM efficiency at steady state was roughly similar.

I am confident that these claims are at best mis-leading, but I am still learning and would like to hear from you before I make any final judgement.

Thank you!

 

[JackFlorey]

(when discussing the switch from AC induction to PMM) - Efficiencies are gained by the magnets working to spin the rotor, as opposed to a standard induction motor that requires additional electricity to induce the magnetic field into the rotor.
- Is this even remotely correct?

Basically yes. Induction motors have to excite a field in the rotor, and that takes power. A small amount of power in a well designed motor, but it still takes power. PM motors have that field built in.

I thought the advantage of PMM was better starting torque with the draw back of poor high speed efficiency due to back EMF

Well, keep in mind that with phase advance/field weakening you can get around that. But then you are doing the opposite of what induction motors do - using some of the power into the motor to _oppose_ the PM field. Weaker field = lower back-EMF = higher top speed. That takes energy as well.

PMM motor design is much more energy efficient when compared to the standard induction motor, achieving efficiency ratings of 90% while the average AC Induction motor will have efficiency ratings between 30% and 70%. - This seems completely incorrect based on my understanding.

You can always make an arbitrarily bad motor, but larger induction motors are usually way more efficient than that. I've seen 95% efficient three phase induction motors.

 

[dforesi]

Thanks for the response Jack!

So would you say that at steady state operation under 3500rpm a PMM and AC induction would be roughly similar in terms of energy efficiency for a given output power? I was expecting the two to be within 5%. Are single phase induction motors significantly less efficient than 3 phase induction?

 

[JackFlorey]

So would you say that at steady state operation under 3500rpm a PMM and AC induction would be roughly similar in terms of energy efficiency for a given output power?

I think it depends completely on design of the motors. In an "identical" motor (not sure what that means since they are so different) I'd expect efficiencies to be close, with the BLDC coming out on top (as long as motor is run below base speed.)

Are single phase induction motors significantly less efficient than 3 phase induction?

True single phase induction motors are very rare, since you cannot reliably start a single phase induction motor.

You may be referring to a single phase shaded pole motor, which has a copper ring around part of the stator. That causes enough phase shift that you get a pseudo second phase, which is out of phase enough from the main field that you get a rotating field. They are horrendously inefficient since that shaded pole wastes a lot of power to create that delay; it's basically a shorted winding.

Or you may be talking about a single phase motor with a start winding. These have two phases - a "start" winding and a "run" winding. The start winding is connected via a capacitor so that you get some delay in the current; this creates a rotating field to start the motor. Sometimes a centrifugal switch or potential relay then removes the start winding once the motor is running. (Once the motor is running, the single phase creates a field in the rotor that's not aligned with the single phase, so the motor can keep running.) Those can be very efficient.

 

[dforesi]

Sounds like I have some research to do...

Thanks Jack, this is a good push in the right direction!

 

[ronaldpottol]

As I dimly understand it, an AC induction motor like Tesla used is more efficient across most of the rpm range than the typical DC motor used in electric cars, but the DC will win in a narrow rpm range.

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