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Nissan LEAF EV and What is a Permanent Magnet AC Motor?

November 23, 2012 2 Comments
Nissan LEAF Permanent Magnet AC Motor PMAC production

Stator Winding in Nissan LEAF Permanent Magnet AC Motor PMAC production

In my earlier post about the Nissan LEAF rare earth electric motor innovation, I promised to provide some more detail on the motor itself which is rather interesting. Car and Driver notes in its specifications rundown, “MOTOR TYPE: AC permanent-magnet synchronous electric motor, 107 hp, 207 lb-ft”. Of course, the vast majority of AC induction motors use no magnets, so the logical question is how do rare earth magnets come into the picture? That’s because the Nissan LEAF utilizes a variation of induction motor known as the Permanent Magnet AC Motor (PMAC). To better understand it, let’s first look at the rotor of a conventional AC induction motor.

ac induction electric motor rotor construction

Typical AC induction electric motor rotor construction

The cutaway above shows the typical construction elements of an AC induction motor’s rotor — the kind found in 90+% of applications over one horsepower. It uses what is called a “squirrel cage” rotor because aluminum conductor bars are die cast into the slots of the stack of steel laminations that make up the rotor. The diecast conductor bars, which can also be copper, can be eiher parallel or skewed as in the above example. When you take away the laminations, it becomes obvious where the name comes from.

squirrel cage AC induction electric motor rotor

The squirrel cage AC induction motor is a proven, reliable design that offers good performance and reliability at a very good price, as in the case of the Dunkermotor AC induction motor shown below.

Dunkermotor Three-Phase AC Electric Motor

Three-Phase Fractional Horsepower AC Electric Motor

A Permanent Magnet AC Motor has permanent magnets either attached to the surface of or embedded within the rotor lamination stacks of an induction motor. This allows for synchronous operation of the motor, which eliminates the rotor “slip” inherent in conventional AC induction motors and produces better efficiency over a wider speed range, while offering more precise speed control. In addition, losses inherent in typical AC induction aluminum conductor bars are eliminated. The net result is a more power-dense, higher efficiency motor as in the illustration below.

permanent magnet ac compared to induction motor

All of these benefits make the PMAC motor desirable for an electric vehicle, especially when you consider that the increased efficiency also permits cooler operating temperatures for the motor, which in turn improves bearing life and extends overall motor lifetime. And because of increased power density, a physically smaller motor can be used and will have power output comparable to the non-PMAC version. We’ve focused on the rotor of the PMAC motor because for all intents and purposes the stator is quite similar in both the PMAC and the conventional AC induction versions.

Further to the Nissan LEAF application, this video will show you more. It’s fascinating to think that each motor contains over a mile of copper wire in its stator windings.

According to Nissan, currently these motors are assembled in Japan. Soon, however, the eMotor will be produced in Decherd, Tennessee, about 70 miles away from where Nissan will build the 2013 Nissan LEAF in Smyrna, Tennessee. This is in keeping with my previous post noting that automotive companies consider their powertrains a core competency, whether based on internal combustion or converting electrons to do work.

That’s the Permanent Magnet AC Motor in a nutshell. Another day, we’ll discuss its similarities and differences in terms of Brushless DC motors.

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About the Author:

John Morehead is an experienced marketing, sales and business development leader. He’s passionate about motion control and advanced motor technology and shares that enthusiastically with those he encounters and many more through his myriad marketing initiatives. This blog is one more tool to be able to spread the word about news, information and insights on the motion control, electric motors, drives and automation fields. Your comments or questions on posts are welcome.

Comments (2)

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  1. peter says:

    Hi John,
    First I would tell you I am retired and have no dog in this hunt financially. I have been studying AC induction motor automobile powertrains for many years. Most of what I know is not likely to be found in a typical college textbook. I would make the case for ACIMs as follows.

    First what you say about slip is perfectly true BUT it is usually quoted as a percentage because text books assume a fixed frequency of 60 HZ and this percentage usually varies according to loading. Slip is, however, more strictly a FIXED value at max torque whatever the input frequency happens to be.

    If an induction motor is allowed to accelerate with input frequency rising from 50Hz to 400Hz @ 12000 rpm – assuming a fixed torque and constant V/Hz – then the power increases by eight times but the copper loss remains constant. If we could ignore iron loss a 92% efficient motor would achieve a 99% efficiency at 400HZ.

    This is probably difficult to comprehend at a first reading. Another way I have found to explain this: The input copper loss is constant at the rated stator current WHATEVER the rpm, whether 1 rpm or a million rpm. Furthermore the rotating magnetic flux by its nature is not effected by its velocity so the stator current need not increase to maintain the original torque.

    Therefore if the copper loss is constant per unit torque we can predict that the faster a motor rotates the more efficient it should be. So yes a PMSM is more efficient than an ACIM at the low frequencies of 50/60HZ but the differential disappears as the two machines accelerate beyond 100Hz.
    See the AEVA website “14000rpm” thread in the motor section. I’m T2 there.
    Interested to see your rebuttal.

  2. Jackie Wong says:

    Good article to learn some information for Permanent magnet AC motor. They are Neodymium magnets in this motors. Some of our customers produce this kind of AC motors. And we are also facing some issues on Neodymium Magnets, that when the motor works for many years, the permanent magnets will weak. It is a good article for us to learn some knowledge for AC motors.

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