Is it possible to use different magnet lengths in the same motor?

From: Lawrence Harris <>
Sent: Friday, January 5, 2018 12:27 AM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] EVLN: GM's Multiple-Magnet-Length Electric-Motor Patent

Interesting from a production point of view.  If they have a need for low 
quantities (relative) of motors with different HP then it might make sense to 
tools once and then be able to vary the characteristics during assembly.  I was 
thinking it might be more interesting if they put the rotor on a lead screw and 
allowed the positioning of the magnets w.r.t. the coils to be adjusted.  Would 
provide something like field weakening on a permanent magnet motor.

Lawrence Harris<>

On Jan 4, 2018, at 15:09, ROBERT via EV 
<<>> wrote:

I do not see the advantage except having the capability to use one motor core 
for multiply HP motors.  Does anyone think GM is going to use different magnet 
lengths in the same motor?  Does this have some efficiency improvement?  Do you 
think they are going modify the controller to allow different magnet lengths in 
the same motor?  Anyone have any thoughts on this subject?

From: EV <<>> on 
behalf of brucedp5 via EV <<>>
Sent: Thursday, January 4, 2018 2:43 PM
Cc: brucedp5
Subject: [EVDL] EVLN: GM's Multiple-Magnet-Length Electric-Motor Patent

Anyone who thinks General Motors isn’t serious about electric vehicle
leadership doesn’t have a clue.

Despite Silicon Valley’s derogatory ideology regarding conventional car
companies like General Motors, the dinosaur from Detroit has been at the
forefront of electrifying personal mobility. GM’s flirtation with electric
vehicles began in earnest back the early ’60s. It started with the
Electro-Vair and Electro-Maro programs in the ’60s, then came a
battery-powered Chevette in 1977, followed by production of the EV1 in the
late ’90s, before culminating with the Chevrolet Bolt, the industry’s first

But the company isn’t resting on its laurels, as Tesla Model 3 reviews begin
to hit the internet, GM is busy working on a new family of electric cars due
in 2021. While advancements in battery technology have long been heralded as
the key to consumer adoption, GM engineers haven’t forgotten that a motor is
still what propels a vehicle forward, electric or not.

Published on December 19, 2017, by the USPTO, GM has filed a patent for an
electric motor with multiple magnet lengths which could totally change how
the company thinks about manufacturing electric propulsion systems.

For example, the Chevrolet Bolt uses a permanent magnet brushless motor,
where a magnetic field is produced by the spinning magnet and rotor assembly
which then transfers to the stator core and interacts with flowing current
to create torque. Differing magnet lengths will change the torque output,
smaller magnets decrease torque, while longer ones increase torque,

What the company is proposing is a new “modular” lamination sheet which
would be capable of accepting multiple magnet lengths. Instead of being
forced to re-engineer the lamination stack each time a change in magnet
length is required, GM proposes a series of tabs within the apertures of the
lamination sheets which, when layered, can be assembled to delineate the
magnet slots.

Effectively, the tabs will allow the stacks to accept either short or long
magnets–the tab will support the shorter magnet halfway down the aperture or
get pushed out of the way upon inserting a longer magnet. GM claims there
will be at least a 25-percent difference in magnet lengths.

It’s helpful to think about GM’s work with modular lamination stacks almost
like powertrain sharing–take GM’s naturally-aspirated 6.2-liter small-block
V8, which is offered in LT1 and L86 guise. As the high-performance version,
the LT1 is equipped with a shorter intake runner for better high-rpm
breathing, different exhaust manifolds, and unique cam timing; while the
trucks make use of longer intake runners in order to fatten up the mid-range
torque curve.

What will be of interest moving forward is how GM plans to implement the
respective magnet lengths, will the smaller magnets be used for efficiency,
while the bigger ones left for high-performance or hauling? Possibly, but
there are also drawbacks to simply increasing magnet size; larger magnets
may create more torque, but they also force the coil to fight through more
resistance as the higher torque values lead to an increase in eddy and

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