# Why do most car makers prefer PMSM over ACIM?



## jhuebner (Apr 30, 2010)

Most recent electric cars use PMSM. Except Tesla.

They are more expensive and only a tiny bit more efficient.

What am I missing?


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## subcooledheatpump (Mar 5, 2012)

PMSM and BLDC, so I've heard, have a higher torque density. But this is just hearsay


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## kennybobby (Aug 10, 2012)

Is the Leaf motor really a PM synchronous, or is it induction? i've seen the sales brochures but never any really definitive technical data--lots of folks on the websticle spit out the sales lit, but has anyone seen a real spec/data sheet for those motors?


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## Hollie Maea (Dec 9, 2009)

subcooledheatpump said:


> PMSM and BLDC, so I've heard, have a higher torque density. But this is just hearsay


Yep, a bit Better power density, in general. Since most of the OEMs besides Tesla are trying to cram a lot into a small car, they just go with that, I guess.

If the world ran out of Neodymium tomorrow, I think everyone could switch over to induction without much fuss.


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## JRP3 (Mar 7, 2008)

kennybobby said:


> Is the Leaf motor really a PM synchronous, or is it induction? i've seen the sales brochures but never any really definitive technical data--lots of folks on the websticle spit out the sales lit, but has anyone seen a real spec/data sheet for those motors?


Nissan calls it an AC synchronous motor, and they made an announcement that their new motor design reduces rare earths in their magnets, so I think it's safe to say they are using PMs.
http://www.nissan-global.com/EN/NEWS/2012/_STORY/121120-02-e.html


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## Tesseract (Sep 27, 2008)

jhuebner said:


> Most recent electric cars use PMSM. Except Tesla.
> 
> They are more expensive and only a tiny bit more efficient.
> 
> What am I missing?


Well, no one but the engineers at the OEMs really know the answer to this question, but my guess is it's because you can get a lot more "at-the-wheel" power from a given inverter if it doesn't have to squander a good chunk of its output current rating supplying the magnetizing (field) current that an induction (asynchronous) motor would require.

The *slightly* higher efficiency of a PMSM motor when delivering *full* power probably doesn't even merit mentioning; I mean, if automakers were so concerned about a couple points of efficiency they wouldn't make SUVs and other monstrosities, now would they?


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## jhuebner (Apr 30, 2010)

Tesseract said:


> Well, no one but the engineers at the OEMs really know the answer to this question, but my guess is it's because you can get a lot more "at-the-wheel" power from a given inverter if it doesn't have to squander a good chunk of its output current rating supplying the magnetizing (field) current that an induction (asynchronous) motor would require.


Hmm, maybe.
Just for me to understand: on my plots you can see 120A torque current and 50A magnetization current. But the sum of these isn't 120+50 but sqrt(120²+50²) which is 130A. Correct? If so, its those extra 10A (or roughly 10%) that make them switch to PM?



Tesseract said:


> The *slightly* higher efficiency of a PMSM motor when delivering *full* power probably doesn't even merit mentioning; I mean, if automakers were so concerned about a couple points of efficiency they wouldn't make SUVs and other monstrosities, now would they?


The world is full of contradictions


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## Tesseract (Sep 27, 2008)

jhuebner said:


> Hmm, maybe.
> Just for me to understand: on my plots you can see 120A torque current and 50A magnetization current. But the sum of these isn't 120+50 but sqrt(120²+50²) which is 130A. Correct? If so, its those extra 10A (or roughly 10%) that make them switch to PM?


Correct, but you are reporting results from your slip control scheme, aren't you? In slip control schemes, the magnetizing current is roughly proportional to torque load, whereas in pretty much all FOC schemes the magnetizing current remains the same from 0% load to 100% (it is decreased for field weakening, or increased to (attempt to) achieve breakdown torque).

Magnetizing current is typically 30-60% of the full load current, so a motor supplied by an FOC-based inverter will appear to draw that magnitude of current even while lightly loaded. As the motor is progressively loaded the phase angle between the voltage and current waveforms will decrease, showing that power factor - and therefore the amount of real work getting done - is increasing. 

The end result is that the switches in a FOC inverter have to handle a minimum of 30-60% of the full load current at all times, and that is a potential negative of the ACIM over the PMSM.

Now is that sufficient reason? I dunno - this was just my guess, after all. Another important consideration - particularly for hybrids - is that a PMSM is significantly smaller for the same torque/continuous power output, and given that hybrids have to cram two drive systems into one car, an electric motor that requires less volume could be compelling enough reason to choose it.


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## bLdC (Jan 21, 2013)

I think the most popular motors used by the OEMs are actually IM(interior magnet)SMs. 
This are kind of hybrid motors(the difference is in the rotor). They can work as switched reluctance motors(SRM) extending the usable rpm range. 
I think this allows for reducing the magnet mass too. 

I just started experimenting with one of these.

The motor inductance varies as a function of rotor angle.

Simplified version of the torque equation for SRM.
T = i^2/2 * dL/d0 

For the IPMSM
the torque equation has a component +(Ld-Lq)*Id*Iq and Lq > Ld, 
so Id has to be negative if we want positive torque from this component. 

And negative Id corresponds to field weakening at the same time. 
Almost like it was designed to work this way.


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## tomofreno (Mar 3, 2009)

If the permanent magnets are on the rotor like a bldc motor, with wound field, there would be no copper losses in the rotor, probably permitting higher continuous power without rotor overheating in a relatively small mass motor. It's hard to remove heat from the rotor in a totally enclosed motor (which car manufacturers would like for water, dust proof), but the field windings are fairly well heat sunk to the iron permitting effective water cooling of them. I assumed that was the reason the ratio of rated continuous power to rated peak power seems to be significantly greater for these motors than induction motors. Apparently not a problem for Tesla though, so dunno.


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## tomofreno (Mar 3, 2009)

Reference: http://www.ti.com/lsds/ti/apps/motor/permanent_magnet/overview.page

The permanent magnet synchronous motor (PMSM) can be thought of as a cross between an AC induction motor and a brushless DC motor (BLDC). They have rotor structures similar to BLDC motors which contain permanent magnets. However, their stator structure resembles that of its ACIM cousin, where the windings are constructed in such a way as to produce a sinusoidal flux density in the airgap of the machine. As a result, they perform best when driven by sinusoidal waveforms. However, unlike their ACIM relatives, PMSM motors perform poorly with open-loop scalar V/Hz control, since there is no rotor coil to provide mechanical damping in transient conditions. Field Oriented Control is the most popular control technique used with PMSMs. As a result, torque ripple can be extremely low, on par with that of ACIMs. However, PMSM motors provide higher power density for their size compared to ACIMs. This is because with an induction machine, part of the stator current is required to "induce" rotor current in order to produce rotor flux. These additional currents generate heat within the motor. However, the rotor flux is already established in a PMSM by the permanent magnets on the rotor.
Most PMSMs utilize permanent magnets which are mounted on the surface of the rotor. This makes the motor appear magnetically "round", and the motor torque is the result of the reactive force between the magnets on the rotor and the electromagnets of the stator. This results in the optimum torque angle being 90 degrees, which is obtained by regulating the d-axis current to zero in a typical FOC application. However, some PMSMs have magnets that are buried inside of the rotor structure. These motors are called Interior Permanent Magnet, or IPM motors. As a result, the radial flux is more concentrated at certain spatial angles than it is at others. This gives rise to an additional torque component called reluctance torque, which is caused by the change of motor inductance along the concentrated and non-concentrated flux paths. This causes the optimum FOC torque angle to be greater than 90 degrees, which requires regulating the d-axis current to be a fixed negative ratio of the q-axis current. This negative d-axis current also results in field weakening, which reduces the flux density along the d-axis, which in turn partially lowers the core losses. As a result, IPM motors boast even higher power output for a given frame size. These motors are becoming increasingly popular as traction motors in hybrid vehicles, as well as variable speed applications for appliances and HVAC.
The saliency exhibited by IPM motors can also provide an additional benefit in sensorless control applications. In many cases, the saliency signature is strong enough that it can be used to determine rotor position at standstill and low speed operating conditions. Some sensorless FOC designs use saliency mapping at low speeds, and then transition to a back-EMF observer model as the motor speeds up.


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## gunnarhs (Apr 24, 2012)

jhuebner said:


> Most recent electric cars use PMSM. Except Tesla.
> 
> They are more expensive and only a tiny bit more efficient.
> 
> What am I missing?


The main reason is that most electric cars by OEM are city cars. As they have no gearbox and their maximum continuous power must be considered for driving max 140 km/h this means that they use oversized motors which means that most of the time they are been driven in partial load. Under partial load PMSM have much higher efficiency than AC-induction ( about 20% in average I have been told, not tested it myself).
The table turns at full load at high speeds, especially when getting into the field weakening area then the induction motor becomes more efficient (needs less cooling).
The other reason is the better power/weight ratio
PS. I am looking at your inverter data in the other post regarding Id and Iq, I will post there some comments later.


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## jhuebner (Apr 30, 2010)

Yeah just looked at a Tesla motor from the model S in the Tesla store in Munich yesterday.
It seems aircooled? It was cut open and I didn't see any water ducts. The stator is smaller than I thought. Its wound with about 1.5-2mm² copper wire. The slots are like 2cm deep and less than 1cm wide. I didn't count how many of them there were. Then theres a LOT of iron between the windings and the motor case.
The rotor looked nothing like the rotor of a normal induction machine. It has a shiny flat surface whos color points to a lot of copper being in there.
I read in the Tesla blog and here on the forum that it took them ages to end up with this particular rotor.

So maybe the car makers who use PM motors today just couldn't be bothered to develop a good rotor.

I still don't really buy the power density argument, it seems to only be a 10% difference.
With FOC you can lower the flux producing current in low load conditions by means of changing the id setpoint.

There is a point a point to the low speed argument, as the rotor losses are the same at 100rpm as at 10000rpm.


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## jhuebner (Apr 30, 2010)

Why words if theres pictures:


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## JRP3 (Mar 7, 2008)

Model S motors are liquid cooled, and actually have a hollow rotor shaft to allow rotor cooling. That cutaway mockup doesn't seem to show that, which could be because it's proprietary and patented. But there is a screen cap from the Tesla Megafactories video


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## jhuebner (Apr 30, 2010)

Ah, thats literally cool 
So they liquid-cool only the rotor or the stator as well?


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## JRP3 (Mar 7, 2008)

I assume the stator as well but I don't know for sure.


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## haldensagar (Mar 25, 2014)

However, unlike their ACIM relatives, PMSM motors perform poorly with open-loop scalar V/Hz control, since there is no rotor coil to provide. The more pins available would allow for more flexibility..


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