Are there issues with having an electric motor in "neutral"/unpowered while driving?

3112 Views 18 Replies 3 Participants Last post by shackf16, Aug 3, 2021

shackf16

Discussion Starter · Jul 15, 2021

I am continuing to consider options to create a performance hybrid by powering the rear wheels of a FWD ICE vehicle. The best options so far are either a full Tesla Model 3 rear suspension/small drive unit swap (best for power, but worst for battery voltage requirements) or mounting a Hyper9 directly to the rear differential that has a 3.5:1 final drive ratio (noticably less power than the Tesla DU but possibly cheaper and lower battery voltage needs). I know I have some significant integration challenges between the two systems (ICE and EV) and that will come with time, so please limit the feedback to the motor question. There will be opportunities to help on the integration side later.

When the vehicle is not actively using the EV components and motivated by the ICE only, will there be any negative side effects on the electric motor? I know the Hyper9 is a permanent magnet motor (SRIPM?) and I believe the Model 3 drive units are similar. Will the permanent magent type motor cause any issues to "freewheel" in an unpowered state? My guess is a pure induction motor wouldn't have any issues, but the spinning magnets might create an undesirable field that could be a problem? Would there be any back EMF to be concerned about if the motor isn't receiving current? (Again, I think the answer is no but I am not an expert.) What else am I missing or not considering???

I want to make sure there would be no damage to the electric motor on the rear axle while the car is driving on the freeway at 70 MPH using the ICE components. If this is all good, then I'll continue looking at methods of controlling the electric motor such that it "plays nice" with the ICE stuff...

Thanks for the input!
Shack

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67BGTEV

· #2 · Jul 15, 2021

There's no damage turning off or disengaging motor from the drive train. I've been doing it with my converted MGB, 2011 Nissan Leaf, 2018 Tesla Model 3. Close to 200k miles between them.

I'm sure you are aware that brake pump, if you have one will stop working if you turn the ignition off.

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shackf16

Discussion Starter · #3 · Jul 16, 2021

If I set things up for "EV only" driving, I would probably keep the combustion engine running with the transmission in neutral to avoid the entire DC-DC converter requirement. My main driving modes would be hybrid with the ICE driving the front axle and EV driving the rear, as well as ICE only once up to speed but with regen braking. EV only mode would probably be pretty short distance due to a modest sized battery pack, probably in the 15-18kWh size.

shackf16

Discussion Starter · #4 · Jul 23, 2021

Anyone else have thoughts on this?

I am now leaning very strongly towards a Model 3 full rear suspension with drive unit for packaging as well as power. I can get the voltage I need (around 350V) without excessive weight, and the EV Controls T2C controller now is able to control the Model 3 drive units. As I have done my research, the Model S small drive unit appears to be a pure induction motor with the same power rating while the Model 3 is a permanent magnet unit. Mounting the Model S unit would be more problematic than the Model 3, but worth it if sustained "zero torque" use of the permanent magnet Model 3 motor will cause problems or reduce the life of the motor.

Thanks guys!
Shack

brian_

· #5 · Jul 23, 2021

shackf16 said:
As I have done my research, the Model S small drive unit appears to be a pure induction motor with the same power rating while the Model 3 is a permanent magnet unit.

Yes, the original Model S/X small drive unit has an induction motor. Recently Tesla switched to putting a PM motor in the front of the AWD Model S/X (with reportedly the same motor core as the rear of a Model 3), but the available salvaged "small drive units" are the original induction design.

The Model 3 rear drive unit has a PM motor. Essentially the same motor core is apparently used in the front of current AWD Model S/X, and in the rear of the Model S Plaid (the Plaid rear drive unit includes two motors).

All EV motor power ratings are dependent on the battery in the car, the inverter, and the programming; the same motor could have very different power ratings in different cars, because the motor itself is often not the power limiting factor.

shackf16 said:
Mounting the Model S unit would be more problematic than the Model 3, but worth it if sustained "zero torque" use of the permanent magnet Model 3 motor will cause problems or reduce the life of the motor.

As you seem to be aware, the big layout difference between the original Model S drive units and the Model 3 rear drive unit is the placement of the motor relative to the axle: ahead in the Model 3, behind in the Model S/X. The Model S small drive unit can be turned around and run in reverse, if the oil flow for lubrication and cooling is addressed.

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shackf16

Discussion Starter · #6 · Jul 23, 2021

Brian, thanks for that info. I still haven't heard anyone point out that the permanent magnet motor would have issues if I were to use it in this setup. It seems there is zero concern with the induction motor spinning in a zero torque state ("neutral") at freeway speeds for extended times. I have seen passing mention of "not wanting to spin a permanent magnet motor" that same way but without any explanation of why or what the issue is or what the actual problem would be. Is it going to overheat the motor, damage the inverter, cause degredation of the magnets, swap the Earth's electric poles, etc???

If the concern over the Model 3 permanent magnet motor is warranted then I'll have to look more closely at the physical dimensions of the Model S small drive unit to see if it could be mounted backwards in the current Model 3 rear suspension without destroying the subframe. I'd be fine with cutting out existing mounting points and welding in suitable replacements on the subframe to match the Model S unit if needed, but it needs to be pretty close in size and shape or its just not an option. I am not interested in fabricating an entirely new rear subframe (and suspension) to make this project happen. However, a quick look at the shape/size/mounting points makes me think that "simply" swapping out the Model 3 motor for the backwards Model S small unit isn't likely...

Cheers,
Shack

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67BGTEV

· #7 · Jul 23, 2021

Both induction and PM motor works the same way. While you are driving with ICE only, EV drive needs to be in either mechanically or electrically neutral. Otherwise, Regen will kick in.

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shackf16

Discussion Starter · #8 · Jul 23, 2021

67BGTEV, thanks for the clarity! The motor controllers I am considering have the ability to select Drive, Neutral, and Reverse as separate modes for the Model 3 unit. This would allow me to force the drive unit into Neutral for ICE only driving, and then use Drive for hybrid mode.

Question: Is regen braking available with the drive unit in Neutral? I'd like to have that option to charge up the battery while in ICE only mode. I'm hoping to add a paddle/lever to the steering column for "on the fly" variable regen much like Sasha did in the Lotus Evora Tesla LDU conversion using a MoTeC motor controller.

brian_

· #9 · Jul 23, 2021

shackf16 said:
I still haven't heard anyone point out that the permanent magnet motor would have issues if I were to use it in this setup. It seems there is zero concern with the induction motor spinning in a zero torque state ("neutral") at freeway speeds for extended times. I have seen passing mention of "not wanting to spin a permanent magnet motor" that same way but without any explanation of why or what the issue is or what the actual problem would be. Is it going to overheat the motor, damage the inverter, cause degredation of the magnets, swap the Earth's electric poles, etc???

True, spinning an unpowered induction motor is not an issue, as long as the bearings are still suitably lubricated.

The potential issue with a permanent magnet motor is that the magnets on the rotor spinning in the stator windings induce voltage in the stator windings. If the windings are connected to anything, current flows because the motor acts as a generator. Even if no current flows, the induced voltage might be a problem for the inverter (but shouldn't be, because it won't be any higher than the voltage from the battery that the inverter is designed to handle). It should be harmless to the motor itself (... and Earth

). The inverter should be able to handle this properly; the details of how to do this would depend on the inverter. The interaction between the magnets and the stator core also causes cogging torque, which could be annoying... but again the inverter might be able to minimize that.

67BGTEV said:
Both induction and PM motor works the same way. While you are driving with ICE only, EV drive needs to be in either mechanically or electrically neutral. Otherwise, Regen will kick in.

Sort of yes... they both need to be suitably controlled to avoid unwanted regeneration, but they don't work the same way internally, and the key difference is that the induction motor has no magnetic field unless there is stator current.

shackf16 said:
The motor controllers I am considering have the ability to select Drive, Neutral, and Reverse as separate modes for the Model 3 unit. This would allow me to force the drive unit into Neutral for ICE only driving, and then use Drive for hybrid mode.

Question: Is regen braking available with the drive unit in Neutral? I'd like to have that option to charge up the battery while in ICE only mode. I'm hoping to add a paddle/lever to the steering column for "on the fly" variable regen much like Sasha did in the Lotus Evora Tesla LDU conversion using a MoTeC motor controller.

No, regen braking should not be available with the drive unit in (electrical) Neutral. Regenerative braking is applying torque in reverse while moving forward, by controlling the motor as a generator. That's "drive", not "neutral". If you want to be able to regen, you would presumably need to keep the controller in Drive, but tell it to apply zero torque.

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brian_

· #10 · Jul 23, 2021

shackf16 said:
If I set things up for "EV only" driving, I would probably keep the combustion engine running with the transmission in neutral to avoid the entire DC-DC converter requirement. My main driving modes would be hybrid with the ICE driving the front axle and EV driving the rear, as well as ICE only once up to speed but with regen braking. EV only mode would probably be pretty short distance due to a modest sized battery pack, probably in the 15-18kWh size.

That would have a substantial fuel economy penalty, burning a litre or more of fuel per hour. A DC-to-DC or simply a larger 12 V battery seem like better solutions to me. If you are depending on the engine running to operate accessories, then giving up on a pure EV mode and just using battery power to supplement engine power (in hybrid mode) would be more efficient.

shackf16

Discussion Starter · #11 · Jul 24, 2021

brian_ said:
No, regen braking should not be available with the drive unit in (electrical) Neutral. Regenerative braking is applying torque in reverse while moving forward, by controlling the motor as a generator. That's "drive", not "neutral". If you want to be able to regen, you would presumably need to keep the controller in Drive, but tell it to apply zero torque.

The EV Controls T2C has a real time adjustable motor power output slider in their iPad controller software which seems like just the trick to maintain regen braking while driving in the ICE only mode. In all honesty that could simplify the driving modes in that I only have a single drive mode which would be the hybrid mode (EV powertrain in the "drive" setting) and I could simply adjust the motor power slider up when I want to have EV+ICE power and adjust to minimum/zero torque for ICE only mode. I still need to get a few questions answered from EV Controls about what the controller does when the battery pack voltage drops to a low SoC. Will adjusting the power slider to zero allow me to continue to operate on ICE with regen available or will it enter a reduced power/limp mode and trip multiple warning codes? I'm really hoping the former and not the latter! I'll update once I hear back from them. The good news is that this project looks more and more doable with minimal impact on the ICE side of things. 😃

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67BGTEV

· #12 · Jul 24, 2021 (Edited)

With what you have explained, I wouldn't go with this solution. Changing the EV controls manually in ICE only mode is a disaster waiting to happen.
Before going ahead with this solution, think about scenarios that are not normal - what happens while you are driving in hybrid mode and change to ICE mode, but forgot to reduce the torque to zero.

If I were to do this conversation, I would have a switch to go between ICE and EV. Do Regen only when applying brake in either mode. Keep it simple.

67BGTEV

· #13 · Jul 24, 2021

Shack, I saw your message in another thread. I think you have been doing enough research and thinking through the solution. Just take my previous post as a additional factors to consider. I think you'll come out well with this project.

Cheers

shackf16

Discussion Starter · #14 · Jul 29, 2021

Okay, new question specific to the Model 3. Does anyone know if we've cracked the code to mount it backwards and run it in reverse? I know that has been sorted pretty well for the Model S with oil pump changes.

Next question. What are the concerns for mounting a full Model 3 suspension backwards? Bearings, loads, suspension alignment, etc?

My issue is that mounting the Model 3 full suspension/drive unit as I originally planned will have significant conflict with the existing unibody for the vehicle. When I was originally looking at space and fitting, I was looking at the pictures backwards - not sure how I missed that! I'd love to just drop in a full Model S rear suspension with the small drive unit as that would alleviate most of the space/fitment problems, but the track width might be an issue. Not sure how great it would look with the tires sticking out of the wheel well by a couple of inches??? Although that would avoid all of the permanent magnet motor concerns in my hybrid setup...

Thanks,
Shack

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brian_

· #15 · Jul 29, 2021

shackf16 said:
What are the concerns for mounting a full Model 3 suspension backwards? Bearings, loads, suspension alignment, etc?

In general, mounting any suspension in the opposite orientation has problems. In this case, the response of toe angle to drag, driving force, and braking force will not be as intended and may not be stable, although that may be acceptable with static adjustment changes... although there may be no adjustment provision. Also, the effective caster angle (which is normally thought of as a front suspension parameter, but will apply to some extent anything capable of toe changes) will be in the wrong direction for stability - there's no reasonable way to fix that, but the magnitude of the issue hasn't been quantified. I can't see how it would matter to the hub bearings or loads on the subframe mounts.

shackf16

Discussion Starter · #16 · Jul 31, 2021 (Edited)

I knew there would be issues, but really couldn't guess at the severity. I'm now considering the Model S full suspension and small drive unit option, and need to do a mock up to see how much the wider rear track will impact the look as well as any wheel/body issues over the suspension travel. If it's not too bad then this will be the easy fix and hopefully be a cheaper route than the Model 3.

Although now that I have said that, Mountain Pass Performance carries all of the necessary control arms in an adjustable format that allows tons more control than the factory fixed components. It appears that stock, the only adjustment is for toe. MPP offers adjustable arms for both toe and camber. Wonder if that would be enough to keep things in check? I'm going to reach out to them and see what their thoughts are...

shackf16

Discussion Starter · #17 · Aug 2, 2021

I think I may have finally figured out my way forward, but want to get a few other folks to sanity check this idea.

From a packaging perspective, the Model 3 is ideal for track width but since the motor is forward of the axle line it will not fit into the target vehicle "as is" due to unibody interference. From a motor perspective the Model S is better as it is an induction motor rather than a permanent magnet motor and has the motor behind the axle line, but is about 4.5 inches wider which will lead to the wheels sticking out of the wheel wells. I have been trying to figure out how to minimize the track width issues with offest wheels and wider fender flares but none of those are really ideal.

What if I narrow the Model S subrame by a few inches? In this image, the left side of the subframe could be shortened moving the entire suspension mounting points towards the center, narrowing the track width by that same amount. The issue with this is I would need an equally shortened half-shaft on that side and the sway bar would no longer fit. I know there are shops that can make the half-shafts and sway bars so those are far from show stoppers.

I cannot shorten the right half of the subframe as there is a motor mount that is difficult to see and this would change the mounting point and also require 2 custom half-shafts. I think just shortening one side will minimize the extended track width without changing any suspension geometry.

This does seem like the ideal option for me as it would resolve the motor/axle orientation problem as well as the concerns with the permanent magnet motor.

What am I missing???

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brian_

· #18 · Aug 3, 2021

shackf16 said:
What if I narrow the Model S subrame by a few inches? In this image, the left side of the subframe could be shortened moving the entire suspension mounting points towards the center, narrowing the track width by that same amount.

Yes, narrowing the subframe can work. As an example, the Corvette C5 suspension is a popular choice for custom cars, and there are companies which offer narrowed subframes to fit it into narrower cars (or cars with wider tires) - others just mount the suspension parts on their own frame or subframe, and they don't necessarily stick with the stock width. Like the Model S, it has a subframe fabricated from cast and extruded aluminum.

Changing the width will change the suspension geometry, but probably not enough to be a concern, especially since this suspension is not optimized for this vehicle anyway.

Tesla appears to have very deliberately kept the differential at the centre line for equal halfshaft lengths, and there are consequences of unequal shaft lengths, but transverse engine cars routinely have different lengths - it's obviously tolerable. Too short is a problem - look at the front shafts in AWD vehicles with the front differential beside the engine for examples of short shafts that are still workable.

Structurally, the front of the subframe (the side shown in the photo) appears to have a straight extrusion welded into cast side frames. I would want to cut and splice only the extrusion, because once the weld between the casting and the extrusion is ground away enough to release the extrusion, I assume that the casting would not be sound. I don't recall how the rear of the subframe is constructed.

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shackf16

Discussion Starter · #19 · Aug 3, 2021

You can see the rear portion here with the space available to narrow on the right side. There is a 3rd cross member under the motor assembly that you can't see well in this photo, but it is similar. My plan is to remove the front and rear motor mounts, narrow the middle portion of the three cross members and weld those parts back together with the appropriate strengthening of the frame, then add the motor mounts back on keeping the original right side geometry for that additional motor mount spacing. I'm thinking it will result in between a 3-4 inch reduction which will translate into the new shortened half-shaft by that same amount. I don't have the half-shaft length in front of me at the moment, but hopefully it won't be too much of an issue.

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