Field weakening & gearboxes

[rstevens81]

I have moved on to the over thinking stage of planning

I have an outlander rear motor (with gearbox diff thing) a Prius inverter (plus a Yaris spare) and the project car 1999 mx5).

Due to life circumstances (moving house) I am only able to bench test stuff at the moment, but I'm trying to 'get my ducks in a row' .
I have been working on BMS stuff with bigpie, but now I really want to get something spinning even if it can't go in the car yet.

Originally I was planing to do the most frowned upon thing in this forum.... Use a clutch!

Now am realising that this is probably very stupid idea.

Gearboxes for mx5are about £40 so insanely cheap... Based on johu's recent video about energy effiency between gears and bextenders pain of field weakening...
I was wondering what are the real world power losses in field weakening?

What I'm thinking is can I remove unused gears from the gearbox (5th, and reverse look easy based on teardowns of mx5/rx7's).
Can I weld the gearstick in second gear as that would give me 80mph at 9000rpm at my current diff and gearbox. Would look cool wulity the zero ev r n d switch

Originally with an 80s and OEM inverter the torque curve goes flat at 3000-6000rpm, which makes me suspect the field weakening point is about 4500rpm similar to the leaf ... Using a 96s would push this up a little...

Is there likely to be much power left at 7000-9000 rpm?
What I would like is good acceleration upto about 70, beyond that becomes a little academic due to the wall of air sapping my precious battery

I know that the real answer is just build it and decide if 2nd or 3rd is best, 3rd would technically give me a top speed of over 100mph which I don't need acceleration is the target.

Just wondered if there was any real world data or experience to analyse/think about

[arber333]

Can I weld the gearstick in second gear as that would give me 80mph at 9000rpm at my current diff and gearbox. Would look cool wulity the zero ev r n d switch

I am not sure gearbox is meant to spin at 9000RPM for extended time. You might see a problem there...

Originally with an 80s and OEM inverter the torque curve goes flat at 3000-6000rpm, which makes me suspect the field weakening point is about 4500rpm similar to the leaf ... Using a 96s would push this up a little...

Yeah... NO. OEM power curve is dependent on battery power. Leaf Gen1 motor has FW in the area of 8000RPM so about 100kph. I run it with Volt inverter and Lebowski brain... Outlander is 10pole motor so you would need more voltage/Hz in theory.
Outlander charger. DCDC and inverter stop functioning at 400Vdc! So 96S is as far as i would go.

I know that the real answer is just build it and decide if 2nd or 3rd is best, 3rd would technically give me a top speed of over 100mph which I don't need acceleration is the target.

I have an outlander rear motor on desk here. I can start a sensorless drive at 360Vdc (Lebowski) and force the motor to its final RPM with FW disabled. I will try to fit a sensor tape to its diff so i get direct output RPM for the driveshafts.
That should settle the matter for good.

However i did spin it to some 12000RPM with 360Vdc and Lebowski with 100A of FW enabled. If you use a different inverter, say Prius or Volt you will be fine to dictate any RPM within reason. I am not sure what will happen with Outlander native inverter.
You might be in luck and inverter is not limited by final RPM but by voltage. Outlander uses up to 320Vdc and if you go by 360Vdc you have some BackEMF advantage...

[rstevens81]

As you can see I have gone full bonkers and working out motor powers at different gears diffs!

Luckily I am using a Prius inverter so it should allow me to rev as high as I want! Assuming field weakening (hoping I don't get issues like the Lexus mgr, although not likely)

A permanent magnet motor should in theory give a fixed torque till field weakening, so assuming I can match 195nm that the OEM inverter and motor produce then I can continue this till the point of field weakening?

Edit corrected excel table
Also I realise the fixed torque till field weakening is a simplifcation an neglects that fw is always present to allow for .Reluctance torque

[arber333]

Well for Mazda MX5 there is no easy choice as TX also serves as body torsion brace. I am not sure what happens if you would replace TX with Outlander rear motor diff. Maybe if you would make additional bracing in place of TX and mount some battery modules in its place...

[celeron55]

In the Leaf the motor is geared to spin 10krpm at 150km/h.

Of course as arber said you probably don't want to input 10krpm to an ICE gearbox for extended periods of time, so if you plan on driving 150km/h for extended periods of time, maybe gear a bit higher.

With OEM control with the 80kW limit, the inverter transitions from constant torque to constant power at about 3000rpm. (250Nm at about 3000rpm equals 80kW)

I'm using a 72S battery with OEM control and with this the sweet spot for the motor is from 2000 to 5000rpm.

EDIT: 3rd gear standard diff looks good to me if you do at least some cruising and don't need the absolute fastest off the line acceleration.

On the other hand if you need the fastest possible acceleration, then you could gear it as low as you can imagine. That will make more noise at speed and you better have your coupler well balanced.

And then on the third hand, if you leave your gear selector in, you will probably end up cruising in 4th gear, because if 3rd requires 5000rpm, that will still make noise.

[arber333]

In the Leaf the motor is geared to spin 10krpm at 150km/h.

Of course as arber said you probably don't want to input 10krpm to an ICE gearbox for extended periods of time, so if you plan on driving 150km/h for extended periods of time, maybe gear a bit higher.

Well you might still use two gears. 2nd gear for start from still and driving up to 60km/h and 3rd gear to cover every other speed. You might reengineer your gear selector lever so all you need is forward - rearward movement. Couple that to Openinverter BMS input pin to inhibit regen while shifting and you have more or less seamless transition.
https://insideevs.com/news/360896/zf-2- ... ore-range/
https://interestingengineering.com/star ... ngle-speed

[rstevens81]

It looks like I'm going to go with a 3 speed dog-leg arrangement,
5th and reverse look like an easy gear to remove (aft section of gearbox), 1 st can be removed, 2nd & 3rd are needed, 4th doesn't actually cause any mechanical losses as it's 1:1.
Probably just put a microswitch on the clutch pedal to disable regen.
I'll add a fwd/reverse switch in the ashtray as I don't smoke it's not a major loss
Tell the better half to only drive it in 3rd to prevent confusion.

Now to phone up the local scrappies to find out if they have some gearboxes

[celeron55]

If you have a gearbox with a 1:1 gear, that gear will definitely be the most efficient and most quiet.