[schmidtbag]

I've been working on converting a 1977 VW Rabbit with a 4-speed manual transmission for the past 3 years. Here are the specs:

• AC-20, since at 96V it has a little less HP than the original 1.6L engine the car came with.
• I'm using 4x Tesla Model S batteries in series, which gets to about 94v fully charged.
• Curtis 1238E

I'm well aware these batteries do not continuously provide the ~650A this motor needs in order to achieve 108lbs/ft of torque. According to the motor controller, it has drawn a max around 430A, which already seems off. The battery output is rated for 1000A for 10 seconds, so I wouldn't think they're the problem, and the motor controller is rated for 650A. According to the BMS, the batteries are in good condition, so even if I were to get 500A for 10 seconds, I should still be able to accelerate faster than I do now.

Is there anything that might be limiting the power?

 

[remy_martian]

At 140 ftlb on the axle, in 1st gear at 40 ftlb, your acceleration will be the equivalent of your girlfriend pushing it with all her weight. So, no, it's not even going to squeak the tires.

You need to get the torque up. What's the motor rpm at 50mph and what's its rated rpm?

 

[brian_]

I've been working on converting a 1977 VW Rabbit with a 4-speed manual transmission...

1st gear has a ratio of 3.45, so even at 40lbs/ft, I imagine that would be enough torque to make the wheels slip (most of the weight is now in the rear).

If I found the right specs, first is 3.46:1, and the final drive ratio is 3.89:1. That would make the torque to wheels 13.5 times the motor torque, or 538 lb-ft. The stock tire size is 155/80R13, for a radius of a bit under 289 mm (11.4"). With that radius, 538 lb-ft (729 Nm) of torque would produce a tractive force at the road surface of 566 pounds or 2519 newtons (force = torque / radius)... equally split between the two front tires. I doubt that most of the weight is in the rear, even with the battery back there, so that force will be much less than the weight on the tires and with any reasonable tires they won't slip.

 

[schmidtbag]

At 140 ftlb on the axle, in 1st gear at 40 ftlb, your acceleration will be the equivalent of your girlfriend pushing it with all her weight. So, no, it's not even going to squeak the tires.

You need to get the torque up. What's the motor rpm at 50mph and what's its rated rpm?

Ah right, I guess it makes sense when you put it that way hah. Perhaps I am close to the 2/3 power then. In either case, if the 1000A for 10 seconds is true, I should be getting a higher peak amperage, right? Even if I got half the amperage for 10 seconds, I should still get more.

I don't have exact numbers but the motor RPM should be around 3000RPM in 4th gear at 50MPH. The motor really starts to lose power at around 5000RPM. I haven't pushed the motor that fast yet.

If I found the right specs, first is 3.46:1, and the final drive ratio is 3.89:1. That would make the torque to wheels 13.5 times the motor torque, or 538 lb-ft. The stock tire size is 155/80R13, for a radius of a bit under 289 mm (11.4"). With that radius, 538 lb-ft (729 Nm) of torque would produce a tractive force at the road surface of 566 pounds or 2519 newtons (force = torque / radius)... equally split between the two front tires. I doubt that most of the weight is in the rear, even with the battery back there, so that force will be much less than the weight on the tires and with any reasonable tires they won't slip.

4th gear is 0.97:1.
The front is pretty light. The motor, controller, transmission, DC-DC converter, washer fluid, and some fuses are all you'll find up there. The "engine" bay is pretty empty. When the car gets moving, you can steer the wheel with your pinky (obviously, no power steering).

 

[brian_]

4th gear is 0.97:1.

Okay... but I thought you were talking about the lack of ability to spin the tires in first gear; obviously it won't be able to spin the tires in fourth gear, with only about one-quarter as much torque to the tires for the same motor torque.

Perhaps you don't understand what "final drive ratio" is? It's not the top gear ratio. In the transaxle power flows from the input shaft to the next shaft through one of the gear sets (ratios 3.46:1 in 1st, 1.94:1 in 2nd, 1.29:1 in 3rd, and 0.97:1 in 4th). That next shaft has a small gear it that drive the large gear around the differential; this final stage of reduction gearing is called the "final drive". The final drive ratio is 3.89:1. As a result, the ratio between motor speed and axle speed is the product of multiplying one of the four gear ratios (or the reverse ratio) by the final drive ratio, or 3.46 x 3.89 = 13.5 in first gear.

I got those gear ratios from Transmission Ratios; see "4 speed to '80 (Rabbit)" line of the table.

The front is pretty light. The motor, controller, transmission, DC-DC converter, washer fluid, and some fuses are all you'll find up there. The "engine" bay is pretty empty. When the car gets moving, you can steer the wheel with your pinky (obviously, no power steering).

No, it's not. The stock Rabbit has perhaps 60% of its roughly 2000 pound weight on the front tires, and the majority of that is not the engine. Even replacing the engine with the electric motor, there is still half a ton of car on the front tires. Try running over your toes with a front tire, and let us know if it's really light.

My first car didn't have power steering, it was a stock front-engine front-wheel-drive Toyota which was about the same weight as a Rabbit, and one-finger driving wasn't difficult... but the front tires (same size as the Rabbit) were still being squashed by over half a ton between them.

Because the car is front wheel drive, when acceleration shifts load from the front tires to the rear tires, load on the driven tires is reduced and thus traction is reduced; however, there isn't enough drive force available here to make the front end actually light - this isn't a drag racer lifting the front tires as a thousand horsepower churn the sticky slicks in the back.

 

[brian_]

I don't have exact numbers but the motor RPM should be around 3000RPM in 4th gear at 50MPH. The motor really starts to lose power at around 5000RPM. I haven't pushed the motor that fast yet.


4th gear is 0.97:1.

This confirms the gearing: 3000 RPM divided by 0.97 and by 3.89 is 795 RPM. That speed with the stock tires (877 revolutions per mile according to published specs) gives 0.907 miles per minute, or 54 MPH. So the final drive ratio is at least close to 3.89:1, and you can use these 1st through 4th gear ratios and final drive ratios with the tire size to calculate motor speeds in any gear at any speed.

 

[brian_]

The motor really starts to lose power at around 5000RPM.

Yes, with about 96 volts from the battery, power drops off rapidly past 5000 RPM, as the HPEVS performance chart from the HPEVS AC-20 page shows:

For best short-term performance you would want to shift to keep the motor speed as close to 5000 RPM as possible; you won't reach that motor speed until 25 MPH in first gear (and 45 MPH in second).

 

[schmidtbag]

Okay... but I thought you were talking about the lack of ability to spin the tires in first gear; obviously it won't be able to spin the tires in fourth gear, with only about one-quarter as much torque to the tires for the same motor torque.

Well yeah I know, but you said final gear (which I assumed meant 4th, but apparently not), so I wanted to clarify it's 0.97 and not 3.89. At this point, I don't even necessarily care about spinning the tires in 1st gear, but would like to be able to tap into more of the power these batteries supposedly have. Even if I were to get half the amperage for half the duration, I should still get better results.

Perhaps you don't understand what "final drive ratio" is? In the transaxle power flows from the input shaft to the next shaft through one of the gear sets (ratios 3.46:1 in 1st, 1.94:1 in 2nd, 1.29:1 in 3rd, and 0.97:1 in 4th). That next shaft has a small gear it that drive the large gear around the differential; this final stage of reduction gearing is called the "final drive". The final drive ratio is 3.89:1. As a result, the ratio between motor speed and axle speed is the product of multiplying one of the four gear ratios (or the reverse ratio) by the final drive ratio, or 3.46 x 3.89 = 13.5 in first gear.

Gotcha. Thanks for the clarification.

No, it's not. The stock Rabbit has perhaps 60% of its roughly 2000 pound weight on the front tires, and the majority of that is not the engine. Even replacing the engine with the electric motor, there is still half a ton of car on the front tires. Try running over your toes with a front tire, and let us know if it's really light.

I replaced the suspension, where the front and rear springs appear to be the same (same thickness, same number of winds). The rear sagged significantly more than the front. Thankfully, these, coilovers are adjustable, but the rears are about as high as they can go. I can grab the tire and turn it relatively easily while the car is still on the ground. While I'm not saying the front is light, I think this is enough evidence to suggest it is lighter than the rear. Perhaps not by a lot.
In any case, I acknowledge it isn't light enough to make a difference.

Because the car is front wheel drive, any acceleration shifts load from the driven tires to the rear tires, reducing the load on the front tires and thus reducing traction; however, there isn't enough drive force available here to make the front end actually light - this isn't a drag racer lifting the front tires as a thousand horsepower churn the sticky slicks in the back.

Well yeah, but I would have assumed the immediate torque in 1st gear would be enough to chirp the tires, as opposed to an acceleration rate akin to grandma driving and realizing her laxatives have kicked in on her way to church: slow, but not annoyingly slow.

 

[brian_]

Well yeah, but I would have assumed the immediate torque in 1st gear would be enough to chirp the tires, as opposed to an acceleration rate akin to grandma driving and realizing her laxatives have kicked in on her way to church: slow, but not annoyingly slow.

I could chirp the tires in my old car (I used it in autoslalom competition so it did get pushed hard), but that was with a rated torque about the same as the AC-20 with 650 amps... at half of that, the drive would be quite "relaxed".

Keep in mind that torque is roughly proportional to motor current, but battery current is not motor current. The controller divides the voltage from the input (from the DC link to the battery) to the output (as 3-phase AC to the motor), while multiplying the current by almost the same ratio (of course considering the RMS value of the total current in the three phases). That's why the HPEVS graphs show current rising up to the "knee" (around 4500 RPM in the 96 V / 650 A chart); with increasing speed the battery current is rising while the battery voltage stays constant, but the motor current is constant (limited by the controller) while the motor voltage increases as it must with speed. It would be nice if both the performance chart and operating information from the controller showed the RMS value of motor current, because looking at DC link (battery) current by itself doesn't tell you what is going on at the motor.

Below the knee in the curve, the DC link current is limited by the motor (controller output) current; above the power peak, the DC link current is limited by the battery voltage available to drive the motor. This means that peak DC link (battery) current is only observed near the knee, as the curve shows; observe the motor under any other condition, and battery current will be lower.

 

[schmidtbag]

Keep in mind that torque is roughly proportional to motor current, but battery current is not motor current. The controller divides the voltage from the input (from the DC link to the battery) to the output (as 3-phase AC to the motor), while multiplying the current by almost the same ratio (of course considering the RMS value of the total current in the three phases). That's why the HPEVS graphs show current rising up to the "knee" (around 4500 RPM in the 96 V / 650 A chart); with increasing speed the battery current is rising while the battery voltage stays constant, but the motor current is constant (limited by the controller) while the motor voltage increases as it must with speed. It would be nice if both the performance chart and operating information from the controller showed the RMS value of motor current, because looking at DC link (battery) current by itself doesn't tell you what is going on at the motor.

Understood, and I agree that the motor current in the chart isn't quite enough. But here's the main reason I posted:
The battery can provide 1000A for 10 seconds and 225A continuously. 10 seconds is enough to get me to in-town cruising speed, and I assume 225A ought to be sufficient to maintain cruising speed on a flat road. Since the motor controller is limited to 650A, that buys me more time for acceleration. The problem is, the motor controller reports max power draw nowhere close to 650A. It's as though something is limiting how much power it can use.

The motor controller has a pin for economy mode. It isn't clear whether it's engaged or disengaged when 12v is applied, but when toggling it, there wasn't any significant change. I'm not sure if there's something else I'm supposed to do.
On that note, the motor controller's clutch/shift switch doesn't seem to do anything either (regen brake always seems to be on). These are the only two 12v functions that don't seem to do anything; all other functions work. But, my current priority is the lacking power.

 

[brian_]

The problem is, the motor controller reports max power draw nowhere close to 650A.

That makes sense, but nowhere close to 650 A (from the battery)... at what motor speed?

 

[schmidtbag]

That makes sense, but nowhere close to 650 A (from the battery)... at what motor speed?

I'm referring to the max amperage reported by the motor controller, which from what I recall was around 430A. I assume this is the amperage it outputs to the motor. The motor reached around 4000RPM during that test (I don't recall that it logs max RPM). So, I should have got much closer to 500A, based on what those charts said.

But also... shouldn't the stall current be much higher? I've done a bit of work with much smaller motors (ones that weigh less than 1kg) and the stall current was often the highest value.

 

[BAstereo]

The motor controller has a pin for economy mode. It isn't clear whether it's engaged or disengaged when 12v is applied, but when toggling it, there wasn't any significant change. I'm not sure if there's something else I'm supposed to do.
On that note, the motor controller's clutch/shift switch doesn't seem to do anything either (regen brake always seems to be on). These are the only two 12v functions that don't seem to do anything; all other functions work. But, my current priority is the lacking power.

I'm not super familiar with the 1238e, but on the 1238, there is no connection to the 12 volt system. The Curtis 1238 only knows your traction pack voltage, so all inputs/outputs must be referenced (grounded) to your traction battery, not 12v battery.

 

[schmidtbag]

I'm not super familiar with the 1238e, but on the 1238, there is no connection to the 12 volt system. The Curtis 1238 only knows your traction pack voltage, so all inputs/outputs must be referenced (grounded) to your traction battery, not 12v battery.

My controller not only has a connection to the 12v system but provides a low-amperage 12v output (and 5v, for that matter). The 12v is working for several other functions.

 

[BAstereo]

Is your traction battery negative is connected to your 12v battery negative?

If your are using the 12v, 5v, and IO ground outputs from the controller, those are referenced to your traction battery negative.
Switching them and returning that signal to the controller then you're probably doing it right.

 

[schmidtbag]

Is your traction battery negative is connected to your 12v battery negative?

If your are using the 12v, 5v, and IO ground outputs from the controller, those are referenced to your traction battery negative.
Switching them and returning that signal to the controller then you're probably doing it right.

No, my DC-DC converter is isolated so there no continuity to the traction battery or anything 12v.
The 5v, 12v, and ground pins provided by the motor controller have no continuity to the traction battery either; they are also isolated. All I/O pins for the controller are only using the voltage and ground sources that the controller provides.
Both ends of the traction battery are only connected to a contactor, a fuse, the motor controller, and the DC-DC converter. The BMS reads 100% isolation.

 

[BAstereo]

Ok, sounds like you've got a good handle on the situation. I was just grasping at threads when you mentioned the 12v and motor controller input.
On the 1238, I believe IO ground isn't isolated.

Is your motor an induction motor? If so the stall current I've seen from Curtis is low. It seems like they limit below even 650 amps AC at 0 rpm.

 

[schmidtbag]

Ok, sounds like you've got a good handle on the situation. I was just grasping at threads when you mentioned the 12v and motor controller input.
On the 1238, I believe IO ground isn't isolated.

Haha yup, the IO ground on my controller is also isolated. Though, it wouldn't since I don't have it connected with the rest of the 12v system.

Is your motor an induction motor? If so the stall current I've seen from Curtis is low. It seems like they limit below even 650 amps AC at 0 rpm.

Yes, it is an induction motor. That is definitely useful to know that they limit the stall current. So, I guess that means the only way for me to reach 650A is if I reach 5000RPMs under load. I haven't tried pushing it that hard.

 

[BAstereo]

Ok, sounds like you've got a good handle on the situation. I was just grasping at threads when you mentioned the 12v and motor controller input.
On the 1238, I believe IO ground isn't isolated from traction negative. It is isolated from 12v negative.

Is your motor an induction motor? If so the stall current I've seen from Curtis is low. It seems like they limit below even 650 amps AC at 0 rpm.

Just clarifying myself here...

 

[wstein25]

I've been working on converting a 1977 VW Rabbit with a 4-speed manual transmission for the past 3 years. Here are the specs:

• AC-20, since at 96V it has a little less HP than the original 1.6L engine the car came with.
• I'm using 4x Tesla Model S batteries in series, which gets to about 94v fully charged.
• Curtis 1238E

I'm well aware these batteries do not continuously provide the ~650A this motor needs in order to achieve 108lbs/ft of torque. According to the motor controller, it has drawn a max around 430A, which already seems off. The battery output is rated for 1000A for 10 seconds, so I wouldn't think they're the problem, and the motor controller is rated for 650A. According to the BMS, the batteries are in good condition, so even if I were to get 500A for 10 seconds, I should still be able to accelerate faster than I do now.

Is there anything that might be limiting the power?

The first thing that came to mind if you are really seeing current limiting is perhaps your wiring - the conductors from battery to motor controller are undersized, or the connectors are inadequate. Have you tried measuring the battery voltage at the battery terminals and simultaneously at the controller, while under full load, that is accelerating from zero at full throttle? If you have even a tenth of an ohm in the cable and connectors, 1000 amp would cause a 100 volt drop.