[madderscience]

looks like the specs for an ac20 are about 80hp max and 65 foot pounds.

The torque might be a little low for direct drive, even with a vehicle sub 2000lbs all built out. Looking at hpevs.com, The AC23 might be a better bet, the tradeoff being you won't have as much high end power. Or, look at the AC35 or AC50. In any case, run the highest voltage system you can (144V with the curtis 1239 controllers it looks like)

As point of comparison, The AC55 in my xB has maximum torque of about 200ft-lbs, and I have an overall direct drive gear ratio of 4.8:1, and the whole thing weighs 3000lbs dry and empty. However I rarely hit max accel, I figure I probably keep it at or below 120ft-lbs most of the time going by the position of the throttle limiter (which limits peak motor amps). So proportionally speaking if you have a 2000lbs car, you'd want minimum 80ft-lbs with similar gear ratio (and wheel diameter) to get driveable performance. I think the 175/60r15 tires on my xB are pretty close to 23" diameter so your 24" tires would mean slightly less torque all else equal.

 

[dr.diesel]

Thanks!

I've been studying those graphs for a few days now. One question I can't come to grips on, why does the higher battery voltage reduce torque?

But you might be about right with the AC23, and at 96v I already have enough Leaf batts for 13s2p.

EDIT:

Though I don't know if the Leaf packs at 13s2p can do 650 amps.

 

[Kevin Sharpe]

My preference would be direct drive with a 60MPH cruise capability

Goal is for a min range of 50 miles, but 75 or so without a deep discharge would be ideal.

I already have 38x Leaf batteries

Even if you can replicate the Leaf's weight, aerodynamics, and efficiency, you will probably struggle to achieve 50 miles with 38 Leaf modules...

 

[dr.diesel]

Yeah, not surprised, but thanks for the graphs, that gives me a rough point to gauge against.

More Leaf modules are already on my list to score.

 

[madderscience]

I'm not a total expert but this is my understanding of torque on 3 phase AC motors.

torque on an AC motor is a function not just of the amps going through it, but also the number of poles, and whether its wound delta or wye. More poles (multiples of 3 always; you will probably not see any EV motors with other than 3 or 6 poles) == more torque and proportinally less RPM, and I think but everybody else correct me if wrong, delta will have more RPM and less torque relative to wye all else equal) A lot of the smaller traction motors are wound in ways to give them more torque since they are originally designed for low speed utility vehicles and golf carts without multi speed transmissions.

The larger motors are wound with fewer poles usually meaning less torque for the same amount of amps, but the higher voltage means they can hold that peak torque up to a proportionally higher RPM, all else being equal, meaning more power overall and freeway capability when suitably sized to a vehicle. So with a bigger motor if necessary you could adjust gear ratios to get more wheel torque if you have more RPM than you need.

Of course larger diameter motors will have more torque and less RPM for the same input current/voltage, and capacity to shed heat in the motor and controller will limit torque when you are below the torque curve knee.

my xB is geared so the torque curve 'knee' is at about 45mph. It still has about 60% of peak torque at 60mph, plenty to maintain speed on anything except a mountain pass. On mountain passes (snoqualmie, only one I've been on) my xB can hold about 55mph on the steep sections. A lower gear ratio (like 4.4:1 or 4:1) would actually improve top speed in these circumstances, but I would lose needed low speed torque. Top speed on level ground is about 75mph.

 

[brian_]

I've been studying those graphs for a few days now. One question I can't come to grips on, why does the higher battery voltage reduce torque?

It doesn't, for the same motor. What are you comparing?

If you're looking at just one graph, such as that one for the AC-23... the voltage isn't being actively controlled, and the company doing the test apparently doesn't have the resources to provide a stable power supply. This means that when the motor draws more current, the inadequate battery drops in voltage due to internal resistance.

 

[dr.diesel]

Between these two, motor appears to be the exact same, just two different voltages, graph nor website state which controller is used for the test.

As an example, ~0 RPM, standing start:

144v, 87ft lbs @ 19 amps

96v, 127ft lbs @ 33 amps

Information came from here:

http://hpevs.com/hpevs-ac-electric-motors-power-graphs-ac-23.htm

Lots of inexperience here, but looks like I'd be sacrificing quite a bit of starting torque by going with the higher battery voltage. Though I'd have considerably more torque at my top end cruise of 4500RPM @ 144v, the 96v is higher from 0 to 3500RPM.

 

[brian_]

Between these two, motor appears to be the exact same, just two different voltages, graph nor website state which controller is used for the test.

As an example, ~0 RPM, standing start:

144v, 87ft lbs @ 19 amps

96v, 127ft lbs @ 33 amps

Information came from here:

http://hpevs.com/hpevs-ac-electric-motors-power-graphs-ac-23.htm

Lots of inexperience here, but looks like I'd be sacrificing quite a bit of starting torque by going with the higher battery voltage. Though I'd have considerably more torque at my top end cruise of 4500RPM @ 144v, the 96v is higher from 0 to 3500RPM.

These motors follow a general characteristic of torque climbing linearly with speed to the peak (why not constant torque like other induction motors? who knows...), then dropping off as the supply voltage is unable to drive the required current at higher speeds (and resulting higher back EMF of the stator coils)... so the speed of that peak increases with higher voltage (2400 rpm @ 96V, 4800 @ 144 V).

Onee problem is that there is a lot going on to obscure the motor performance, including:

• supply (battery) voltage is not constant (sags at higher currents)
• current is limited (presumably by the controller) at different levels in different tests

The current limit for the "96 V" test is stated as 650 amps, and reaches 601 amps; the current limit for the "144 V" test is stated as 500 amps, and reaches 486 amps.

Both graphs show data in that linearly climbing torque region around 1500 rpm. At that speed

• in the "96 V" chart, 80 volts drives 400 amps (for 32 kW) and produces 120 lb-ft of torque which is 34.3 HP (25.6 kW) by calculation or 35 HP read from the chart
• in the "144 V" chart, 140 volts drives only 160 amps (for 22.4 kW) and produces 82 lb-ft of torque which is 23.4 HP (17.5 kW) by calculation or 22 HP read from the chart

The combinations of these sets of data points for the same motor at the same speed (in both stall and 1500 rpm cases) are nonsensical if they are for the same motor. In the low-speed region where torque is limited by available current, they should be the same. This leads me to conclude that one of two things is going on:

1. either the controller is not working properly and it is not maintaining appropriate slip speed so it is unable to use higher voltage to drive higher current, or
2. the dynamometer is not controlled properly and is not providing the highest mechanical load which the motor can drive at each speed.

I wouldn't be surprised if the tests at different voltages used different controllers, or a single controller which was not tuned appropriately for the higher voltage. The dynamometer may be just a flywheel, so there is no control of load or speed, just observation of how rapidly speed changes (which implies torque).

The performance charts from the DC series-wound motor suppliers (such as Netgain) appear to have been generated with a simple friction brake and are worthless. These charts for the AC- series initially look much better because they cover a meaningful range of speeds, but they appear to me to be of little value in actually describing the performance of the motors.



Physical reality is that with more supply (battery) voltage available, a properly working controller will be able to drive more current at a given shaft speed and thus produce more torque, but limited by

• the battery current capacity,
• the controller current limit, and
• heating of the motor

Whatever the current limit of the system, a higher supply voltage will be able to maintain that current (and resulting torque) to a higher shaft speed, unless an electrical power limit is reached (due to some component heating, for instance).

 

[brian_]

my xB is geared so the torque curve 'knee' is at about 45mph. It still has about 60% of peak torque at 60mph, plenty to maintain speed on anything except a mountain pass. On mountain passes (snoqualmie, only one I've been on) my xB can hold about 55mph on the steep sections. A lower gear ratio (like 4.4:1 or 4:1) would actually improve top speed in these circumstances, but I would lose needed low speed torque. Top speed on level ground is about 75mph.

With your existing overall drive ratio being 4.8:1 (from your published specs), 4.4:1 or 4:1 would be numerically lower ratio or "taller gearing"... terminology can be confusing in final drive ratios.

I think you're saying that in this scenario of climbing a steep pass at 55 mph, you would need more power to go faster, but your motor is in a region where it would have more power available at a lower rotational speed. This seems to be a common characteristic of these low-voltage induction motors for EV conversions (and it's clearly shown in the HPEVS graphs), but not a problem for the higher-voltage systems in production cars (whether induction or PM AC) which have essentially constant power capability to almost the upper shaft speed limit. It does make gearing choice a challenge, but your setup in the xB looks like a very effective compromise.

 

[dr.diesel]

Thanks to all for the great info. The AC23 might be a tad more than I need, but the cost to do so is minimal, so might as well be sure.

 

[madderscience]

yes it is confusing, I may have written wrong. a "lower gear" is generally a higher ratio, and "higher" gear is a lower ratio. For example a typical 5 speed manual for a car, 5th gear is usually an overdrive at about 0.8:1 and 1st gear is usually someplace around 6:1.

Yes, I could get significantly more amps (and thus torque) at a given freeway speed by running a lower gear ratio (higher gear) since the motor would be either running in the flat part of the torque curve (constant amps; controller limited). More power would make it to the wheels vs. if it going at a higher rpm and thus less amps could flow due to back emf.

The downside of the lower gear ratio is at lower speeds where amps are controller limited, I have less torque to the wheels since the higher gear does not multiply the motor torque as much.