# Axial flux motor dragster



## tropes (Jul 24, 2011)

Thanks to those who messaged me. I was able to make contact with Paul at P&S Power Electronics http://www.pandspowerelectronics.com/

tropes


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## tropes (Jul 24, 2011)

This is a drawing of the inverter/controller that I envision for a dragster with an AC motor. Please ignore the crudeness of the drawing and I realize the complex differences but I wanted to point out how few functions were needed with my Junior Dragster DC controller. Feel free to criticize or comment.


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## kennybobby (Aug 10, 2012)

It's an interesting concept.

There are large axial-flux motors used in oil well drilling that provide lots of torque, not sure about how the acceleration or speed would play out for a dragster due to the high inertia of the pancake design. But it avoids needing a gearbox so that's a plus.


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## brian_ (Feb 7, 2017)

tropes said:


> High inertia and torque should only affect the axle and speed is determined by motor RPM and gear ratio.


Okay... but if I understand your illustration correctly you're showing this motor mounted directly on the axle, so there is no gearing and the drive ratio of motor to wheel is 1:1. You can't choose the gear ratio when you have no gears. 

Inertia of any rotating component does matter. You could compare the energy stored in the rotation of this high-inertia/low-speed motor driving the axle directly with a more common lower-inertia/higher-speed motor driving through a reduction gear, to see if this configuration is forcing you to put more energy into the flywheel effect of the motor.



tropes said:


> The motor I am considering is Yasa 750 http://www.lightswitchracing.com/YASA-750-Product-Sheet.pdf and the battery is two of these Lone Star 100V in series.


Although the sheet's file name says "750", it's actually for the 750R... I assume it's the 750R that you want.

Even at only 200 volts - the blue line on the right-hand chart in that sheet - you will be limited by the motor's maximum torque and current up to 600 rpm (axle and motor speed)... are you expecting to be traction-limited to this speed, so that reduction gearing would not help performance (or not enough to justify the complication)? With 18" tall tires, this would be 30 mph.

I suppose this gearing issue doesn't really matter too much to the controller, since you will need to handle over 200V and at least 400 ARMS, although it looks like the motor can actually handle at least 450 ARMS in any case, so a controller which can support that much current would give you more torque for the launch.


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## kennybobby (Aug 10, 2012)

i was hoping it would be an AC induction motor, but looking at the patent,
https://encrypted.google.com/patents/EP2773023A1?cl=und , it appears to be a permanent magnet motor. These would probably make a better generator than a motor due to the high back emf generated. At 200 V the back emf causes the torque to start rolling off at only 625 RPM and the maximum power is on the order of 56 kW. If you have to add a gearbox then you probably lose any possible benefit to the axial flux package.

Their website had no information about the motor technology or any details, and i'm suspicious of any technical abilities when their board has engineers who started companies to trade in carbon credits. Two thumbs down for me.

As far as electrical controllers, you could probably find a used variable frequency inverter that would work to drive the motor.


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## tropes (Jul 24, 2011)

kennybobby
I will look deeper into this Yasa company. Thanks for the info.

tropes


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## brian_ (Feb 7, 2017)

kennybobby said:


> i was hoping it would be an AC induction motor, but looking at the patent,
> https://encrypted.google.com/patents/EP2773023A1?cl=und , it appears to be a permanent magnet motor. These would probably make a better generator than a motor due to the high back emf generated


Synchronous permanent magnet motors are preferred over induction by every current manufacturer of production road-going EVs (now including Tesla Motors, starting with the Model 3), so I don't think the PM design should be a surprise. I'm pretty sure it makes a good motor. 



kennybobby said:


> At 200 V the back emf causes the torque to start rolling off at only 625 RPM and the maximum power is on the order of 56 kW.


Yes, at low voltage the motor hits the power-limited (rather than torque-limited) mode at a relatively low speed... but it keeps working at that constant power to the top of it's operating range. This is not substantially different from an induction motor. The Tesla Model S motors hit this transition about one-third of the way up their speed range.

The plan here is to use the motor at relatively low voltage, which works if the idea is to operate at constant power through much of the run. This voltage would not be so suitable if the idea were to run at constant torque for most of the run.



kennybobby said:


> If you have to add a gearbox then you probably lose any possible benefit to the axial flux package.


I agree that this "pancake" and axial-flux motor design is intended for high-torque and lower-speed applications; this makes it appropriate to directly drive the dragster's axle, but if a reduction drive were used a more common lower-torque/higher-speed radial-flux motor would be a more obvious choice.

With no differential and no suspension, a dragster like this with motor directly on the axle is about the simplest possible four-wheeled vehicle... which has appeal for reliability. It also avoids the mechanical loss of a reduction drive, although at the expense of motor inefficiency if components are not carefully selected for the conditions.



kennybobby said:


> Their website had no information about the motor technology or any details, and i'm suspicious of any technical abilities when their board has engineers who started companies to trade in carbon credits. Two thumbs down for me.


That's an interesting observation. I wouldn't usually look to the Board of Directors for technical expertise in a corporation. Whatever one might think of the technical qualifications and motivations of the people involved, this isn't a startup offering vapourware - they appear to have built motors which have worked in at least project (concept, prototype, race) cars by legitimate manufacturers.



kennybobby said:


> As far as electrical controllers, you could probably find a used variable frequency inverter that would work to drive the motor.


Yes, every controller to drive this motor - like every AC motor including induction designs - will be a 3-phase variable frequency inverter. I'm pretty sure that tropes is looking for more specific information than this.


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## brian_ (Feb 7, 2017)

tropes said:


> The final ratio is determined by tire diameter. Minimum tire diameter for NHRA Jr Comp class is 22"


Thanks - all I had found was the 18" minimum for Jr. Dragster (not Comp). To make the best of the limited torque, I assume that you'll run the shortest allowed tire... and you're still going to use less than 2/3 of the motor's speed range.



tropes said:


> Now could you build for me an inverter/controller that would handle 400V 600amps. I would prefer heat sink rather than liquid cooled.


Sorry, that wouldn't be me. Hopefully someone in that business will notice this discussion.


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## brian_ (Feb 7, 2017)

kennybobby said:


> There are large axial-flux motors used in oil well drilling that provide lots of torque, not sure about how the acceleration or speed would play out for a dragster due to the high inertia of the pancake design.


A valid concern (rotational inertia stores energy which then isn't kinetic energy of the car's speed), but a Jr. Comp run takes about 8 seconds (fast for a car down the 1/8th mile, but not much time to spin up a motor), so I don't think accelerating the motor will be a concern.


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## kennybobby (Aug 10, 2012)

To run 130 mph in 8 seconds with a 1000 lb car would require 130 HP (97kW) without consideration of whether the motor could actually accelerate the inertia of itself and both tires, and whether it can reach 2000 RPM under that load. With a 200 V pack it would require about 500 Amps.

At 400 Arms the motor makes 553 ft-lbs of torque off the line, so on 22" tires that would provide about .5g acceleration. But .5g for 8 seconds only gets you to 87 mph. Without aero loads or friction included. And the torque starts falling off above 650 RPM so that acceleration would also drop.

we can't find any motor winding resistance or inertia data on their website to actually make additional sanity check calculations.

With proper thermal considerations and parts selection it may be possible to do this with a limited cooling system--just let it heat up during the run and then cool down off-track. Quite a sporty approach.


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## brian_ (Feb 7, 2017)

tropes said:


> Drag racing is about E.T. not mph. Jr Comp cars are restricted to 6.90 seconds or slower based on either an e.t. dial-your own or heads-up basis; breakout rules apply. In qualified events, no racer can qualify quicker than 6.900. Any racer running faster than 110.00 mph at any time during an event is disqualified


True, but kennybobby is just running through the physical reality of the work to be done by the motor, which depends on the change in velocity. The 130 mph was the result of an axle speed which I calculated for 18" tires, but with 22" tires, so it's not relevant... and I realize that even 110 mph is not necessarily the trap speed.



tropes said:


> Most Jr Comp cars weigh 550 - 600 lbs. plus driver.
> Calculate E.T.: 755 pounds and HP of 114 (85 kW)


If you try to calculate elapsed time based on a constant power level, you will also need to define a maximum rate of acceleration (or tractive force), since otherwise you will be assuming infinite (and thus impossible) acceleration at zero speed.

If you have a target trap speed, it's easy to simplistically approximate the required constant power, ignoring the acceleration impossibility, as well as rolling drag, aerodynamic drag, and tires losses:
mass: 342 kg (755 lb)
trap speed: 49 m/s (110 mph)
kinetic energy at end: E = 1/2mv^2 = (0.5)(342 kg)(49 m/s)^2 = 411 kJ
duration = energy / power = (411 kJ)/(85 kW) = 4.0 s​
There are many problems with this, so the actual time is much longer.

kennybobby also mentioned the relationship between the rate of acceleration and run time...
To run an 1/8th mile (201 m) in 6.9 seconds at constant acceleration (which is still not quite right), there's only one equation to solve:
x = 1/2 at^2 (distance proportional to acceleration and time squared)
so a = 2x/t^2 = (2)(201 m)/(6.9s ^2) = 8.44 m/s (or 0.86 g)

That doesn't seem like an unreasonable rate of acceleration for a dragster (of any size), using drag slicks and having most of the load on the rear tires while accelerating. The challenge is that after 6.9 seconds at 8.44 m/s the car will hit a trap speed of 58 m/s or 130 mph. That's too fast, so to run a 6.90 second ET legally the run needs to accelerate harder in the early part, then fall off in acceleration near the end... which is what is naturally going to happen as the limit of power is reached.

Again ignoring the power needed to overcome drag, while accelerating 342 kg at 0.86 g (so 2890 N or 650 lb), 85 kW would be reached at 29 m/s. That's 65 mph, and reached after 3.4 seconds, so if you're limited to this power level then there's still about half of the time of the run of constant-power acceleration left after that... and so the initial acceleration will need to be substantially higher than 0.86 g.

So... 


kennybobby said:


> At 400 Arms the motor makes 553 ft-lbs of torque off the line, so on 22" tires that would provide about .5g acceleration.


Fortunately the car with driver is about two-thirds of the expected 1000 pounds, so this 533 lb-ft of torque, ideally producing 581 pounds of force at the road, is good for about 0.7 g of acceleration... and 10% better with a change of tires to 20" tall. That's not going to be enough... which is the second problem with the constant-acceleration calculation.

Is about 0.7 g initially and up to the motor's power limit (e.g. 84 kW given 300 volts), then constant power from there to the end of the run close enough? You can do the calculation in two parts (constant acceleration then constant power), with some estimated drag values, and it might work out.


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## brian_ (Feb 7, 2017)

tropes said:


> Are you referring to the motor RPM ?


Yes. Motor rpm, which is shaft rpm in this configuration.



tropes said:


> 2000 RPM, 22" tire, 1:1 final drive ratio = 130mph


I had calculated roughly 2000 rpm assuming a smaller tire than 22".



tropes said:


> The shortest allowed tire is actually 20" but the Jr Comp car I am looking at is wearing M&H 9.0 x 22.0 -13 tires...


It might be worth a tire change, since with the direct drive you've lost the ability to trade off between torque and speed with gearing.



tropes said:


> ... the Jr Comp car I am looking at
> ...weighs about 600 lbs. with a Suzuki engine without driver.





tropes said:


> Most Jr Comp cars weigh 550 - 600 lbs. plus driver.
> Calculate E.T.: 755 pounds and HP of 114 (85 kW)


Realistically, how will the weight of the motor, controller/inverter, batteries, and wiring, compare with the weight of the engine and drive (chain and sprockets or whatever) and fuel system?


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## tropes (Jul 24, 2011)

Thanks Brian. Unfortunately, my prospective Junior Comp contributor has backed away because of the high estimated cost of conversion. However, if you wish we I could pursue this discussion in the hopes that some other owner will take me up on my offer to supply the motor required for a Junior Comp conversion.
I do have numbers acquired from the runs made with my former dragster ( 98 runs over 6 years) which I have attempted to extrapolate and extend by inferring unknown values from these known numbers.
I do understand if you are reluctant to go further with this thread since it now becomes hypothetical.


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## brian_ (Feb 7, 2017)

It's all sort of hypothetical even when real cars are involved, when they're at the other end of an internet connection.  Drag racing is a reasonable application for electric drive (which is why quite a few have done it), and these small dragsters might be very reasonable place to try it, so there will likely be someone else to pick up the idea.

Get a bit further, and the whole thing would even be ready for a serious look by a controller/inverter builder, as originally intended. 

I would be interested in speed versus time (or distance versus time) data for an actual (although gas-engined) car of this class, if the data is in a convenient form to share. 

There is a good online calculator that has been used before in other discussions in this forum. It makes realistic calculations with reasonably available data, and is well-suited to a single-speed drive configuration. I've seen it used with Tesla data, but I haven't found it yet in my searching. If anyone has a link, I would be interested in working through the input data to see if this motor might work.


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## Karter2 (Nov 17, 2011)

Single speed is possibly an advantage,.... but i seriously doubt that direct drive (low motor rpm) is advantageous.
The huge increase in torque from ratio multiplication is a big advantage for drag racing.
These Yasa motors have been used many times in race applications, and most every time a geared or chain reduction drive was used.


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## brian_ (Feb 7, 2017)

tropes said:


> Drag racers have been collecting data for decades. I have at least four sites favourited on my laptop. They are however meant for ICE vehicles and most are for 1/4 mile.


I'm thinking of a calculator which is specifically for electric vehicles. If I can find it again, I'll share a link to it.


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## brian_ (Feb 7, 2017)

tropes said:


> The Yasa motor produces about 550 ft/lbs. of torque. I was hoping this would be enough to launch the dragster.


That's the easy part. Motor torque multiplied by gear reduction (1 in this case) divided by tire radius is force (under 600 pounds in this case). Force divided by mass is acceleration (2/3 of one g in this case).

Of course you need a target for acceleration to find a motor and choose gearing, or a way to assess whether a proposed motor and gearing is enough, which was the point of my calculations. Experience with competitive vehicles works, too. You don't need to know about their engine power characteristics, just how fast they go.



tropes said:


> One obstacle I had to overcome 10 years ago when converting ICE to electric is the idea of replacing the engine with a motor. I now think more about replacing the engine with a battery and adding to the drive train with a motor.


Sure, you can think of the fuel tank plus engine as the power source, transmission and final drive as the transmission system. Then in the electric version the battery is the power source, the motor and whatever gearing you use is the transmission system. There are other ways to think of the equivalency, but that one can work well for weight and packaging.



tropes said:


> I realise that large motors do not produce more power; higher voltage does. Now, as you point out, I must deal with the thought of losing torque by eliminating gear reduction.


Well, larger motors do generally produce more power, if they are set up to run at their optimal speed. This is why my concern with a configuration that runs a motor designed to go to over 3000 rpm at only 2000 rpm and less. It will never take full advantage of the motor's capabilities.

More specifically, motors with a large working radius and large stator/rotor gap area produce more torque.

While the short duration of drag racing means you can push components harder than in other forms of racing, and much harder than in practical street use, there are still limits. You can't just add voltage to add power endlessly... and even in the range of voltage you can use, more voltage (for a given motor) means more current (it isn't just voltage).

Eliminating gearing doesn't so much lose torque, as lose the opportunity to choose your tradeoff between torque and speed. Since available motors are all capable of higher speed than the car's wheel speed (even at the end of the run), hitting the desired tradeoff will always require some gear reduction.


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## tropes (Jul 24, 2011)

brian_ said:


> Eliminating gearing doesn't so much lose torque, as lose the opportunity to choose your tradeoff between torque and speed. Since available motors are all capable of higher speed than the car's wheel speed (even at the end of the run), hitting the desired tradeoff will always require some gear reduction.


I am thankful for all members of this forum who have done the math and lead me to Plan B which is to revert back to a design which requires some gear reduction.http://www.lightswitchracing.com/round.MOV


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## Karter2 (Nov 17, 2011)

I realise you are not new to the EV drag world, but If you didnt notice them, there are several build threads for Electric Jnr dragsters in the "EV Performance" forum below.
Are you specificly set on going with AC drive, or just maximising the performance of an jnr ev ?
Whilst not strictly a Jnr dragster, this thread has much relavent detail, though using a DC drive..
http://www.diyelectriccar.com/forum...k-therapy-racings-golf-cart-based-104481.html


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## tropes (Jul 24, 2011)

Karter2 said:


> I realise you are not new to the EV drag world, but If you didnt notice them, there are several build threads for Electric Jnr dragsters in the "EV Performance" forum below.
> Are you specificly set on going with AC drive, or just maximising the performance of an jnr ev ?
> Whilst not strictly a Jnr dragster, this thread has much relavent detail, though using a DC drive..


Yes, one of the contributors has set two NEDRA records and built the only electric to have won an NHRA Wally. Very impressive.


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## kennybobby (Aug 10, 2012)

Since the car weight is much less than 1000 lbs, it will accelerate much quicker. For 1/8 mile run, the motor would only turn 114 revolutions, and if an ET was 6.9 seconds then speed would be on the order of 65 mph. The motor RPM would be ~1000, so it would be running up higher on the torque band than previously thought and do it for most of the run.

i would still be concerned about direct drive accelerating the inertia of the tires and wheels, i doubt their dyno testing was done with any significant load inertia. By contrast, a 4:1 differential reduces the reflected inertia seen at the motor by a factor of 4.


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## tropes (Jul 24, 2011)

Since we are no longer discussing AC controllers perhaps we should start an " EV Drag Racing" forum. 
NHRA and IHRA presently allow EV bracket racing but regulate only one class of EV; Junior Dragster. My interest in Jr Comp is based on the premise that the next class to be opened to EV's will be Jr Comp. The present rules would require minor changes to allow EV's.
I have very little technical knowledge but I do recognize it's value.


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## major (Apr 4, 2008)

Hi tropes,

Been watching this thread from the sidelines. The admin here is friendly to EV racers unlike some other EV groups. I'd like to see more EV racing talk here: http://www.diyelectriccar.com/forums/forumdisplay.php/ev-performance-29.html

You'll notice John Metric and some land speed guys post there. 

Regards,

major


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## tropes (Jul 24, 2011)

major said:


> Hi tropes,
> 
> Been watching this thread from the sidelines. The admin here is friendly to EV racers unlike some other EV groups. I'd like to see more EV racing talk here: http://www.diyelectriccar.com/forums/forumdisplay.php/ev-performance-29.html
> 
> ...


Hello major
Good to see we are both still alive. I am being educated by brian, kennybobby and Karter2 but still have plenty to learn. If they are willing I will open a new thread " EV drag racing" in EV Performance.
Regarding President Metric, he should be working with NHRA and IHRA to allow EV's to compete in their classes.
tropes


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## brian_ (Feb 7, 2017)

kennybobby said:


> The motor RPM would be ~1000, so it would be running up higher on the torque band than previously thought and do it for most of the run.


I'm not sure what is meant here. Assuming the tires don't break traction, the motor speed with go from zero to the terminal speed, in direct proportion to the car's road speed. It won't be at some particular speed for "most of the run". It will be in the torque-limited low-speed range for much of the run.

"Shock Therapy" (from the other linked thread) ran a 1/8th mile ET of 6.7 seconds with a trap speed of 101 mph. That's a bit too quick for Jr. Comp, but close, and with a legal trap speed, so it might be a reasonable model of the target performance. At 101 mph a 22" tall tire would be turning (not accounting for circumference growth at speed) at 1500 rpm... so it would pass through 1000 rpm, but it would never get to half the top speed of the motor.

At the originally planned 200 volts, the motor would be torque-limited for about the first half of the speed range; at 400 volts it would be do most of the run in that condition... except that it wouldn't be producing enough torque to get close to a 6.7 sec ET. There's a good reason that the plan has changed to a geared drive.


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## tropes (Jul 24, 2011)

What is your thought on moving to a new thread?


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## tropes (Jul 24, 2011)

*I started a new thread: Hypothetical Jr Comp Dragster under EV Performance*


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