[kennybobby]

[EDIT 1/8/2018]

Index of calculation results:

An aero and friction power load chart is found in post #152.

A chart showing aero and friction effect on ET and speed for 700lbs and 3.3GR is found in post #161.

------------------------------------------------------
Initial post
We need some performance requirements:
Is this a 1/8 mile or 1/4 mile run? Answer: 1/8 mile = 660 ft
How quickly do you want to run, what is your target ET? Answer: min ET = 6.9 sec
Is there a maximum speed limit for this class? Answer: 110 mph
Do you want to include a cooling system or do without? He wants one.

We will get the answers from the previous thread:
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.
Most Jr Comp cars weigh 550 - 600 lbs. plus driver.
Calculate E.T.: 755 pounds and HP of 114 (85 kW)

[edit]

Constants and Constraints:
race distance: 1/8 mile = 660 ft
max speed: 110 mph = 161.3 ft/sec
minimum ET: 6.9 sec
tire diameter: 22" , Circumference = 5.76 ft
weight: 755 lbs; Mass = 23.4472 lb-sec^2/ft
pulley ratio: 10:3
YASA 400 motor torque at 450A: T~360 N-m = 265 ft-lbs up to 3000 RPM at 500V


Preliminary Calculatus Eliminatus:
Acceleration based on ET(min) and distance from x = .5 A t^2 ;
A = 27.72 ft/s2 = 0.86 g
Velocity from A and ET(min): v = At = 191.3 ft/sec = 130 mph exceeds max speed limit, therefore too high.
Acceleration based upon V(max) speed and ET(min):
A = v/t = 23.38 = 0.726 g

Power based upon Force and speed:
P = M A V(max) = 88405 lb-ft/sec = 161 HP

Energy of mass at V(max) from 1/2 M V^2:
E = 304956 lb-ft

Energy of pack: 500V at 500A
E = 250000 kW

Average Power from Energy over ET(min):
P = dE/dT = 44197 lb-ft/sec = 80.4 HP

Bottoms-Up Look:
Need 548 Lbsf to accelerate and meet 6.9 sec ET and not break out Vmax

Torque at axle = r x F = 11/12 * 548 = 502.5 ft-lbs * 3/10 pulley ratio = 150 ft-lbs at motor
And current will be 150/285 * 450A = 238 Amps, easily provided by the 500V/500A pack.
This is all without aero, friction and other losses.

So the pack and motor size appear to have a good margin to meet the requirements with some margin for losses.

 

[kennybobby]

Using 6.9 sec for 1/8 mile run is consistent with your sizing of the motor power and gear ratio for the vehicle weight. The 85kW at 450 Amps would only require 188 Volts, so the 200 V pack might be plenty, but the back emf of the 400 motor is about 165 V/krpm and you will need 3krpm, so that gets you to 500 Volts for the battery pack.

So the cooling answer depends if this motor can survive 6.9 seconds at 450 Amps. The graphs are for operation with coolant at 65C. That's where a decent motor datasheet would list the winding resistance or a thermal time constant--to determine how long it can run at peak torque. Usually the peak torque rating is for 5 or 10 seconds of operation, which would be great in this situation meaning no cooling required.

i wonder if they could make a custom winding for you, larger gauge wire to lower the resistance and reduce the voltage and also allow an increase in current.

 

[tropes]

We need some performance requirements:

Is this a 1/8 mile or 1/4 mile run?

How quickly do you want to run, what is your target ET?

Is there a maximum speed limit for this class?

Do you want to include a cooling system or do without?

This was from the previous thread:
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.
Most Jr Comp cars weigh 550 - 600 lbs. plus driver.
Calculate E.T.: 755 pounds and HP of 114 (85 kW)

1/8th mile.
Target ET is 6.9 sec.
The motor has a liquid cooling system but no radiator.
Maximum driver age is 19 years.

Estimated weight
rolling chassis 460 lb
Yasa 400 motor 55 lb
battery 60 lb(500V)Lone Star Sleeper Cells
cable electronics 40 lb
615 lb
driver 140 lb
755 lb.

 

[tropes]

 

[tropes]

The weight is based on a Mcgee Cams Jr Comp car with a Suzuki engine. I will ask John Metric the weight of his Sleeper cells. Most Jr Comp drivers are former Junior Dragster drivers. Our last driver weighed 95 lbs. The electronics weight is just a WAG. The attraction of the Yasa 400 is the weight and physical dimensions- 4" wide which eliminates the need for a jackshaft and narrows the rear axle. The rule on brakes is difficult to define: "Two rear-wheel hydraulic disc brakes mandatory. With a total car weight of 1,000 pounds or less, and a one-piece rear axle, may use a single brake rotor with dual calipers."
Minimum tire diameter is 20" but I chose 22" because neither Hoosier or M&H make a wide enough 20" (I will stand corrected).

Top/Bottom View:

 

[Frank]

IIRC the 6S sleeper cells are about 2#/brick ~ 1kg/brick. I'm sure you know there's lots of brushed-DC motor solutions that would be a lot more economical than the PM approach. Which "other" thread are you referring to?

thx
Frank

 

[tropes]

IIRC the 6S sleeper cells are about 2#/brick ~ 1kg/brick. I'm sure you know there's lots of brushed-DC motor solutions that would be a lot more economical than the PM approach. Which "other" thread are you referring to?

thx
Frank

Thank you Frank.
I have asked John to join the conversation re his sleeper cells.
Yes, as you know I have had some experience with series wound brushed DC motors.
The attraction of the Axial Flux PM motor is the weight and physical dimensions- 4" wide which eliminates the need for a jackshaft and narrows the rear axle.
The other thread is in Controllers Forum - Axial flux motor dragster.
tropes

 

[brian_]

Seeing the two-stage belt drive in the previous thread, I wondered if you might take advantage of the narrow width of the "pancake" Yasa motor to simplify that to a single stage... and you did. The narrow motor makes the packaging work, and the reduction ratio needed for the relatively high-torque and low-speed motor can be reached in one stage. Looks good!

 

[brian_]

If you can use up to 7500 rpm, with the maximum allowed trap speed of 110 mph corresponding to 1680 rpm at the axle with 22" tires (not accounting for tire growth with speed), then you could use up a drive ratio up to 4.4:1. It may be that a greater ratio is not mechanically practical, due to the minimum size of the motor pulley (to maintain at least the minimum bend radius for the belt) or the maximum size of the axle pulley (for component clearance or other factors)... I'm sure that's already been considered.

There's no need to go with more reduction if you can't use the drive force (due to limited traction), and even with the 3.33:1 reduction you may need to set a lower current limit to prevent wheelspin (assuming that you want to electronically control that rather than depending on the driver's control of the accelerator pedal).

I didn't see a link to the motor: it appears to be the YASA-400 (datasheet)

At 500 volts, the constant-torque range ends at 3000 rpm motor speed, which would be 900 rpm axle speed with this gearing, and 59 mph with the 22" tires... well down the 1/8 mile, but not to the end so the run would finish in the constant-power zone.

If the tires have enough traction to use all 360 lb-ft of drive torque at low speed, that would be 1309 lb of forward thrust (with 22" tires, assuming 11" loaded radius and no losses in the belt drive or tires). That's 1.73 g of acceleration (assuming 755 lb total weight), so 59 mph would come up in only 1.55 seconds after about 20 metre or 48 feet, without rolling or aerodynamic drag. Due to drag it will take longer and cover more of the track... perhaps roughly around the 60-ft mark (reached in 1.633 seconds by Terry in a timing slip that he posted for his "golf cart" drag racer at Ozark). If you have the traction to handle the thrust, this is looking good for a Jr. Comp run.

To finish the 1/8 mile, another 4.2 seconds of constant-power acceleration at 85 kW (well under peak power for 500 volts, but above the cooling-limited continuous power), minus a wild guess of 15 kW (average over the speed range) for drag, adds about 280 kJ of kinetic energy to the 116 kJ of energy at 26 m/s (59 mph), to reach 48 m/s or 107 mph. Calculating the distance covered to see if the 1/8 mile is reached in this time is past my one-line-on-the-calculator-on-Sunday-afternoon level, but it looks promising... it's faster than Terry's time for the 1/8th.

If anyone remembers the link for the online calculator (as I mentioned in the other thread) which produces acceleration given various motor and car characteristics, the data could be plugged into that to predict performance. Both aero drag and rolling drag will be pretty rough guesses, unless someone has suitable data - drag slicks are way off of average tires, and dragsters are way off of typical cars.

 

[brian_]

The motor has a liquid cooling system but no radiator.

I assume that this will mean a coolant reservoir and a circulation pump. The coolant is oil; the flow rate would presumably be the 12 litres/minute specified by YASA. With only a few seconds of operating time under load and only 0.2 L/s of flow, the oil will get only one pass through the motor so essentially the motor inlet will always be getting cool oil.

The coolant temperature will rise to absorb the heat generated; if the motor uses 100 kW average and puts 15% of that into the coolant as heat, that's about 15 kW of heating. If the heat capacity of the oil is about 2 kJ/kg-K, or 1.5 kJ/L-K, that flow rate can absorb 0.3 kW per degree K of temperature rise... suggesting a 50 K temperature rise. The motor is spec'd for 65 °C inlet, so if the oil starts at "room temperature", it will still be almost cool enough to feed right back in the inlet, even after coming out of the outlet after one pass. Add a couple litres of oil capacity, in a metal tank that acts as a slow radiator, and it seems like there should be margin for warm days and repeated runs.

 

[brian_]

From the earlier topic, but still relevant:

... Perhaps the back EMF could be used for regen since this is concern I have. Without a clutch towing would cause the motor to become a generator.

The motor can freewheel, and will if the controller is not trying to regenerate.

You could deliberately use this as a way to get a head-start on recharging while towing back after a run, but my guess is that it would be better to just let the battery cool a bit first. Some regenerative braking could certainly be used to slow down after completing a run, reducing mechanical brake heating and wear.

 

[tropes]

The motor can freewheel, and will if the controller is not trying to regenerate.

Before digesting your valuable information allow me a nostalgic moment.
A 40year quest.

I can assemble a 1350 cfm Holley with my eyes closed but have some trouble with electronics.

 

[kennybobby]

...
If the tires have enough traction to use all 360 lb-ft of drive torque at low speed, that ...

The units for torque on the datasheet were in N-m.

torque is a vector cross product = r x F, and the units are ft-lbs. Lb-ft is an energy unit, just to avoid confusion. The SI units should be m-N to be consistent with the math definition, but they do it wrong and i don't care to try to get them to change...

 

[major]

torque is a vector cross product = r x F, and the units are ft-lbs. Lb-ft is an energy unit, just to avoid confusion. The SI units should be m-N to be consistent with the math definition, but they do it wrong and i don't care to try to get them to change...

The proper Imperial unit for torque is the pound foot or lb.ft., where it is assumed that it is a pound force and need not be abbreviated lbf.ft. The energy unit is foot pound or ft.lb.

In S.I. units I've never seen the meter Newton (mN) used. Nm is used for torque and sometimes for energy or work although the Joule is the common unit for energy.

Even though torque and energy basically have the same units, they are different physical quantities and cannot be equated.

It is easy to find references to support either ft.lb. or lb.ft. for either. I've stated the convention I've seen used in academia and industry for 4 or 5 decades. Really not worth an argument.

Regards,

major

 

[brian_]

The units for torque on the datasheet were in N-m.

Yes, as they should be. I consistently use pound-feet for torque, for consistency with the SI unit and distinction from the energy units. While I agree with major that it is not very important, anyone interested in standard usage might want to consult the Guide for the Use of the International System of Units (SI) from the U.S. National Institute for Standards and Technology. They are not the world's only authority on this, but I note that newton-metres is the usage consistently followed for torque in the automotive industry and every other area that I have seen.

This does mean that both energy and torque (or moment) have the same units... but that's not unusual. There are many cases where two different physical quantities are dimensionally equivalent. If "lb-ft" means energy to you, while "ft-lb" means torque, then you need to consider the context. This is not like the novice problem of confusing power and energy units (or the scam artists practice of deliberately convoluting them).

And yes, pound of force (rather than pound of mass) is implied by context. The stupidity of the pound-slug-poundal mess of force and mass units is hopeless, and context must always be considered when "pound" is used.

 

[tropes]

Back to my level. If the center of the rear axle is less than a foot away from the circumference of the tire, do I measure a gain in torque?
Radius of the tire is less than a foot (11 inches).

 

[tropes]

Rear Axle Parts

 

[tropes]

Gates Poly Chain Belt

 

[tropes]

I assume that this will mean a coolant reservoir and a circulation pump. The coolant is oil; the flow rate would presumably be the 12 litres/minute specified by YASA. With only a few seconds of operating time under load and only 0.2 L/s of flow, the oil will get only one pass through the motor so essentially the motor inlet will always be getting cool oil.

I was hoping to use antifreeze and a small expansion tank. Little heat is generated in 8 seconds.