4000W Compact Mid-Drive Build

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cozy35   1 W

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4000W Compact Mid-Drive Build

Post by cozy35 » Feb 01 2019 2:06am

This is my effort in creating an affordable, powerful mid drive to enable me to enjoy the wilderness sort of effortlessly.

Criteria:
Powerful
Lightweight
Quick Install
Benefit of standard gearing on most bikes.
Reliable

I will be using the Alien Power Systems APS-6384-100KV Out-Runner. They have this motor in both sensored and sensorless. The ESC will be the Open Source Vedder VESC for it versatility. Standard E-Bike Controllers can also be used if the sensored motor option is used.

The APS 6384 100KV has massive torque of 6.9NM at the rated power of 4000W. I used the excel spreadsheet by Jaroslaw „Jaros” Berezowski for a more accurate simulation of power, torque, speed, inclines and range. I have a copy of a couple of years ago and used this successfully on many other electric vehicle projects. It does bit seem to be available anymore ad I had to go back in my backups to find it again.

My mid drive is primarily designed utilizing a two stage reduction drive of synchronous belts and pulleys and I selected the belt sizes with room to spare for stresses.

The drive will have two mounting options:
In the "V" above the bottom bracket for conventional hard tails and soft tails to the down tube and seat stay.

On carbon bikes or full suspension bikes the option will be under the down tube with side plates to anchor on the bottom bracket.

Reduction ration to output sprocket is 27.3:1 driving the 3rd front sprocket of 28T on my bike. I still have the 38T and 48T for front selection and from 12T to 28T on the rear wheel.

In high gear the ratio to the wheel is 6.8:1 which should provide a top end speed of 65KmH at an incline of 5%. Current draw at 58A @ 2110W. On the level at 65KmH, the current draw will be 19A @ 700W.

Pedal cadence will be 135 RPM, so that begs a front freewheel crank. I think for short bursts the average cyclist can achieve 120 RPM. I any case with 4000W power who needs to pedal?

In the low gear the ratio to the wheel is 20.1:1 and a maximum incline of 18% is achievable at a speed of 20KmH. Current draw at 37VDC is 43A @ 1585W.

All the above performance calculations was done using very conservative drive train, motor and battery efficiency numbers and tyre rolling and drag co-efficients.

My initial estimate of weight is around 1.58Kg without the battery pack. The 10S Lipo pack will ad another 2.4Kg.

I am still contemplating twist throttle or thumb, but field test will show the best option.

I will update my progress from time to time.
CompactDriveAss-left.jpg
CompactDriveAss-Right.jpg
If once you have tasted flight, you will forever walk with your eyes turned to the sky.


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cozy35   1 W

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Re: 4000W Compact Mid-Drive Build

Post by cozy35 » Feb 01 2019 8:40am

Here is an example of under the down tube with side plates to the bottom bracket.
Attachments
FrameMount.jpg
If once you have tasted flight, you will forever walk with your eyes turned to the sky.

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galp   100 W

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Re: 4000W Compact Mid-Drive Build

Post by galp » Feb 01 2019 10:35am

You'll have issues cooling your motor. It would be better if front part (where you'll mount your motor) was made from aluminum instead of CF to help dissipate heat from motor stator. I used two similarly sized motors on my longboard build and they got quite hot (>60°C) climbing hills or just constantly cruising at ~30 km/h. Problem with those small motors is that they are thermally designed to be under airplane propeller with huge airflow through windings on stator. Don't expect your motor to output 4 kW constantly without proper thermal management. We could calculate estimated losses in your motor but sadly there's no data about motor's winding resistance. Copper losses are usually dominant over iron losses for this type of motors so later can be disregarded for this estimation.

If we take SK3 6374-149KV for example, the estimated power losses at full current are:

P=Imax^2 * IR = (70A)^2 * 0.021 Ohms = 100W.

If your motor was mounted on this heat sink with 100mm length SK3 motor would heat up to 230°C above ambient temperature. Of course we are talking about constant maximum current. In reality average motor currents are 2-4 times smaller that max.

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cozy35   1 W

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Re: 4000W Compact Mid-Drive Build

Post by cozy35 » Feb 01 2019 11:12am

So what you're saying is that 100W of heat must be dissipated to be safe? (in the case of a SK3 6374 149KV)

This motor is rated 4000 Watts and at most in this case the motor will draw 58 A @ 2150 Watts. I take it that efficiency is lost by way of heat? I have seem some guys 3D printing a centrifugal fan to draw cooling air through the stator. This may be a way to dissipate some heat.

I'm not sure what the efficiency of this particular motor is.
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galp   100 W

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Re: 4000W Compact Mid-Drive Build

Post by galp » Feb 01 2019 12:49pm

(Motor) efficiency is defined as input electrical power / output mechanical power. Their difference are losses that show up as heat.

Using a fan would definitely help. Also using heat conductive material for motor mount (aluminum).

Fastest1   10 W

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Re: 4000W Compact Mid-Drive Build

Post by Fastest1 » Feb 04 2019 9:57am

120 rpm is pretty easy to maintain. Just have to spin an easier gear.


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cozy35   1 W

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Re: 4000W Compact Mid-Drive Build

Post by cozy35 » Feb 04 2019 11:07am

Fastest1 wrote:
Feb 04 2019 9:57am
120 rpm is pretty easy to maintain. Just have to spin an easier gear.


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Yeah, but at the top end that this motor/ratio combo is capable of, (165 RPM) will require a freewheel crank.
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cozy35   1 W

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Re: 4000W Compact Mid-Drive Build

Post by cozy35 » Feb 08 2019 5:14am

OK, so I have decided that reduction drives built out of synchronous belts and pulleys is not the way I want to go, simply because I cannot justify reliability. It just seems that you will forever spend time on maintenance.

My new drive thread is here:
https://endless-sphere.com/forums/viewt ... =6&t=98618
If once you have tasted flight, you will forever walk with your eyes turned to the sky.

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