Optimizing DD motor and moped rim for 60km/h and 2.5kW (and other questions)

[chadez]

Hi all,

I'm planning a non-bike project involving hub-motors and found this forum to have the most knowledgeable and helpful members regarding this half of the topic, the other half being The Forestry Forum. What I'm thinking of is a hub-motor powered bandsaw sawmill. I have a fairly good understanding of basic electronics and I'm aware of the issues regarding constant power supply and delivery needs for such a high-amp, continuously running device. What I'm not so familiar with is how to match a motor/rim/controller for optimal performance.

The simplest example of a similarly built machine I could find is from a Youtube video
the difference being that the wheels are built using spokes (I’m planning on casted rims) and driven by an induction motor:

My specs for the time being:

-2 identical motors and controllers running the blade
-Average continuous motor power estimate 2 - 2.5kW per motor
-Peak power about 3kW per motor
-Tire diameter 400mm+
-(blade) speed max 17m/s (60km/h, 38mph). In practice this might be lower as 5kW might not utilize the cutting capabilities of the blade sufficiently

I have been thinking about this motor https://a.aliexpress.com/_mrrLnj4

Specification:
Material: Aluminum alloy
Size: 16 inch
Maximum Efficiency: 88
Voltage: 48v/90v
Power: 500W to 3500W
Speed : 50KM/H to 120 KM/H
Sensor: For Hall
Waterproof Rating: IPX5
Maximum Torsion: 75N.M
Magnetic Poles: 26

for the following reasons:
-the shaft thread claims to have M16 thread whereas many motors seem to have a thinner axle
-the motor vs rim ratio seems larger than in some similar category motors
-the dimensions and specs are provided (assuming they match the product), I could design further before I get the wheels


Some of the questions I still have in my head are:

1. What tires would fit the rim? 2.25X16? some metric size maybe?
2. Are there better known motors with similar specs that I could use to estimate the performance of this one?
3. Is heating likely to limit the average power from 2kW?
4. Is this motor just not going to cut it? Any better suggestions? Do I need to gear down to meet my specs?
5. Do I need a driver matching the pole count (26)?
6. Should I have the beefiest driver or is there a benefit (other than price) for opting for a more tightly specified one?
7. How does the Voltage/Power/Speed specs add up, or is it just a marketing spec? (Going from 48V to 90V shouldn't double the wheel speed, yet speed spec is 50km/h to 120km/h)
8. How much power can I expect to get at about 60km/h?
9. Can I get the power (of previous question) by setting the speed to 60 (assuming the voltage would allow more) and just feeding the blade until I reach the wanted power, or would I need to throttle up as the load increases? (Now that I rethink it, I’m asking if there are controllers with speedometer)
10. If I need to slow down from my maximum speed, is there a difference between using lower supply voltage and full throttle vs. higher voltage and limiting the speed/power with the driver? Of course the speedometer function wouldn't work if there wasn't any room for added power.

And some more general questions:

• Where could I find dimensions (mainly OD) of different inch sized moped/scooter tyres? Metric ones I have understood I can calculate.
• What's the maximum pressure for a moped/scooter tire? (Will eventually limit the blade tension: a typical blade tensioning on steel wheels is around 15000 psi!)
• Can I make or buy a completely solid tire and get it balanced well enough?
• What’s the typical thickness of an e-scooter hub-motor shaft?
• What's the size of the shaft bearings on a hub-motor with M16 thread?
• Am I asking the right questions?

A lot of questions, any help would be greatly appreciated!

BR, Pauli

 

[Vbruun]

Why do you want to use hub motors? Induction motors are way cheaper and run on the right voltage for grid use.

 

[nicobie]

Maybe for off grid use?

Why do you think you need two motors/controllers? Just get a stronger hub motor. My big bandsaw only powers one wheel. Are you planning on using a bicycle wheels?

 

[chadez]

I'd like to use hub motors (electric scooter wheels) mainly for mechanical simplicity. They would be a lot lighter than an induction motor of similar power, doubling as band wheels. I would get rid of the drive belt and associated tensioning mechanism. Most often these machines are built using a small petrol engine for its high power/weight ratio and portability but those are noisy and smelly to work right next to and add greatly to the mechanical complexity of the saw unit.

I thought by using 2 motors instead of a single beefier one I could manage the heat (am I wrong in this?) and currents a bit easier, plus maybe in the lower power range I could find common and cheap motors more easily. This being said, I'm open for other suggestions too. I also am not sure if two driving motors even is feasible, or does blade tracking require an idling wheel.. That's probably a question to ask elsewhere.

Powering will clearly be where the complexity shifts to. I have a pile of 48V server PSU's I could use, or maybe modify a forklift battery charger for constant voltage output. For possible off grid use I could build a little less noisy petrol generator unit by modifying high power alternator(s), buy a used forklift battery or try to step down voltage from an electric car.

 

[amberwolf]

Since the two motors are directly connected via the band, the wheels will spin at the same speed at the tires if the tires are identical. But if they are not, or the motor characteristics are slightly different, then whichever one is a bit faster will have to provide most of the power, but you can manage the speeds so they are equal.

If using brushless motors you'll need separate controllers to properly drive them...except that if they are physically locked together so there can be no slippage, and the phases are aligned correctly (rotor magnets vs stator teeth/windings), a single controller could run both in parallel, reading the hall sensors from only one of them. A sensorless controller *might* work on this, too,

If using dual controllers you can use speed sensors on both wheels and a feedback circuit that compares them and adjusts one of the throttle inputs to match the other one's speed. (arduino or similar could probably do it cheaply, just needs one analog output and two digitals).

If using brushed motors you can wire them both to one controller so they either share current or share voltage, so if they are identical windings/etc they can be run at the same speed; dual controllers could be used with a speed sensor feedback to make them both the same speed regardless. Brushed are less efficient, get hotter and wear out brushes and commutator and bearings, whereas brushless only wear out bearings someday.


In either motor type you could create a forced-air ventilation system with opened up side covers for extra cooling, as long as you can keep the wood dust/etc out of them. (put the intake somewhere outside the area the unit is used).

 

[chadez]

I think we can concentrate on brushless options. I was thinking to simply measure current fed to the controllers, compare them and adjust throttle accordingly (manually or automatically) to keep motor loads symmetric.

I'm afraid the wood dust will be everywhere... The motors are literally in the middle of the dirtiest compartment of the machine, no way to seal them off.