Mid-mount Hubmonster Madass

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Re: Mid-mount Hubmonster Madass

Postby John in CR » Wed Feb 15, 2017 11:34 pm

You can't run 30kw at 20s without running into heat issues. Heat is the enemy and it compounds, since the resistance of copper goes up by 0.4% per degree C of temperature increase.

Heat is difficult and slow to disperse to the environment, so the one best solution in the battle against heat is to create as little as possible. Steveo runs 400A combined from his controllers and surpasses 100°C with a couple of hard pulls of the throttle. Hillsofvalp did the same thing. At 400A you're creating over 5kw of heat with a cool room temperature motor, and almost 7kw of heat with a 110°C HubMonster. When you can shed maybe 1kw of heat continuously with a stock motor, it's easy to see how heat problems are such a problem for every forum member pushing performance. With winding heat going up by the square of current, heat created decreases much faster than performance as you lower current.

While HubMonster has a lower phase-to-phase resistance than other motors, which makes it better able to handle high current, there's still a finite limit. OTOH HubMonster has such a well designed and constructed iron core and slot and pole combination, that going to higher power via higher voltage costs only hundreds of watts of heat, not thousands of watts like pushing current creates.

I may sound like a broken record about this topic, but going up on current without appreciating the true consequences is a widespread mistake that's easily avoided, especially with HubMonster in a mid-drive since it's rpm limit is way way above the reach of even a 150V controller.
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Re: Mid-mount Hubmonster Madass

Postby Ohbse » Thu Feb 16, 2017 4:36 am

John in CR wrote:You can't run 30kw at 20s without running into heat issues. Heat is the enemy and it compounds, since the resistance of copper goes up by 0.4% per degree C of temperature increase.

Heat is difficult and slow to disperse to the environment, so the one best solution in the battle against heat is to create as little as possible. Steveo runs 400A combined from his controllers and surpasses 100°C with a couple of hard pulls of the throttle. Hillsofvalp did the same thing. At 400A you're creating over 5kw of heat with a cool room temperature motor, and almost 7kw of heat with a 110°C HubMonster. When you can shed maybe 1kw of heat continuously with a stock motor, it's easy to see how heat problems are such a problem for every forum member pushing performance. With winding heat going up by the square of current, heat created decreases much faster than performance as you lower current.

While HubMonster has a lower phase-to-phase resistance than other motors, which makes it better able to handle high current, there's still a finite limit. OTOH HubMonster has such a well designed and constructed iron core and slot and pole combination, that going to higher power via higher voltage costs only hundreds of watts of heat, not thousands of watts like pushing current creates.

I may sound like a broken record about this topic, but going up on current without appreciating the true consequences is a widespread mistake that's easily avoided, especially with HubMonster in a mid-drive since it's rpm limit is way way above the reach of even a 150V controller.


Totally agree with everything you're saying John, just to clarify - I will not be running this on the highway for protracted periods (registered as a moped, so that's a quick way to big fines) and the 30kw *potential* power peak will only be utilised whilst drag racing and perhaps the very occasional blat. In the case of drag racing I will be fully cooling off the motor between runs, which obviously will be limited in duration for <15 seconds (all going well)

I'm very familiar with the numbers around resistance and heat generation and fully anticipate heat issues if I was attempting to run at this for anything other than limited bursts.

My reasons for sticking with 20s are many, but primarily it's around controller selection. With the new generation of Mobipus controllers I do not require the voltage to get the RPM (at the expense of further heat generation of course). I don't really want to compromise the day to day function, safety, charging practicalities etc for a requirement that's only going to come up a handful of times a year. Like my other bike, this is a practical, primary means of transportation FIRST, toy second!

Rest assured I won't be cooking this hubmonster!
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Re: Mid-mount Hubmonster Madass

Postby John in CR » Thu Feb 16, 2017 10:35 pm

Ohbse, I figured you were on top of it based on previous discussions. My post was more for everyone else, because the general thing on ES for the non-pedalists is to tread current settings like a di#k measuring contest. Another irritant is treating phase current peak limits as something belonging in a power calculation, where only battery current (or RMS phase current) is something to multiply by voltage.

Imagine if we could get cheap 250V controllers. I'd cut my current limit down to 100A per controller for a cool running 50kw. :shock:
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Re: Mid-mount Hubmonster Madass

Postby Animalector » Fri Feb 17, 2017 2:09 am

Am I really missing something? I thought the torque was determined by the phase current, if you want acceleration it's torque that is required? A 250v motor running 100A will have less torque at low rpm.

Or are you basing your assertion that a high V is better only in situations where you can get the toque multiplying benefit of some sort of gear reduction?

For a hub motor we're stuck right? So long as we design the peak voltage of the system so that we can reach our target max speed it's only current that's going to get us there faster...right? Taking this same hub motor and throwing 250v across it might give us 50kW but benefits are only going to be higher max speed, at the cost of acceleration torque... Right?

Thanks

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Re: Mid-mount Hubmonster Madass

Postby Ohbse » Fri Feb 17, 2017 4:27 pm

Animalector wrote:Am I really missing something? I thought the torque was determined by the phase current, if you want acceleration it's torque that is required? A 250v motor running 100A will have less torque at low rpm.

Or are you basing your assertion that a high V is better only in situations where you can get the toque multiplying benefit of some sort of gear reduction?

For a hub motor we're stuck right? So long as we design the peak voltage of the system so that we can reach our target max speed it's only current that's going to get us there faster...right? Taking this same hub motor and throwing 250v across it might give us 50kW but benefits are only going to be higher max speed, at the cost of acceleration torque... Right?


That's correct. In my case I can change ratio very easily, with a hub motor this is only achievable by varying the diameter of your wheel.

With my current gearing (30 tooth front, 49 tooth rear, ~600mm tall tire) the wheel torque required to lift my front wheel off the ground (with my weight shifted all the way forward etc) takes about 600 phase amps. As RPM increases you get into your battery current limit and you can no longer produce 600 phase amps required for peak acceleration.

Were I to swap out my 80v pack for a 160v pack with no other changes, it would have zero effect on the amps required to produce this torque, however it could produce that torque to a higher RPM before running into battery current limitation. It would also have the ability to spin the motor twice as fast, but without adjusting gearing to suit that's not really a win. If I doubled my gear ratio at the same time as I doubled voltage, I would also require half the phase amps to produce the same wheel torque, but with my original limitations around speed. In reality, things aren't that simple - as motor RPM increases so do RPM related losses. While low in the hubmonster, they still affect things. You would also have controller issues, as with current technology the sweet spot for switching efficiency is around 80v. As I can't magically fit double the number of cells that fit on the bike, re-configuring for double the voltage I'm halving the AH. That effectively halves my ability to produce amps as well, so there's no inherent power advantage there.

All in all, a hypothetical double voltage/double gearing build would see the same effective acceleration (because I'm limited by lifting the front wheel) and then would run into the same limitations with battery power. It would produce less copper loss heat in the motor, however it would do that at the expense of higher iron losses, bearing losses, noisier, less chain engagement if using a smaller front sprocket or much more expensive gigantic rear sprocket, potentially lethal pack voltages, higher controller switching losses, increased pack complexity around monitoring, charging etc. I also can't buy a super power dense, cost effective, highly reliable controller for 40s voltages - but I can for 20s :)

I have tried to optimise the whole system, not just one or two facets. What I've done here might be completely different for your requirements. This system is pushing the hubmonster to its current based limits, but that's intentional because it makes everything else better.

Here's what a sim looks like for my bike configured with an 84v pack, 16mOhm pack resistance, 30/49 gearing, 600mm tire, 185 total weight (me included), 400a battery current limit, 600a phase current limit for an example.

Madass graph example.gif
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Re: Mid-mount Hubmonster Madass

Postby John in CR » Tue May 16, 2017 11:51 pm

It's mid May and you need to be out riding. You've invested so much time in a great build, and I assure you that once you're going that you'll regret every spare second that you used doing something other than finishing your Madass beast.
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Re: Mid-mount Hubmonster Madass

Postby Ohbse » Wed May 17, 2017 5:28 pm

John in CR wrote:It's mid May and you need to be out riding. You've invested so much time in a great build, and I assure you that once you're going that you'll regret every spare second that you used doing something other than finishing your Madass beast.


Absolutely agree John - the trouble is just life getting in the way :) I am still riding every day on my other bike, because I don't *need* to finish this it's taking substantially longer.

I have also been distracted by changing jobs, starting a CNC business, leasing a building, purchasing machinery etc. I'm also travelling a bit with new role initially so that's pushing pause for a while as I'm on the other side of the planet.
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Re: Mid-mount Hubmonster Madass

Postby macribs » Thu Jun 15, 2017 11:22 am

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