Why Chris Decided on a High Voltage System.

jonescg

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Many folks across several forums have expressed concerns about the high voltage system I plan on using for my electric race bike. Your concerns are not unfounded, as anything over about 300 V DC is dangerous territory. But there are a few good reasons why I have gone this way, and rather than reply to every post around the shop, I hope to explain to you why I made the choices I did.

Motor Power.

I want lots of power for two reasons - more power means faster acceleration and more fun, and secondly, it means even if I only use a fraction of the available power, I won't risk incinerating a rather expensive component. The most power dense motors out there today are permanent magnet AC synchronous motors. Induction motors are bigger, heavier and bulkier, and they all tend to be radial flux which limits how they fit in a bike. So the options were EMRAX, Evo, Carbon, YASA and UQM. All have their upsides and downsides, but I settled on the Evo because it was cheaper than a YASA, more powerful than an EMRAX, smaller than the Carbon. UQM was another option but they effectively stopped selling one-offs to people like me.

The Evo came in two sizes, the AFM130 and the AFM140. Both of these came in two viable winding configurations. Given peak power from the AFM130 wasn't quite enough to sustain the racing duty cycle, I opted for the AFM140 which is 40 kg in weight and 380 mm in diameter.

So with the motor size selected, I needed the right windings. Malte from Evo suggested the #4 winding would be better as it would have more inductance, use lower currents (due to the higher voltages) and generate less heat:

Malte from Evo said:
You are also right that you could potentially use a AF-140-3 machine instead. This would mitigate the need for field weakening but would have two big drawbacks: Firstly, the inductance of the machine is much lower, which translates into a choppier ac waveform and, consequently, magnets which will overheat when operated at higher speeds for a longer period of time. This is difficult to quantify but the risk is very real. Once demagnetised, you will see an irreversible loss in performance, i.e. torque and power will drop significantly. Second, since you have a lower voltage constant on the AF-140-3, you also have a lower torque constant, which means that you need more current to produce torque. For example, the AF-140-4 needs about 350Arms to produce 600Nm whereas the AF-140-3 needs 465Arms to achieve 600Nm.

OK, so the inverter becomes the big issue - the motor is useless without an inverter supplying it with power. Lots of inverters are out there, but I had to settle for one which delivered enough volts to the motor. In the case of the Evo AFM140-4, it needs 565 V AC rms to get to top speed (5000 rpm). Thanks to field weakening, it can achieve the same thing at lower voltages, so the "base" speed is about 4000 rpm, or 450 V AC rms.

How do you produce 450 V AC rms? From an inverter running a DC bus of sqrt(2) times the AC voltage, or about 678 V DC. The Rineheart Motion Systems inverter, PM150D- series can operate from either 360 V DC Bus maximum (PM150DX), or 720 V DC (PM150DZ). In the case of the AFM140-4 motor, the high voltage inverter is needed to achieve a respectable speed.

So how about a motor which can be spun up using a lower DC Bus? Well the PM150DX would be able to produce a maximum motor voltage of 240 V AC rms from a maximum battery voltage of 360 VDC. The #3 wound Evo still needs 424 V AC rms to achieve top speed, but with field weakening it could be achieved at ~240 V AC, but at what current? The PM150DX is limited to 450 A peaks, or 109 kW. Wasn't this inverter supposed to be able to deliver 150 kW? Where did the power go? How about a Tritium Wavescupltor 200? Would it work with an Evo motor? Well yes, but it can't seem to employ field weakening like the RMS inverter, to the top speed and ultimately motor power would be restricted to about 100 kW max.

In the end, I needed a motor which could deliver 80-100 kW continuous. In racing, continuous power is what matters, since you are on the throttle all the time. Getting the battery, inverter and motor package right was my biggest conundrum. Either it couldn't handle the DC bus required, the correct winding wasn't suitable, or the motor wasn't powerful enough.

The AFM140-4 was the best motor option, as it could be driven by an RMS inverter running the maximum DC voltage limit allowed in the rules - 700 V top of charge. Oh, and trying to find a voltage which relies on a number of cells divisible by 4 with the resulting product being an odd number to satisfy the terminations... The combination of all of these factors lead me to believe that the option I went for gave the best power for the least compromise, at the expense of a high voltage.
 
Hey Chris, sounds like a plan. For me anything above 12V gets treated with respect, and definitely above 48V. So you won't catch me bucking higher voltage, since anything useful is high voltage to me. Go as high as you can afford.

You mentioned motor weight and diameter, but what's the weight and volume of the other stuff between the battery and motor? Out of curiosity, what does a race bike weigh, sans the power system (batts, control, motor, cooling system if any)? I'm trying to get a frame of reference of how little a superbike could weigh compared to my 55kg all up beast of an ebike that's all I can handle albeit at much lower top speed.

Just thinking about electric racing is interesting as hell with so much new stuff to explore, since we're not bound to the ICE limitations. Does anyone have regen braking on the front yet...the biggest argument in favor of 2wd? How smooth are the tracks, ie are any the Perfect Road where the unsprung weight of hubmotors is a non-issue? How much better are the wind drag coefficients, since we don't need near the amount of cooling that an ICE requires, or does most of an ICE's waste heat go out with the exhaust?

I hope your bike turns out like you plan.

John
 
Hi John,

My bike is expected to top out at ~205 kg. The motor is 40 kg, inverter is 11 kg, battery will be about 85 kg fully assembled, and the rolling chassis will be about 70 kg. At >150 km/h speeds, suspension is a very good thing. All of our circuits in Australia are pretty smooth, but you still wouldn't do it without suspension. Some epic bumps at the end of the straight at Queensland Raceway were finally removed after 10 years or so, plus the big hump at the end of the pit straight at Winton Raceway... all testaments to the need for some shock absorption.

Volumetrically, getting everything to fit is a hassle. The motor in my case will be situated inside the pivot point of the swingarm. Not concentric (waste of engineering time and money I believe) but placing the drive sprocket just in front if the pivot. This frees up about 100 mm of space where the battery sits. The battery box in my case will be 220 mm by 320 mm, and about 700 mm high. It will slide down into the top of the bike for convenient removal. Ideally it would be tapered at the bottom so you can fit a few more cells in there, but that's for another design. It's also going to be built from flame and arc resistant plastic, I don't want anything shorting on anything!

Areodynamics are no different to any other bike - shithouse. The power I plan on running through this motor will still demand a hefty cooling system for both the inverter and the motor, but not as big as that required for an ICE.

It shall be fun and expensive... And a little bit dangerous.
 
Interesting. One question, why in front of the pivot instead of behind it? Behind means the motor is on the swingarm and chain pull no longer affects the suspension, and at a short distance the effective unsprung weight can't be much.

11 kg inverter + hefty cooling system....you say that and it makes me want add another motor instead. All this talk is making me need to build a bike with 2 of my HubmonsterHE's to see what happens. With half the battery of yours I think it can still come in under half the weight even with a good strong frame. Being a fatty I have to come in way under weight on the bike. Then geared to only to 110-120kph I'd have enough power to match the acceleration you get to experience.

Thinking that way is gonna get me killed though, but I'd at least be able to go hunt up some sports bikes. All the regular motos hide from me now, so riding is getting boring, and I don't have enough to go over to the nearby race facility. It has practice time every morning for just $4 to get in the gate.
 
The motor wouldn't fit on the swing arm. Unless the swing arm was about a foot longer, and the shock would be in the way... Yeah, not worth it.

Hub motors are great for commuters because they free up space in the middle of the bike for batteries. They aren't good for racing. The unsprung mass, the lack of cooling, the PITA changing tyres presents... and they're just slow compared to a decent frame mounted AC motor :lol:
 
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