Hubmotors, Volts and acceleration

I´m trying to get my head around how this really works.

Lets say you have 2 identical bikes, both with a 4 turn motor, same sine wave controller, everything the same except the battery. Bike A is set at 80V 100A, top speed 90 km/h, and bike B is 50V 100A. top speed 65.

The use is hooning around in the bush, lots of acceleration and breaking. speed mostly around 50 km/h max speed 65. Since we will never use the extra speed Bike A is capable of, Is Bike B the better more efficient choice? Will bike A accelerate faster up to 50-60 because of the extra Volts?

Not like I will go for bike B But I´d like to know how it works

VA=W, so bike A is 8KW and bike B is only 5KW. So A will accelerate and go faster.

Your guess is as good as mine. But one rule of thumb will always be, going faster is less efficient than going slow.

Bottom line, enough power to hoon around, lots of grab brake, then grab throttle, is never going to be anything remotely close to efficient. What's the difference?

However, you can easily cross a line, and feed a motor far more than it can handle, at which point it's saturated, and cannot possibly provide more torque. Since we have no idea what motor you are planning to use, we can only guess if either one will be enough to smoke the motor in a few min. So if the smoke comes out, well that one wasn't efficient.

So here's my guess. a huge hubmotor like a cromotor, will handle the higher voltage and amps. But can you handle the bike? I rode a 10,000 watt dirt bike this spring, and it seemed only it's owner could really ride it. He looked a bit funny, having to constantly throw his body weight forward so hard to keep the front wheel on the ground. Me riding it, I could only give in a touch of throttle, or I'd wheelie right off it.

So think a bit about this question. Do you want a fun ride? Or do you want a bike fun for watching your buddies crash it trying to ride it? Efficiency is hardly the question to ask.

I'd think the more fun bike to ride would be one that can be handled a bit easier. 5000w perhaps? 80v of course! But maybe only 60 amps of it. Ride that, then increase amps till you are happy with the way the bike rides.

It's not as easy as Wes states. Peak power is a single point in time. I ran my bike for over a year at 16kw peak input, and the past year and a half at 27kw peak. Low speed acceleration was virtually identical when running the same current limits. That's because acceleration comes from torque, and torque is determined by current...more specifically, phase current. While power input is Vots X Amps from the battery, power output is Torque X RPM.

What will happen in a side-by-side race is that on launch both motors will see the same phase current and have an identical launch. Phase current tapers off all the way to top speed, and up to 20kph or so the lower voltage motor will only see slightly more tapering of it's phase current making early acceleration almost identical with only a slight edge to the higher voltage bike. The gap will then start to widen quickly.

With that type of riding your real limitation will be heat in the motor, and when an identical motor is making more torque it is also making more heat. Because heat created in the copper is current SQUARED times resistance, the difference in heat grows quickly. That makes the lower voltage probably the better choice in your example.

The question, however, that you should be asking yourself is "Do I have that motor in the smallest wheel I can live with?" Put it in say a 15% smaller wheel running the higher voltage, and you'll have markedly greater acceleration across the board, not just in the mid range. Plus, you'll still have a somewhat higher top speed along with greater efficiency (less heat).

Thank you for great answers.

The bike is as of now a Stealth Bomber with a 5403 with an original battery that is not taking the abuse I am giving it at 80A Battery current and 200A Phase amps with the Adaptto E-max. So the reason I ask is because I am getting a new battery built and I´m wondering what voltage to pick. I am also changing to a Mxus 4t motor. It is in a 24" wheel. I know I should run a 20" for the motors sake, but I need the ground clearance, and it would look stupid Maybe a 18" mc rim is acceptable.

The average speed I go is around 50 kph, sp top speed over 90 is not that important, but If the set up that reaches 90 kph is just as fast/has the same torque, and won't heat the motor more than the one that reaches 60 kph, the choice is very easy... But as usual it is not a clear cut answer...

At first, you gain acceleration upping the watts, no matter if it is by upping volts or amps. Then, you reach the limit of power that the motor can pull. A controller is usually set to block current after a very short time, extending that block time does let the motor pull its maximum power for a longer period, thus giving better acceleration. Yet, there is a limit to this, when the motor gets too hot and will fry. After you have achieved the best acceleration from your motor, I mean just before it will fry, the only way to gain better acceleration is to save weight. Every extra pound requires power, as every pound saved is giving you extra power.

arcticfly said:

I´m trying to get my head around how this really works.

Lets say you have 2 identical bikes, both with a 4 turn motor, same sine wave controller, everything the same except the battery. Bike A is set at 80V 100A, top speed 90 km/h, and bike B is 50V 100A. top speed 65.

The use is hooning around in the bush, lots of acceleration and breaking. speed mostly around 50 km/h max speed 65. Since we will never use the extra speed Bike A is capable of, Is Bike B the better more efficient choice? Will bike A accelerate faster up to 50-60 because of the extra Volts?

Not like I will go for bike B But I´d like to know how it works

Click to expand...

Simple generalizations are useful but the devil is in the details, and some understanding of those details is important to answer these types of questions. Perhaps the best way is to spend some time with a good simulator, such as the one at ebikes.ca, our forum owner's business website.

The Motor Controller is a power converter, it converts the battery power (voltage and current) to the (mostly lower) effective voltage and current to drive the motor, as well as other secondary things like making AC and commutating the motor, which we will ignore for this discussion.

The most important value was not mentioned, that is the motor current (also called phase current) limit. Motor current does essentially two things, it creates heat (via I squared R of the windings), and it creates the magnetic field that produces torque. At low speeds the back EMF of the motor (that's the voltage the motor makes as it turns that opposes the battery power) is low, so the power required to make motor current is low. Assume for a moment that the motor current limit is 250 amps just to have a value for this discussion, and using the Clyte 5403 mentioned later in the thread with the simulator.

The operation of the motor can be divided into different regions where different parameters limit the performance. In the lowest speed region, from 0 up to some value, the limiting factor is the motor current limit. This is determined by the motor current limit setting in the controller. This is the highest torque region. If we look at the simulator in this region we see that this particular simulator doesn't implement the motor current limit feature (Justin you might want to add that to the simulator, it is important in the higher power systems). So the reality is that the torque from 0 to some speed the motor current and the torque produced will be flat at this maximum torque value. This is the constant torque region. Increasing battery voltage or battery current will not change the torque in this region, but the battery voltage and current, or power limit will change the speed at which this region ends. Higher voltage has no effect here. This region ends around 5 mph for this setup, when the motor current drops below the motor current limit, assumed to be 250 amps here.

In the next region the torque falls as the limiting factor is power. This is the constant power region, and it extends from about 5 mph to 41 mph in this example. In this region the battery voltage and battery current limit set a maximum power, and this limits the system performance. As the back EMF of the motor increases with speed, the motor current drops, so the motor power input stays essentially constant as the torque drops. This region ends when the motor back EMF rises to the point that the battery voltage can no longer push the battery limit current into the controller. You can clearly see this on the simulator as a small inflection in the torque curve, as well as the peak motor power point. The speed at which this occurs can be raised by increasing the battery voltage or lowering the battery current limit.

The third region is the diminishing power region where the system doesn't have enough battery voltage to fully utilize the battery current limit. The actual maximum velocity typically occurs in this region at the point where the motor output power crosses the system load line. This occurs just over 48 mph in this example.

Now if we drop the voltage to 50V and change nothing else in the simulator we can see what happens.

Now the motor current at zero speed is only about 210 amps, so the constant torque region disappears altogether. The constant power region extends to about 23 mph (instead of the previous 41). The maximum speed drops to about 33 mph (previously 48).

It is no surprise that an 8KW system has higher performance than a 5KW system. It also stresses the controller more, in both voltage and current. The losses at higher voltage and higher current in the controller are slightly higher, but these are not a significant fraction of the system losses, most of those are in the motor at the high torque end of things that occur at low speeds.

Making torque requires current which makes heat in the motor. The battery voltage and current don't change the heat the motor makes for a given torque. Only the motor or gearing can change that. There is a detailed discussion covering the heat to make torque factor here on ES, if you want to dig deeper into that.

Simple answer for now is, get a thermometer on the motor.

Then you can see which one overheats faster. Run the cooler way. You want to cooler way, because the type of ride is going to make it heat more than cruising. You'll see immediately if you are gaining performance you need, or just heating up the motor too fast to have a decent ride time. John is definitely right though, running cool will not be near as much torque. Find the happy middle, between having fun performance, and the ride length you want without overheating the motor. Don't be afraid to run a warm motor of course, just don't got to 400f in 15 min is all I'm saying.

Try many combinations if possible, using a CA to limit amps. Find the right combo to run your battery out, riding the way you do, and keep the motor under 250f, or at least that cool for much of the ride. See what I mean, if you reach 250F in 5 min, that is not right. Ideally, you'd reach 200 fairly quick ( having good performance that way), but then a much slower climb in temp past that.

Again the idea is to run your battery dead, before you'd smoke a motor. Your motor is big and tough, so it should take awhile. I'd definitely be looking at 80v with that motor. So I think that's a bit of a no brainer, just find the amps at that voltage that works good for you.

In my own crude way doing the same thing with dinky motors, I found I could run 72v 40 amps fine all winter, but in summer I needed to limit amps to 20 amps, to avoid smoking the motor. So weather does play a part too. Go for the 80v I say. Then limit amps as needed, whether for motor heat, or for it being ride-able. If you can't keep the front wheel down, turn it down. Make it ok to ride normal, and ride cool enough to the end.

Thank you, get as much voltage as the controller/motor can take and monitor this with the motor temp, sounds doable

Very enlightening Alan!

Running a higher than necessary voltage results in excess heat. Inadequately addressing heat results in failures in the worst case or having to interrupt your rides to let the system cool down, which takes a long time, in the best cause.

But then again more Amps than necessary is causing even more heat right? So either way I loose...

Higher voltage causes no extra heat in the motor (current causes heat in the motor, not voltage).

Higher voltage causes very small heating in the controller. Not really significant heat until higher voltage FETs are required, then it causes more since the higher voltage FETs have higher R values.

The heating in both the controller and the motor come primarily from current via I squared R.

Excess voltage makes it easy to allow too much current or power. But if the current limits are properly set, you won't have excess current or power. That's what the settings are for. If you raise the voltage and don't change the current limits, then, yes, you are going to get more heating. But it is from the current, not the voltage.

...And higher voltage results in higher current for a longer period of time. At WOT it results in higher current from 0 all the way up to top speed, and the difference is most pronounced in the mid range. It's a result that's virtually impossible to control by more conservative use of the throttle. If you don't believe me, just look at the sims, and everywhere there's more torque, and that means there's more current and therefore more heat. The OP wants to go "hooning around the bush", which will put him operating most of the time in the rpm region where excess heat is easiest to make when running a higher than necessary voltage.

Exactly. But I think for his motor, 80v is not an unusually high voltage. So I'm saying go 80v, but watch thermometer in the motor, and find the sweet spot for amps.

But if he has no need for the speed 80v will give, then sticking to 48v makes sense. I run 48v 40 amps through old 5300 clytes for a cargo bike, and never see a hot motor ever. Not even on a big mountain pass with a full load. It's tuned for efficiency, and I don't want to run 40mph with a full loaded cargo bike. But it could climb that pass just about as good on 72v 30 amps. What I get from 48v is a more controllable throttle when riding 15 mph.

How much making too much heat he can stand, and keep the motor cool enough depends on the battery size. Small enough, he runs out below 250f. But obviously, any combination that shoots to 250f in the first five min is too much. Look for something perky enough to shoot to 150f immediately, but then go into a slow climb towards 250. Running at 300 is possible, but again, look for that slow creep up after the initial heat up. Ideally, run a setup that reaches equilibrium below 300f, and just sticks there after that.

Weather plays a part. with the common 28mm motors, I ran 40 amps in winter, but had to drop to 20 amps in summer. Summer riding did not suck, but for sure I got up hills faster in winter. Every year I'd smoke the halls in a motor, running close to 300f in the spring. I'd just want one more day on 40 amps.

As Dave says, you control the heating with the battery current limit setting of the controller, don't depend on the throttle for that.

If you raise the voltage and lower the current setting of the controller so the product is the same, then it will deliver the same power to the motor, it will just be able to so at a higher speed before the power falls off due to back EMF (no need to try to control that with the throttle). If you don't change the battery current limit settings, then yes higher voltage will increase power and heat in the motor.

Note also that the sims don't model the phase current limit, so they don't show the flat torque section starting at 0. Click on the graph and you can see the motor current there, the model has no input value for phase current limit so it doesn't represent that part of the operating curve properly.

John - a lot of the experiences you're referencing (while valuable) are artifacts of controllers that do not measure or limit phase amps correctly/at all. The pack voltage being higher in the case of the Max-e will allow for a higher RPM, however below that point it's all about the phase amps. The power source being at 40v or 80v will have no impact at low speeds. Given that you're spending a good amount of time >30km/h high pack voltage will be beneficial.

Given the constraints of the Adaptto hardware you're going to be best off building a 20s pack for your usage. It makes for the most power delivery while keeping a safe voltage margin on the FET's and it makes the BMS wiring very simple. Dropping the pack voltage will have negligible impact on motor temperatures, reducing the phase amps you've programmed will have a big impact - regardless of pack volts. You can also ensure that you've got a temperature probe in the motor and configure current roll back in the controller, this will ensure you can find and maintain some sort of equilibrium. If you can improve cooling, the controller will allow a bit more fun all while ensuring you don't melt things.