does acceleration change with more voltage?

[jimmyhackers]

i use brushless hub motors, some of my friends now too.

im wondering something, as the general rule of thumb with them is more voltage is more top speed and more amps is more torque.

but say if i run my 1000w hub at 48v (2x6s) then at 72v (3x6s). will the 0-30mph time be the same?

another way to put it is, am i loosing any umph (torque) from running a lower voltage?

 

[john61ct]

No, for a given motor, the lower top speed winding means more power is available for torque

the whole curve is "compressed" so each segment is "denser".

 

[pwd]

I'd recommend playing with the motor simulator to help understand the differences:
https://ebikes.ca/tools/simulator.html?bopen=true&batt_b=B7208_DT

or

https://ebikes.ca/tools/simulator.html?bopen=true&batt_b=B7208_DT&cont=C40&cont_b=C40

It depends if the controller phase amps remain at the same level and if the limit was being hit in the first place

 

[Chalo]

Most controllers can send higher current to the motor (phase amps) than what comes out of the battery (through the magic of electronics and P=I^2R). Typically this is limited to about twice the battery amps.

In principle, this means a higher voltage system should accelerate at least a little bit better at the same battery amps. Initial acceleration will be the same, but it will last longer before tapering off.

 

[E-HP]

i use brushless hub motors, some of my friends now too.

im wondering something, as the general rule of thumb with them is more voltage is more top speed and more amps is more torque.

but say if i run my 1000w hub at 48v (2x6s) then at 72v (3x6s). will the 0-30mph time be the same?

another way to put it is, am i loosing any umph (torque) from running a lower voltage?

It will accelerate harder longer. If the motor is only capable of 30 mph, it will only inch it's way up once you get past 25 mph or so, since the torque curve plummets. If you up the voltage so the bike is capable of 40 mph, it will hit 30 mph a lot faster (double the torque at 30 mph in the example below), provided your battery can feed it.


https://ebikes.ca/tools/simulator.html?motor=Leaf%205T&batt=B4823_AC&cont=cust_70_200_0.03_V&hp=0&axis=mph&frame=mountain&autothrot=false&throt=100&grade=0&bopen=true&cont_b=cust_70_200_0.03_V&motor_b=Leaf%205T&batt_b=cust_72_0.05_24&hp_b=0

 

[donn]

Something that may be missing here just because it's so elementary that our illustrious panel just takes it for granted:

the voltage from the battery, is not the voltage to the motor.

The controller takes whatever from the battery - 36V whatever, doesn't matter as long as the controller supports that input voltage - and converts it to whatever motor voltage is indicated by the throttle setting. Voltage = speed.

Give the system the same current limit, say 30A, with a 36V and a 72V battery, and you get twice as much power - but you didn't mention any current limits.

I didn't really follow Chalo's story about phase amps vs. battery voltage, that may be a wrinkle on my possibly over-simplified battery current conversion.

 

[Chalo]

That's phase amps versus battery amps, and what you're talking about is what I'm talking about. The controller can trade volts against amps up to a point (usually 2x battery amps), and those increased amps create increased torque (and heat, and energy consumption).

Bottom line is, higher volts provide more torque for at least part of the motor's output curve, and should result in better acceleration. Generally speaking, they'll also have a negative effect on efficiency by running the motor in a lower part of its RPM range for more of the time.

 

[E-HP]

That's phase amps versus battery amps, and what you're talking about is what I'm talking about. The controller can trade volts against amps up to a point (usually 2x battery amps), and those increased amps create increased torque (and heat, and energy consumption).

Bottom line is, higher volts provide more torque for at least part of the motor's output curve, and should result in better acceleration. Generally speaking, they'll also have a negative effect on efficiency by running the motor in a lower part of its RPM range for more of the time.

Combining the concepts, using a lower throttle position on the higher voltage system, to match the same speed of the lower voltage system (so that the phase current is the same), the efficiency should be about the same as well. Using the same example above, and throttling down the higher voltage system to 31.6mph, yields about the same Wh/mile (a little more efficient in this example):



One downside of the higher voltage is a very touchy throttle, which translates to more unusable torque down low. Even with only partial throttle, you can see the higher voltage system will out accelerate the lower voltage system, to arrive at the same speed.

 

[MadRhino]

Higher voltage will improve acceleration some, but at the cost of lower efficiency. The faster the bike, the lower its efficiency at low speed. You don’t want to build a 70 mph bike that you will ride within 0-30 mph. High power is a much better way than high voltage, to build acceleration performance.

 

[john61ct]

Would it be practical to buy a powerful motor spec'd for say 80+ V, and speed-limit it in a low speed high-torque use case, by using a controller designed for say 48V?

So you could repurpose it for a higher speed use case later on by swapping in a higher voltage controller and battery?

Would the former rig still get that efficiency advantage?

 

[Chalo]

Partial throttle always costs efficiency, like for like.

 

[MadRhino]

Would it be practical to buy a powerful motor spec'd for say 80+ V, and speed-limit it in a low speed high-torque use case, by using a controller designed for say 48V?

So you could repurpose it for a higher speed use case later on by swapping in a higher voltage controller and battery?

Would the former rig still get that efficiency advantage?

Motors are not voltage limited. If you feed it 48 v and its speed is enough, that is the right voltage for your usage. Torque is another story, directly related to watts and wheel size. Feed more watts or use a smaller wheel (or both) to have better torque.

 

[john61ct]

Sorry if my question was not phrased clearly, or my noob brain isn't grokking

Higher voltage will improve acceleration some, but at the cost of lower efficiency. The faster the bike, the lower its efficiency at low speed. You don’t want to build a 70 mph bike that you will ride within 0-30 mph.

Trying to break down above use of "bike", disregarding torque.

Bike A at 48V, needs only 20mph

Bike B at 72V to go 40mph

Obviously the battery and likely controller will differ.

But can I buy just one motor for both?

You say that using the higher voltage means lower efficiency, but is that still true in this scenario?

 

[john61ct]

Partial throttle always costs efficiency, like for like.

So does that translate to

Design the bike so it **can't** go much faster than your desired top speed?

not just for efficiency but that also optimises torque?

 

[ZeroEm]

by john61ct » Aug 12 2021 5:43pm

Sorry if my question was not phrased clearly, or my noob brain isn't grokking
MadRhino wrote:
Higher voltage will improve acceleration some, but at the cost of lower efficiency. The faster the bike, the lower its efficiency at low speed. You don’t want to build a 70 mph bike that you will ride within 0-30 mph.
Trying to break down above use of "bike", disregarding torque.

Bike A at 48V, needs only 20mph

Bike B at 72V to go 40mph

Obviously the battery and likely controller will differ.

But can I buy just one motor for both?

You say that using the higher voltage means lower efficiency, but is that still true in this scenario?

They answered correctly

Let me have a crack at it, once it clicks it's easy with tools.

For Bike A and B. Build for bike B 72V, motor and controller will not have a problem switching back to 48V. In the real world it will not double speed. More like 25mph and 35mph

Example with out details: I have a 48V-72V 40a controller and 1500w motor in 26" wheel. Say I have a 48V battery that I can pull 20amps. Say the bike's top speed and efficiency is ok but I want more torque/acceleration.

Best option here is to double the battery amps up to 40a. Better torque/acceleration and efficiency does not change.

Another option is a 72V 20a battery (just raising the volts for example) This battery will work with the same motor and controller. You will get better torque/acceleration, not as much as you think or as more amps provides. Here is where you take a efficiency hit. You may only use the lower half of the motors rpm range and may not have enough amps to hit your top speed.

A 72V 40a battery will raise both and get the best torque/acceleration with lower efficiency unless you make other changes.

To get the most out of higher volts, bring your speed down with a smaller wheel if that is not enough higher Kv motor to your top speed.

The best bang for your buck is amps. You may need a new controller, wiring mods. still easer, cheaper than more Volts. 48V equipment is cheaper than 72V.

Hoped this helped?

 

[jimmyhackers]

those graphs help a lot.

some gains in power and torque...minimal loses in efficiency.

im contemplating adding my random spare 6s to my other two for a 72v on my bmx thing.

it has a 3-pos speed function now. so i can turn it to mid or low and bypass the twitchy throttle problem if it happens.

 

[ZeroEm]

I don't worry about efficiency outside the fact that efficiency is usable power and not just turning watts into heat.
I do focus on torque to pull me up the biggest hills here at speed, without pedaling to hard.

When I built my trike I wanted to keep my 26" wheel but wanted more torque and did not want to go above 40amps and around 28mph but not below. So I went with 72V and ordered a motor with a 7T to lower my speed and raise torque within the 40amp limit. It worked out well and still don't want to change it.

Now i'm on a new build and want to use parts I have so i'm going with a smaller wheel to raise torque/efficiency and lower my speed but i'm not happy, the speed is still to high and wheel is to small.

My choices are new batteries with lower volts to run a bigger wheel or new motor with a higher T count. This will be a road bike looking for 100+ miles at cursing speed at 15-28mph but may want to ride up 35 or 40 on stretches' of road not friendly to bikes.

Its all about what you want. You could up your Volts and get a small wheel and or motor that better fits what you want. Motors are not all the same Kv so use a simulator to figure it out.

 

[E-HP]

those graphs help a lot.

some gains in power and torque...minimal loses in efficiency.

im contemplating adding my random spare 6s to my other two for a 72v on my bmx thing.

it has a 3-pos speed function now. so i can turn it to mid or low and bypass the twitchy throttle problem if it happens.

That's sort of what I have on my bike. Main pack is 52V 28Ah. Series pack is 6s 24Ah of lipos (three 6S 8Ah paralleled). Even in low the throttle is twitchy, but you can get used to it (without taming it with the Cycle Analyst). Middle or High, pretty uncontrollable. I use the CA custom settings, plus the three way switch to control how much power I want. I have Unlimited mode on the CA set with pass through throttle (have to lean forward and slowly apply throttle), and all other modes with CA controlling the ramp.

It's really the mid range torque that make the bike fun to ride in either medium or high, and even to some extent on low.

 

[Chalo]

Partial throttle always costs efficiency, like for like.

So does that translate to

Design the bike so it **can't** go much faster than your desired top speed?

not just for efficiency but that also optimises torque?

Running WOT at cruise speed is not only a good way to maximize efficiency, but it makes a maximum amount of torque available from a limited (some might say reasonable) amount of current.

I mean, that's what I do. Some others here drive their motors to near saturation at high voltage, and only have partial throttle as a way to make their bikes usable for normal riding. I like to decide how fast I want to go, how much power I'm willing to spend, and reconcile the two things.

I'm heavy and tall, my bikes are heavy and tall, so it's an energy intensive thing for me to get around the city. With all the stop and go, I usually end up near 40 Wh/mi consumption just tootling around at about 20 mph top/cruise speed. So I guess I have more incentive than most to keep efficiency high and only have as much performance as I want to use.

 

[MadRhino]

Yep. Build a fast bike to go fast. If your goal is range and economy, you don’t want a fast bike. It would cost more and set you further away from your goal. That is because fast bikes are consuming a lot, not only when they go fast but also when riding slower than their efficient zone. When you ride at 50% efficiency, you double the power required, because half of it is only making heat. It is not going fast that kills an electric motor. It is loading it out of its efficient speed zone.

People are not conscious of their average speed. They remember the fastest they got. But, most would be surprised of how slow their average is.
A friend shows me the fastest speed reading on his Cycle Analyst: 103! Ahh! I set it to show average speed: 21! Ooh! You are a rabbit my friend.

 

[jimmyhackers]

that 50% efficieny turned into heat made me go and look back at the graphs and ponder stuff.

from stand still.....it says 0% efficiency..... or at 5mph its only 21% efficient?

i guess that why manufactures only give you peak numbers.... like bhp, wattage, mpg, mpw. (graphs need to be seen for fuller pictures)

i wonder how that translates to things like telsa etc.....that say things like their cars over 90% efficient.

when in actual fact a tesla in 5mph traffic might be less efficient than a petrol car.

 

[ZeroEm]

when in actual fact a tesla in 5mph traffic might be less efficient than a petrol car.

That does not hold water. believe me ice is not efficient at any speeds. They are best at speed.

 

[E-HP]

that 50% efficieny turned into heat made me go and look back at the graphs and ponder stuff.

from stand still.....it says 0% efficiency..... or at 5mph its only 21% efficient?

i guess that why manufactures only give you peak numbers.... like bhp, wattage, mpg, mpw. (graphs need to be seen for fuller pictures)

i wonder how that translates to things like telsa etc.....that say things like their cars over 90% efficient.

when in actual fact a tesla in 5mph traffic might be less efficient than a petrol car.

If you're looking at the graphs above, to look at efficiency, at anything but top speed it's the efficiency during acceleration WOT, so you spend about a 1/2 a second at those speeds. So where the Leafbike 5T is about 83% efficient at top speed (42.3mph) and WOT, it's around 67% efficient at 5mph and over 80% at 15mph. I usually ride at around 20mph, at which point it's more efficient than WOT due to wind resistance. That to me is a decent trade off for having ~40% midrange torque advantage over running a lower voltage.

84% is likely to be more efficient than my MXUS motor is at any speed, so upgrading to the Leaf motor will definitely extend my range, and still have some killer torque, without having to go with a wider heavier motor.

(these graphs are using my battery setup and controller)





PS. If the load is fixed, then the motor is most efficient at top speed WOT, but load isn't' fixed and increases with speed, so in the real world, the bike/system is more efficient at speeds around 20mph where wind resistance is less of a factor.