How important is volts vs amps?

[apmorgan93]

On a hub motor, what would be the difference between a 48v 40a setup when compared to a 72v 30a. They're both roughly 2000w (give or take a little bit). I from what I've read while lurking here over the last few months that higher volts lead to top speed and amps lead to torque, but I don't know to what extent.

This is my first post here BTW! I'm building what will hopefully be my first EV (well second I suppose) and am trying to answer questions I havn't been able to answer through google and lurking around on here. I love this site, so much helpful content.

 

[Drunkskunk]

What really matters is watts. 10 volts and 100 amps makes the same power as 100 volts at 10 amps. The watts are the amount of work the motor is doing, and are directly translatable to horsepower. 750 watts = 1 horsepower.

The idea that more voltage increases speed, or more amps increases torque is true, if you are using a specific motor as your baseline. For instance, a 9X7 wound 9C will do 22mph at 36 volts. If you increase the voltage, you increase the speed. So the same motor would do 27mph at 48 volts.
However, you could also have changed the winding count to get the same results. a 11X5 motor would do the 27mph at 36 volts.

The same holds true with Amperage. if your motor makes 60 pounds of thrust at 36 volts, 30 amps, and you increase the amp limit to 40, (1440 watts) it will make 80 pounds of thrust. However, it's still about the watts. You could also have left the amperage at 30, and increased just the voltage to 48 volts, 30 amps,(1440 watts) and gained your wattage increase. now you have 80 pounds of thrust without increasing the amps.

 

[Supertux1]

Welcome to the Forum, it might be helpful to update your profile to include your geographical location since what can be done with ebikes legally and parts wise can be location specific.

Motors are wound to run at a particular Kv, which is RPM per volt and have an associated torque spec as well. More Kv is not necessarily better, it depends on the application.

Some motors have more turns of wire on the stator (or rotor), resulting in a slower speed and a lower Kv, but they pull harder than a motor with fewer turns and a higher Kv.
These motors will accelerate faster, are good for pulling loads and going up hills, but will have a comparatively lower top end speed.

Some people like to take a motor wound for torque and put higher voltages through it to get something that has both torque and more speed, but there are diminishing returns and it's not as fast as starting with a fast wind in the first place if speed is your goal.

This applies to all phase motors no matter what the configuration is, geared hub motor, direct drive, bottom bracket etc...
Bottom bracket motors can utilize the bikes' gears for torque conversion so you can efficiently climb a hill or whip down one at top speed while keeping the motor at its optimal RPM. They are quite popular now.

All else equal, the higher the voltage the better if your system components can handle it.

The reason is that amperage creates heat and heat is wasted battery energy and destructive to components.
As you have discovered, it is possible to have equal amounts of power in the system with less amperage.

Most parts specified for operation at 48v will not handle 72v, especially controllers and the capacitors and FETs in them.

Most beginners are usually looking for speed, but you might be more successful with your first build if you put speed aside and look at other factors such as cost and how to assemble things on the bike for your first build.

 

[apmorgan93]

While speed is important to me, I cant necessarily afford the components to produce the kind of speed I'm looking to attain in the long run. So to make sure I understood correctly, achieving higher wattages with minimal amperage is ideal to reduce heat, but the trade-off is that components cant handle the higher voltage? So in that case do you have to compromise between having a voltage that isn't too high and an amperage that isn't too high to squeeze out the maximal watts?

I'm thinking of starting out with a 36-48 volt system initially, and eventually moving it up to a 72v or higher system once I gain hands on experience and knowledge working on it. I finished assembling a 36v system on a trike back in december and got to take it for a spin. It was fun, but I think the 3 disgustingly heavy SLAs were really holding it back from its full potential. I think it probably was only getting around 15mph or so. Id like to be able to achieve 25mph, and maybe upgrade one day to 35-40. Here is a shot of it in action:
[youtube]https://www.youtube.com/watch?v=jCPgMnBuPvw&list=UUZWL3Opsv_tU_cPk-PvLS0A[/youtube]

Sadly the trike isn't mine. It belongs to the school. Otherwise I'd still be scooting around on it.

 

[d8veh]

The most import thing is to know the speed of the motor at any voltage (KV or rpm per volt). You then match your motor and voltage to get the speed you want, bearing in mind the capability of the motor. The efficiency curve of the motor is also very important. As a general rule, efficiency increasingly decreases as the motor drops below 50% of its maximum rpm (which depends on the voltage). Having loads of current will only make the motor heat up quickly if you're at say 30% of maximum speed.

Increasing the voltage also increases the torque because each pulse of power sent to the motor has more energy in it.

If you have a play with the simulator at www.ebikes.ca/simulator, you'll see what effect voltage, current and speed have on output power and melt-down times.

 

[dogman dan]

There is nothing cut and dried about how much you can overvolt a particular motor. Most of the medium size direct drive motors can tolerate 2000w or so. But pushing them to 3000w can fry them. This is the motor type sold as 9 continent, conhis, Yes.com, muxus, etc.

I'm told the crystalyte H model can take more, but I can't say where the wattage line is on them. Much disagreement as to what motor heating is reasonable. Certainly not above 3000w.

The mac and bmc motors seem to be advised to be run at about 1500w or less.

However, if your needs are for a 5 mile ride rather than 15, pushing 3000w into the 9 continent type motor may work fine for you. It's inefficient, yes, but if the motor runs perky and a 5 mile run doesn't use up your battery who cares? The motor is frying, but you stop before it gets bad. Just don't go for a longer ride is all.

If you want to run 72v, and go farther, then a 25 or 30 amps controller can work fine. Still a decent top speed, but much more sluggish leaving the stop sign. Now you are back into the range the motor is happier with, and could run longer distances with less wasted energy heating the motor to the melting point.

But if you really want a flyweight motorcycle rather than a moped, you need to start with a honking big motor with huge magnets. Often they won't fit into a normal 135 mm wide bike frame. It's stuff that will mean some welding or fabricating will be done. Then you can have a 5000-10000w ride.

 

[MadRhino]

Consider electricity like gravel in a wheelbarrel on a sidewalk :wink:

Volt is speed, going faster with the wheelbarrel will carry more gravel (watts) at the end of the day.
Amp is the size of the wheelbarrel, so carrying more gravel for any given speed.

Then there is work optimisation...
There is a limit to the convenient wheelbarrel size. If it is too big it will need a wider sidewalk or even a street, that is bigger wiring and electronic components. There is also a speed limit, the speed that you don't want to (or can't) do any faster with your wheelbarrel. Optimisation is to find the sweet spot, the combination of speed (volt) and load (Amp) that will give you the best result in work (watts), considering the availability, price, weight, size and reliability of the components required. That is how important is volts vs Amps to best achieve your goal, would it be efficiency, performance, or any other that you may seek.

 

[r3volved]

What really matters is watts. 10 volts and 100 amps makes the same power as 100 volts at 10 amps. The watts are the amount of work the motor is doing, and are directly translatable to horsepower. 750 watts = 1 horsepower.

The idea that more voltage increases speed, or more amps increases torque is true, if you are using a specific motor as your baseline. For instance, a 9X7 wound 9C will do 22mph at 36 volts. If you increase the voltage, you increase the speed. So the same motor would do 27mph at 48 volts.
However, you could also have changed the winding count to get the same results. a 11X5 motor would do the 27mph at 36 volts.

The same holds true with Amperage. if your motor makes 60 pounds of thrust at 36 volts, 30 amps, and you increase the amp limit to 40, (1440 watts) it will make 80 pounds of thrust. However, it's still about the watts. You could also have left the amperage at 30, and increased just the voltage to 48 volts, 30 amps,(1440 watts) and gained your wattage increase. now you have 80 pounds of thrust without increasing the amps.

That was very well articulated

 

[Supertux1]

While speed is important to me, I cant necessarily afford the components to produce the kind of speed I'm looking to attain in the long run. So to make sure I understood correctly, achieving higher wattages with minimal amperage is ideal to reduce heat, but the trade-off is that components cant handle the higher voltage? So in that case do you have to compromise between having a voltage that isn't too high and an amperage that isn't too high to squeeze out the maximal watts?

I'm thinking of starting out with a 36-48 volt system initially, and eventually moving it up to a 72v or higher system once I gain hands on experience and knowledge working on it. I finished assembling a 36v system on a trike back in december and got to take it for a spin. It was fun, but I think the 3 disgustingly heavy SLAs were really holding it back from its full potential. I think it probably was only getting around 15mph or so. Id like to be able to achieve 25mph, and maybe upgrade one day to 35-40. Here is a shot of it in action:
[youtube]https://www.youtube.com/watch?v=jCPgMnBuPvw&list=UUZWL3Opsv_tU_cPk-PvLS0A[/youtube]

Sadly the trike isn't mine. It belongs to the school. Otherwise I'd still be scooting around on it.

Cool trike!

Motors are chunks of metal with wire wrapped up in them and it's fairly safe to run them beyond specification for short periods of time depending on how over-engineered it is and the quality of the parts. I can run my MAC 500W geared hub motor at 1500W (50V @ 30A) during my 10 mile stop and go commute for about a half an hour before the heat gets out of control. At this power range with the weight centered in the bike, it will _almost_ wheelie from a dead stop and hits 28 mph in a block or so at which point it will stop accelerating and settle down to about 700W.

I'm not getting 3x the speed or torque either, diminishing returns the more you push it, a large percentage of the extra watts is wasted as heat because the motor just wasn't designed for it. But it's fun, it accelerates fast, and my commute isn't 20 miles up hill, so I am inefficient and I don't care.

Other 500W motors might fail sooner or might not actually be 500W to begin with. Beware of Chinese Marketing, which often
deceptively confuses peak power with continuous power in anything that uses or produces... power.

They DO make components to handle the higher voltages and current so you don't have to compromise on anything if you know what you want from the start and just buy those higher rated parts and use them at lower power until you are ready for more. There may be a minimal floor for some components where they will be operating inefficiently or not at all if you underrate them too much. Some high voltage controllers might just decide that your lower voltage battery is just too low and shut down to 'save' the pack from an over-discharge condition etc...

Those higher rated parts are also bigger in some cases, which might mess with your mounting options should you decide to start off small and upgrade later. In most cases it's better to buy parts that will let you expand beyond your power limits than ones that will artificially hold you back (or release the magic smoke when you upgrade your power.)

 

[apmorgan93]

Thank you all for the great info so far, this site is great!
So, I've been playing around with the eBike simulator and I've found something that just doesn't seem right. How accurate are the simulations?

 

[arkmundi]

Batteries are measured in volts.
Wires, controllers & components are measured in amp max capacity.
Motors are measured in watts.
They're all interrelated - fewer volts, the less the max current (amps). Generally, select a motor because it can deliver the speed and torque you want (there are cost/performance trade-offs). Then get a recommended controller for that motor. And size your battery to the demands of the motor, not any more than the motor can draw, because batteries are expensive.

GRIN's (http://www.ebikes.ca/) calculator then adds in other factors like wheels size, weight, and road grade to show what you might expect in the way of performance. Its the best such simulator available. So as to conform expectations to real world delivery.

 

[John in CR]

If you want more speed than you think you can afford, then go with the 72V battery, but there are some caveats. If it's a geared hubbie, then stick with 48V, since they can't handle over-volting as well as direct drive hubbies. Big loads, especially long hills can cause heat problems that are much worse at higher voltages. If you need maximum range, the extra 12%+ of energy in the 72V30ah battery may not be enough to offset the higher per mile consumption of running a higher voltage with the same motor. That's because with higher voltage you'll tend to ride faster, and the higher voltage is less efficient during acceleration with a given motor. Higher voltage using the smallest wheel you can live with is the easiest way to maximize performance for a low price.

<edit>
Regarding motor selection, for a given motor there is no such thing as a "high torque" winding. It's only higher torque per amp, but the speedier wind can handle proportionately higher current and is capable of the same torque, period. At a given rpm both motors are capable of exactly the same power (assuming equal copper fill). That means there is a slow wind, and a fast wind. If you have big hills, ride off road, or push big loads then you need to go slower, so the lower Kv motor can better suit your needs. We are more limited in our voltage choices at reasonable prices for controllers, so speed wind motors are actually higher power motors, because they can handle more current with the same voltage limitations.

 

[nicobie]

At a given rpm both motors are capable of exactly the same power (assuming equal copper fill).

Everyone tends to forget this.

 

[John in CR]

At a given rpm both motors are capable of exactly the same power (assuming equal copper fill).

Everyone tends to forget this.

Even the companies selling motors typically don't get it, or they intentionally mislead with the high torque model nonsense.