High Voltage versus High Current

I'm almost afraid to ask... almost... I searched with several key words and tried to digest various threads, but I don't seem to be searching the right thing. If there are two motors putting out a given wattage. One gets there with high voltage (say 120V/10Amps) and the other with high current (12V/100Amps) what are the ramifications for use in an EV project? I'm sure it can be very complicated (after struggling through some of the threads I've read), but I'm looking for that 90 percentile type generalities like are in the "PM Motor theory - formulae etc."

Thanks.

Wattage is wattage--the result in the motor should be identical, since you are talking about battery voltage and current.

Controller sees battery voltage and current.

Motor sees phase voltage and current.

It depends more on how you have the controller setup (particularly for programmable brushless controllers) than what voltage you run.


The main difference between the two is the size (and weight) of the supply wires needed between battery adn controller--hgiher currents will need much larger wires to keep resistance losses low.


the other difference is that if you already ahve a motor that could do either voltage at that power level, is that you run it at a much higher RPM for the higher voltage, so to get the same wheel speed you have to gear it down, vs a lower voltage with higher currents, at a lower RPM, where you might not have to gear it down at all, dpending on your sytem and requirements.

Since the gearing down makes it a little less efficient, there is some loss form doing the higher RPM...but it can make a system more efficient by not having some of the resistive losses int eh cable, the losses in the controller from heat at higher currents, etc.

if you work it out it is possible the losses in either case could be the same. I'm not a math guy, so you'd have to look up the numbers for your particular case and work them out.

As a general rule, particularly applicable to direct drive motors, a direct drive motor can do higher and higher wattages where those wattages are derived more from the factor of voltage than from the factor of amperage.

This line of thinking underlines a general philosophy which holds that it is better to acquire the required wattage by using high voltages and low amperages. Hence the expression, "Volt up, Gear Down" {ie go for high voltage in a small wheel build eg 24", 20"}

Most DD motors can handle the 72v. Not all of them have a winding, or a lacing, or a wiring, that is 60/70/80 Amp friendly.

Example: When I was a noob, I bought a direct-drive motor off ebay. Here are its characteristics:

1. 8*8 Winding. Excellent. Won't need high amps, can handle all the voltage you fling at it, within reason.

2. 26" wheel build. Not good for high draw applications.

3. Long, skinny phase wires. Again, forget about high amps.

What was the best scenario for that motor? Run it at 72v 30A in a 26" wheel, or re-lace it into a 20" rim, rewire it with some 14G wires, and run it at 60amps for some fun.

There are many matters to consider. Terrain. Top speed. Size of the finished wheel. Riding style. Weight of the individual. All of these will dictate how your setup should be, or how well your existing setup will perform.

If you have an X5305 in a 20" rim, then do what you please. Pump in all the amps you want. If you have an X5302 in a 28" wheel, then better to use higher voltage, and much much lower amps.

However, the matter becomes complicated once the user moves beyond the 100v mark, where issues of reliability and safety, not stemming from amperage, begin to present themselves irrespective of the above factors. This is not to say that the issues mentioned above will disappear, but that new problems will present themselves anyway.

Specifically, the matter of controller-reliability, efficiency etc. Moving from 4110 Mosfets to 4115 Mosfets presents its own issues.

With geared motors, most of them "dislike" both voltages and amperages which go beyond 50 units.

Running MAC motors at 72v is asking for trouble, whereas the same voltage would be nothing to an X5 or a 9C.

I read here on this forum that 55v 50A for some of the original BMC geared motors was pushing the boundaries.

Inquisitor said:

If there are two motors putting out a given wattage. One gets there with high voltage (say 120V/10Amps) and the other with high current (12V/100Amps) what are the ramifications for use in an EV project? I'm sure it can be very complicated (after struggling through some of the threads I've read), but I'm looking for that 90 percentile type generalities like are in the "PM Motor theory - formulae etc."

Click to expand...

It gets more complicated when you take into account things further up the chain but for the motor itself it doesn't make any difference (assuming they are wound to run at the same speed).

If you have a steep hill to climb and the motor can take it then you will need to give it more amps, sure run higher voltage and a smaller rim is best but not always practical, a 20" rear wheel looks daft on a mountain bike imo, but maybe someday I'll be converted. Once someone makes a custom frame for 20" wheels, I'll be sticking to 26" wheels.

I use 60 volts and 80 amps into a conhismotor 1kw, I had to rewire the phase wires, it's defiantly a front wheel lifter and because I Love to take the bike to the woods I need the 80 amps to get me up the hills, sure It couldn't take 80 amps battery, or 110 phase for too long, but It's a great little motor so far.

So many things to consider, such as how fast you want to go and what hills you got and how fast you need or want to get up them.

For Electric car, then higher voltages would be necessary for faster speeds but for a 40 mph E-Bike then 60 volts gets me there pretty quickly with the little conhismotor at 85 amps battery and 110 phase.

The only point of higher voltages to me seems to be for those that run smaller wheels and need to get the speed they want, for instance, 100 volts into my motor in a 20" wheel might make it go 40-45 mph and accelerate and climb just as good on 40 amps, the difference being it would run a lot cooler and is always the better way to go if your bike can take a 20" wheel.

When I build my first Ebike, I chose 28" wheel and 48V. It was good for the first time, and fast too, but then I tried a 18" wheel Ebike with the same voltage and the amps were actually less and it was more powerful, but slower of course. And that's why I chose a high voltage and low amp setup. I'm using a 108V 42A controller and a 104V 33Ah battery pack (117.6V when fully charged) with 20" wheels and damn, it's powerful and fast too. Even 10 amps can do insane torque, because it's already around 1000W, because of the high voltage. To be honest, I'm afraid of high amps, I've burnt some wires before, because of the high amps, so I don't need to worry about it here, and the high voltage won't hurt the battery, but the high current does. On fully throttle, one 18650 cell will only provide 4,2A, because it's a 28S10P setup, so that's not much current. The voltage drop is only a few volts. Also the higher RPM is more efficient, the cooling is better too, so I don't know the reason why people stick to lower voltages and raise the amps. My 42A can provide around 4000W, while a 72V system would need 55A and the lower the voltage the more amp is required and of course more cooling to the battery pack and for the wires (or just use thicker wires). For the latest post, I've build a custom 20" full suspension frame, so it's not a problem for me. And for this rim size, motorcycle tires also available, which are designed for faster speeds and they are thicker to be more puncture resistant.

Lower voltage is usually better if the conductors aren't a problem since you will have less losses in switching.

If you want tons of power in a small package higher voltage becomes attractive since otherwise your wires would get really big even for short runs.

What higher or lower means is up to you. Ebikes up to 10+kw can just as easily run 10S as 30S. Pick the right winding on your motor and you will be in business.

Summary is lower is usually better due to potentially higher efficiency and less lethality.

There are many factors to consider. Diameter of the wheel and the Kv of the motor are huge influences.

So let's consider a chain driven motor to a small rear wheel to allow both competitors to spin at high RPMs.

One motor has many turns of small diameter wire, so it is a slow winding which requires higher volts to spin at "X" RPMs. The second example has a few turns of fatter wire for a fast winding (but same copper mass) which will use lower volts to achieve "X" RPMs.

The Alta motorcycles use 355V at low amps, and the Zero motorcycles use about 105V at higher amps.

If they are both sized to run with the same total Watts, the Zero will have less inherent resistance in its motor, because the fatter wire has a much shorter length in its winding.

Higher volts requires more cells in series, but slightly fewer cells in parallel. Lower volts use fewer cells in series, and maybe more cells in parallel, depending on the size of the cell and it's C-Rate.

90 cells in series requires 90 channels on the BMS, and higher voltage components in the controller and charger are less efficient.

Zero uses 28S, so the BMS only needs 28 channels.

A recent curveball is field weakening to raise the top RPMs of a given motor. This means you can lower the volts of the fast winding motor even more, but of course, you would need to raise the amps to still get the same watts.

While watts are watts, with a given motor you will absolutely not get the same results with 10V at 120A as you would with 120V at 10A. The voltage and current limits of a motor depend upon it's construction and how it is wound. It is generally best to run a given motor at the highest voltage it was made for. That's because for the same wattage the motor needs less current.

Heat is the enemy and the main limit of our motors is thermal, so the less heat you make, the better. The bulk of the heat generated in our motors is due to copper resistance, and that heat increases with the square of current. As a result, relatively small reductions in current, pay big dividends in heat reduction. That benefit doesn't apply to just the motor either, the entire pathway of the electricity has less losses with a reduction in current, from the battery and phase wires to the battery pack and controller.

When running higher voltage you want to avoid a motor that has high iron losses. This is heat created in the steel and magnets that increases in a generally linear manner with rpm. This heat occurs any time the motor is turning whether the motor is applying torque to the wheel or not (so even while coasting or going down hill. The key motor specification that gives you this info is it's no-load current. No-load current X the voltage to spin that rpm is the amount of losses (heat) generated simply to spin the motor at that rpm. It gives you good information about how efficient the motor is, and it's an especially important piece of info if you do a lot of cruising at higher speeds.

Resistive losses are comparatively small to whatever portion of your energy isn't going toward propulsion. What is the efficiency of the motor, under heavy load? 70%? The other 30% is dissipated as heat regardless of the volt / amp proportion. If your resistive losses are a significant fraction of your losses you have a poorly designed system.

To make power the important things, ceteris parabus, are the amount of copper and the winding in the motor. The volt / amp proportion is pretty unimportant, with various impracticalities surrounding particularly low / high voltages.

flat tire said:

Resistive losses are comparatively small to whatever portion of your energy isn't going toward propulsion. What is the efficiency of the motor, under heavy load? 70%? The other 30% is dissipated as heat regardless of the volt / amp proportion. If your resistive losses are a significant fraction of your losses you have a poorly designed system.

To make power the important things, ceteris parabus, are the amount of copper and the winding in the motor. The volt / amp proportion is pretty unimportant, with various impracticalities surrounding particularly low / high voltages.

Click to expand...

Struggling to find a correct statement here.

John in CR said:

Struggling to find a correct statement here.

Click to expand...

You're the one who posted an essay last time, so go ahead and correct me.

The Mighty Volt said:

If you have an X5302 in a 28" wheel, then better to use higher voltage, and much much lower amps.

Click to expand...

Only if you want it to peak at 30% efficiency, with the rest dumped as heat.

To get the motor running most efficiently, you're looking to operate at 80% of its free speed. High voltage/fast wind/large wheel doesn't give you a chance of doing that.

I lean towards high voltage.

Pros:

- Fewer losses due to resisitve heating in wiring
- or thinner wires possible, saving weight
- Slower wound motor possible, reducing amps needed for same torque, again reducing resisitve losses (remember those are R*I²)

Cons:

- More complex BMS
- Controller needs higher voltage FETs, increasing switching losses

Anything I missed?