High Voltage on DC motors

I have a “working understanding” of ohms law.....
But don’t have a good grasp on this:

If I “lug” a motor, ie leaving my ebike in a high gear and heading up a hill, is it more likely to burn up with a higher voltage.

My thoughts are that low voltage would cause higher amps, amps means heat..... so to get more power with less heat more volts is good.

But..... with more volts comes more watts, motors are usually stamped with a watt limit, not a volt or amp limit....

Of course high volts is very hard on the controller and apparently above 58vlts DC very bad things can happen to humans.....

Behavior of a system under various conditions depends on multiple factors.

The easiest way to see how they interact is to go to http://ebikes.ca/simulator , and play with systems and conditions that are close to what you have (if yours are not listed), and see what results you get, then vary one piece of the system and see if it gets better or worse, etc., until you get the result you're after.

Regarding "watt ratings", there's a good article on that same website, in the Learn section.

it depends.

Brushless, or brushed? what kindof motor, and what kindof controller? obviously some motors will just pull back on assist, like some mid drives with sensors. Likewise, some battery packs will cut out long before the motor gets damaged.

Holding watts the same (not always possible with one motor and varying voltages), the lay understanding is that upping the volts will allow most motors to run cooler.

all your energy comes from watts, not amps or volts, so independent of what voltage you choose, to go a given speed, up a given hill, in a given gear, will generally use the same watts. If your motor was designed with the lower voltage in mind, it probably has wires big enough, and can dissipate the heat to do what you want. You'd need to look at a motor rating chart, sometimes specifications in a manual. They can be rated for huge amps for short periods of time. Then they get "heat soaked". Sometimes, the magnets pull off from the outer casing long before overheating hapens, just due to poor design, old glue, whatever the case may be.

Upnorth said:

I have a “working understanding” of ohms law.....
But don’t have a good grasp on this:

If I “lug” a motor, ie leaving my ebike in a high gear and heading up a hill, is it more likely to burn up with a higher voltage.

My thoughts are that low voltage would cause higher amps, amps means heat..... so to get more power with less heat more volts is good.

But..... with more volts comes more watts, motors are usually stamped with a watt limit, not a volt or amp limit....

Of course high volts is very hard on the controller and apparently above 58vlts DC very bad things can happen to humans.....

Click to expand...

Are you after a technical answer, or a simple one?

Motors are not a straight resistive load. So it's not that a "1000W" motor run at 50V, means 20A all the time, and a 20V motor means 50A all the time.

The amount of power that can be "sent" to a motor depends on the back-EMF - I.E. the electricity that is generated in the opposite direction while the wheel is spinning. Think about it logically - If a wheel can spin at 1000rpm with 50v applied, then if it was rolling down a hill at 1000rpm, it will generate 50v back the other direction. At that point, there's no potential difference, and no current flows.

On the other hand, when the wheel is stationary, there is no back-emf, and so the motor is effectively a short circuit - it will consume as much power as your power source will give, and since no battery is "ideal", the current will pull the voltage down.

So how does this work? Let's say you have a very high voltage source. 100V. This should get you to 100km/h with no load. However, going up a hill in the wrong gear, you get slowed to 20km/h. Because at that speed, you'd be generating "20v" worth of Back-EMF (This is definitely NOT the right way to define back-EMF, but I don't understand the actual maths enough to give you the right answer, just enough knowledge to be dangerous). This leaves you a potential difference of 80v - At 80v and the resistance of the motor, you can pump in a huge amount of current, and if your controller doesn't limit this, something will burn out.

If you used a 24v battery, your top speed would only be 24v. Lugging it up the hill, you're already very close to your top speed, your back EMF will be 20v, leaving only a 4v potential difference. 4v at the resistance of your motor will be very, very few amps, and so even if your controller should be able to pump out more amps, the motor simply won't take it.

There is a chance that I am wrong, and if I am, I hope someone comes here to correct me, and educate me. But based on a bit of high school physics and watching some university lectures on open education websites, this is my simplified version of how I understand it.

So it isn’t simple as voltage decreases heat.....

I am working towards a 60vlt setup on a BBSHD.

Anyone have one that they have installed a temperature probe?
A few test runs to gather data would be interesting.

Upnorth said:

So it isn’t simple as voltage decreases heat.....

Click to expand...

Only for the components that are straight resistors.

Something like a BBSHD, unless you're towing a lot of weight, it's almost impossible to be in so far wrong a gear, that you damage anything. It can be pretty inefficient if you're in the wrong gear though.

Increasing the voltage beyond stock, may require you to resize the gearing to remain efficient.

In general terms, the more unused wattage you system is producing, the more heat its making.

So if your motor still lugs with more voltage applied, it will heat up faster. If the motor rpm increases enough, it may get up the hill cooler.

There is also some kind of limit, at some point, to the motor itself. In the simplest terms, more current than the actual wire in the winding can carry. Then it will produce a ton of heat even at higher rpm.

We have some ballpark ideas here, what is practical and what is not for various bike motors, based on the experience of us here that have run them to melt down.

For example, the typical cheap " 48v 1000w kit" hubmotors have been pushed to 3000w, on 72v. But they can melt then, even at higher rpm. At 60v, or 48v limited to 2000w, they can go for a very long ride. And they can climb pretty good, if you zap one with 2000w, like 48v 40 amps. Those motors are actually rated 500w, but because they have pretty good heat radiation, they can run hot a bit longer than others. So at 2000w, they do ok. Other motors also rated 500w, are best limited to about 1500w, 48v and 30 amps.


All that was kind of aimed at other readers. You say you want to run 60v. That should work ok, if you limit amps enough to keep it also in that 1500w max at zero rpm ballpark. The stock controller though, might not like 60v, if it has 63v capacitors, it will not enjoy 72v at top of charge much.

Sunder mentioned its hard to be in so wrong a gear you wreck stuff. That is true for an average person on a bike, meaning the total load is under 300 pounds, you, the bike, the battery and motor, and cargo.

But, EVERYTHING changes when you overload. If you are overloaded enough to run lugged up the hills in your lowest gear now, you don't need to increase your volts. You need a second, front hub motor that is running 1500w. You don't need 12 more volts, you need 1500 more watts. 2000w will run up steep hills with a 400 pound total load pretty good. If you weigh even more, then consider putting a huge front hub motor on your ride. Or going to something extreme, like a 3000w motor geared real low, running to a huge sprocket on your left side of your rear wheel.