dogman wrote:Yes, but how many of us lower the amps when we increase the volts to keep the watts the same? Not many, as seen by the number of people that have ridden 5 min too long when jumping to 72v 40 amps, which often was increasing both volts and amps, tripling the watts. Fun though. Even melting one down is fun in a perverted way.
I'll generalize now, using the 9 continent 2807 motor as an example. It's designed with the idea that you will run it on a bike with 36v and 20 amps. You can still melt one at that watt level if you stall it hard enough, but in my road tests I never could make it actually melt on paved roads with max grades of 10%. Max speed is about 23 mph. 26" wheel btw.
Same motor but now running on 72v 20 amps. Max speed is now about 30- 35 mph. I've never melted a 9c motor with this wattage either, but I have not tested this setup as extensively. Never took it to Emory pass, to ride 10 miles of 8-10% grade continuous and find out if it would melt. Riding around closer to the house, I've never seen one overheat with 72v 20 amps.
Same motor now running 72v 40 amps. Really perky now, and able to just touch 40 mph before the battery voltage drops a few v. But a long ride, say 30 min can get a motor dangerously hot. I've melted one with a 40 min ride. the solder on the phase wires connecting to the windings melted and the motor stopped. Didn't melt the winding, though it sure is black now. Motor still runs after repair. I have also toasted hall sensors at this power level, and then replaced them on the same motor. Toasting the halls happened after a 30 min ride.
Same motor at 90v 40amps. 47mph possible, but 43mph most of the ride. The motor gets very hot very fast. Didn't finish a race, and on mile 11 burst into flames, having a temp reading of 450F inside. The flames was the string inside that ties the windings in place combusting. That motor now is burnt, windings shorted, the varnish on the copper wire melted and shorting.
So what happened? As I undersand it, at some point the copper in the motor just can't use more current. It's maxed out. From then on, more current just makes heat, and at 90v 40 amps, clearly a LOT of heat. Then the motor melts. I would guess this point is somewhere between 1500w and 3000w, based on how much hotter the motor gets if you give it up to 3000w. In use of course, the motor doesn't draw 3000w continuous. But enough 2500- 3000w spikes can do the trick.
Because you keep seeing more speed as you increase the wattage, clearly the motor gets to use some watts above 1500w. But my guess is that the efficiency gets worse and worse. Once you have 1000w going into heat, that's just like an electric space heater, and it warms up quite fast. Too fast for holes in the hub cover to help you, except to let the smoke out.
The major (maybe 90% plus) cause of heat in a motor comes from the I²R losses, in other words the square of the current x the motor winding and wiring resistance. Double the current and you get four times the heat. Double the voltage and keep the current the same and you'll get only a little more heat.
parajared wrote:Yes, this was my hypothesis when I wrote this thread. I found it strange that people were running 50 amps down their phase wires, and began to study ohms law, and the rules of resistance. I figured the 18ish gauge coils in everyone's motor abided by the same rules and presumed that our forum's attempts to modify controllers, battery cables, phases wires ect.. in an attempt to improve performance was all an exercise in futility because internal coil wire resistance just sheds off the extra amperage in heat.
I understand why manufacurers don't build their controllers high voltage, because increased voltage = increased speed, and they are building e-bikes, not e-motorcycles, but I was wondering why with so much knowledge of how electricity works on this forum we don't see more 200+ volt builds to accomodate for our tiny coil wires. I presumed people were running into uncomfortable ride, or high voltage problems?
Spinning a motor faster does mean that you get more power from a given size of motor and a given amount of heating, but spin a motor too fast (in the sense of the number of pole commutations per second) and you start running into problems with core losses. These are a function of switching rate - higher means more loss. Hub motors can run into this problem if pushed, because they have a high pole count, so have a higher commutation frequency with respect to rpm than a smaller diameter motor, like a big RC outrunner, for example.
dogman wrote:Seems to explain why I don't get motors much hotter running at 72v 20 amps, than I do running 48v 20 amps.
amberwolf wrote:It is *also* a function of the wattage, as there is a somewhat proportional amount of power that is lost in the motor as heat for any given power input (vs motor characteristics and load and speed, etc.). So if you put 10KW into the motor but at only 10A, if the motor is only 70% efficient at those particular conditions, it's still going to melt it because you have 3KW of heat in there.
But apparently, because of the core losses in laminations, and resistive losses in windings, higher winding currents are more of an immediate problem than the total power input. But winding (phase) currents aren't constant under a given load, like battery current essentially is. They are pulsed with the PWM (for throttle control) and commutation (usually trapezoid on our ebike controllers) steps.
So this makes another issue: depending on the controller design and/or programming, you may have a 20A controller that could still melt the motor down because it's programmed for very high phase currents, so it is sending say (WAG example) 150A pulses down the line. You lose some as heat in resistance of the phase wires getting to the motor, then more as heat in the resistance of the actual windings of the phases, then still more in the laminations because of the changes in current flow as PWM is switched on and off and commutation steps occur to spin the motor, where current flow actually reverses thru any given set of windings for various combinations--that current flow has to slow and stop before reversing, and the magnetic fields in the stator laminations create heat when forming and falling because of eddy currents inside each lamination. Thinner laminations help, up to some point I don't yet understand.
Those core losses, from the laminations, are part of why it gets so much hotter above some certain speed, because after that point it's switching commutation steps fast enough that the eddy currents in the laminations are so high that it makes a lot of waste heat in the process of cancelling them out and changing the field direction, for each step in spinning the motor. So thinner laminations for faster motors, as I understand it, makes them more efficient at those higher speeds (but I am not sure what that does at the lower speeds, as there is always some trade-off).
I'm still just beginning to actually grasp what's going on inside these things, so my explanations might not make sense yet (or even be partly incorrect in detail).
About 600 rpm?dogman wrote:Would 47 mph on a 26" rim qualify for fast enough to cause some core losses worth worring about?
Jeremy Harris wrote:High voltage also brings safety problems, at least for commercial products. Anything that is going to be CE marked (the EU approval mark) will need to be certified against the Low Voltage Directive if it runs at more than 75 V DC, or 60 V AC.
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