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teddillard
02 June 2012, 0755
Every time I cycle back around to some basic concepts I find more that I thought I knew, but really don't have a good handle on. What's bugging me now is system voltage.

I was told early on that higher voltage gives you more RPM, so more speed. I ran everything at 72V, and got good results. So. Why is it that the same motor, run at higher gearing at a lower voltage won't be capable of the same speed? Is it simply a matter of the motor curve? You get more horsepower at higher voltage? Higher torque, higher RPM so more HP?


oooOOOoOOO my haid hurtz

podolefsky
02 June 2012, 1019
Higher voltage basically shifts the torque vs RPM curve to the right. So you maintain the constant torque (current limited) part to higher RPM. That's assuming you have the same current limit.

The power peak is at *about* the knee, where current is max and motor voltage = battery voltage. If you lower voltage, you have less power, hence lower top speed. But that assumes you geared so that motor power = friction losses right at that RPM. So like, maybe you can get 20 HP at the knee, you need 20 HP to go 100 mph, so you gear it exactly for that. In that case, lowering voltage will lower top speed since you have less power.

But most of the time, top speed is above the knee. My bike is geared 5:1, makes around 40 HP at ~2500 RPM, but the top speed of ~90 mph is at more like 6000 RPM. At that point, power output is lower, more like 15-20 HP, but still enough to hit 90 (ish).

Anyway - say your top speed is 60 mph and it takes 10 HP. That means your gearing is such that at 72V, you make 10 HP at the RPM for 60 mph. If you reduce your voltage to 48V, then you hit 10 HP at a lower RPM, so you need to gear higher so that RPM is also 60 mph. If you can still get 10 HP at 48V, you should be able to find a gear ratio that will still hit 60 mph. If you can't make 10 HP at 48V, no gear ratio will get you to 60.

Not sure I answered your question...but I did spew a bunch of b.s. your way, so that ought to help :D

picaroon
02 June 2012, 1253
Not sure if this will answer your question either but here is a bit more b.s spewing your way :)

Here is how I understand it works in a brush type DC permanent magnet motor like an Agni or Mars ME1003.
The voltage will set the maximum rpm the motor can spin. Voltage itself won’t damage the motor but exceeding the rpm limit of the motor can! The agni 95R spins at 71rpm for every 1v and has a max limit of 6000rpm, meaning 84v is ok but 85v could over rev it and cause damage.
Now amps can damage your motor by making things a bit melty, which is why the motor makers will state a limit for it. For example the ME1003 rate it at 200 amps continuous or 500 amps for 1 minute. I believe ‘continuous’ actually means 1 hour.

So for example, let’s say a bike needs 20kw (about 27HP) to overcome the wind, weight of the bike and friction to travel at 90mph.
You could use 72v and 300amps to ride at that speed. 72x300= 21.6kw
If you were to only use 36v the motor rpm would be lower and you would actually need twice the amount of amps to push the higher gear ratio. 36v x 600amps = 21.6kw

I’m still learning, but by my understanding is the example above shows that by using a lower voltage you will exceed the motors amp limits much sooner and it will be damaged, and by using a higher voltage you can still be within the rpm and amp limits.

podolefsky
02 June 2012, 2105
Yeah, that's basically right. It's actually the same for brushless motors - they all have BEMF which counters applied voltage and limits max RPM.

Continuous rating is the current at which the motor can dissipate heat at the same rate it generates it, so the temperature doesn't rise. In theory, you should be able to run at the continuous rating forever (or until your batteries die).

Power is volts * current * efficiency. Efficiency varies between motors, and also with current and RPM - generally between 60-90%. So in your example, a brushed PM motor might be something like 72*300*0.8 = 17.3 kW (or about 23 HP). Actually, 72*300 is going to be the battery voltage and current. In general, motor voltage will be lower, and motor current will be higher...but power will be the same (ignoring controller efficiency).

podolefsky
02 June 2012, 2107
Why is it that the same motor, run at higher gearing at a lower voltage won't be capable of the same speed?

I guess it would help answer this question with more context. Is this something you've tried and found you couldn't go as fast with low voltage / high gearing? Or something you heard or read?

teddillard
03 June 2012, 0233
So for example, letís say a bike needs 20kw (about 27HP) to overcome the wind, weight of the bike and friction to travel at 90mph.
You could use 72v and 300amps to ride at that speed. 72x300= 21.6kw
If you were to only use 36v the motor rpm would be lower and you would actually need twice the amount of amps to push the higher gear ratio. 36v x 600amps = 21.6kw

Iím still learning, but by my understanding is the example above shows that by using a lower voltage you will exceed the motors amp limits much sooner and it will be damaged, and by using a higher voltage you can still be within the rpm and amp limits.

That, actually is exactly what I was trying to grasp. Thanks for putting it into words for me!