Motor rpm

[hoppy]

When a motor has a quoted rpm is that for the no-load speed for a given voltage, or what?

 

[Deleted member 4366]

It's its maximum speed at the nominal voltage. Bear in mind that a 36v battery goes up to 42v, which is 17% higher, but then it sags down to about 38v under load, by the time all those things have affected it, you end up with the quoted rpm about what you get on the road on the flat with a bit of pedalling.

 

[hoppy]

It's its maximum speed at the nominal voltage. Bear in mind that a 36v battery goes up to 42v, which is 17% higher, but then it sags down to about 38v under load, by the time all those things have affected it, you end up with the quoted rpm about what you get on the road on the flat with a bit of pedalling.

Thanks. So if you have a 200 rpm motor in a 26" wheel it will do 15.5 mph at max revs.Peak power would be at about half this speed, so best hill climbing would be at about 8 mph?

 

[Deleted member 4366]

Don't get power and torque mixed up. You need torque for climbing. maximum torque comes at zero RPM. Efficiency drops off once you go below 50% of the maximum rpm, so you can climb steadily at 8mph, but once you go below that, your motor starts to heat up. The slower you go, the lower the efficiency and the faster it heats up, so now motor life depends on the length of the hill. You can keep the speed up by pedalling to stop the motor over-heating, but then it doesn't make so much torque.

 

[jerrysimon]

That's intersting Dave. So if you use a Tongxin 260RPM motor in a 700c wheel and go up a steep hill, is it more likely to heat up because it has low torque and will thus drop in speed quicker than say a 180RPM motor in the same wheel size ?

Won't keeping the current restricted by the controller to 12A, which is recommended for the Tongxin, stop it over heating ?

Jerry

 

[Deleted member 4366]

Limiting the current to even 5 amps won't stop it from over-heating if you slow it down enough. At zero RPM, any motor will have zero efficiency and will be drawing the maximum current that the controller will give, assuming full throttle. Therefore at 5 amps,it would have to dissipate 200w, which it wouldn't be able to.

In your example, the 260 rpm one will be less efficient than the 180 rpm one when going slowly. There's a range of rpm from zero, where the controller will give maximum current. At higher rpm, the current will gradually reduce to zero. That band will be longer with the 260 rpm one. The torque curve is more or less a straight line from maximum at zero rpm to zero at max rpm, and lower speed motors make slightly more torque, so in the mid-range, they'll have about the same torque, at low rpm, the low-speed one has the most, and at high speed the high-speed one has the most.

 

[Geebee]

That's intersting Dave. So if you use a Tongxin 260RPM motor in a 700c wheel and go up a steep hill, is it more likely to heat up because it has low torque and will thus drop in speed quicker than say a 180RPM motor in the same wheel size ?

Won't keeping the current restricted by the controller to 12A, which is recommended for the Tongxin, stop it over heating ?

Jerry

Going off several hundred tests on the road a 24 volt Tongxin limited to 12 amps crawling up a long steep hill at 5~6 kph will stay cold, 15 amps it will get a hairs breadth above ambient.
On a 20" wheel 210 RPM motor.

 

[jerrysimon]

Thanks guys I guess I will find out

Jerry

 

[wehey]

So is there an optimal speed to ride up hills to increase battery range?

 

[Deleted member 4366]

Yes - as fast as possible. Best efficiency comes at about 2/3 maximum RPM, ut torque is much lower there, so you'd need to pedal hard.