What limits an E-bike power?

Post by **Peter C in Canberra** » Fri, 15 Jan 2010, 17:12

Hi,
After looking at the various hub motors on ebay, shops and elsewhere I am coming to suspect that the motors from any particular vendor are the same internally and it is only the controller sold with it and the recommended battery voltage that provides different current or voltage limits. IE if you buy the kit with a controller that limits to 8 amps and use a 24V battery you have bought the "200W" motor. If you buy the same kit but they say to use 36V and the controller limits to 10 amps you have the "350W motor". Does anyone know if my suspicions are correct?
Also, if some motor really is only able to do 200W of pedal assistance, I wonder if the controller is really necessary. I suspect a pedalling sensor and an extra on/off switch would do as much. IE would it be better just to have the full battery voltage applied to the motor when one pedals and when one doesn't pedal it is not applied and leave it at that. It would save having to hold the throttle on all the time. It might also be more easily adapted to my bike which has dropped racing style bars whereas all the kits seem suited to mountain bike style bars.
....just fishing for people's thoughts on the matter.
thanks,
Peter.

Post by **Johny** » Fri, 15 Jan 2010, 17:17

Peter C in Canberra wrote:....would it be better just to have the full battery voltage applied to the motor when one pedals and when one doesn't pedal it is not applied and leave it at that.

Hi Peter. You can't do away with the controller as my understanding is that these motors are usually BLDC motors which require a small but sophisticated system to actually make them run - not just DC. I can't comment on the 200 vs 350W power ratings.

Post by **Peter C in Canberra** » Fri, 15 Jan 2010, 19:02

Thanks, that makes sense that I would need a controller then. I guess the main question remains though: is the only difference between different power rating actually in the controller not the hub motor? Anyone else know?
Thanks,
Peter.

Post by **antiscab** » Sat, 16 Jan 2010, 02:48

yes, the power rating difference is really all in the controller.
it has to be that way to comply with legislation.
the controller can limit the motor to 200w.

if you were to direct connect a motor (assuming it is brushed, almost none are) than you will get *much* more than 200w when going less than top speed.

without the legislated power limit, you can just fit a more powerful controller and higher voltage battery for more power and speed (as it is with all motors).

Matt

Post by **Richo** » Sun, 17 Jan 2010, 00:02

Most motor "manufacturers" have different model motors which look almost identical.
And the power ratings are for the motors.
Some will also show the increase in power with different voltages eg 24V/36V

So it could be true that a "supplier" is buying one motor and using different controllers with current limiting to get different power levels or by using different voltages to get the increase in power.

Post by **Richo** » Sun, 17 Jan 2010, 00:05

Peter C in Canberra wrote:IE if you buy the kit with a controller that limits to 8 amps and use a 24V battery you have bought the "200W" motor. If you buy the same kit but they say to use 36V and the controller limits to 10 amps you have the "350W motor". Does anyone know if my suspicions are correct?

So you would be correct.

Post by **Richo** » Sun, 17 Jan 2010, 00:09

Peter C in Canberra wrote: It would save having to hold the throttle on all the time. It might also be more easily adapted to my bike which has dropped racing style bars whereas all the kits seem suited to mountain bike style bars.

Some fancy pants expensive ones I have heard use a sensor to detect how hard you push to adjust the motor power rather than a throttle.

Post by **Richo** » Sun, 17 Jan 2010, 00:12

Another option is to use the cable from a brake lever and connect it to a pot box.
That way there is no twist grip.

Post by **D1Sniper** » Sat, 08 May 2010, 07:14

isnt the power to the motor controlled by the shunt resister, the more amps the wires can take the more power gets to the motor, so a 200w shunt will be smaller wire than a 450w shunt, but both can have the same motor, the 200w has the current limited in the esc in some models

Post by **Gregb** » Thu, 29 Jul 2010, 19:47

d1sniper has a good point. without knowing the wire gauge used in the windings you could be in trouble.

Website · Post by **Simon** » Thu, 29 Jul 2010, 21:02

Peter. Golden Motors sell a BLDC motor controller that has cruise control!

Post by **rjws** » Thu, 16 Jun 2011, 01:40

"What limits motor Power"? The answer to that, as for most things electronic/electrical, is heat. You can up the voltage to the point just below where the insulation starts to melt and the motor will run, but it gets hotter the more load (= more current) you put on it until something melts or burns internally. As for BLDC motors running on pure DC, as someone further on in the forum states, it won't work because the electronic controller and the internal position sensors in the motor replace the brushes and commutator usually found in a 'normal' DC motor.
So ...a '48volt' motor made to run on 60 volts will run with proportionately more power until it overheats. This is why the higher power BLDC motors have a 'thermistor' (or some other) temperature sensor inside them which is used to monitor internal motor temperatures and progressively shut down the drive current from the controller if things get out of hand.

Post by **SamD** » Fri, 23 Sep 2011, 21:51

You're right in assuming up front it is the controller - when it comes to cheaper hub units. Eg bafang geared internal hubs are pretty much the same. The controller is usually limited to a given number of amps (torque), so you up the voltage to get more speed.

A 24v 20 amp controller is obviously much nicer delivering 36v 20 amps to the motor.

But once you go over about 500W with the cheaper geared hubs then things like the plastic internal gears start to give out. People cut metal gears and wind their own hubs to go beyond. People like Hyena develop offroad units to 4kw.

Another example is my Aprilia Enjoy. Not a hub motor - a crank motor. Purchased stock second hand they have a 24v based 250W limit, but the put in a cheap $10 throttle and $35 controller and the motor will happily deliver 1200W. Motor doesnt get hot enough to demagnetise. Cheap motoring with enough change to buy a 40W panel and some SLA's to charge from.