justin4u wrote:I have a question about the review at the top of the thread. On level ground both motors could hit 40mph. Why only 11/13 mph going uphill? What is the limiting factor battery, controller, or motor.
In my experience with electric motors in the RC world they will draw as many amps as required to spin at the rated KV
100KV motor (100 rpm per volt)
This system will draw 20Amps and spin the prop at 1200 rpm
If you increase the size of the prop to 15Ã¢â‚¬Â the rpms will remain the same but the Amp draw will increase to 40Amps.
This is exactly true only for an ideal motor with zero internal resistance and other load dependent losses. It can be approximately true for a very good motor. However, likely you have a small rpm drop as well as a small voltage drop.
justin4u wrote: If the system is over loaded three things can happen
1) the motor moves outside of its efficiency curve and generates heat instead of power
2) as the amp draw on the battery goes up the voltage starts to drop until you hit a low voltage cut off
3) The controller limits the system to XX Amps
With very good (low internal resistance) battery packs and motor number three is likely the cause as Ypedal suggests. However, in practice all factors play a role. For example top speed of my 9C with 60x 8Ah NiCd cells (nominally 72V) is 50km/h on the flats, but drops to 30km/h up the hill near my house. (Which is of course not the same hill tested on in the previous post but serves as an example). On the flats the Cycle Analyst measures 700-800W at top speed. Up the hill it is about 1600W. Voltage on the flats is about 70V and up the hill about 60V.
Quantifying the impact of your three limiting factors:
1. Motor loss: Amperage more than doubles. Hence losses in the motor due to the internal resistance more than quadruples
(p=I^2*r; this would be a simplistic model of the motor, but seems quantitatively a reasonable approximation here). I don't know the Ohm value of the internal resistance of the 2709, but a reasonable guess is that internal resistance losses at 800W on the flats would be in the range 100-150W, and hence up the hill at 1600W losses would be 400-600W.
2. Voltage drop is 10V, a 14% decrease. This generates about 300W heat in the battery packs (so they get warm to the touch).
3. My controller is 30A, and amperage up the hill is about 26A, so the controller shouldn't be limiting. I'm not sure if due to production variances it is limiting anyway. Haven't modified the current shunt to see if it makes any difference.
Bottom line is though that of the 1600 W the CA sees put into the motor, maybe 1000-1200W gets turned into useful mechanical work pushing the bike forward. Motor losses at 400-600W and battery losses at 300W are not insignificant.
To improve performance uphill, one could get a stronger motor, better battery pack, or higher current controller if it is current limiting. Which makes most sense would depend on the particular setup and what part is the weakest link in it.