pickworthi
100 W
I'm trying to get a better understanding of what goes on between controller and motor in terms of power, voltage and current. My motor is a geared hub Shengyi, close-ish to the SX1 that Grin stock.
The SX1 standard winding (according to the site) is 7.4 rpm/V and a 4.87:1 reduction ratio.
(reference: https://ebikes.ca/product-info/grin-kits/shengyi-sx-geared-hubs.html )
I am particularly looking at a slow speed scenario - hill climbing at 100% throttle at around 5 mph.
My controller has a 15 amp limit, and my battery is nominal 36V (10s). Based on my own measurements in this scenario, the highest wattage drawn from the battery is around 560W, typical is around 500W. (measured by watt meter between battery and controller.)
Armed with this, I set up the simulator (https://ebikes.ca/tools/simulator.html) as follows
- Selected Shengyi SX STD motor
- Custom controller - limited battery amps to 15A, Effective Resistance (Ω)=0.03, Phase Current Limit (A)=70 (last two are defaults).
- Set throttle to 100%
- Set Wheel size to 700C
- Left everything else on default
- Moved the slider until wheel RPM was 62.1rpm which on my bike is approx 5mph - simulator has it as 5.2mph - close enough.
The result is in a screenshot below. The puzzling bit is that this shows:
- Battery Power = 554W .... looks about right
- Mtr amps = 37.1A ..... looks very wrong - see below
Using Ohms law (P=VI), I calculate that (assuming 550W makes it to the motor) the motor voltage will be 550/37.1 = 14.83V
But, the motor Kv is 7.4 rpm/V, which means the motor will be rotating at 2 rpm, which means I'v got something wrong!
I have worked out the the motor will actually be rotating at 290 ish RPM, assuming a wheel diameter of 2155 mm, and wheel rotation at 62 ish rpm.
Based on a lot of posts here, the Grin Simulator is not to be questioned......
....so I clearly don't understand what a motor Kv measured in rpm/V means!
Can anyone help me make sense of this?
Many thanks
The SX1 standard winding (according to the site) is 7.4 rpm/V and a 4.87:1 reduction ratio.
(reference: https://ebikes.ca/product-info/grin-kits/shengyi-sx-geared-hubs.html )
I am particularly looking at a slow speed scenario - hill climbing at 100% throttle at around 5 mph.
My controller has a 15 amp limit, and my battery is nominal 36V (10s). Based on my own measurements in this scenario, the highest wattage drawn from the battery is around 560W, typical is around 500W. (measured by watt meter between battery and controller.)
Armed with this, I set up the simulator (https://ebikes.ca/tools/simulator.html) as follows
- Selected Shengyi SX STD motor
- Custom controller - limited battery amps to 15A, Effective Resistance (Ω)=0.03, Phase Current Limit (A)=70 (last two are defaults).
- Set throttle to 100%
- Set Wheel size to 700C
- Left everything else on default
- Moved the slider until wheel RPM was 62.1rpm which on my bike is approx 5mph - simulator has it as 5.2mph - close enough.
The result is in a screenshot below. The puzzling bit is that this shows:
- Battery Power = 554W .... looks about right
- Mtr amps = 37.1A ..... looks very wrong - see below
Using Ohms law (P=VI), I calculate that (assuming 550W makes it to the motor) the motor voltage will be 550/37.1 = 14.83V
But, the motor Kv is 7.4 rpm/V, which means the motor will be rotating at 2 rpm, which means I'v got something wrong!
I have worked out the the motor will actually be rotating at 290 ish RPM, assuming a wheel diameter of 2155 mm, and wheel rotation at 62 ish rpm.
Based on a lot of posts here, the Grin Simulator is not to be questioned......
....so I clearly don't understand what a motor Kv measured in rpm/V means!
Can anyone help me make sense of this?
Many thanks