Grin Motor Simulator and the back of my fag packet don't agree

pickworthi

100 W
Joined
Oct 1, 2020
Messages
129
Location
UK - Oxfordshire
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

Screenshot_2021-07-17_17-41-13.png
 
For those who don't know

a packet of fags means cigarettes, "seat of the pants" calculations
 
That s not quite how that works... V=IR.. relativity, I have not much idea as to what you know about how a BLDC motor is driven. Using the Pulse width modulation and the code to cycle it.

You must use every unit and step of the equation to assume the function of the simulation.... YOu confuse the output voltage and the input voltage, current. How the motor is driven through its PWM commutation of the (phases) (polarity) in the (inverter) system.

I can try to tell you how a voltage relates to a RPM with the wave form. It is complicated how the controller pulses.

It is as follows: simulator_model.gif

I have no idea as where teh "2rpm" you have above relates to. If the motor ( 7.4kV) sees 14.8(v) the motor will turn 109.7 RPM~.

Reduced at a 4:1 ratio, this will yield a rotation of 27.43 RPM.

I do not kjnow if this is relative for I do not know its orgins... the "2rpm" number.

7.4(kV)14(v)=109.7 (rpm)
/
4.0:1 (reduction)
=
27.43 (rpm)

pickworthi said:
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!

C
 
Justin from Grin has some great lectures up on YouTube about motors… well worth watching and might help explain, or at least give you a better understanding of brushless DC motors.
https://youtu.be/c96n0Ma2rLY

Also only because it’s a pet hate of mine - P=VI is the power equation. It’s not Ohm’s law, which relates current to voltage with a constant of proportionality that we know as resistance (V=IR).
 
DogDipstick said:
I have no idea as where teh "2rpm" you have above relates to. If the motor ( 7.4kV) sees 14.8(v) the motor will turn 109.7 RPM~.

Reduced at a 4:1 ratio, this will yield a rotation of 27.43 RPM.

I do not kjnow if this is relative for I do not know its orgins... the "2rpm" number.

7.4(kV)14(v)=109.7 (rpm)
/
4.0:1 (reduction)
=
27.43 (rpm)

Apologies, I was looking at my calculation to derive Voltage from RPM, and divided 14V by 7.4 rather than multiplying. Dyslexia is a wonderful thing :).

However, if the motor is rotating at 109 rpm, as you show, that means the wheel is rotating at 28 (ish) rpm - since reduction from motor to wheel is 4:1.

But, the simulator (and my calculation) has the wheel rotating at 62 rpm. With a 4.78:1 reduction ratio, the motor will be spinning at 296 RPM.

What I can't see, is how the motor can consume around 30 amps spinning at 296 rpm. To spin at 296 rpm requires 296/7.4=40V. And at 40V the current can't be more than 550/40=13.75A

I know my calculation based on P=VI is gross and rough, but watts can't be created in the system.

Applying a bit more grey matter (it hurts!) I see I need to recognise that the voltage from controller to motor is oscillating. Keeping it rough and ready, I'll assume that both views are correct:
- Simulator is showing the *peak* current needed
- My rough calculations are showing the *average* current needed (or mean or median - never sure about which).

If the maximum current is 30 amps, at 550W the voltage should be 18V.
My rough *average* puts voltage at 40V.
So assuming that is the "middle" of the curve (I know, I know) the peak voltage would be around 60V.

Although I intend to deep dive into how brushless DC motor work in due course, for now my intention was to treat it as a black box, and come up with some rules of thumb for setting the phase amp maximum in the controller.

And I appear to have arrived at roughly twice the calculated "average" based on motor RPM and input power.

Apologies for all this thinking out loud.
 
pickworthi said:
- Battery Power = 554W .... looks about right
- Mtr amps = 37.1A ..... looks very wrong - see below

Motor ( phase ) amps are going to be very different than battery amps because controllers are basically giant DC-DC convertors and will push higher amps, lower volts to the motor itself at lower RPMs in order to produce the desired torque.. so 'motor amps' will bare little relation to battery amps. That's normal and expected.

kv = how many rpm you get per volt.. motors tend to have different turn counts, so a fast winding will have a high kV ( for example, the Leafmotor 1500w 4T will almost do 40mph on 48v, and suck down 40A; the 7T will crawl along at maybe 25mph on the same voltage but only consume ~20A while doing it )
 
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