# How to read motor data plots?



## Qer (May 7, 2008)

Xringer said:


> I'm not sure how to read these plots. What is that 0 to 260 scale across the bottom?


Since voltage is fixed, RPM decrease and current and power increase along the X-axis I'd say that the 0-260 is torque. However, you better find out if it's Nm or some weird kilopoundfeet, millistoneinch or other non-SI-unit before you draw any conclusions from the diagram. 



Xringer said:


> I see these plots are at fixed DC voltages. Since controllers drive EV motors with pluses of power, shouldn't run-times and plots for EV motors be based on PWM drive voltages & currents?? (I feel like such a newbie)!


Forget about PWM, it's just your magic wand to create a variable voltage out of pack voltage, think of it as a big knob on the pack that magically let you transform the size of it. But yeah, sometimes a graph with a voltage ramp would be nice, but it seems that that's not how motor manufacturers do it.


----------



## major (Apr 4, 2008)

Xringer said:


> http://evdrives.com/Manuals/Mars_Electric/ME0709 Performane Data.pdf
> 
> I'm not sure how to read these plots. What is that 0 to 260 scale across the bottom?


Hi Xzringer,

Yeah, the x-axis is not labeled. If you take the power out and the RPM for a particular x-value, you can back calculate the torque. I did this and it shows that the x-axis units are lb.in. That is pound-inches. Or inch-pounds as gearheads call it.



> I see these plots are at fixed DC voltages. Since controllers drive EV motors with pluses of power,
> shouldn't run-times and plots for EV motors be based on PWM drive voltages & currents??


Forget "pulses of power". Consider the controller as a voltage reducer. The motor curve is plotted at the full battery voltage (usually). If the controller is set to 50%, then assume the motor voltage will be half of the battery voltage. You can then use the curve to determine the motor parameters at half voltage. The RPM will be half of the value shown on the "full voltage" plot. The current vs torque will be unchanged. The power out for a given torque will be half of the full voltage value. Efficiency will change, but not by half. Don't worry about that. 

Using the motor voltage ratio to proportion the RPM is an approximation. It is more accurate for small change relative to the plotted voltage. It is very accurate for PM motors. A little less so for series motors. And not so good for compound and shunt motors because the field excitation varies with voltage.

Sometimes you will find a motor curve plotted with a family of RPM vs torque curves for various voltages. But it is impossible to supply a plot showing all possible RPM vs voltages.

Hope that explains it,

major


----------



## Xringer (Oct 12, 2008)

Thanks Major. It's starting to get a little clearer. 
I just printed out a bit of info that might be helpful..
http://www.westerndynamics.com/Download/MechanicalFormulae.pdf

Not being a gearhead and having missed out on a good education 
is a bit of a handicap, when trying to understand the end result
of all those amps and volts (power in watts is easy to grasp).. 

That PWM controller explanation is interesting. I guess it has to do with the ratio of ON time relative to OFF time (duty cycle?).

So, if I had a 48v supply and the controller was hitting the motor with
50% on and 50% off time, (48V symmetrical square wave) the motor would turn like it was getting 24VDC.. 
That sounds overly simplistic! 

Thanks,


----------



## Qer (May 7, 2008)

Xringer said:


> So, if I had a 48v supply and the controller was hitting the motor with50% on and 50% off time, (48V symmetrical square wave) the motor would turn like it was getting 24VDC..
> That sounds overly simplistic!


Na, it's not. It's actually pretty accurate. Think of it this way:

If it takes you 5 minutes to completely fill up a bath tub for your son (just go with the fantasy here, no matter if you don't have kids or it's a/they're daughter(s)) when he's not in it, then in 5 minutes it'll only be half full if he's in it as it fills up since he'll be turning the water on-off-on-off-on-off-on-off-on-off all the time just for fun.

Lousy analogue, I know, but it was fun writing it.


----------

