# Driving Techniques



## Frank (Dec 6, 2008)

I don't know for sure but I would think you could use 4500 or so rpm, look on the Net gain site it's probably posted there. As motor speed climbs so does it's ability to draw current (due to back emf rising) which is why less current is drawn. You are generally better to keep motor rpm higher because it is air cooled with an internal fan. If you don't have some sort of tach you can look up gear ratios, tire diameter, etc and calculate max safe speed in each gear. Most folks use 2nd and 3rd gears only.


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## twright (Aug 20, 2013)

I've got a tach so I can run it wherever I want. I see that the motor efficiency curves have the peak efficiency at around 3200 rpm, but on my car, the current keeps dropping the higher I go. 

So: should I be at the max efficiency point (3200 rpm) or keep it above that RPM?


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## elevatorguy (Jul 26, 2007)

I have tried different motor speeds and I seem to have the lowest draw at 2800-3500. I would pick a speed that gives you the best efficiency. 
I think netgain recommends 6000 rpm max on the warp 9, but I would check their website to be sure.


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## onegreenev (May 18, 2012)

Drive the DC motor at higher rpms than you would an ICE engine. Keeping the R's up lowers the current draw from the pack. Makes it easier on the pack. Don't go over the recommend rpm of the motor. 5500 rpm is pretty much it. Don't run your motor at 5500 rpm all the time. Run it in the sweet spot of power and efficiency. You will find it. Enjoy your EV.


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## Ziggythewiz (May 16, 2010)

The motor will have a range where it's most efficient, but your car will too. Mine is 20% more efficient in 2nd than in 3rd...don't know if it has to do with the tranny or what, but it's a big difference for me.


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## dougingraham (Jul 26, 2011)

Are the amps you are watching the battery amps or the motor amps? You want to watch the battery amps. The WarP9 wants to be spinning about 3000 rpm for the fan to be operating well. The RPM should be limited to somewhere between 5 and 6k rpm. I believe the armatures are tested to 8k.

With the voltage you have on your pack the motor probably hits the back EMF at about 3000 RPM and so the current will drop off quickly above that point. But so does the torque. What you want to do is watch the battery amps while holding speed constant in several different gears. While there will be some difference, it shouldn't be huge unless there are efficiency issues in other parts of the drive train. The WarP9 motor likes higher voltage than the 120 you are feeding it now. When you decide to switch to Lithium think about running the voltage up to 170 sagged. This may require a lot of other changes depending on what you have for controllers and chargers.


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## twright (Aug 20, 2013)

The amp reading that I have been reporting is the motor amps. The volt reading is the battery voltage. What's the difference between motor and battery measurements? Aren't they almost the same?


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## dougingraham (Jul 26, 2011)

twright said:


> The amp reading that I have been reporting is the motor amps. The volt reading is the battery voltage. What's the difference between motor and battery measurements? Aren't they almost the same?


They are the same only at full throttle. The rest of the time the motor current will be higher than the battery current. This is because the motor controller works by reducing the voltage the motor sees. At its most basic level this is all the motor controller can do. Everything else is done as a function of controlling the voltage. What you are interested in is the power used. You can either watch the battery current or compute watts by multiplying the motor current by the motor voltage. The motor current alone is not usable for comparison with different gears. Battery current is because the battery voltage is almost unchanging.


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## major (Apr 4, 2008)

dougingraham said:


> They are the same only at full throttle.


Clarification Doug. Motor current is the same as battery current only at full throttle above base speed; that is after it is out of current limit and at 100% PWM duty cycle. For example, full throttle from standstill will result in much higher motor current than current from battery.

But yes, for efficiency comparison at different motor RPM (different gears), you need to look at battery current. Because battery voltage is relatively constant, battery current is an indicator of power. So when the battery current drops as motor RPM speeds up, the power is decreasing. 

For energy usage comparison (which some call efficiency), you need to compare power (or battery current) with different RPM at the same vehicle speed (steady state) and in equal conditions. This means not accelerating or decelerating and on level ground with no wind and the car similarly loaded and the battery at similar SOC and temperature.


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## twright (Aug 20, 2013)

In another post I have been noting "high" energy usage. I have been calculating my usage by using the motor current X Battery voltage:

170 amps X 109 Volts = 371 Watt Hour/Mile at 50 mph.

Does this mean that this calculation is inaccurate because I am using motor current instead of battery current?


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## major (Apr 4, 2008)

twright said:


> In another post I have been noting "high" energy usage. I have been calculating my usage by using the motor current X Battery voltage:
> 
> 170 amps X 109 Volts = 371 Watt Hour/Mile at 50 mph.
> 
> Does this mean that this calculation is inaccurate because I am using motor current instead of battery current?


It very well might be wrong. It could be close to correct if your motor controller was at 100% PWM duty cycle at the time you took the reading. Otherwise, the motor current is higher than battery current and the product of motor current and battery voltage is meaningless.

Just wondering  Why did you wire the sensor or shunt to measure motor current?


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## twright (Aug 20, 2013)

I didn't do the wiring on the car. I bought it like that.

Does this mean I'd have to know the duty cycle of the controller at my 50 mph point to know how much current I'm really using? Is my Open Revolt controller just using PWM to limit the output of the controller to control motor speed/power?


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## twright (Aug 20, 2013)

I wonder if my ampmeter responds so slowly that it gives a pretty good average of the high and low PWM signal? (and really is accurate at less than 100% duty cycle.)


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## Frank (Dec 6, 2008)

The best thing to do is wire the shunt into the battery side; motor current is less important. Is there no other instrumentation in the vehicle?


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## twright (Aug 20, 2013)

I just have a Voltmeter and an Amp meter.


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## Frank (Dec 6, 2008)

If the charger can work off of 120 volts you could use a kill-a-watt type device to measure energy for some typical driving. This would include charging losses but would probably be more accurate. You can only guesstimate from your gauges if you have them both on the same device I.E. both on motor or both on battery.


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