# Overvolting a motor - How does it affect efficiency?



## major (Apr 4, 2008)

EscapeVelocity said:


> I'm planning on building a BLDC hub-motor driven scooter.
> 
> The motor is 7kW, 72V, and 11.5Kv.
> 
> ...


Since your curve is normalized (in percentages), it will look about the same. Maybe a percent different here or there.


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## EscapeVelocity (Nov 12, 2010)

major said:


> Since your curve is normalized (in percentages), it will look about the same. Maybe a percent different here or there.


Are you sure? You are the master of motors...

Say you have two of these motors, one running off a 72V controller and battery pack, and the other running off a 96V controller and battery pack. Would the 72V setup have a higher efficiency at 25MPH than the one running at 96V? It seems to be what you're suggesting, but wouldn't they each be sending the same voltage and amperage to the motors? I was thinking that a difference wouldn't show up until you reach 45+ MPH.

Any thoughts on this?


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## major (Apr 4, 2008)

EscapeVelocity said:


> Are you sure? You are the master of motors...
> 
> Say you have two of these motors, one running off a 72V controller and battery pack, and the other running off a 96V controller and battery pack. Would the 72V setup have a higher efficiency at 25MPH than the one running at 96V? It seems to be what you're suggesting, but wouldn't they each be sending the same voltage and amperage to the motors? I was thinking that a difference wouldn't show up until you reach 45+ MPH.
> 
> Any thoughts on this?


Hi EV,

I was hoping you'd take the simple answer. There are a lot of variables. The normalized graph made the answer easy. Now you want to compare at equal vehicle speed  So O.K. Equal vehicle speed means equal motor load (power) if all other conditions are equal. If you have the same gear ratio in both cases, then the motor RPM and torque will be the same and motor efficiency will be the same (assuming it is the same motor). The controller will be operating differently due the difference in battery voltage (72 vs 96). There might be a slight difference in controller efficiency, but likely not much. Depending on the battery and battery cables, maybe some difference there, but likely not much.

Now you can get into all types of "what ifs". What if you changed the gear ratio? What if you changed the motor winding? And so forth. But with a slight voltage change, you'll not make a huge efficiency change. When you hit very high speed, or very high load, or other extremes, then maybe yes. And as always, you change the voltage to go faster, you load the motor more and that will likely cause the motor efficiency to drop at those higher speeds.

Hope that answers what you're lookin' fer 

major


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## EscapeVelocity (Nov 12, 2010)

Thanks, Major. I was using a normalized graph when I shouldn't have. Which of the following two graphs would most likely represent reality? My guess would be graph #1.


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## EscapeVelocity (Nov 12, 2010)

I believe I've made a noobish mistake. All of these graphs are sampled at 100% throttle, maximum acceleration. However, decreasing the throttle compresses the efficiency curve, effectively raising the motor's efficiency at lower speeds.

Using the hub motor simulator at http://ebikes.ca/simulator/ helped me understand this.


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## EVfun (Mar 14, 2010)

EscapeVelocity said:


> I believe I've made a noobish mistake. All of these graphs are sampled at 100% throttle, maximum acceleration. However, decreasing the throttle compresses the efficiency curve, effectively raising the motor's efficiency at lower speeds.


Wait a minute now. Motor efficiency tends to decline a little at lower voltage. That is because a major loss is winding resistance. The resistive losses are amps squared time the resistance. If you raise the voltage you raise the power but the resistive losses stay the same (if the current is unchanged.)

It is important to consider that the actual efficiency doesn't follow any one voltage curve, as the power level is varied by varying the motor voltage. I find the easiest way to read these is to find the motor current and then go horizontally to the AMPS line. Where you meet the line draw a vertical line. From there you find a data set at that current (horsepower, efficiency and rpm at different voltages, plus torque.) Here is some graphs of the Prestolite MTC motor at different voltages:


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## EscapeVelocity (Nov 12, 2010)

EVfun said:


> Wait a minute now. Motor efficiency tends to decline a little at lower voltage. That is because a major loss is winding resistance. The resistive losses are amps squared time the resistance. If you raise the voltage you raise the power but the resistive losses stay the same (if the current is unchanged.)


Right. As long as the controller is in torque mode the throttle will control the amount of amps going to the motor. For a given voltage, a lower amperage will produce less heating and thus better efficiency, right?

Even though my graphs show that efficiency is only 50% at 10MPH, that's if I was giving full throttle. If I wanted to simply maintain 10MPH, then I would be sending a LOT less amperage to the motor, and I would probably be able to attain 80% efficiency at that reduced throttle.


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## EVfun (Mar 14, 2010)

EscapeVelocity said:


> Right. As long as the controller is in torque mode the throttle will control the amount of amps going to the motor. For a given voltage, a lower amperage will produce less heating and thus better efficiency, right?
> 
> Even though my graphs show that efficiency is only 50% at 10MPH, that's if I was giving full throttle. If I wanted to simply maintain 10MPH, then I would be sending a LOT less amperage to the motor, and I would probably be able to attain 80% efficiency at that reduced throttle.


Most likely not 80%. The problem is that resistive losses are based on amps, but power is based on voltage times amps. Efficiency is output power divided by input power. In the motor graph I showed the motor resistance is 0.0153 ohms. If the motor is running on 20 volts at 200 amps the resistive loss alone reduces efficiency to 84.7%, and there are other losses in a motor. Just the brush drop (in a brushed motor) should push that case to under 80%. 

If the motor is running on 100 volts at 200 amps the resistive loss alone reduces efficiency to 96.9%. There will be a lot larger power window where the motor efficiency is over 80%. Look at how much of the efficiency curve is over 80% efficiency at 100 volts vs. how little is above 80% at 48 volts. At very low speeds, like 10 mph, the required motor voltage is likely to be smaller still. You can also see how the efficiency falls off at all voltages under 100 amps. 

The points will be different for different motors. The curves will even change some for different types of motors, but the basics shape tends to remain the same.


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