Hub motor efficiencies - Amp limit vs voltage limit

[Cowardlyduck]

Question: For the best hub motor efficiencies; Is it better to limit amps whilst running at a higher voltage, or run a lower voltage through either a lower voltage battery pack or DC-DC voltage converters?

This is a lengthy post, so I'll give the detailed explaining version below, and the short version with the question above.

So I'm planning on building a LiPo pack for my Stealth Fighter some time down the track, and one of my main motivations for doing so was to gain an increased range.
As such I was thinking of making an 18S pack to give 72V hot off the charger. My current pack is 58.4V hot off the charger.

My current pack gives around 55kmh at full throttle, at which it pulls around 1500w to maintain that speed on the flat.

The LiPo pack I plan on building should give around 65kmh at full throttle. I imagine with the increased wind resistance this will mean a higher amp draw in order to maintain that top speed.

At the moment I limit power usage by limiting the amp draw, as do many others. This seems to work ok, but I'm wondering how efficient it really is. According to Justin's Ebike simulator http://www.ebikes.ca/simulator/ hub motor efficiency is at it's greatest when nearest the battery pack output voltage.

So I was thinking, instead of using the CA to limit amps to conserve power, wouldn't it make more sense to use 1 or 2 DC-DC voltage converters to step the voltage down so that the motor can reach it's peak efficiency sooner and easier. This would also create the effect of electronic gearing.

Good idea/bad idea? What are your thoughts?

Cheers

 

[amberwolf]

By limiting amps, you already are limiting voltage, because that is exactly how controllers feed current to the motor--by varying the average voltage. Controllers *are* DC-DC converters.


If you want to use separate DC-DC units, remember that they will have to handle the PEAK currents the system will draw. If they are between the battery and the controller, that's going to be some hefty DC-DCs.

If between the controller's phase outputs and the motor phase inputs, you'll need at least one for each phase, and they will probably have to be able to handle hundreds of amps, and be able to handle being turned on and off *at least* thousands of times a second, which probably they can't do, even if they can handle the current (which I doubt).

 

[liveforphysics]

The motor see's no difference in the way the power is delivered.

A 1000v pack at 1amp or a 100v pack at 10amps both give the motor the exact same phase current after it's passed through the controller. The difference is that one would be able to spin to a higher speed (if it had the power to reach that speed), if either option reaches it's power limit before it's BEMF limit, than it would be exactly identical performance and heating and power and efficiency, between the 1000v 1amp pack and 100v 10amp pack.

 

[Cowardlyduck]

ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh... I understand.

Lol :lol: Thanks guy's for filling that gap in my knowledge. I totally didn't 'get' it....now I do.

Cheers

 

[DeaninMilwaukee]

The motor see's no difference in the way the power is delivered.

Your not quite right here. The controller provides varying average volts,( and with it current and rpm), but if you ran the controllers output on an occiliscope, you would see that the peak volts, ( and with it current ) are close to full battery voltage, about the same number you would see if the controller was simply wide open throttle.

Since every motor has an amperage its most efficient at, going to a higher voltage battery and using the speed control to throttle it causes the motor to be less efficient and therefore heat up more. Model airplane guys make this mistake by using props that are too big for their motor, and trying to compensate by setting max throttle to not exceed the motors max rating. This works poorly and often results in a burned up motor.

If you were trying to get best possible motor efficiency, you would use a battery voltage that would cause the motor to run right at its most efficient amperage with wide open throttle. Picture a motor thats best amperage is 10, and a hypothetical battery with variable voltage. You would see best motor efficiency by going to full throttle and then adjusting battery voltage to achieve exactly 10 amps. Performance might not be what your looking for, but the motor would be at its best.

In real life, the best you could do, ( for range), is to pick a battery voltage that can get you just barely fast enough when you're wide open, and hopefully the resulting amperage isn't too much higher than the motors best amperage.