Why is HV better?

[xcntrk]

Hey all, I’m trying to better understand motor selection (kv), versus wattage and voltage, to determine which motor is best suited for my rig. I’ve been using BrianG’s calc and don’t understand why higher voltage coupled with lower KV motor is a more ideal setup? Or at least that seems to be the consensus around here.

For example I crunched the following two configs.
The first is a 1521/1y on a crazy 8s (way too much power for me, just an example):

Differential Ratio:  3.3076923076923075 
Transmission Ratio:  1.7222222222222223 
Other Ratio:  1 
Spur Tooth Count:  51 
Pinion Tooth Count:  20 
Total Voltage:  29.6 
Motor KV:  1577 
Tire Diameter (inches):  5.75 
Tire Ballooning (inches):  .1 
Motor Current Draw:  75 
Motor coil Ω:  0.008 
Spur/Pinion Ratio:  2.55 : 1 
Total Ratio:  14.52628 : 1 
Tire Circumference (inches):  18.69 inches (474.79 mm) 
Total Motor Speed:  45733 RPM 
Vehicle Speed:  55.73 mph (89.52 km/h) 
Effective Speed Rate:  1.88 mph/V (3.02 kmh/V) 
Estimated Adjusted Speed:  52 mph (83 km/h) - 7% loss 
Effective KV Value:  1545.03 
Motor Torque:  0.33 ft-lbs 
Final Torque:  4.86 ft-lbs 
Final Power:  2220 watts (3 HP)

The above produces a top speed of 52mph with 4.86ft-lbs torque and overall 3hp with the motor spinning at a comfortable 46k rpm. The next config is a much more realistic 1515/1Y on 6s. (Great config for me as it doesn’t require a HV ESC, plenty of 6s compatible out there):

Differential Ratio:  3.3076923076923075 
Transmission Ratio:  1.7222222222222223 
Other Ratio:  1 
Spur Tooth Count:  51 
Pinion Tooth Count:  20 
Total Voltage:  22.2 
Motor KV:  2200 
Tire Diameter (inches):  5.75 
Tire Ballooning (inches):  .1 
Motor Current Draw:  110 
Motor coil Ω:  0.006 
Spur/Pinion Ratio:  2.55 : 1 
Total Ratio:  14.52628 : 1 
Tire Circumference (inches):  18.69 inches (474.79 mm) 
Total Motor Speed:  47388 RPM 
Vehicle Speed:  57.75 mph (92.76 km/h) 
Effective Speed Rate:  2.6 mph/V (4.18 kmh/V) 
Estimated Adjusted Speed:  53 mph (86 km/h) - 8% loss 
Effective KV Value:  2134.59 
Motor Torque:  0.35 ft-lbs 
Final Torque:  5.11 ft-lbs 
Final Power:  2442 watts (3.3 HP)

This setup produces 53mph top speed, 5.11ft-lbs torque and 3.3hp with the motor spinning at a comfortable 47k rpm. Nearly identical figures (with more torque too) on less voltage.

So I don’t understand why the higher voltage setups with larger motor (lower KV) is more ideal?

Please help set me strait, obviously I'm missing something???

[brushlessboy16]

High voltage is a better etup because it pulls less amps, say you want to make 100 watts of power, you could do high amp setup which would be 100amps and 1 volt of power. or the high voltage route say 100volts and 1amp of power, the lower the kv motor, the higher the voltage can be.... The higher voltage it is, means the less amps it draws. so you can run very high voltage setups that pull very little amps and have wayy better run time. hope this helps

[xcntrk]

That does help, thanks… Is there a formula for calculating wattage? I know that wattage/voltage will = the motor amperage, but how do I calc the wattage?

[BL_RV0]

Amperage X Voltage= Wattage.

[joeling]

Hi,

Here's my take. Higher voltage with lower current draw produces similar power to lower voltage with higher current draw (P=IV). Higher current draw results in higher heat (P=IsquaredR, where R is resistance of wires & stuff). Heat is wasted energy & the enemy of the esc. This is similar to why electricity is distributed in many Kilovolts instead of the normal household voltage (something like that - my high school physics is a dim memory...)

Regards,
Joe Ling

[Finnster]

Motor efficiencies also increase with lower kv motors (and thus higher V setups.)

If you are making 2000W on a 4S 90% eff system, you are losing 200W as heat.
If you are making 2000W on an 8S 95% eff system, you are losing 100W as heat. That's 100W more going to the wheels.

Also, the higher turn motors have more TQ, and combined with the eff, can be comfortably geared higher and ran harder w/o danger fo thermals. Couple this with the lower resistive losses, its a far superior system.

[lutach]

HV rules. Please vote: http://www.rc-monster.com/forum/show...t=10609&page=3 .