stunthanger.com
Electric Stunt => Gettin all AMP'ed up! => Topic started by: John Rist on October 27, 2010, 02:35:42 PM
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I am fairly new to serious electric power. I have bench mounted a TURNIGY TR38-48-900. I am running 11x5.5E prop. Power source is a 12 cell Nicad (will be replaced with 4-5C Lipo). the 12 cell Nicad should be around 14 volts. Speed controller is a MYSTERY 60A unit.
As I understand it the 900 rating is 900 RPM / Volt. Is it true that this is the no-load rating? At 14 V x 900 the rpm would be 12600. Run test shows the motor to be turning the 11x5.5 prop at just over 7,000 rpm or around 56% of rated rpm. My experience is with old brush motors. 50% rpm of no-load was the norm for them. Does this hold true for brushless Out-Runners also? If so I am about correct on the prop.
??? ??? :)
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50% of no-load RPM gives you -- more or less -- worse than 50% efficiency. You can expect far better than that from a brushless motor. Unless the mystery speed controller is throttling the thing down then something is wrong, even if it's your motor/prop combo.
What's getting hot?
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I will make some more measurments, It is old nicads. My voltage may well be well below 14 volts. Ran it very little but every thing was ice cold.
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Try running it with a little flat balsa board (think double-wide Popsicle stick) and tach that. You really should see full speed out of the thing.
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Motor RPM is Kv * effective voltage
while the effective voltage is battery voltage lowered by current * resistance (battery + ESC + winding resistance)
means motor RPM = Kv*(U-Ri*I)
so if your motor needs 40A and its internal resistance is 50mohm, then effective voltage is at least 2 volts lower. Means it makes 20% lower RPM on 10V battery
BTW it has exactly same rules like DC brushed motors
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Ok here is the real deal. I think I am OK. I recharged the old Ni-cad pack. I re-ran the test with my volt/amp meter installed between the battery pack and the controller. The readings were 10V, 19 amps and 7800 rpm. At 900 V/rpm no load would be 9000 rpm. Says it is running at 86%. Of course it is probably higher than this because I am measuring the voltage at the battery. The 10 volts does not allow for speed controller voltage drop and motor resistance drop. Obviously the Ni-Cad pack is no good. When the hobby budget recovers I will order the battery charger and 4S 3000 lipo batteries. Looks like a good project for Santa. LOL
LL~
It will be awhile but I will post the results.