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Old 01-06-2010, 04:30 AM
  #27  
kyleservicetech
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Location: Wisconsin, USA
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Originally Posted by alaursen View Post
It may seem low, but if the manufacturer says the KV=800 and my measured RPM = 7300, doesn't that mean the motor must be "seeing" 9.1 volts? Maybe I don't understand KV correctly, however, one thing I know for sure is that battery voltage into the ESC is certainly not what the motor will get from the ESC output even at full throttle; right?

Oh I meant to ask; where is your wattmeter connected?
Those wattmeters are connected in between your battery and the ESC.

Back in the brush type motor days, you could easily measure the voltage at the battery pack, and also at the brushes of the motor. In these brushless motors, that is a whole different story.

Very little voltage drop exists inside the ESC itself. (If the ESC did have even one volt drop at rated current, the mosfets in the ESC would quickly melt from the heat.) These Electronic Speed Controls are in effect a device that converts the batteries Direct Current (DC) power to three phase AC power. And, you get to deal with RMS (Root Mean Square) voltage, average voltage, effective voltage, and the output of these ESC controls, which is a three phase square wave.

So, if you tried to measure the voltage on two of the motors three input wires, you'd be measuring a square wave voltage at very high frequency. Since these digital multimeters have been designed to assume the user is measuring a sine wave voltage, the voltage the meter reads will likely have serious errors. And, most digital multmeters have a real problem in trying to measure frequencies over a few thousand hertz (cycles per second) A typical brushless ESC switching frequency is 5 or 10 times that.

The only way to measure this voltage is to use an oscilloscope, something I've done with my Tektronix 2236 scope. Believe me, that voltage wave shape is a real mess, especially when you are running that ESC at less than full throttle, where the pulse width modulation of the ESC is used to reduce full power to the motor.

(Before retiring, I measured the input to one of my brushless motors with our departments $6000.00 Tektronix four channel 400 Mhz oscilloscope. Using three channels of that scope allowed me to see those three phases in three different colors. I could also watch how the pulse width modulation operated to vary the speed of the motor, while using the throttle on the transmitter.)
Hope that helps.
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