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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Dennis Toth on December 30, 2012, 09:53:03 AM
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Guys,
While flying yesterday a question came up from my IC friends about how the electric senses load and what it does in responce. I am an ME by training and understand the basics about motors by not the details. I told them that as load is added the motor starts to lag behind the rotation magnetic field and this causes a the motor to pull more current to increase it's field strength and stay in phase.
What is the real explanation?
Best, DennisT
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That what you described is valid for AC motors, but we have DC motors (with electronic comutator, thus brush less, but still DC as any other with permanent magnets), so the primary self regulation is, that slower motor will make smaller back EMF and thus the higher voltage difference between battery voltage and BEMF will push more current to winding and it will make proportionally stronger torque. Means motor with 0 internal resistance will keep constant rpm, little internal resistance will allow little slowing under load and higher internal resitance will allow more slowing down under load.
But beside that, we can use governor, which manage PWM regulation and as it sees lower RPM, it simply add throttle. It is slower close loop regulation but it can compensate internal resistance and keep constant RMP also under load with internal resistance.
And beside that we can use active regulation which can compensate gravity or drag of model during flight :- )))
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Well you see, there's this little man inside the motor and... Oh, wait, you said "what's real", didn't you?
A motor acts like a generator, even when it is "motoring". The faster the motor spins, the higher the voltage that the motor generates. This generated voltage (called the "back EMF", with EMF meaning "electro-motive force" which is an old phrase for "voltage") opposes the voltage being applied to the motor. The current flowing through the motor coils is a consequence of the applied voltage less the back EMF, and the coil resistance.
So when the motor is going at the speed where the back EMF exactly balances the applied voltage, then no current flows (and the motor's armature generates no torque). As the motor spins slower, the current increases and the torque (which is proportional to current) goes up.
This is all complicated by the fact that you probably have a governor in the system. In that case, the governor senses the motor speed, and increases the applied voltage (or causes the applied voltage to increase, if it's a KR timer). This higher applied voltage makes for higher current, which makes for higher torque, which makes the motor spin faster, which is why a governor works.