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Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
i plan on using an outrunner motor for a project in the future, but i don't know about the three pins. normally they would be positive negative, and signal. in a motor however, that doesn't make sense. i believe they are like stepper motors, and you have to time the pulses into the three combinations of wires, as would the stater in a brushed motor. if i knew what wave-forms looked like, i would program a micro for this. i would rather build or buy a cheap esc though. can anyone tell me how to control these? thanks, ben
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
They require a three phase input, much like a HDD or some CD player motors. You would need a special driver, or you could make one. You can use 3 op amps as a phase shift oscillator, which gives you 3 outputs each 120 degrees out of phase with each other, IE 3 phase. It would have to be amplified with some transistors or mosfets. Also you should note that the speed relies on the frequency of the 3 phase source.
Go to page 12.
Here is a simulation of the same circuit. I am unsure what purpose the pot serves, it did not seem to do much in the simulator. If the resistance was high in relation to the grounded pin, it clipped the wave, but it seems to be required to get the oscillations to start. Just set the pot all the way to the right after the oscillations start. The circuit required a startup voltage, so just click the L and the oscillations should start, then you can just press the H to turn it off and it will continue to oscillate. I built some other circuit with less resistors and it did not need the pot, but i think this one is supposed to be more stable.
You can make it variable frequency by using a 3-gang pot as the resistors. This: would work, and it has a switch that could be used to turn the circuit on and off. Also, the value of the caps should all 3 be the same, but you can use a different value to set the frequency, or frequency range in the case of a circuit using pots.
Edit: This is the one i made before, it starts up by itself, and after playing with that sim i posted above i realized it takes forever and a day to start up. The only difference is that it is squarewave output, it seems i forgot about that. That would be perfect for mosfets though, to ensure they don't run linear, and i am pretty sure an outrunner runs just fine on squarewave 3 phase.
The best way to drive outrunner motors is just to buy an ESC for them, hobbyking sells loads at very good pricees (plus motors). They are driven by this waveform, in which only two phases are powered at any one time. The third phase is allowed to float, from which you can measure the back EMF across that phase, meaning the measured voltage across a phase looks like this The important point is where the back emf crosses 0V, as this marks the middle of a commutation cycle, meaning by timing this and doubling it you know when to next switch to the next drive commutation.
/edit I should add, this is for the more common sensorless control. Larger motors use hall effect sensors that accurately read the motor position, making driving them a lot simpler.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
both both the op amp way, and the sequenced pulses work fine, i will either program an avr to sequence those pulses, or build the op amp circuit with some small transistors. the only thing is, the circuit has to be light as it one or more will be used to control the main motors on a uav i am building for school. i cant put very much weight on it as it will be loaded with batteries. the other thing is that i need to control speed. a pot is too big and bulky, and anyways, how can a computer turn a pot? i need i to respond to electronic pulses.
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can anyone tell me about outrunner motors?
