DKNguyen
Senior Heliman
Location: Alberta, Canada
My Posts This: Topic Forum | Yes, I know how the BEMF causes the resistance measured at the terminals of a running motor to appear higher than the actual internal resistance, and what you say is not true. THe motor does not draw as much as power as it needs to stay in synch. It draws as much current as it needs to develop the voltage drop across the internal resistance to balance out the BEMF at the current RPM with the supply voltage. The current in the winding makes a magnetic field and *IF* the winding happens to be near a magnet, then torque will also be generated. If the winding is not near magnet like in miscommutation, you basically get a stall-like condition.
And this leads us to this core of a direct generator-motor setup won't work:
A motor powered by a fixed voltage source will slow down when torque is applied. Yes, it will draw more current to produce that torque, but it will slow down. No amount of current draw is going to speed up the motor. Motor speed depends on voltage and torque (but not current even though the two are directly proportional), and motor torque depends only on current.
Your generator is going to be spinning at a pretty constant speed isn't it? Being driven by the bottomless pit of energy that is the engine? THe generator is not going to slow down with the tail rotor. It's going to be spinning at the same RPM, commutating at the same speed, outputting the same voltage. When the tail motor becomes loaded and slows down because it's input voltage is not increasing to compensate, it's going to require a lower commutation rate but the generator is going to be commutating at the same speed as it was before. Miscommutation is inevitable.
Yes, a higher generator kV than the motor kV would compensate for this...but only at a single motor torque value. As soon as the motor torque changes, guess what? THe motor speed changes because the input voltage has not changed, but the generator commutation rate stayed the same.
Blind commutation is used, but only for sensorless startup. And when it is used, the torque conditions across the entire range of operation where blind commutation is used must be known (for example, a fan just starting up in static air requires almost zero torque and is easily predictable).
Or try thinking about it this way:
If your reasoning was true, there would be no sensored brushless ESCs. In fact, sensorless brushless ESCs wouldn't even need the "sensorless" BEMF detection at all to commutate the motor- they could just commutate the motor blindly. The motor would always smoothly follows the commutations of the ESC as long as the ESC output enough current (which also completely ignores the effect that voltage has).
Test it out if you wish, but you are going to see jerky rotation and overheating (from miscommutation sending current being the wrong windings - windings not moving near a magnet and generating no BEMF thus producing stall level currents). You need an ESC (and all associated 3-phase rectifier stuff)if you are going to get this to work properly.
I will admit that my initial suspicions about a direct generator-motor connection were more intuitive than cold-hard fact, but after your challenged me to think about it some more, the setup's shortcomings have become much more obvious to me, and I hope to you too. |
