Over Propping - It Works For Me

[Dickw]

50 minutes ago, RottenRow said:

I agree that the Maxon controllers have many more features and operate with much more precision (they are sensored for a start) than our model ESCs but I used their example as I couldn’t find one specifically for model ESCs.

 

However they do operate on the same principle and they do not have a separate DC:DC converter, it is all done in the output MOSFETs and motor windings. The diagram you have reproduced above is an equivalent circuit and isn’t supposed to represent what is in the controller component for component.

 

The switching sequence of the MOSFETs in a brushless motor controller is such that the applied voltage across any individual winding gets reversed during motor rotation. This, along with the voltage induced into the winding as it passes the magnets, means that the motor voltage and current waveforms are not simple or constant. This is complicated even further by the fact, as the motor windings are (usually) star connected, two windings are being powered simultaneously (in series) by the controller but a fraction of a second later one of those two is replaced by the third winding as a different MOSFET switches on. It is this that creates the rotating magnetic field that the rotor then follows.

 

My main reason for raising this in the first place was to doubt the comments about the use of a wattmeter in the battery leads being accurate at anything other than full throttle. I still believe this statement to be untrue.

 

DC (battery) power in must equal motor power out plus losses.

 

Brian.

If that diagram is meant to be an equivalent circuit for our ESC operation, it is incredibly badly drawn. It shows both an input and an output, whereas for our application the output is the inductor (i.e. the motor winding). If you replace the output (Vo) in that diagram with a short circuit it would better represent our ESC/motor operation. I am not sure then how the DC-DC conversion process is supposed to be working.

 

Yes, I am well aware of the relationship between the applied square(ish!) voltage pulses and the sinusoidal back emf generated in the windings and the effect this will have on instantaneous currents in the circuit. (incidentally, most of my motors are delta wound rather than star therefore giving an even more complex series/parallel powering of the windings).

 

This sort of discussion should probably be taken to a separate thread as it has headed far beyond what the OP queried, or indeed most peoples' interest 😀

 

Dick

 

[PatMc]

The pics below show two test runs that I mentioned yesterday.

The tachometer (top of each pic ) shows rpm to the nearest 100, I think the Wattmeter readings are self explanatory.  

 

IMO no deep knowledge of how an ESC works is needed, beyond that the battery is connected to it at one end, it then controls the motor rpm by varying the RMS volts that it feeds to the motor via 3 wires. 

 

 

 

Note that in both images the wattmeter displays higher volts during the 1/2 power run than during full power but the tacho displays higher rpm at full power. Bearing in mind that the RPM is proportional to the voltage & Kv, it's obvious that the Wattmeter must be displaying the battery terminal volts not the RMS voltage being fed to the motor. Note that in each case Watts is taken to be = Volts x Amps from the respective display.

In summary the above indicates that the wattmeter can only give a true indication of the power being consumed by the motor when the throttle is at full power.

 

Mr Kirchhoff can rest easy in the knowledge that his laws are being obeyed. The voltage may vary in our series circuit but the current is constant from end to end.

 

BTW IIRC, Delta connected windings produce root 3 x the power of Star connected windings. I can't think of any reason we would want a motor to be Star wound for our applications but I believe there are few. 

 

 

[RottenRow]

I’m sorry Pat but your test results prove nothing of the kind, other than that the wattmeter measures the battery voltage, which I already agreed with, and that there is some sag in the battery voltage as the current it is supplying is increasing (to be expected). I have never stated that the wattmeter reads the rms voltage being fed to the motor.

What I did state, and stand firmly by, is that the power drawn from the battery, that is the reading given by the wattmeter, at any motor speed setting, equals the power supplied to the motor (plus a small amount for losses in the ESC).

 

Luckily, as the majority of users of wattmeters will be taking their readings at full ‘throttle’ anyway the subject is somewhat academic.
 

As this discussion is going around in circles, and I really don’t like disagreeing with people on the forum, I will bow out at this point.
 

I concede to your better knowledge with respect to the winding arrangements (star or delta) within the small brushless motors that we use in our models, as I haven’t taken one apart for a while.

 

Brian.

[PatMc]

Sadly Brian I'm not too surprised by your response.

You have completely ignored the anomally of how a lower recorded voltage can result in higher rpm.