2 replies to this topic

[elossam]

Hi friends,
I need to know the voltage value a brushless engine I own is working at when the stick is at 1% (or the one that starts the engine in the lower part), 50%stick and 100% stick. To do it I need to get some answers if anyone of you know them.

When a brushless engine manufacturer sets a certain rpm/v value for an engine, does he use the voltage value from the battery to the ESC or the voltage that arrives from the ESC to the engine? Could be wrong but the ESC converts DC into AC and have no idea if the same voltage value is present before and after de ESC suposing the Thr stick is set at 100% (may be there is a voltage loss in the conversion).

The second cuestion is: At 50% stick the engine is working at 50% of the max voltage value? or the rpm are dependable of a matter of switching frequency of the coils while the voltage value keeps constant?

Thanks guys

[Kilrah]

At 50% stick the engine is working at 50% of the max voltage value? or the rpm are dependable of a matter of switching frequency of the coils while the voltage value keeps constant?

All a common R/C brushless ESC can do to control the motor is switch the direct battery power on or off, with both polarities, to each of the motor phases. At full power it's supposed to apply the battery voltage for as long as possible, with as small a dead time as possible between the phase switches required to create motion.
At lower powers, what it does is reduce that time, so instead of switching on phase A, then phase B,... it will turn on phase A for let's say 70% of full available time then turn it off and wait, then phase B for 70% too and so on (simplified).
The actual motor voltage is pretty much unpredictable. Firstly it depends on the ESC's software, different models will often have different throttle curves that may be non-linear, then on the motor's inductance, on how appropriately the ESC's timing is set, etc...
Then my mechatronics theory is becoming rusty, but the Kv value is a theoretical value, that links input voltage and output RPM supposing no losses. So to calculate the RPM from a given input voltage you'd have to know the motor's internal resistance and inductance as well, its magnetic losses that depend on the construction and nobody gives in the specs, and mechanical power load/torque as well. Not to mention that motor manufacturers will of course have different methods for calculating/measuring the Kv value they give...

So basically, the only good approach would be to take that motor, with the ESC that you want to use with it, the load you intend to drive attached, and measure the throttle setting/RPM curve yourself on the actual setup...

I need to know the voltage value a brushless engine I own is working at when the stick is at 1% (or the one that starts the engine in the lower part)

As you've probably understood by now this is 100% dependent on the ESC, how its startup procedure works, how much it considers the lowest speed for proper operation to be, etc...

Edited by Kilrah, 22 January 2011 - 04:57 AM.

[elossam]

Thanks Kilrah, it was very helpful.

Edited by elossam, 25 January 2011 - 06:45 AM.