Motors Motor Continuous Watt Calculation

[Robair]

Hi Tony,

I am in the process of upgrading a simple foam trainer from stock powertrain to a more powerful setup but have run into a specification question for which I need and solution.

Using your very helpful method of selecting a Motor, ESC and battery, there are certain parameters required for all the calculations.

All up weight carrying a 2.2 3cell battery is 1600gr

The motor that I will probably settle on is a 905KVA but I can only find a Max current rating of 18Amp in available specifications.

I want to determine Cont. and Max Watts for this motor for which I believe I require Battery Volts and Motor Amps to do the calculation.

Question: The 2.2 3Cell battery is rated at 11.1V but as far as I know, this means that “Volts under continuous load” is +-9.91V and “load under Max load for limited time” is 9.69V

Which of these Volt values do I use to calculate Motor Continuous Watts (W) = volts x amps?

Ultimately, I also want to calculate the theoretical Watt/Pound Ratio.

Thanks in advance

Rob Davidowitz

 

[Smoggie]

Maximum current (Amps) will occur when the battery voltage is at a maximum (i.e when it is fully charged). A decent battery fully charged will easily hold 3.7V per cell under load, so I'd use 11.1V for the maximum Power (Watts) calculation. As the battery depletes voltage in the battery drops and current (Amps) will drop too. So you will have less watts/lb at the end of the flight compared with at the start of the flight, but that's just the nature of electric power.

It's important when first using a new power setup to test it with a wattmeter to make sure you are staying within component ratings.