View Full Version : power or watt formula

01-21-2011, 08:18 PM
I am about to replace the 6-pole motor in my aging Stryker F27C; the old motor was 1850kv 11 volt; the replacement is 2000kv (not a big difference; should I use my existing 25amp speed controller, or use a 33.3amp controller sitting in a drawer,; and batteries; I was using a 2600ma 20C, have used an 1850ma 20C when running low on batteries; but I also have a 2600ma 30C. I just don't know how to figure out what is good, and bad. I have another flyer with a Watt meter, but I wouldn't know a good reading or a bad reading; there must be a relatively easy way of figuring this out. I won't be in the air again until next week, but would apreciate any help. Waiting for new motor mount. I have four iarcraft flying, but I always go back to the stryker for just pure high speed fun. TKS.

01-22-2011, 06:57 AM
I'm still pretty new at this plank game but huge amounts of info at http://www.rcgroups.com/forums/index.php and at http://www.rcuniverse.com/ . What I can tell you is this, the 30c battery and new motor will most likely fry that 25 amp controller especially if you use a decent sized prop with a lot of pitch say a 6x4 or 6x5.5. The c rating on the battery is how many amps it can supply steady. You might get by with it for a while if you only use full power for a few seconds at a time. The current cold weather would help the controller survive a bit longer too. My Stryker is an EDF, and I have a 30 amp el cheepo ESC and only use 20c batteries in it. I have 25 or so flights and no problems the ESC, motor and batteries are barely warm after a flight. So yes use the 33 amp if it is just lying around or stay away from the 30 c batteries. Hope this helps.

the collective
01-22-2011, 07:49 AM
The Kv rating of the motor has essentially NOTHING to do with the speed control requirement. You can't determine amp draw from Kv.

If you need to find a speed control for a particular motor, look at the motor's max wattage rating, then divide the max wattage by the typical voltage of the pack you'll be using to get the max amps you'll likely see with the motor...

Here's one example: Say you have a motor rated for 400 watts maximum on 3 cells... Divide 400 watts by the voltage you'll have in flight... at the beginning of a flight when the 3 cell pack is fresh off the charger. it'll be at 12.6 volts, dropping to around 11.8 or so under heavy load. Dividing 400 watts by 11.8 volts you get 33.9 amps.

This is a close approximation, but to know for sure, put a watt meter inline and measure the actual current and voltage under load to be certain. In most cases the approximation will get you there however. Bear in mind though that some motors ratings are very conservative... I had an ARC motor that ran easily at more than double its rated wattage in short bursts...

01-22-2011, 10:18 AM
I knew someone would have the correct answer. I know it is usually recommended to have an ESC that is 50-100% bigger than the motors max wattage under load. Got to get a power meter of my own. Oh and yes my Stryker is my go to plane for fun quick flight.

01-22-2011, 10:47 AM
The right answer is to get a $20-$25 watt meter (http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10080) to ensure that the current and power are within the ratings of your components, and that the battery voltage isn't sagging too much under load. A good watt meter will let you use full throttle for a couple of seconds and then read back the peak current, peak power, and minimum voltage that they measure.

The prop is critical to this question, but so are all the details. You can't look at the pieces and figure out anything: you need to include the battery, esc, motor, prop, and even the air, as air temp and pressure/altitude will change the thickness of the air and the effective load on the system. You can use a calculator to estimate performance (I like www.ecalc.ch) but it could mislead you if you don't really understand what you're doing.

As the collective mentioned, some manufacturer's ratings may be conservative, but you should abide by them for best practice. The Turnigy B2835-3900kv motor in my stryker is rated for 250W peak but I'll burn 750W briefly, with the expectation that it may die young, or even burn up and take out the plane.

01-22-2011, 11:04 AM
X you changed your picture! Thanks guys for the info. Even though it wasn't my question I learned a few things from you.