motor formulas - RC Groups
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Mar 22, 2004, 03:58 AM
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motor formulas


I'm new to electric flying and would like to know more about the formulas used when predicitng performance of electric power systems. Is there any homepage that is explaining this in detail?? It would be nice to know the formulas used in most motor prediction programs.

By the way could someone please eplain to me the definition of the no load current I0? Is it the noload current at motors labeled voltage ?
When looking on motordata I0 is written as a constant and in formulas I have found it is used as a constant, but in real life I0 must bee depending of input voltage. Am I missig something here? Shouldnt I0 be defined as a no load curent versus input voltage? like X.X Amps/volt?

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Mar 22, 2004, 04:32 AM
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vintage1's Avatar
No load current is approximately constant. It represnents the amount of energy needed to 'charge' and 'discharge' the magnetic circuit each revolution. Plus frictional losses. That energy varies with RPM, which varies with voltage, so whilst lost power is proprtional to voltage, the current remains approximately the same. There are other effects - eddy current loss - that do go up with voltage, so idle current does indeed increase a litlle with voltage, but its nearer a constant than a variable.

Thats for brushed motors: Brushless motors with thei controllers have an added dimension in that the controller itself may interact to produce other losses.

Like all formulae in engineering, those used to predict motor peformance are partly quite accurate, and partly rule of thumb,.

Copper losses are very predictable, idle current losses are less so, but if measuremnets at or near expected voltage are taken, are sufficiently constant for a good appropximation.
Mar 22, 2004, 05:49 AM
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Do a search on the main ezonemag page for "Understanding Electric Power Systems". You'll find a series of articles containing all the formulae you need.

There's also some useful basic information on motor constants (and many other things) in the Electric Flight FAQ, link at the bottom of every page.

Mar 22, 2004, 08:30 AM
Registered User
"Vintage" and steve. Thanks for your replies.

Just for fun i measured how constant the "I0 constant" is on a speed 300 6V. I've attatched an image of the result. Still i wonder at what voltage the I0 is defined. Accordingly to the sp300 specs (graupner #3306) I0 is 0,7 A. That corresponds to 3v in my graph. i.e I0 is taken at 1/2 the motors labeled voltage. Is that normally how it is done?

Mar 22, 2004, 12:47 PM
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vintage1's Avatar
It looks as though you had some kind of speed dependent load on there somehow.

Its normal on graupner to take Io at 7.2v I think.

Interesting result.
Mar 22, 2004, 08:52 PM
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mmormota's Avatar
I measured a Pot03 system: 280-like motor with plastic gearbox.
5V: 0,41A
10V: 0,48A
Mar 23, 2004, 02:52 AM
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Bruce Abbott's Avatar
As you have discovered, Io is not really a constant. However, it doesn't usually vary much over the range of voltages used in practice (a 6V Speed 300 should not be operated on 11V - as it may disintegrate at over 50000rpm!). Compared to typical operating currents of 5 to 10 Amps, an Io variation of around 0.3A isn't worth worrying about.

If you want an accurate Io value, don't believe the manufacturer's numbers or Motocalc! Measure it yourself and use the value closest to the intended operating voltage.

Here's my Io measurements for a GWS EM300H. The variation between 6V and 10V is only 0.12A. Even a slight load (eg. gearbox) can increase current quite a bit. With a 2 inch diameter flat disc attached (for measuring rpm), current increased by 15%.

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