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[Geoff C]

Hi Guys,       Is there a formula to calculate the revs, without using a rev counter, by the amps used, wire diam, and number of turns and number of poles.     I know I could just read the specs list, but how do manufacturers work it out.?    Just a thought, maybe its one of my senior moments again.

[NFMike]

Not really. You'd also need (at least) the flux density of the magnets and some idea of friction.

[malcolmfrary]

I have a feeling that the manufacturers have, over the years, managed to learn that after you have designed a motor, you still have to test it to see if what has been designed is, in fact, what is being produced. There are plenty more variables involved over and above those mentioned - brush quality and air gap being another couple.  The empirical method, while messy, is far more reliable than using a manufacturers details that might well reflect a perfectly bedded in motor with an unaged magnet.  Better to guess, get a bit of surplus "go", and run with the wick turned down a bit than select a motor that might, under perfect circumstances, only just do the job.

[Geoff C]

Thanks a lot for your replies chaps.

[nick_75au]

You can calculate the Kv or RPM per volt fairly easily by driving the motor at a known RPM such as a drill press, measure the voltage a quick calculation and hey presto


Quoted from Patmat on RC groups


You drive the [/size]motor[/b][/color][/size] at a known speed (with the help of a drill press)... measure the generated voltage. Will likely be be a volt or three on say, 3600 rpm. Just set your VOM accordingly.[/size]Then calculate the motor constant ('kV" value) as RPM divided by measured volts.Measured 2v at 3600rpm? 3600/2 = 1800 ... 1800 is the motor's "kV" value.How to use kV? Just mutliply it by the voltage you will apply to get the FREE RUNNING speed of the motor. 7.2v RC pack? 1800 x 7.2 = 12,960rpm (free running speed on 7.2v)

[Martin (Admin)]

 
http://www.ebay.co.uk/itm/RC-Plane-propellor-speed-rev-counter-2-and-3-blade-Super-Tacho-tachometer-/321561846712?pt=UK_ToysGames_RadioControlled_JN&hash=item4ade9463b8

http://www.hobbyking.co.uk/hobbyking/store/__28413__Turnigy_Multi_Blade_Micro_Tachometer_UK_Warehouse_.html

                 

[Subculture]

Manufacturer specs may be a little bit out, but they give you a very good base to work from.

I used to struggle working out what RPM to drive props at and what sort of a motor to use. This became a lot easier once I found some literature on the subject, and I was then able to decide what RPM to drive a certain prop at to get a specific speed.

That still doesn't help with knowing whether the motor can drive it or not however. Experience helps a great deal, but a further boon was the introduction of data loggers, specifically the Eagle Tree series.

First time I saw those i thought 'wow an onboard diagnostic for a model'. They were very expensive though, but later the micro logger came out, which was much cheaper. They're fantastic units that really help to dial in motors and props and to see how efficiently things are working.

[nick_75au]

Ack what happened to the text in my post

[Tug-Kenny RIP]

Is that what it is.          Please try again

Ken

[derekwarner]

Easy to read in OZ language  ..... Derek


"at a known speed (with the help of a drill press)... measure the generated voltage. Will likely be be a volt or three on say, 3600 rpm. Just set your VOM accordingly Then calculate the motor constant ('kV" value) as RPM divided by measured volts.Measured 2v at 3600rpm? 3600/2 = 1800 ... 1800 is the motor's "kV" value.How to use kV? Just mutliply it by the voltage you will apply to get the FREE RUNNING speed of the motor. 7.2v RC pack? 1800 x 7.2 = 12,960rpm (free running speed on 7.2v)"

[nick_75au]

Thanks Derek,


It showed up much bigger originally when I posted it ..... Martin   whats up with that??

[malcolmfrary]

Thanks Derek,


It showed up much bigger originally when I posted it ..... Martin   whats up with that??

Glad to know that the resize goblin is not just biting me when I mark something as a quote.