Motor KV

[Frank Skilbeck]

Helping a friend re-motor an old electric glider, currently has a geared 600 brushed motor but it's kaput, I found this 28mm diameter outrunner, but looking at it it's got 6 magnets and 9 coils, would I be correct in think this is a pretty high KV motor, probably best suited to a ducted fan?

 

 

[Simon Chaddock]

Frank

I am not sure you can interpret a kV from the number of magnets and coils but if the original motor was geared then you are going to need a pretty low kV motor for an ungeared substitute. You will need to examine the motor specs to find one that is happy to turn an appropriate diam and pitch prop at the chosen power (Watts).

You will likely find a suitable out runner will be quite a bit more powerful and lighter than the original geared brushless.

 

My own experience substituting a brushless out runner for for an ungeared 480 brushed whilst retaining the same prop and voltage gave a 25% increase in max Watts coupled with a 40% reduction on motor weight. 

[PatMc]

Simon, IIRC from electric tech studies, kV is inversely proportional to both the number of pairs of poles, to the number of wind turns & to the strength of the magnets. The standard brushed 600 motors we used to use were pretty high KV hence the benifit of being geared or only driving a small prop at high rpm.

 

Frank I'd say that the brushless motor you have is likely to be high KV. If you have access to a tachometer & wattmeter (or voltmeter) it's easy to determine the KV (Actually revs/volt).

Stick a couple of strips of light coloured adhesive tape aproximately opposite each other down the length of the motor, connect the esc battery & wattmeter as normal, run the motor full power with no prop. Measure the rpm by pointing the tacho at the side of the motor, then do the maths.  

[Frank Skilbeck]

Pat, thanks, I'll give  that a go on a 2s lipo.

 

The original was a geared brushless,

[Shaun Walsh]

https://www.4-max.co.uk/ppi-3660-1105.htm

[Simon Chaddock]

frank

I think your original post said it was "geared brushed".

Any chance you could use the old gear box & prop? Although a direct drive with a suitable kV motor will be rather more efficient.

[Frank Skilbeck]

Don't know, but we don't have the original prop anyway. Not sure if the motor would fit the existing gearbox either, if this motor isn't suitable we'll source something suitable. 

[Peter Christy]

Difficult to tell from the pix, but that motor looks like an inrunner? They tend to be higher kV than an outrunnrer of the same physical size.

 

If it is an inrunner, you could always attach a small prop that won't load it too much to measure the RPM. Unlike an I/C engine, you won't damage it by under loading it. It won't over-rev, as the back-EMF will rise to limit the current draw.

[Frank Skilbeck]

Peter, no it's a long can outrunner.

[Frank Skilbeck]

Ran the motor on a 2s this afternoon, 24,300 rpm at 7.15v (lipo wasn't fully charged), so around 3,400kv 😜 probably not a good glider motor! It's probably out of an old EDF, it did run very smoothly.