Motor Kv

[Tosh McCaber]

My usual ignorance with electric motors.  I have bought a 2730/1700KVmotor that I bought from AliExpress.  The motor had the option of several Kvs- 1300, 1700, 2300, and 3000.  Looks good, however it has absolutely no markings on it.  Does any one know how I can check that it is indeed 1700Kv, if that is possible?

 

 

Edited March 28, 2023 by Tosh McCaber

[Dickw]

kv is the relationship between motor rpm and volts, so there are two options.

 

Run the motor at full throttle without a prop on using the lowest voltage battery you have so it doesn't rev too fast. Measure the battery volts and the motor rpm at the same time. kv is rpm divided by volts. You can do this with a prop on, but will have to estimate by how much the prop is slowing the motor down - so a bit of a guess!

 

Alternatively, spin the motor up on its own (e.g. using something like an electric drill) and measure both the rpm and the volts generated by the motor. Again, kv is rpm divided by volts.

 

Dick

[Geoff S]

Measuring motor (or drill) rpm is the hard bit on the few occasions I've attempted to measure kv.  I made a 'prop' out of wood with zero pitch so minimal load and used an optical tacho to make the measurement - taking care not to use mains powered lights, of course, but a battery torch does the job.

[Cuban8]

I've checked a motor's KV by fitting it with a small blank prop - flat piece of 1/8 ply cut to about 6 inches long and about 3/4 of an inch wide. Central hole drilled for a standard prop driver. Driven by a 2S lipo and RPM measured by optical tach.

Best done with motor fixed in a model and well secured or a suitable and safe bench mount - please wear a pair of decent safety glasses or goggles.

To get an indication of the motor's KV, work on a 2S input voltage of 7.5V and divide that into the measured rpm at full throttle.

e.g 4000 rpm /7.5V gives a tad over 500KV. Test figures I've had tally very well with advertised KVs.

With a blank or very small prop, little power is being drawn from the system so by experimentation in the past I allow about a reduction of about 30% in no load rpm when a suitable sized prop is used on the model. Use one of the on line prop performance calculators to make a sensible selection. A Watt meter is essential to monitor what's going on - dont be tempted to guess or play it by ear, so to speak. Guaranteed to free the magic smoke from ESCs and motors.

Edited March 28, 2023 by Cuban8

[Tosh McCaber]

Thanks all.  I have the tachometer, left over from my ic days.  But how do I connect the motor to  the lipo- do I need to go through an esc, or can I connect directly (three wires motor- two terminals lipo......)?

 

Thanks again- pardon my ignrance!

[Dickw]

Run the motor as normal with your ESC.

Don't connect a lipo direct to the motor wires - that would be a dead short and a big bang 😨

 

Dick

[Simon Chaddock]

The motor MUST go through an ESC and some form of ESC control. A servo tester will do fine..

Connect a LiPo directly to a motor and at best you will burn out the motor, at worst start a LiPo fire! 

[Cuban8]

You'll need to run brushless motors through an ESC. Quite a lot of electronic switching of the battery DC and feedback from the motor back to the ESC going on to make it run correctly.

Not a bad article here https://en.wikipedia.org/wiki/Brushless_DC_electric_motor

[Cuban8]

Just a thought but do you have the spec for your new motor's max wattage and current ratings? A lot of attention is quite rightly paid to not overdriving ESCs, but the poor old motor is often forgotten until the lovely smell of cooked varnish wafts up from your models cowl. Heat is the clue here, electric motors should not run hot in fact the cooler the better. As a rule of thumb(!) if you can't comfortably keep a finger on a motor case after a spirited flight then the motor's too hot - the windings at its core could be considerably hotter than the motor's outer case.

I have an 800W  motor in an aerobatic model. The motor is correctly specified and has plenty of cooling, its case never gets more than luke warm.

[PatMc]

5 hours ago, Geoff S said:

Measuring motor (or drill) rpm is the hard bit on the few occasions I've attempted to measure kv.  I made a 'prop' out of wood with zero pitch so minimal load and used an optical tacho to make the measurement - taking care not to use mains powered lights, of course, but a battery torch does the job.

I stick 2 strips of adhesive tape to the motor body positioned roughly diametrically opposite & measure the rpm using an optical tacho directed at the side of the motor. If necessary I remove (at least temporarily) any labels & use either white or black tape depending on which is the best contrast with the motor's colour.    

Edited March 28, 2023 by PatMc

[PatMc]

1 hour ago, Cuban8 said:

You'll need to run brushless motors through an ESC. Quite a lot of electronic switching of the battery DC and feedback from the motor back to the ESC going on to make it run correctly.

Not a bad article here https://en.wikipedia.org/wiki/Brushless_DC_electric_motor

Notice that in that article the Kv is (correctly by convention) the rpm for the motor to produce 1 volt. We almost universally wrongly refer Kv as rpm attained per volt running with no load.

[Tosh McCaber]

Thanks again for your suggestions.  I've bought the motor on the advice of someone on line, who's flying a similar model to the (small) one I'm building.  I've bought it via Ali Express- (very good delivery service- only a couple of weeks to arrive).  But, on their website,I'm not sure re their info:

https://www.aliexpress.us/item/32914145472.html?spm=a2g0o.productlist.main.1.abc713386DLyJJ&algo_pvid=eaa54b81-90d4-4c76-abbb-b710de87e897&algo_exp_id=eaa54b81-90d4-4c76-abbb-b710de87e897-0&pdp_ext_f={"sku_id"%3A"12000029305750672"}&pdp_npi=3%40dis!USD!12.44!10.7!!!!!%4021021f7b16789047281031313d06d1!12000029305750672!sea!US!0&curPageLogUid=tgwUUQR0BL21&gatewayAdapt=4itemAdapt

 

There's no paperwork at all with the motor.  They specify 4 different Kvs- I just want to confirm that I have been sent the right one-  I wanted the 1700Kv option- I've written to their chat line- so far no response.  I guess I'll set things up, and measure the RPM with a dummy no load prop, along with wattmeter readings- and see how it turns out?  I wasn't sure how things would be using, say the prop specified for 1700kv, if it turns out- perish the thought- they've sent a 3000Kv version.  I don't want to overload the motor/ esc!

Edited March 29, 2023 by Tosh McCaber

[PatMc]

Tosh, forget about a dummy no load prop, just use two pieces coloured tape on the "rim" of the motor. Don't fit anything to the shaft, point your tacho at the rim from the side & take the rpm & voltage reading.

I do this with every new motor I get.

 

[Cuban8]

58 minutes ago, PatMc said:

Tosh, forget about a dummy no load prop, just use two pieces coloured tape on the "rim" of the motor. Don't fit anything to the shaft, point your tacho at the rim from the side & take the rpm & voltage reading.

I do this with every new motor I get.

 

Good Idea.

[Geoff S]

23 hours ago, PatMc said:

I stick 2 strips of adhesive tape to the motor body positioned roughly diametrically opposite & measure the rpm using an optical tacho directed at the side of the motor. If necessary I remove (at least temporarily) any labels & use either white or black tape depending on which is the best contrast with the motor's colour.    

 

You're right, Pat, but IIRC the motor I was trying to measure was a brushed one and the case remained stubbornly stationary 🙂

[PatMc]

Was that this century, Geoff ? 😄

[Geoff S]

1 hour ago, PatMc said:

Was that this century, Geoff ? 😄

 Perhaps not 🙂

[John Minchell]

I've done it in the past by putting the motor shaft in the chuck of a pillar drill (where I know the revs from the manufacturers info plate and gear ratio).  Hold the motor body on its mounting in a machine vice (not the body of the motor though).  Then measure across two of the wires the voltage, then repeat for the other 2 combinations of the 3 wires to get 3 readings.  Average them out and do the maths to check on the Kv figure.  You could also do this using a lathe.

John M

[Tosh McCaber]

Thanks for all the very helpful and explanatory replies. After a week's wait, Ali Express have now sent a reply message to confirm  positively that I have been sent the 1700Kv version of the motor.  However, I'm still going to go through the testing procedures that you have recommended, once I have the motor hooked up!

Thanks again.

[Sparks]

If you are going to use the drill/measure AC voltage method, you need to multiply the rms voltage by 1.414 to obtain the peak voltage.

Some folks also advocate multiplying the peak voltage by 0.95 to approximate the motor's efficiancy.  

 

 

Edited March 31, 2023 by Sparks

[PatMc]

2 hours ago, Sparks said:

If you are going to use the drill/measure AC voltage method, you need to multiply the rms voltage by 1.414 to obtain the peak voltage.

Some folks also advocate multiplying the peak voltage by 0.95 to approximate the motor's efficiancy.  

 

 

If that method is used it will give to the rpm required to generate one volt. This not what is commonly quoted as the Kv by the model world who actually give the rpm achieved per input volt. 

[Phil Green]

I would adjust the ruminate/fly ratio 😄

Edited April 1, 2023 by Phil Green