Finding out motor RPM?

[neptronix]

Hey guys; i wanted to measure the RPM of a few motors coming in, which all have a keyed axle.

http://www.hobbyking.com/hobbyking/store/__10783__Turnigy_Multi_Blade_Micro_Tachometer.html

Wondering if something like this could work?

or is there a more sophisticated device that can read pulses from the phase lines?

[Ypedal]

got a CA handy with magnet sensor ?

[neptronix]

Got a CA.. no magnet sensor tho..

These motors are gonna be in the 1000-4000rpm range BTW. Can the CA handle that if i bought the appropriate stuff? i'd think that maybe the magnetic sensor couldn't click on/off fast enough.

[Miles]

I use the Eagle Tree sensor - this senses the commutation frequency via one of the phase wires. Knowing the number of pole pairs, this gives you rpm. I expect there are cheaper alternatives by now..

[Miles]

Maybe investigate this?
http://www.hobbyking.com/hobbyking/stor ... tment.html

[neptronix]

I looked at that, and others. I don't understand how the connections would work, makes it look like the ESCs have a special plug for RPM sensing.

Yeah, the eagle tree stuff is rather expensive.

[neptronix]

( evil genius mode )

What about hooking a large speaker in between 2 of the phase wires, recording the audio frequency, and then multiplying the HZ by 60, to turn it into pulses per minute?

Since there are 3 phase sections, would i then need to multiply that figure by 3?

So:

pulses/sec x 60 x 3 = Motor RPM?

[flathill]

you can also just use a microphone

[Miles]

Frequency * 60 / no. of pole pairs.

[Ypedal]

you can also just use a microphone

can you elaborate on that .. a lot more ? lol.. ( got me curious )

[flathill]

Attach something to shaft so it clicks once per rotation
Use mic and view signal in frequency domain (freeware fft)

[amberwolf]

If you don't have the magnet sensor for the CA, you can use any hall sensor for this, if you run 5V to it from the CA as well as the signal and ground lines. Then just stick any magnet (like from a namebadge, harddisk, speaker, dead motor, etc.) on the wheel or motor, and there you go. All you must do after that is the appropriate math to recalculate the CA's KPH or MPH to RPM. I do recommend gluing or otherwise securing the magnet down, however--at about 70-ish MPH the ones I had stuck on my 9C steel spoke flange flew off and were lost in my front room somewhere.

If you want a dedicated RPM detector, there are reasonably cheap laser-tach units. I have used one borrowed from a friend a few times. It just requires a lighter spot on the wheel or motor somewhere for it to detect against a darker background. It comes with reflective stickers you can use, but it works with anything of sufficient contrast to the background of the wheel or motor.

[neptronix]

If you don't have the magnet sensor for the CA, you can use any hall sensor for this, if you run 5V to it from the CA as well as the signal and ground lines. Then just stick any magnet (like from a namebadge, harddisk, speaker, dead motor, etc.) on the wheel, and there you go. All you must do after that is the appropriate math to recalculate the CA's KPH or MPH to RPM.

OK - here is a hybrid of your idea
I could run a tiny speaker off the halls i bet, or maybe even run that line through a microphone jack itself?

Then i need to do the math that miles mentioned - this would be identical to measuring off the phase lines - correct?
Except in this case, i'd be getting an low voltage and current, rather than many amps off the phase line - which is so many times more easier to deal with.

Barking up the right tree here?


Also, how do i find the number of poles in a BLDC motor? is that the number of wound sections on the stator?



^-- here i am seeing something like 36..

[Miles]

Also, how do i find the number of poles in a BLDC motor? is that the number of wound sections on the stator?

No, they're the "slots" or "nuts".

The poles are the magnets, so the no. of pole pairs is the no. of magnets / 2.

[amberwolf]

OK - here is a hybrid of your idea
I could run a tiny speaker off the halls i bet, or maybe even run that line through a microphone jack itself?

No need for all the complications. If the motor already has halls in it, just wire the hall to the CA just as if it were a speed sensor (supplying 5V from the CA just as for an external hall) and tell the CA how many poles, exactly as if you were using it on a bike.

[neptronix]

o'rly, amberwolf.. i like it, i like it. I guess i could just hook up the negative voltage of the hall harness + one of the sensor leads straight into the speedo, huh?

So the number of poles is half the number of magnets because they are aligned alternating from north to south, right?

[amberwolf]

I guess i could just hook up the negative voltage of the hall harness + one of the sensor leads straight into the speedo, huh?

Yes, plus 5V on the hall harness to the CA's 5V. You might also need a pullup resistor from hall signal to 5V, not sure if that is already inside the CA; should say in the manual. Pretty sure it already has this though, as I think the reed switch type just shorts that to ground when it activates, to generate the pulses.

[zappy]

I use a laser tacho from ebay, $15 delivered. You can use it on everything just apply a little reflective sticker. Very easy and cheap.

Zappy

[neptronix]

AHA!!!!!

http://www.ebay.com/itm/LCD-Digital-Laser-Photo-Tachometer-Practical-Non-Contact-Automatic-RPM-Tach-New-/260920256155?pt=LH_DefaultDomain_0&hash=item3cc00f269b#ht_5449wt_954

Like one of these, Zappy?

[zappy]

Yep, It seems to work well and very easy to use on anything that rotates.
definately a bargin

Zappy

[neptronix]

Zappy, you win this thread. I didn't know these existed, esp. at such a low price. Ordered!

[acuteaero]

Yes, that's good stuff, I have one and have used it for all kinds of things. For $15 it can't be beaten. I have read speeds above 9500 rpm on outrunner motors, I'm sure the limit is well above that. Note: mine killed the 9v battery in several months without use- I don't know if it's drawing constantly (it really shouldn't be...) or if it was a defective battery- either way I have been disconnecting the battery for storage now. Anyone else had this experience?

[Ypedal]

I recently for the Lumen meter similar built as this, just orderd the tachometer.. thx for the link z !

[amberwolf]

Zappy, you win this thread. I didn't know these existed, esp. at such a low price. Ordered!

FWIW, I did mention them in a previous post in the thread.

[neptronix]

Zappy, you win this thread. I didn't know these existed, esp. at such a low price. Ordered!

FWIW, I did mention them in a previous post in the thread.

True. You both helped. I guess you are going to have to split the prize money.