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        Help! how to find motor rpms

#1 dnchvs Apr 17, 2009 07:49 AM

how to find motor rpms
i'm a little confused on what setting to to use on me voltometer..i'm using an old radioshack analog meter with the sweeping needle...i tried lastnite chucking the motor to my drill press but i know i didn't have it on the right setting

#2 moparherb Apr 17, 2009 09:32 AM

I do not believe that you can find RPM's using a volt meter. Volt meters are for electrical current not RPM's

#3 dnchvs Apr 17, 2009 09:44 AM

i thought you could use the motor as a genorator running off a drill with a known speed then xthe speed to 12v

#4 patmat2350 Apr 17, 2009 10:44 AM

You drive the motor 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)

Of course, a prop will slow the motor down some.
- A tiny prop won't be noticed by the motor, and it will run near the no-load speed.
- A huge prop might slow it down to 50% of free running, in which case you'll suck amps and want water cooling.
- A well matched prop will slow it down to no less than 75-80% no-load speed, and you'll likely be ok without water cooling.

#5 mfr02 Apr 17, 2009 10:53 AM

Figuring out what the motor generates at a given speed will do little to tell you what it will do as a motor.
On the other hand, using a motor as a generator, once it has been calibrated as suggested above, it will be a serviceable head for a RPM indicator.

#6 dnchvs Apr 17, 2009 11:02 AM

what do you set the vom to....thats where i'm really confused

#7 patmat2350 Apr 17, 2009 11:12 AM

mfr- au contraire, my friend. this method will give you the kV value of a DC brushed motor quite nicely.

dnch- you're measuring voltage, set it to one of the volt scales... probably something like "0-5v".

#8 Aerominded Apr 17, 2009 11:19 AM

Great explaination, Pat! :)

#9 dnchvs Apr 17, 2009 12:03 PM

thanks pat....i'm pretty dumb when it comes to electronics

#10 patmat2350 Apr 17, 2009 12:17 PM

And then, on your meter, read on the appropriate scale. In the example below, there's a 0-50 and 0-100 scale for DC volts... if the meter knob is set to 0-5v, you would read 50 on the scale as "5 v".


#11 dnchvs Apr 17, 2009 12:42 PM

using the dc-rms volts pat (black numbers?

#12 patmat2350 Apr 17, 2009 01:00 PM


#13 dnchvs Apr 17, 2009 01:06 PM

thankyou verymuch pat
i appreciate your help

#14 dnchvs Apr 17, 2009 01:40 PM

pat i got 600 rpms at 5 volts so 1333 seem about right at 12 volts?

#15 patmat2350 Apr 17, 2009 02:25 PM

So, you drove it at 600rpm (with a hand drill, maybe?) and measured exactly 5v across the motor's terminals?
Then that says its free running (no load) speed should be 600/5 x 12 = 1440 rpm... but that is REALLY slow... unless the motor was designed to run on, say, 110vDC (like a treadmill motor).

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