How to measure rpm on an electric motor?

Ask questions about projects relating to: aerodynamics or hydrodynamics, astronomy, chemistry, electricity, electronics, physics, or engineering

Moderators: MelissaB, kgudger, Ray Trent, Moderators

How to measure rpm on an electric motor?

Postby brendaj88 » Tue Apr 17, 2007 7:00 pm

Hello, I am working on a science fair project with my son, who is in fifth grade. We need help in the research part of the project. My son built a simple electric motor (reed switch). His scientific question is: If the voltage is increased, will the electric motor go faster? So, we need help in making something that will measure the rpm. How do we make a simple mini tachometer or measure the rpm on a motor this size? Any ideas or suggestions will be greatly appreciated. My son is still looking online for answers and we both cannot find a solution yet.

Thank you so much for your time and energy!

Brenda & Robbie
Involved Mom & son/Student
BLJ
brendaj88
 
Posts: 3
Joined: Tue Apr 17, 2007 6:05 pm

Re: How to measure rpm on an electric motor?

Postby davidkallman » Tue Apr 17, 2007 9:38 pm

Hi Brenda and Robbie,

If you input "how to measure rpm on an electric motor?" to answers.com, you'll get several sites answering your question.

Notes:

1. The exact wording of your input to answers.com, or any other search engine, may have a big impact on your result. So, some experimentation may be needed.
2. Beware of sites that are just trying to sell you something. In answers.com, these are supposed to be segregated under "Sponsored Links." But, they occasionally leak into "Web Results."
Cheers!

Dave
davidkallman
Former Expert
 
Posts: 675
Joined: Thu Feb 03, 2005 3:38 pm

Postby Craig_Bridge » Wed Apr 18, 2007 9:05 am

How do we make a simple mini tachometer or measure the rpm on a motor this size?


Unfortunately, this might be far more difficult than making the motor!

If you have access to an oscilliscope or counter with a built in frequency reference, you can measure the frequency of the electrical pulses at the reed switch easily. Without this kind of test equipment, it comes down to how can you build something to do a similar function. Maybe it is time to network and see if there are any electrical engineers or technicians around that might provide the test equipment and knowledge and help.

If you have a video camera and a frame by frame playback mechanism, you might try using a small circular piece of white cardboard and drawing a single thich black line from the center to one spot on the circumference. Attach this wheel on the motor shaft and aim the video camera at it. An NTSB video camera will take 60 half frames per second per second which may allow you to measure some multiple of the RPM by seeing how far the line rotates between frames. Video tape using only florescent lights to act as strobes.

Another way to approach this qualitatively might be to come up with some sound maker driven by the motor. I remember kids attaching a baseball card with a clothspin to a fender support on a bicycle so that it flapped as the spokes went by. The frequency of the sound changed as the speed of the bicycle changed. You might be able to come up with something similar and be able to judge the relative frequency by ear. If you have a way of slowly changing the voltage while the motor is running(again another piece of test equipment), it isn't too hard to judge whether the frequency is going up or down by ear.

Often coming up with a way of measuring something is far more difficult than the whole rest of the experiment. This hypothsis is quite ambitious for a fifth grade science fair project because of the measurement aspects.

Eighth grade physical science or Highscool teachers might be helpful in terms of knowledge and equipment.
-Craig
Craig_Bridge
Expert
 
Posts: 1297
Joined: Mon Oct 16, 2006 11:47 am

Postby zzzzdoc » Wed Apr 18, 2007 9:08 am

I've used the video camera approach many times. Works quite well. Also used a laser counter, but you had to have one around.

You could take gears and reduce the speed of the motor to something easily measurable too. Easier to measure 30rpm than 600 rpm.
Alan Lichtenstein, MD
Anesthesiologist

Mens et manus
Veritas

He who laughs last...Thinks slowest.
zzzzdoc
Former Expert
 
Posts: 238
Joined: Thu Jan 05, 2006 6:28 pm

Thank you for your suggestions!

Postby brendaj88 » Wed Apr 18, 2007 12:07 pm

Hello! Thank you for your insightful suggestions! Your replies allowed for more research and helped to find the right solution for this science experiment. For future reference and for anyone else working on a science fair with a motor this small, this is what we found:

for a simple (ie. reed switch) motor, you can oder online from Tower Hobbies an optical tachometer (usually used on model airplanes). This website is:
https://www2.towerhobbies.com and model # LXPT31. This can be used to measure the rpms on this size of a motor.

Thank you again for your helpful replies & take good care!

Brenda & Robbie :)
BLJ
brendaj88
 
Posts: 3
Joined: Tue Apr 17, 2007 6:05 pm

Postby zzzzdoc » Wed Apr 18, 2007 3:50 pm

I have actually used that tachometer on RC airplanes I built. It works well with a propeller (you will need something like a prop to break the optical beam).

If you do, take great care working close to a propeller. They can be exceedingly dangerous if contacted when rotating (depending on the speed/torque of the engine - losing fingers kind of danger.) If you need to use one, dull the edges as much as possible before using it.

Instead of a true propeller, if you could use something that will break if contacted (light balsa wood, foam, etc.) if would be much safer.
Alan Lichtenstein, MD
Anesthesiologist

Mens et manus
Veritas

He who laughs last...Thinks slowest.
zzzzdoc
Former Expert
 
Posts: 238
Joined: Thu Jan 05, 2006 6:28 pm

Propellor and/or prop info...

Postby brendaj88 » Wed Apr 18, 2007 5:57 pm

Dear Alan,

Thank you very much for taking the time to respond. We will definitely be using this information for the project!

Keep up the great work on educating and know we appreciate you!

Have a good night/day!

Brenda & Robbie
:)
BLJ
brendaj88
 
Posts: 3
Joined: Tue Apr 17, 2007 6:05 pm

Re: How to measure rpm on an electric motor?

Postby tonyandjulie2002 » Tue Jan 10, 2012 9:28 am

My daughter was doing a similar experiment with the Beaman type motor. After reading the above and doing similar research I stumbled upon another method for measurement. My son had a snap together electronic experiment kit that came with a probe that plugs into the mic on your PC. The software converts the signal into something resembling an oscilloscope, which can easily be used to measure frequency of the motor.

I have some technically background and familiarity with this kind of stuff, but even if you don't, with a little research you could probably figure it out. There are several programs that can do this, I used Zeloscope, free 14 day trial, enough for the project, but only $10 to purchase anyway. The probe that plugs into your mic input is just alligator clips with a 10 M Ohm resistor inline (can't plug signals directly into the mic input, it is way too sensitive for most applications that would be measuring voltage, such as this one).

Just hook up the probe leads across the motor contacts. On the beakman motor, each revolution causes a drop in the voltage as the coil conducts, so you need to measure the timing of one full cycle to get the frequency of the motor. The wave won't be anything uniform, but that's ok. Eg, if one complete cycle lasts 100ms, you have a frequency of 10 cycles/sec (hz).

You'll have to play around a bit with the settings to get the wave to fit on the screen, so if you don't have and experience using a scope, it might be a little intimidating at first, but with a little research, you can do it (as a friend remarked, "Science fair projects really show how much the family learns :-)".

It took me a while to figure it all out (and my tech skills are a little rusty), but in the end, I think it was less time consuming than the other approaches (at least for me), a little fun, and cost nothing (I had the probe already, but it can be easily made for a couple dollars).

Hope that helps.
tonyandjulie2002
 
Posts: 1
Joined: Tue Jan 10, 2012 9:05 am
Occupation: parent
Project Question: n/a
Project Due Date: n/a
Project Status: Not applicable


Return to Grades K-5: Physical Science

Who is online

Users browsing this forum: Yahoo [Bot] and 2 guests