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  #1  
Old 06-08-2008, 03:41 PM
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Default How to Make a Tachometer and Motor Dynometer

How to Make a Tachometer and Motor Dynometer
by DaveJr


To calculate the power of a small D.C. Motor you need to know the Stall Torque and the Maximum RPM.

To find the maximum RPM of the test motor, make a disc approximately 1” in diameter that will fit the armature shaft snugly. Use a magic marker to blacken half of the disc.

Connect the motor and a DVM to a variable power supply. Under fluorescent lighting, increase the motor’s RPM slowly from zero and watch the disc spin. The first time the pattern on the disc stops turning is at 3600 RPM, the second time it stops turning is at 7200 RPM. Read More...
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Old 06-09-2008, 05:14 AM
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Default Tachometer & Dynomometer

Do the new Compact Fluorescent bulbs have the same strobe effect as the old long-tube fluorescents? We don't have any of the old ones in the house. I have to try the tach effect. Sounds like a great idea, and cheap. I think we have a sponge, too!
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Old 06-09-2008, 09:08 AM
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Robert

I only had a 14 Watt Compact bulb available to use. The pattern was visible, but you have to look quite closely to see it. It is more visible using a long tube fluorescent light.

I upgraded the motor mount to a piece of 2"x2"x3/4" Pine with felt pads.

Dave
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Old 06-12-2008, 01:46 PM
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Default Strobe Light For Your Tachometer

Using fluorescent lighting to see a spinning disc isn’t always easy.

I made a strobe light using a high intensity led (slot car xeon headlamp) and a 3Volt A.C. transformer.

This will give a sharper light beam and make it easier to see the bi-colored disc stop at 3600 and 7200 RPM when using a DVM for a tachometer.

The photo show the led and transformer set up to measure the RPM of the motor.

Dave
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File Type: jpg STROBE LIGHT.jpg (69.9 KB, 82 views)
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  #5  
Old 06-17-2008, 06:32 PM
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How did you determine the RPM of the motor? You said you attached this disk and when it appeared stationary under the light the RPM was 3600. Then the second time it appeared station the RPM was 7200. I didn't see anything to tell me where those RPM numbers came from.
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Old 06-18-2008, 01:46 PM
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ohioroyce

Fluorescent lights and the A.C. transformer strobe light flicker at 60 cycles per second, or 3600 cycles per minute. This is caused by the duty cycle of the A.C. house current.

The bi-colored disc appears to stop spinning at 3600 and 7200 RPM because of the stroboscopic effect. The disc appears to spin backwards as it approaches 3600 and 7200 RPM, then spins forward as it passes that RPM.

Incandescent lights don’t cycle fast enough to show this effect.

Dave
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Old 06-18-2008, 04:56 PM
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Thank you Dave! It all makes sense to me now.
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  #8  
Old 08-01-2008, 01:49 AM
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I liked this idea for making a simple basic tachometer so much, that I made up this simple spreadsheet:

I re-jigged Dave's formula to suit either 60Hz or 50Hz power to your light source.

If you don't know your power specs, or you're interested in electricity around the world - refer here:

http://users.pandora.be/worldstandards/electricity.htm

Cheers, Tony.
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File Type: xls Fluro Light Tachometer.xls (16.0 KB, 29 views)
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  #9  
Old 11-26-2010, 09:35 AM
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Hi all, I have been meaning to try this for ages. Just never got around to until now. Great idea and cheep.
We have a long tube fluorescent light in our kitchen.
Doing a test with a Falcon motor, I just could not see the disc stop. Not having an AC transformer, re DaveJR thread, any other ideas on getting this to work.
I'm in the UK, so 240 @ 50
Cheers
Andy
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Old 11-26-2010, 09:42 AM
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You need an adjustable DC-output power supply so you can dial in the voltage until the disc appears to stop rotating. I did this the other day using a CF bulb, and checked it with a real tach. It works. Voltage must go down to 3v. With a normal Scalextric motor, the disc appeared to stop at 3.4 volts. I expect a Falcon would need less voltage to rotate at 7k, which is the RPM you are looking for.
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  #11  
Old 11-26-2010, 11:34 AM
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Quote:
Originally Posted by Robert Livingston View Post
You need an adjustable DC-output power supply so you can dial in the voltage until the disc appears to stop rotating. I did this the other day using a CF bulb, and checked it with a real tach. It works. Voltage must go down to 3v. With a normal Scalextric motor, the disc appeared to stop at 3.4 volts. I expect a Falcon would need less voltage to rotate at 7k, which is the RPM you are looking for.
Yes, got that, I'm using a Skytronic 0 - 20 Volt 2Amp APSU.
Disc is half black and half white.
Starting off at 0V and rising slowly, under a Strip Light, 240 Volts 50Hrz I do not get the disc to show. Could be that the light is way to bright, need something that is not so bright and maybe use in a darkened room, or in a shaded spot, so light on disk is all you see.
What I need is what Davej set up. But not having an adjustable AC transformer is a bit of a set back.
Just wondered if there was another way of getting a strobe?

Cheers
Andy

Just thought of something. I have a adjustable DC 12 Volt PSU, 6V 9V 12, is there anyway I could, say tap off the 6V and put that through a small AC converter, Diode square. What else would be needed?


Andy
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Old 11-26-2010, 11:53 AM
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So, is the way that this could be a dyno, as well as a tach, is to know the moment of the disk and then time how long it takes to get from 3600 to 7200 rpm? (You'd assume a flat torque curve, I guess, since it's an electric motor.) If so, how do you precisely measure the delay from 3600 to 7200 rpm?
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  #13  
Old 11-27-2010, 07:45 AM
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Quote:
Originally Posted by Iowa999 View Post
So, is the way that this could be a dyno, as well as a tach, is to know the moment of the disk and then time how long it takes to get from 3600 to 7200 rpm? (You'd assume a flat torque curve, I guess, since it's an electric motor.) If so, how do you precisely measure the delay from 3600 to 7200 rpm?
Theoretically yes, but practically no. You would need a rev counter that trips a timer at one speed, and trips it again to stop it when it reaches the higher speed. The elapsed time would be very small fractions of a second under no load, or longer under a higher load. You would have to take into account the torque absorbed by the rotating mass of the armature, since you are directly measuring acceleration, not torque.

On the other hand, it is relatively simple to measure stall torque at a low voltage using a lever attached to the motor, pushing against a scale. If the lever is exactly 1 cm long, and the scale read in grams, you get a direct read of g-cm, the traditional measure of torque. Since we use crude three-pole motors, I've found it useful to take three measurements on each pole, for a total of nine equally spaced measurements around a single rotation of the armature. Each pole develops slightly different torque from the others.
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  #14  
Old 11-27-2010, 08:14 AM
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Quote:
Originally Posted by 9000CSE View Post
Yes, got that, I'm using a Skytronic 0 - 20 Volt 2Amp APSU.
Disc is half black and half white.
Starting off at 0V and rising slowly, under a Strip Light, 240 Volts 50Hrz I do not get the disc to show. Could be that the light is way to bright, need something that is not so bright and maybe use in a darkened room, or in a shaded spot, so light on disk is all you see.
What I need is what Davej set up. But not having an adjustable AC transformer is a bit of a set back.
Just wondered if there was another way of getting a strobe?

Cheers
Andy

Just thought of something. I have a adjustable DC 12 Volt PSU, 6V 9V 12, is there anyway I could, say tap off the 6V and put that through a small AC converter, Diode square. What else would be needed?


Andy
Andy, I can't answer all your questions but I can answer one.

You need to see that shadowy spot on the disc. The spinning disc should look like an airplane propeller in an old movie, as if it is rotating very slowly. If you get that effect, it is easy to vary the motor voltage until the spot of shadow on the disc appears to stop hunting around, no longer rotating CW nor CCW. I used a three-loop 25W household lamp bulb, CF (compact fluorescent) in a dark room. There was no other light source. Even under those conditions the shadow or spot was difficult to see. Possibly, by blocking the light from two of the three loops of fluorescent tube, I'd get a more distinct visual signature. I'll have to try that. If you are using a strip light, you might be getting a series of light impulses that cancel each other.
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  #15  
Old 11-27-2010, 03:33 PM
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Quote:
Originally Posted by Robert Livingston View Post
Andy, I can't answer all your questions but I can answer one.

You need to see that shadowy spot on the disc. The spinning disc should look like an airplane propeller in an old movie, as if it is rotating very slowly. If you get that effect, it is easy to vary the motor voltage until the spot of shadow on the disc appears to stop hunting around, no longer rotating CW nor CCW. I used a three-loop 25W household lamp bulb, CF (compact fluorescent) in a dark room. There was no other light source. Even under those conditions the shadow or spot was difficult to see. Possibly, by blocking the light from two of the three loops of fluorescent tube, I'd get a more distinct visual signature. I'll have to try that. If you are using a strip light, you might be getting a series of light impulses that cancel each other.
I'll have another go. I get the airplane propeller in an old movie bit, but stopping it, well. I'll go buy a
three-loop 25W household lamp bulb, CF (compact fluorescent).
Cheers
Andy
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