looking for an RPM meter ( revolution per minute meter) for a dc motor !!?

Question

i gotta build an RPM meter for a dc motor for which the range of speed is from 30 to 3000 revolutions per minute.... i gotta make this speedomter with less than 0.1% error in metering....

Answer 1

Do you have to build it or can you buy it? Radio Spares/ Allied Components supply these things off the shelf.

Years ago tacho generators were made using a dc generator (hence the name) that produced a certain number of volts per number of rpm, usually expressed in volts/100 rpm. This may not give the accuracy you require.

Modern tacho generators are basically pulse encoders, that may be optical or proximity switch based, and the pulses are counted by some electronics and converted to an rpm reading. The actual design depending on the application. Do you want a display or a signal (0-10v, 0-20mA etc) that can be used in control? Optical based encoders can give you much more accuracy than you want but I could never understand wanting to make, or being able to make something of this complexity.

Answer 2

Actually the easiest way to do this is to get yourself a small motor that is rated as high as 3000 RPM and use it as a voltage generator. The voltage from an unloaded armature is very accurate and matches closely the RPM of the motor from the equation relating CEMF, RPM and flux if flux is constant. Apply a fixed field to the motor - if the field leads are not available then you may have to butcher it to separate them from the armature. Keep the field current constant at all time. You may need to build a circuit for this. (Field current == Flux) Connect a precision voltmeter across the armature. Put some kind of rubber cone on the motor shaft so you can press it against the motor you are measuring. Volts on the meter will be a fixed ratio of the RPM if the Field is constant. For accuracy at the low end you may want to gear up the shaft by about 3:1, but that will require a 9,000 RPM motor for measuring.

Another alternative is to use a stepper-motor as the measuring device since it will be very accurate at the low end - you will need to energize the coils of this with a fixed voltage but should be able to detect the current change using some electronics or an oscilloscope. Even a fluke-meter with a Hertz readout could be used to get frequency back from the motor coils, if you have batteries and resistors energizing them. The measured frequency will be a direct ratio of the RPM you are measuring.

Answer 3

There's three speed sensor manufacturers listed below.

Chances are, they've got what you're looking for.

But you'll have to be pretty specific about what you request ... even more so than how you've worded your question for Yahoo!Answers.

For example ... is it 0.1% accuracy (based on a full scale reading)? Or is it 0.1% accuracy REGARDLESS OF ACTUAL SPEED? You'd also have to define whether that was +/- 0.05%, or some other deviation.

A 0.1% FS for a 3000 rpm device would be about 10% accuracy (3 rpm) at 30 rpm.

Answer 4

The RPM meter shows the RPM of the engine. All the engines are rated at certain RPM. It also helps you in selecting the right gear at any given speed.

Answer 5

First responder has a good idea. Another possibility is to use a photocell arrangement with a counter/timer.

Answer 6

It's called a tachometer. Do some searches under that word.

Answer 7

You need a chip, that's a very high accuracy.

Answer 8

try a RC plane tach sold by any large hobby chain.