RPM measurement

The speed of the motor is usually measured in RPM (revolutions per minute). It is one of more difficult tasks but it is one of the most important things to know about your motor. There are several ways to do it.

  • A strobe light (stroboscope) may be used to measure RPM. Some strobe lights have active counters showing the flash rate.
  • Optical tachometers may be used to determine spin rate, however they require something like a propeller attached to the motor which may slow it down.

These methods require special expensive equipment that is not easy to find. In his science project Stan built an electronic decade counter that was counting the number of times the magnets on the rotor were passing by the sensor in a certain period of time (15 seconds). Similar counter is available as a kit on our site. Note that while it is an excellent electronic circuit project by itself it could be recommended only to experienced users. It also does not provide a real time speed measurement as average RPM is calculated for measurement period.

After investigating all of these methods and based on our customers feedback we came out with the solution that is very accurate, relatively inexpensive and provides real time measurement.

The RPM measurement tool (available as an attachment or as a full kit) is based on frequency measurement. It works great on all brushless motors available at our site.

RPM Kit#1

Frequency in electronics is measured in Hertz (Hz). One Hertz is equal to one cycle (or one event) in a second. What this tool is counting is number of times a magnet on the rotor is passing by a sensor each second. This is similar to the counter Stan used in his project but you do not need to use a stopwatch for measurement as you will get a real time data on the multimeter display in frequency measurement mode.

The picture at the top of this page shows how to use this simple tool. Just insert the batteries, connect alligator clips to the test leads of the multimeter, set it to Hertz measurement, and hold the Hall Effect sensor close to rotating magnets. Note that branded side of the sensor should be facing the magnets on the rotor.

So what is the speed in RPM?

The multimeter display shows how many times a magnet passed by a sensor in one second. We need to divide this by number of magnets on the rotor and multiply by 60 (seconds in a minute). Typically there are 4 magnets so the display reading should be multiplied by 15 to get RPM.

On the picture the speed of the motor is about 4100 RPM on 6 Volts. This is one of the fastest motors (Hall Effect motor). As the speed of the motor constantly changes slightly you probably should round the readings to the whole number or ignore the digits after the decimal point.

Most of multimeters today do not measure the frequency so if you buy just an attachment you should make sure that your device is capable of doing it. If you buy RPM measurement kit you can use included multimeter for all other measurements.

Can the multimeter in frequency mode be connected directly to electronic circuitry of the motor without an attachment? It is possible for the motor on a Hall Effect IC and the motor with an optical control but for all reed switch and conventional motors the answer is no. Signal from these motors is very noisy and not suitable for frequency measurement without additional electronic circuitry. Excellent information about it is available in Frequency Measurement article by Giorgos Lazaridis.

Watch Easy Speed Measurement for Simple Brushless Electric Motors video on our YouTube channel.

Measuring the speed of the conventional motors is even more difficult task. In those motors, such as motor built of the Kit #15 or any other popular design on the Internet, the coil spins and permanent magnets do not move. For these motors we developed another special attachment that allows measuring RPM.

RPM measurement attachment #2 (assembled)

This measuring tool, also available on our site as an attachment or a full kit that includes a multimeter with the frequency mode, is based on optical measurements. For this purpose the coil has to be modified slightly.

Bend the ends of the coil as shown. Make sure that the coil is still balanced.

Coil with bent ends

Attach a tiny piece of opaque material to one end as shown. You may use a piece of electrical tape or glue a small piece of folded aluminum foil.

Coil with piece of electrical tapeCoil with piece of aluminum foil

This is how the speed is being measured for conventional motors:

RPM measurement with attachment #2

The multimeter set for frequency measurement counts how many times your piece of material is passing through the optointerrupter slot in one second. The display reading should be multiplied by 60 to get RPM.

This measurement attachment comes unassembled. It includes all electronic parts, battery holder, and test leads with alligator clips. No special tools or soldering is required. It could be assembled in minutes.

Once again, this tool could be used not only on the Kit #15 available at our site but on most of other motors where you have a rotating coil. If you buy a standalone attachment make sure that you have a multimeter that can measure frequency.

Watch How to Measure Speed of Rotating Coil in Simple Electric Motor video on our YouTube channel.

January 19, 2016.
There is another device available at our site which has been added recently. It is an optical tachometer that uses a laser light to accurately measure the speed in RPM. Technological advancements in recent years finally made this measurement instrument very affordable.

It is extremely easy to use – just place a piece of included special reflective strip on a rotor and point the laser beam!

Optical speed measurement for kits #1-10

How to choose which RPM measurement tool you need?

   For all brushless motors (kits #1-14):

Optical tachometer is the easiest to use and provides quickest results. However it is just a plain tool for RPM measurement. If you plan to measure voltage, current etc. in your project you might consider RPM measurement kit #1.
Also methods based on frequency measurement add some fun and learning experience as RPM has to be calculated – it might be a part of your project!

   For conventional motors (kits #15-17 and any other designs with rotating coil):

We recommend RPM measurement kit #2. Optical tachometer can be used as well but placing a reflective strip on a small light coil might be a slightly challenging task. Reflective strip with adequate surface for measurement might slow down this type of the motor.

Good luck with your RPM measurements!

5 Responses to Motor Speed Measurement

  1. simplemotor

    Recently we discovered that some multimeters such as Radio Shack 42-range do not work well with measurement attachment #1. They might be not sensitive enough, or require only clean sine/square wave signal.

    Some models can register the signal if the output is connected to a load (resistor across the alligator clips), which is now provided with stand alone attachments.

    Multimeters we provide as a part of the full RPM measurement kit were tested extensively and they are guaranteed to work without any additional circuitry.

  2. timoteo

    Another way is to attach a small magnet to the rotating part (helic for istance) and with a coil and a multimeter measure the frequency of the inducted emf.

    • Simple Motors

      True. However for brushless motors on our site there is no need to attach the magnet as rotor magnets serve the same purpose. And Hall Effect IC in measurement tool acts the same as the coil for frequency measurement.

  3. In optical tachometers what reflective material is used, and where can we get it?

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