The speed of the motor is usually measured in revolutions per minute, ie RPM. It's one of difficult tasks but also one of the most important things to know about your motor. It is essential to figure out the speed a motor rotates. This can be measured using a variety of tools. The easiest way is to use a tachometer. In some cases, this can be done using just a smartphone and an app. Skilled manufacturers and hobbyists can build their own devices to accomplish this task.
Motor speed measurements in revolutions per minute (RPM) are usually obtained optically or by contact with the motor shaft. Devices that use optical measurements require a reflective sticker to be placed on the rotating part of the motor. One device illuminates the motor, reads the reflection of the sticker and displays the RPM on the display.
Other devices require a mechanical connection to the motor shaft, usually by simply pressing the reading device onto the motor shaft. A tachometer designed to measure motor speed can be purchased for about $40 or more, depending on the type and brand.
If a device specifically designed to measure RPM is not available, many other techniques can work. One DIY method is to use a strobe light that can be adjusted to a known value. In the absence of any tools, the easiest method (though not always the most reliable) is to use a phone app that will strobe the light on the phone at a specific rate to measure RPM.
In some cases, a computer with a sound card and appropriate software can be used instead of a frequency meter for this measurement. The goal is to receive a set of pulses to calculate one revolution of the motor. A Hall effect sensor, light blocker, or even just a microphone is also needed.
The Hall effect sensor works on a DC motor and uses the magnetic field variations of the motor to make the measurement. The light interrupter has two sides; one is the light source and the other is the receiver.
The microphone is less accurate due to ambient and other motor noise. To use the microphone, place a piece of tape on the motor shaft and place something, such as a finger, so that the tape touches it every time it turns, resulting in a humming sound that is picked up by the microphone. The output of one of these sensors is read as a pulse. If you have a multimeter with a frequency counter, the sensor can be connected directly to it. An oscilloscope will also work.
For computers that have neither a sound card, frequency counter, or oscilloscope, software that displays waveforms can also be used. Study the waveform of one of the sensors and look for a string of pulses; measure the space between the two pulses, i.e., the time taken for one revolution. To determine how many revolutions occur in a minute, divide 60 by the time of one revolution. This simple calculation will give you the RPM of the motor.
Using one of the above methods, it is perfectly possible to obtain a motor speed value with reasonable accuracy.
WingFlying offers various motor test stands to meet different test tasks for small size, medium size and heavy size of propulsion systems. Feel free to click the product you are interested in or directly contact us for further inquiries:
▶ LY-10KGF Thrust Stand
▶ LY-30KGF Thrust Stand
▶ WF-CO-30KGF Coaxial Thrust Stand
▶ LY-70KGF Thrust Stand
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.
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.
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.
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.
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.
This is how the speed is being measured for conventional motors:
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!
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!