When buying an electric motor from your hands, you cannot count on the availability of technical documentation for it. Then the question arises of how to find out the number of revolutions of the purchased device. You can trust the words of the seller, but honesty is not always their hallmark.
Then there is a problem with determining the number of revolutions. You can solve it, knowing some of the subtleties of the motor device. This will be discussed further.
Determine the speed
There are several ways to measure the speed of an electric motor. The most reliable is to use a tachometer - a device designed specifically for this purpose. However, not every person has such a device, especially if he does not professionally deal with electric motors. Therefore, there are several other options that allow you to cope with the task "by eye".
The first involves removing one of the motor covers in order to detect the winding coil. There may be several of the latter. The one that is more accessible and located in the visibility zone is selected. The main thing is to prevent violation of the integrity of the device during operation.
When the coil is visible, it is necessary to carefully examine it and try to compare the size with the stator ring. The latter is a fixed element of the electric motor, and the rotor, being inside it, rotates.
When the ring is half closed by the coil, the number of revolutions per minute reaches 3000. If the third part of the ring is closed, the number of revolutions is approximately 1500. At a quarter, the number of revolutions is 1000.
The second method is related to the windings inside the stator. The number of grooves that one section of a coil occupies is counted. The grooves are located on the core, their number indicates the number of pole pairs. 3000 rpm will be in the presence of two pairs of poles, with four - 1500 rpm, with six - 1000.
The answer to the question of what the number of revolutions of the electric motor depends on is the statement: on the number of pole pairs, and this is inversely proportional.
On the body of any factory engine there is a metal tag on which all the characteristics are indicated. In practice, such a tag may be missing or erased, which complicates the task of determining the number of revolutions a little.
Adjusting the speed
Working with a variety of electrical tools and equipment in everyday life or at work certainly raises the question of how to regulate the speed of an electric motor. For example, it becomes necessary to change the speed of movement of parts in a machine tool or along a conveyor, adjust pump performance, reduce or increase air consumption in ventilation systems.
It is practically pointless to carry out these procedures by lowering the voltage, the revolutions will drop sharply, the power of the device will significantly decrease. Therefore, special devices are used to adjust the engine speed. Let's consider them in more detail.
Frequency converters act as reliable devices capable of drastically changing the current frequency and waveform. They are based on high-power semiconductor triodes (transistors) and a pulse modulator.
The microcontroller controls the entire operation of the converter. Thanks to this approach, it becomes possible to achieve a smooth increase in engine speed, which is extremely important in mechanisms with a large load. Slow acceleration reduces loads, positively affecting the service life of industrial and domestic equipment.
All converters are equipped with protection that has several degrees. Some models operate due to a single-phase voltage of 220 V. The question arises, is it possible to make a three-phase motor rotate thanks to one phase? The answer will turn out to be yes if one condition is met.
When a single-phase voltage is applied to the winding, it is required to "push" the rotor, since it itself will not budge. This requires a starting capacitor. After the motor starts rotating, the remaining windings will provide the missing voltage.
A significant disadvantage of such a scheme is a strong phase imbalance. However, it is easily compensated for by including an autotransformer in the circuit. Overall, it is quite complex scheme... The advantage of the frequency converter is the ability to connect asynchronous motors without the use of complex circuits.
What does the converter give?
The need to use an electric motor speed controller in the case of asynchronous models is as follows:
Significant savings are achieved electrical energy... Since not all equipment requires high rotational speeds of the motor shaft, it makes sense to reduce it by a quarter.
Reliable protection of all mechanisms is provided. The frequency converter allows you to control not only the temperature, but also the pressure and other parameters of the system. This fact is especially important if a pump is driven by a motor.
A pressure sensor is installed in the tank, sends a signal when the proper level is reached, due to which the motor stops.
A smooth start is performed. Thanks to the regulator, the need for additional electronic devices... The frequency converter is easy to tune and get the desired effect.
Reduced costs Maintenance because the regulator minimizes the risks of damage to the drive and other mechanisms.
Thus, electric motors with a speed controller prove to be reliable devices with a wide range of applications.
It is important to remember that the operation of any equipment based on an electric motor will only be correct and safe when the speed parameter is adequate to the conditions of use.
Photo of revolutions of the electric motor
Several years ago I urgently needed to measure the engine speed, but there is no tachometer! How to be here? Since I really needed to measure the speed, the option of ordering a tachometer and waiting for it for a month did not suit me. I had to think! And I came up with the idea to use a computer for this purpose, or rather, a sound editor installed on a computer.
I have installed the sound editor "Adobe Audition" for a long time to work with sound. Therefore, it remains to come up with a way to connect the engine to the computer. This issue was resolved literally within 1 minute - IR LED receiver! He reached into the box and took out the LED, as well as the mini-jack plug. I found a piece of microphone cable and in 10 minutes the LED sensor was ready! I glued the diode itself into a bug from a fountain pen.
Cable assembly.
I used a flashlight to illuminate the IR LED sensor. Also LED.
I glued the sensor with a piece of tape on the nose of the model, and just held the flashlight with my hand. The distance between the sensor and the flashlight is 5 ... 7 cm. The light flux from the flashlight illuminates the receiving LED, and the propeller interrupts (modulates) the light flux. As a result, the LED generates pulses. The sensor is connected to the microphone input of the sound card. The voltage required for the operation of the LED is provided by the design of the microphone jack of the sound card. Any sound card is designed to work, including with an electret microphone, since it needs a supply voltage of + 5 Volts. Therefore, this voltage is present at the center contact
the microphone jack and goes to the LED, which ensures its operation. As a result, the impulses arising from the rotation of the propeller go through the microphone input to the sound card, and the "Adobe Audition" editor records all this as a regular sound file.
To measure the engine speed, it is sufficient to record for a few seconds. It's enough. This is what we will see on the screen in the sound editor window.
First of all, I would like to note that at the very bottom of the Editor there is a timeline, and it is from it that the engine speed is determined. In this case, the recording time was 9 seconds. An arrow shows the timeline at the bottom of the Editor window. Now we need to enlarge the scale of the audio file. In order not to count the impulses in one second (they are long to count), we will count them for a time interval of 0.1 seconds, and then multiply by 10. First, along the timeline, select a recording area a little over 0.5 seconds and stretch it to the entire screen.
The selected area ~ 0.5 sec is stretched to fill the entire screen. The timeline is also stretched out.
Now select a period of time on the timeline smooth 0.1 sec - from 3.1 to 3.2 sec.
and also stretch it to full screen. Now you can see clear impulses, which are not difficult to calculate.
We count impulses in a time interval of 0.1 sec. - there are 42 of them.
And now for some simple arithmetic. Every 0.1 sec. we have 42 impulses, which means in 1 sec. they received 420 from the sensor. And in 1 minute 420 x 60 sec. = 25200 impulses. But since the propeller has 2 blades and interrupts the luminous flux twice, the result must be divided by 2 and we get 12600 rpm. What was required to be determined. In the case of a 3-blade propeller, we divide the result by 3. In the case of a 4-blade propeller, we divide it by 4. Such an unusual tachometer - the synthesis of an IR diode, a computer and a sound editor, quite satisfied me! And the question of purchasing an "iron" tachometer in a store,
I have disappeared by itself. And he refused the purchase.
I don't need a tachometer when flying in the field, but at home a computer and a cable with an LED are always at hand.
I think that not all colleagues at home already have a tachometer, but I want to measure the engine speed! In this case, my experience, I hope, will be useful to my comrades. "Adobe Audition" can be downloaded free of charge from http://www.fayloobmennik.net/2293677. You can use another sound editor, whoever you like. My sound file of this engine test, recorded by the Editor, is here. In this article, I wanted to show that, if necessary, if you really want to, in most cases that arise with us, modelers, you can come up with a worthy replacement for the necessary, but absent, device. I hope the Chinese comrades are not offended by me.
Old and used asynchronous machines Soviet production are considered the highest quality and most durable. However, as many electricians know, the nameplates on them can be completely unreadable, and in the engine itself they could be rewound. You can determine the rated speed by the number of poles in the winding, but if we are talking about machines with a phase rotor or there is no desire to disassemble the case, you can resort to one of the proven methods.
Determination of speed using a graphic drawing
There are circular graphical drawings to determine the engine speed. The bottom line is that a paper circle with a given pattern glued to the end of the shaft, when rotating, forms a certain graphic effect when illuminated by a light source with a frequency of 50 Hz. Thus, after going through several figures and comparing the result with the tabular data, you can determine the nominal speed of the motor.Typical installation dimensions
Industrial production in the USSR, like most modern ones, was produced according to state standards and has an established correspondence table. Based on this, it is possible to measure the height of the center of the shaft relative to the landing plane, its and rear diameters, as well as the dimensions of the mounting holes. In most cases, this data will be enough to find in the table desired engine and not only determine the rotational speed, but also establish its electrical and net power.With a mechanical tachometer
Very often it is necessary to determine not only the nominal characteristic of an electric machine, but also to know the exact number of revolutions at a given moment. This is done when diagnosing electric motors and to determine an accurate indicator of the slip coefficient.In electromechanical laboratories and in production, special devices are used - tachometers. If you access such equipment, measure the speed asynchronous motor can be done in a few seconds. The tachometer has a dial or digital dial and a measuring rod, at the end of which there is a hole with a ball. If you lubricate the centering hole on the shaft with viscous wax and press the dipstick firmly against it, the dial will display the exact RPM.
With a stroboscopic effect detector
If the engine is in service, you can avoid having to detach it from the actuator and remove the rear cowl just to reach the centering hole. The exact number of revolutions in these cases can also be measured using a stroboscopic detector. To do this, a longitudinal risk of white is applied to the motor shaft and a light catcher of the device is installed opposite it.When the engine is turned on, the device will determine the exact number of revolutions per minute by the frequency of the appearance of the white spot. This method is used, as a rule, in the diagnostic examination of powerful electric cars and the dependence of the speed on the applied load.
Using a cooler from a personal computer
A very original method can be used to measure engine speed. It uses a paddle fan for cooling from a personal computer. The propeller is attached to the end of the shaft with double-sided tape, and the fan frame is held by hand. The fan wire is connected to any of the motherboard connectors, where measurements can be made, while the cooler itself does not need to be supplied with power. An accurate RPM reading can be obtained through the BIOS utility or a diagnostic utility running under the operating system.Sometimes, in the process of work, it is necessary to determine the number of revolutions of an asynchronous electric motor, on which there is no tag. And not every electrician can cope with this task. But you need to understand this. Determining the number of revolutions of an electric motor is very easy and simple.
We determine it by the winding. To do this, you need to remove the engine cover. It is better to do this with the back cover, since the pulley or half-coupling does not need to be removed.
It is enough to remove the cooling jacket and the impeller and the engine cover will be accessible. After removing the cover, the winding can be seen quite well. Find one section and see how much space it takes up around the circumference of the circle (stator). Now remember: if the coil spans half a circle (180 degrees), this is a 3000 rpm motor.
If there are three sections in a circle (120 degrees), this is a 1500 rpm engine. If the stator accommodates four sections (90 degrees), this motor is 1000 rpm.
This is how it is quite simple to determine the number of revolutions of an "unknown" electric motor. This can be clearly seen in the presented figures.
This determination method is suitable when the winding coils are wound in sections. And there are "loose" windings, and here this method will not work. But "loose" windings are rare.
There is another method for determining the number of revolutions. There is a residual magnetic field in the rotor of the electric motor, which can induce a small EMF in the stator winding if we rotate the rotor. This EMF can be "caught" with a milliammeter. Our task is as follows: you need to find the winding of one phase, regardless of how the windings are connected, delta or star. We connect a milliammeter to the ends of the winding. Rotating the motor shaft, we see how many times the milliammeter needle will deviate in one rotor revolution.
From this table you can see what kind of engine is in front of you:
- (2p) 2 3000 r / min;
- (2p) 4 1500 r / min;
- (2p) 6 1000 r / min;
- (2p) 8 750 r / min.
In the USSR, the TCh10-R device was produced, maybe someone has preserved it. For those who did not see and did not know about such a meter, I am attaching a photo. The set includes two attachments: for measuring revolutions along the shaft axis and for measuring along the circumference of the shaft.
You can also measure the number of revolutions using a digital laser tachometer
Specifications:
- Range: 2.5rpm ~ 99999rpm.
- Resolution / step: 0.1 rpm for the range 2.5 ~ 999.9 rpm, 1 rpm 1000 rpm or more.
- Accuracy: +/- 0.05%.
- Working distance: 50mm ~ 500mm.
- The minimum and maximum values are also indicated.
So, let's start assembling. As already mentioned, a homemade tachometer consists of two main parts: a motor powered by direct current and a voltmeter. If you don't have such a motor, you can easily buy it at a flea market for the price of a loaf of bread or cheaper, for the price of two loaves you can buy a new one in an electronic component store. If there is no voltmeter, it will cost more than a motor, but in the same flea market its price will be quite acceptable. The voltmeter is connected to the contacts of the motor, and that's it, the tachometer is ready. Now you need to test the finished tachometer in operation. When the motor generator shaft rotates, a voltage proportional to the rotational speed will be generated. Consequently, the speed will be proportional to the readings of the voltmeter.
You can calibrate such a tachometer in different ways. For example, build a reference graph of the voltage versus armature rotation frequency or make a new voltmeter scale, on which the number of revolutions is recorded instead of voltages.
Since the graph reflects a linear relationship, it is enough to mark two or three points and draw a straight line through them. Obtaining control points is the most problematic stage in preparing a homemade tachometer for work. If there is access to branded machines, control points can be easily obtained by clamping a rubber tube put on the motor shaft in the chuck of a drilling or lathe and turning on the machine in different gears, fixing the voltmeter readings (the spindle rotation speed in each gear is indicated in the machine's passport). Otherwise, for calibration, you will have to use either a drill or a motor in the operating mode for which the rotational speed is known. And even if it was possible to measure the voltage at the motor contacts only for one speed of rotation, the second point is the intersection of the axes (x) and (y) (that is, the number of revolutions and voltage), although the measurement accuracy based on the dependence based on two points will be low.
To measure the rotational speed, the shaft of the engine under study is connected to the motor with a small piece of rubber tube or using various adapters. If the voltmeter goes off scale when measuring high rotation speeds, a switch with additional resistors is introduced into the circuit. You will also need to rebuild the graph for each switch position.
The capabilities of the device can be significantly expanded. If you make a roller friction adapter with a diameter of 31.8 mm, the tachometer will also measure the linear speed, expressed in meters per minute. To do this, the number of revolutions per minute, determined according to the schedule, is divided by 10.
The measurement accuracy depends practically only on the accuracy of the graph and the voltmeter division price. Such a simple and very cheap home-made tachometer can be widely used wherever you need to quickly determine the frequency or speed of rotation of shafts, pulleys and other parts.
Do-it-yourself digital tachometer from a smartphone
If you own an iPhone, then I strongly advise you to install best app for rpm measurement shown below. And do not stop at the stroboscope from the phone flash, it will just help you understand how the stroboscope-tachometer works. DIY very simple electronic circuits You will receive stroboscopic and laser tachometers that are not inferior (and in some situations superior) to proprietary tachometers. Diagrams, photos and descriptions of tachometers can be found in this application. See a video demonstrating this application below.
DIY homemade stroboscopic tachometer from iPhone
Homemade laser (optical) tachometer from iPhone do it yourself
Comparative measurements of engine speed with laser and stroboscopic tachometers
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