Measurement of engine speed. How to determine the speed of an electric motor. Typical installation dimensions

The rotational speed of an induction motor is usually understood as the angular frequency of rotation of its rotor, which is shown on the nameplate (on the motor nameplate) in the form of revolutions per minute. A three-phase motor can also be powered from a single-phase network, for this, parallel to one or two of its windings, depending on the mains voltage, but the design of the motor will not change from this.

So, if the rotor under load makes 2760 revolutions per minute, it will be equal to 2760 * 2pi / 60 radians per second, that is, 289 rad / s, which is not convenient for perception, therefore they simply write “2760 rpm” on the plate. For an induction motor, this is the slip speed s.

Synchronous speed this engine(excluding slip) will be equal to 3000 rpm, since when the stator windings are supplied with a mains current with a frequency of 50 Hz, every second the magnetic flux will make 50 complete cyclic changes, and 50 * 60 = 3000, so it turns out 3000 rpm - synchronous speed of an asynchronous electric motor.

In this article, we will talk about how to determine the synchronous rotation speed of an unknown asynchronous three-phase motor, just by looking at its stator. By appearance stator, by the location of the windings, by the number of slots - you can easily determine the synchronous revolutions of the electric motor if you do not have a tachometer at hand. So, let's start in order and analyze this issue with examples.

3000 rpm

About asynchronous electric motors (see -) it is customary to say that this or that motor has one, two, three or four pairs of poles. The minimum is one pair of poles, that is, the minimum is two poles. Take a look at the picture. Here you can see that the stator contains two series-connected coils for each phase - in each pair of coils, one is located opposite the other. These coils form a pair of poles on the stator.

One of the phases is shown in red for clarity, the second in green, and the third in black. The windings of all three phases are arranged in the same way. Since these three windings are powered in turn (three-phase current), then for 1 oscillation out of 50 in each of the phases - the stator magnetic flux will once turn at full 360 degrees, that is, it will make one revolution in 1/50 of a second, which means 50 revolutions will be obtained in give me a sec. And so it comes out 3000 rpm.

Thus, it becomes clear that to determine the synchronous revolutions of an asynchronous electric motor, it is enough to determine the number of pairs of its poles, which is easy to do by removing the cover and looking at the stator.

Divide the total number of stator slots by the number of slots per winding section of one of the phases. If you get 2, then you have a motor with two poles - with one pair of poles. Hence the synchronous frequency is 3000 rpm, or about 2910 with slip. In the simplest case, there are 12 grooves, 6 grooves per coil, and there are 6 such coils - two for each of the three phases.

Please note that the number of coils in one group for one pair of poles may not necessarily be 1, but also 2 and 3, however, as an example, we considered the option with single groups for a pair of coils (we will not focus on winding methods in this article).

1500 rpm

To obtain a synchronous speed of 1500 rpm, the number of stator poles is doubled so that for 1 oscillation out of 50, the magnetic flux would make only half a revolution - 180 degrees.

For this, 4 winding sections are made for each phase. Thus, if one coil occupies a quarter of all slots, then you have a motor with two pole pairs formed by four coils per phase.

For example, 6 grooves out of 24 are occupied by one coil, or 12 out of 48, which means you have a motor with a synchronous frequency of 1500 rpm, or taking into account the slip of approximately 1350 rpm. In the above photo, each section of the winding is made in the form of a double coil group.

1000 rpm

As you already understood, to obtain a synchronous frequency of 1000 rpm, each phase already forms three pairs of poles, so that in one oscillation of 50 (hertz) the magnetic flux would turn only 120 degrees, and would accordingly turn the rotor behind it.

Thus, at least 18 coils are installed on the stator, with each coil occupying one sixth of all slots (six coils per phase - three pairs). For example, if there are 24 slots, then one coil will take 4 of them. The resulting frequency, taking into account the slip, is about 935 rpm.

750 rpm

To obtain a synchronous speed of 750 rpm, it is necessary that the three phases form four pairs of moving poles on the stator, this is 8 coils per phase - one opposite the other - 8 poles. If, for example, 48 slots have a coil for every 6 slots, you have an asynchronous motor with synchronous speeds of 750 (or about 730, taking into account slip).

500 rpm

Finally, to get asynchronous motor with a synchronous speed of 500 rpm, 6 pairs of poles are needed - 12 coils (poles) per phase, so that for each network oscillation, the magnetic flux would rotate 60 degrees. That is, if, for example, the stator has 36 slots, while there are 4 slots per coil, you have a three-phase motor at 500 rpm (480, taking into account slip).

When buying an electric motor from your hands, count on the availability technical documentation it is not necessary to it. Then the question arises of how to find out the number of revolutions of the purchased device. You can trust the seller's words, 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 open to sight, 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 slots 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 slightly complicates the task of determining the number of revolutions.

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 or along a conveyor, to adjust the performance of pumps, to reduce or increase the air consumption in ventilation systems.

It is practically pointless to carry out these procedures by lowering the voltage, the speed will drop sharply, and 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 semiconductor triodes (transistors) high power 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 several degrees of protection. 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 case asynchronous models is as follows:

Significant savings in electrical energy are achieved. Since not all equipment requires high rotational speeds of the motor shaft, it makes sense to reduce it by a quarter.

Provided reliable protection all mechanisms. 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.

The 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 for 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

Electric motor - stator winding

From time to time in the process of work, you need to find the number of revolutions of the asynchronous electric motor, on which there is no tag. And far from every electrician can cope with this task. But my worldview is that every electrical fitter should understand this. At your own workplace, as they say - on duty, you understand all the properties of your own engines. And they ran to a new one workplace, and there are no tags on any engine. Finding the number of revolutions of an electric motor is even very simple and simple. Determined by the winding. To do this, remove the motor cover. It is better to do this with the back cover, since the pulley or half-coupling does not need to be removed. Enough to remove the shroud

cooling and impeller and motor cover are available. After removing the cover, the winding can be seen quite well. Find one section and see how many

Engine - 3000 rpm

it takes places around the circumference of a circle (stator). Now remember, if the coil takes half a circle (180 degrees) - this is a 3000 rpm engine.

Engine - 1500 rpm

If three sections fit in a circle (120 degrees), this is a 1500 rpm engine. Well, if the stator accommodates four sections (90 degrees) - this engine is 1000 rpm. This is how it is quite simple to find the number of revolutions of an "unknown" electric motor. This can be seen perfectly in the presented figures.

Engine - 1000 rpm

This is a way of detecting when the winding coils are wound in sections. And there are “loose” windings, which cannot be found in this way. This winding method is rare.

There is another way to determine the number of revolutions. In the rotor of an electric motor, there is a residual magnetic field that can induce a small EMF in the stator winding if we turn the rotor. This EMF can be “caught” - with a milliammeter. Our task is as follows: it is necessary to find the winding of one phase, regardless of how the windings are connected, with a triangle or a star. And we connect a milliammeter to the ends of the winding, rotating the motor shaft, see how many times the milliammeter needle deviates in one rotor revolution, and from this table you can see what kind of engine you are determining.

(2p) 2 3000 r / min
(2p) 4 1500 r / min
(2p) 6 1000 r / min
(2p) 8 750 r / min

These are the usual and I think two understandable ways of determining the number of revolutions on which there is no tag (plate).

In the USSR, the TCh10-R device was produced, maybe someone has preserved it. Who has not seen and did not know about such a meter, I suggest looking at a photo of your own. The set includes two nozzles - for measuring revolutions along the axis of the shaft and the second one for measuring along the circumference of the shaft.

You can also measure the number of revolutions using the "Digital Laser Tachometer"

"Digital Laser Tachometer"

Technical properties:

Spectrum: 2.5rpm ~ 99999rpm
Resolution / step: 0.1 rpm for spectrum 2.5 ~ 999.9 rpm, 1 rpm 1000 rpm or more
Accuracy: +/- 0.05%
Working distance: 50mm ~ 500mm
The lowest and highest value is also indicated.
For those who really need it - just a super thing!
L. Ryzhenkov

When operating any machine, you cannot do without an electric motor. Many people buy an electric motor from their hands without any documentation. In such a situation, there is a problem with determining the speed of the electric motor. To solve this problem, you can use several methods.

All the main characteristics of the electric motor must be indicated on a metal tag located on its body. But in practice, the tag is either missing, or the information has been erased during operation.

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 pulses 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 more than 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 on the timeline, select a period of time 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. Once 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 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.