Determination of electric motor power by current, size, shaft diameter

In practice, it is not always necessary to work with electric motors for which the operating parameters are known. This information is usually indicated on the tag, but it may be overwritten or missing altogether. What to do in such a situation, not to throw out the "motor"? In this article, we will tell you how to determine the power of an electric motor by overall dimensions, current and other indicators. Let's make a reservation that the article will focus more on three-phase asynchronous electric motors, since they are the most common.

Content:

First we look at the tag
Compare overall dimensions
Estimated calculation for no-load current and voltage
Calculation based on speed and torque
Conclusion

First we look at the tag

The simplest is the way to determine the engine power by the nameplate (it is also called a plate or tag). First of all, it is worth remembering that the number indicated on the tag is the mechanical power on the shaft, the so-called. R₂. To find the active electric P₁ (which will be taken into account by your meter), it must be divided by the efficiency (η), and in order to find the total S, then also divided by COSph, you will find them on the same nameplate.

P1 = P2 / η = 180 / 0.68 = 265 (W)

S = P1 / cosF = 265 / 0.78 = 340 (W)

And if only the current is indicated, you can determine the total power according to the standard formula for three-phase circuits:

S = UI * 1.73

If, following the example of the above nameplate, then:

S = 380 * 0.52 * 1.73 = 341 (VA)

Then active:

P1 = S * cosF = 341 * 0.78 = 266 (W)

And mechanical P2 on the shaft:

P2 = P1 * η = 180.8 (W)

As you can see, the results of calculations for current and voltage coincided with the numbers indicated on the plate. From the nameplate, you can determine other parameters of the electric motor, such as nominal voltage, amperage, number of revolutions per minute.

Compare overall dimensions

If there is no plate or it is difficult to read something on it, then it is possible to determine the power of an asynchronous electric motor without a passport by its dimensions, namely by the diameter of the shaft.

This method of determination is used in practice more often than others, since you only need to measure the shaft with a caliper and you do not need to connect to the network. After measuring the diameter, the values obtained are compared with the table and the approximate power is determined. This method allows you to get fairly accurate characteristics without a tag. The table for this is shown below.

This method of determining the power of an electric motor by size (by rotor) is suitable for both three-phase and single-phase asynchronous motors. Note "P" is indicated in kW (kilowatts), as is customary in electrical engineering, and not as in physics - in watts.

If for some reason the data from this table does not suit you, that is, there is another way to find out the power of the electric motor by its overall dimensions, you need to measure:

shaft diameter;
its rotation frequency (number of pole pairs);
mounting dimensions;
flange diameter or width of the fixing feet;
height to the center of the shaft;
motor length (excluding shaft protruding).

And compare these data with the dimensions of electric machines of a single series 4A, AIR, A, AO. They can be found in various directories or catalogs of the companies that produce them.

To determine the engine power of the common AIR series by the mounting holes on the feet, use this table.

To determine the motor power from the flange diameter (D20) and the diameter of the flange mounting holes (D22), use the following data:

With time and practice, you will learn to roughly determine the engine power by its appearance, mentally comparing with those that we have encountered before, but for this you need to know a number of standard denominations electric motors: 0.25; 0,37; 0,55; 0,75; 1,1; 1,5; 2,2; 3,0; 4,0; 5,5; 7,5; 11; 15; 18,5; 22; 30; 37; 45; 55; 75 kW.

Estimated calculation for no-load current and voltage

It is possible to determine the power of an electric motor by current or, as amateurs say, “by amperage”. But measure the current when the machine is under load to find out its rated power is wrong, because you cannot know how it works under rated load, overload, or vice versa underutilized. The stator current depends on the load. This means that you will not measure the rated current, but the current consumption at this moment.

So, you need to measure the no-load current, that is, when the engine is running without load. Before you measure anything, in order to obtain correct data, you need it to work for some time, namely 0.5-1 hour for motors up to 100 kW and 1-2 hours - over 100 kW. After the measurement, according to the table, find out the typical deviations of Iхх from Inom in percent and calculate the estimated Inom.

Let's give an example, let's say you measured the current, it turned out that it is 5 Amperes. We estimate the power of the engine "by eye", let's say that it is quite large, and you assume that it is more than 5 kW. Moreover, it is a "three-thousander", that is, its shaft rotates at a frequency of 3000 rpm. Then the measured no-load current is 40% (or 0.4) of the nominal. To find out the rated current, you need to divide Iхх by percentages from the table:

Inom = Iхх / 0.4 = 5 / 0.4 = 12.5A

Then the total and active power can be determined by the formulas:

S = UI * 1.73 = 380 * 12.5 * 1.73 = 8217 W = 8.2 kW.

Let us assume that the cosF of the motor is 0.85, and its efficiency is 0.8, then the active P1 is equal to:

P = Iav * Uav * 1.73 * cosf * efficiency = 12.5 * 380 * 1.73 * 0.85 * 0.8 = 5.5 kW

True, there are no standard asynchronous three-phase motors with such parameters, the numbers were taken just for example, but in the above way you can find out the motor power, knowing the current and voltage.

Calculation based on speed and torque

To select a motor for a specific mechanism, you can determine the engine power by the torque and number of revolutions that are required on the shaft. To do this, use the formula:

P = M * n / 9550,

where M is the moment, n is the number of revolutions, 9550 is the coefficient.

Conclusion

We examined the main ways to determine the power of an electric motor. There are other methods, for example, on the resistance of the windings, but it cannot be accurate, since after rewinding it may not correspond to the passport data. And in order to accurately measure the resistance of the stator windings of powerful motors, precise measuring instruments are needed, the so-called measuring bridge, or to make measurements by the voltmeter-ammeter method. What to do in practice, no one will, and with a multimeter it will not be possible to accurately make such measurements.

The method for determining the parameters of an electric motor by weight also cannot be called accurate, it consists in the fact that, on average, the weight of an asynchronous electric motor is equal to:

for 3000 rpm - 7-9 kg per 1 kW;
for 1500 rpm - 11-13 kg / kW;
For 1000 rpm - 14-15 kg / kW.

But it cannot be called accurate at all, the housings of modern electric motors are made of aluminum and are lighter up to 30%, according to compared to the old Soviet ones, while a protected electric motor will weigh more than its unprotected analogue. Therefore, this method, although it has the right to life, is more like fortune telling on coffee grounds.

Perhaps the simplest definition of the power of an electric motor is by size, shaft diameter, etc. with subsequent comparison with the catalog data of the engines of the same series.

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