What is the rated power

With the term "nominal power" we encounter almost daily. Whether we choose an electric kettle or an incandescent lamp - this value is indicated everywhere. The unit of measurement is watts or kilowatts. It would seem - what could be simpler in this matter? After all, with the school course of physics, everyone knows that to determine the power (P) it is enough to multiply the values of current and voltage. But what is hidden behind the words "nominal power"?

By the term "nominal" is meant a certain value of something that does not take into account external corrective factors. Thus, the rated power is the value specified by the manufacturer, which can be obtained only with the specified design parameters. This is a general concept. In each specific case, it is necessary to take into account their specific characteristics. Let's give an example with an incandescent lamp. On its glass flask is marked: 230 V, 100 W. That is, 100 W can be achieved only at a voltage of 230 V. Nominal power - these are the same 100 watts. Its value decreases with decreasing voltage and increases with increasing, since these parameters are in direct dependence on each other (P = I * U).

As a rule, for most electrical appliances there is a restriction on the upper boundary, usually 5-10%. In other words, work at 230 V + 23 V = 253 V is permissible. The lower limit may not be indicated, as in the case with a lamp. More complicated equipment is limited by passport parameters both from above and from below.

For example, how to understand the term "rated engine power"? There are two equal definitions - one in terms of electricity, and the other based on the calculated mechanical load on the shaft. Although they are directly interrelated, the second is simpler to understand. We will quote both. A power rating is always indicated on the rating plate. It is numerically equal to that consumed from the electrical network at the design mechanical load, and the hull temperature must be within the permissible limits (a continuous mode of operation is implied). That is, we can assume that the passport value is equal to the nominal value. If the actuator operates in a short-time mode (PV is not equal to 100%), then this correspondence is not fulfilled, since the operating time is not enough to go to steady mode, when the increase in heating is compensated by the ambient temperature. In this case, a loading schedule is required: the nominal power will be equal to the product of the passport value P and the square root of the selected coefficient. All of the above is true for the electrical component.

According to another definition, the rated power is assumed to be equal to the mechanical one, developed by the engine at the design value of the voltage and the temperature regime corresponding to the passport one. Thus, if the voltage (U) decreases, then the force moment also changes, although the rotation speed of the shaft can remain the same. As it was said, the manufacturer puts into the product a certain "margin of safety": U fluctuations within + -5% allow the engine to develop the calculated torque (with the network frequency unchanged). For a frequency such a stock is only 2.5%.

But the rated power of the transformer takes into account only the temperature regime. If you look at the device's passport, there are two temperatures indicated: nominal and ambient air. If during operation the first does not exceed its calculated value, and the second differs slightly from the passport data, then in this mode the transformer gives the nominal power. Any increase in the electrical load causes an increase in current and temperature, so it is enough control of the latter. As in the case of engines, a slight excess is allowed.