With the term "rated power" we come across almost every day. Whether we choose an electric kettle or an incandescent lamp - this value is indicated everywhere. The unit is watts or kilowatts. It would seem - what could be simpler in this matter? After all, from 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 "rated power"?
The term “nominal” means a certain value of something that does not take into account external corrective factors. Thus, the rated power is the value indicated by the manufacturer, which can be obtained only with the specified design parameters. This is a general concept. In each particular case, it is necessary to take into account its specific features. We give an example with an incandescent lamp. On her glass flask marked: 230 V, 100 watts. That is, 100 W can only be achieved with a voltage of 230 V. The rated power is the same 100 W. Its value decreases with decreasing voltage and increases with increasing since these parameters are directly dependent on each other (P = I * U).
As a rule, for most electrical appliances there is a limit on the upper limit, usually 5-10%. In other words, operation at 230 V + 23 V = 253 V is permissible. The lower limit may not be indicated, as is the case with the lamp. More sophisticated 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 from the point of view of electricity, and the other based on the estimated mechanical load on the shaft. Although they are directly interconnected, the second is easier to understand. We will give both. The nameplate always shows the power value. It is numerically equal to that consumed from the electrical network at the calculated mechanical load, and the temperature of the case must be within acceptable limits (continuous operation is assumed). That is, we can assume that the passport value is equal to the nominal. If the drive operates in intermittent mode (PV is not equal to 100%), then this correspondence is not fulfilled, since the operating time is not enough to switch to the steady state, when the increase in heating is compensated by the ambient temperature. In this case, a load schedule is required: the rated power will be equal to the product of the passport value P and the square root of the coefficient selected from the schedule. All of the above is true for the electrical component.
According to another definition, the rated power is taken to be equal to the mechanical power developed by the engine at the calculated voltage value and the temperature regime corresponding to the rated one. Thus, if the voltage (U) decreases, then the torque also changes , although the shaft rotation speed may remain the same. As it was said, a certain “margin of safety” is laid in the product by the manufacturer: fluctuations of U within + -5% allows the engine to develop the calculated moment (with the network frequency being unchanged). For frequency, this margin is only 2.5%.
But the rated power of the transformer takes into account only the temperature regime. If you look in the passport of the device, then two temperatures are indicated there: 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 out rated power. Any increase in electrical load causes an increase in current and temperature, so control of the latter is quite enough. As is the case with engines, a slight excess is allowed.