Power rating
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In electrical engineering, the power rating of a device is a guideline set by the manufacturer as a maximum power to be used with that device. This limit is usually set somewhat lower than the level where the device will be damaged, to allow a margin of safety.
The power rating can actually mean a couple different things. In devices which primarily dissipate electric power or convert it into mechanical power, such as resistors, motors, and speakers, the power rating given is usually the maximum power that can be safely dissipated by the device. The usual reason for this limit is heat, although in certain electromechanical devices, particularly speakers, it is to prevent mechanical damage. When heat is the limiting factor, the power rating is easily calculated. First, the amount of heat that can be safely dissipated by the device, <math>P_{D,max}<math>, must be calculated. This is related to the maximum safe operating temperature, the ambient temperature or temperature range in which the device will be operated, and the method of cooling. If <math>T_{D,max}<math> is the maximum safe operating temperature of the device, <math>T_{A}<math> is the ambient temperature, and <math>\theta_{DA}<math> is the total thermal resistance between the device and ambient, then the maximum heat dissipation is given by
- <math>P_{D,max} = \frac{T_{D,max} - T_{A}}{\theta_{DA}}<math>
If all power in a device is dissipated as heat, then this is also the power rating. On the other hand, if most of the power is converted into mechanical power, then we need to know the efficiency, <math>\eta<math>. Then, the power rating is given by
- <math>P_{max} = \frac{P_{D,max}}{1 - \eta}<math>
Note that this is the real or effective power dissipated in the device.
In devices that primarily convert between different forms of electric power, such as transformers, or transport it from one location to another, such as transmission lines, the power rating almost always refers to the maximum power flow through the device. If an amount of power equal to the power rating were actually dissipated in the device, it would certainly be damaged. The usual reason for the limit is again heat, and the maximum heat dissipation is calculated as above. However, there may not be a direct relationship between power dissipated as heat and power converted by the device; simply put, the power converted depends on the power factor of the load whereas the heat dissipated does not. In this case, the maximum current is calculated and the power rating is given by
- <math>S_{max} = V_{nom}I_{max}<math>
where <math>V_{nom}<math> is the nominal operating voltage. Note that the power rating in this case is an apparent power.
Power ratings are usually given in Watts for real power and Volt-Amperes for apparent power, although for devices inteded for use in large power systems, both may be given in a per-unit system. As the power rating depends on the method of cooling, different ratings may be specified for air cooling, water cooling, etc.
Exceeding the power rating of a device by more than the margin of safety set by the manufacturer usually does damage to the device by causing its operating temperature to exceed safe levels. In semiconductors, irreparable damage can occur very quickly. Exceeding the power rating of most devices for a very short period of time is not harmful, although doing so regularly can sometimes cause cumulative damage.