Power rating
In electrical engineering and mechanical engineering, the power rating of equipment is the highest power input allowed to flow through particular equipment. According to the particular discipline, the term power may refer to electrical or mechanical power. A power rating can also involve average and maximum power, which may vary depending on the kind of equipment and its application.
Power rating limits are usually set as a guideline by the manufacturers, protecting the equipment and simplifying the design of larger systems, by providing a level of operation under which the equipment will not be damaged while allowing for a certain safety margin.
Equipment types
Dissipative equipment
In equipment which primarily dissipate
If all power in a device is dissipated as heat, then this is also the power rating.
Mechanical equipment
Equipment is generally rated by the power it will deliver, for example, at the shaft of an electric or hydraulic motor. The power input to the equipment will be greater owing to the less than 100% efficiency of the device.[1][2][3] Efficiency of a device is often defined as the ratio of output power to the sum of output power and losses. In some types of equipment it is possible to measure or calculate losses directly. This allows efficiency to be calculated with greater precision than the quotient of input power over output power, where relatively small measurement uncertainty will greatly affect the resulting calculated efficiency.
Power converting equipment
In devices that primarily
Power ratings are usually given in
Average vs. maximum
For AC-operated devices (e.g.
Maximum continuous rating
Maximum continuous rating (MCR) is defined as the maximum output (MW) that an electric power generating station is capable of producing continuously under normal conditions over a year. Under ideal conditions, the actual output could be higher than the MCR.[7]
Within
Other definitions
In some fields of engineering, even a more complex set of power ratings is used. For example, helicopter engines are rated for continuous power (which does not have a time constraint), takeoff and hover power rating (defined as half to one hour operation), maximum contingency power (which can be sustained for two-three minutes), and emergency (half a minute) power rating.[9]
For electrical motors, a similar kind of information is conveyed by the service factor, which is a multiplier that, when applied to the rated output power, gives the power level a motor can sustain for shorter periods of time. The service factor is typically in the 1.15-1.4 range, with the figure being lower for higher-power motors. For every hour of operation at the service-factor-adjusted power rating, a motor loses two to three hours of life at nominal power, i.e. its
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.
Power ratings for electrical apparatus and transmission lines are a function of the duration of the proposed load and the ambient temperature; a transmission line or transformer, for example, can carry significantly more load in cold weather than in hot weather. Momentary overloads, causing high temperatures and deterioration of insulation, may be considered an acceptable trade-off in emergency situations. The power rating of switching devices varies depending on the circuit voltage as well as the current. In certain aerospace or military applications, a device may carry a much higher rating than would be accepted in devices intended to operate for long service life.