View source for Physics:Power rating

{{short description|Highest power input allowed to flow through electrical or mechanical equipment}}

In [[Electrical engineering|electrical engineering]] and [[Engineering:Mechanical engineering|mechanical engineering]], the '''power rating''' of equipment is the highest [[Physics:Power|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 [[Physics:Electric power|electric power]] or convert it into mechanical power, such as [[Engineering:Resistor|resistor]]s, and [[Engineering:Loudspeaker|speaker]]s, the power rating given is usually the maximum power that can be safely [[Dissipation|dissipated]] by the equipment.  The usual reason for this limit is [[Physics:Heat|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 [[Physics:Temperature|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 [[Physics:Operating temperature|operating temperature]] of the device, <math>T_{A}</math> is the ambient temperature, and <math>\theta_{DA}</math> is the total [[Physics:Thermal resistance|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.

=== 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.<ref name="AtkinsAtkins2013">{{cite book|author1=Anthony G. Atkins|author2=Tony Atkins|author3=Marcel Escudier|title=A Dictionary of Mechanical Engineering|url=https://books.google.com/books?id=0TjtKmSIL48C&pg=PA269|year=2013|publisher=[[Organization:Oxford University Press|Oxford University Press]]|isbn=978-0-19-958743-8|page=269}}</ref><ref name="Thumann2010">{{cite book|author=Albert Thumann|title=Plant Engineers and Managers Guide to Energy Conservation|url=https://books.google.com/books?id=D7xO-d6gLUgC&pg=PA320|year=2010|publisher=The Fairmont Press, Inc.|isbn=978-0-88173-657-1|page=320}}</ref><ref name="Eccles2008">{{cite book|author=William J. Eccles|title=Pragmatic Power|url=https://books.google.com/books?id=HWntzQ4vY3kC&pg=PA74|year=2008|publisher=Morgan & Claypool Publishers|isbn=978-1-59829-798-0|page=74}}</ref> 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 [[Engineering:Power converter|convert]] between different forms of electric power, such as [[Engineering:Transformer|transformer]]s, or transport it from one location to another, such as [[Engineering:Transmission line|transmission line]]s, the power rating almost always refers to the maximum power flow through the device, not dissipation within it. The usual reason for the limit is heat, and the maximum heat dissipation is calculated as above.

Power ratings are usually given in [[Watt|watt]]s for real power and [[Volt-ampere|volt-ampere]]s for apparent power, although for devices intended for use in large power systems, both may be given in a [[Physics:Per-unit system|per-unit system]]. Cables are usually rated by giving their maximum voltage and their [[Engineering:Ampacity|ampacity]].<ref name="Patel2012">{{cite book|author=Mukund R. Patel|title=Introduction to Electrical Power and Power Electronics|url=https://books.google.com/books?id=Am5CI32itwAC&pg=PA54|year=2012|publisher=CRC Press|isbn=978-1-4665-5660-7|pages=54–55}}</ref> As the power rating depends on the method of cooling, different ratings may be specified for air cooling, water cooling, etc.<ref name="Patel2012"/>

== Average vs. maximum ==
For AC-operated devices (e.g. [[Engineering:Coaxial cable|coaxial cable]], loudspeakers), there may even be two power ratings, a maximum (peak) power rating and an average power rating.<ref name="Whitaker2005">{{cite book|editor=Jerry C. Whitaker|title=The Electronics Handbook, Second Edition|url=https://books.google.com/books?id=FdSQSAC3_EwC&pg=PA314|year=2005|publisher=CRC Press|isbn=978-1-4200-3666-4|pages=314–315}}</ref><ref name="Davis1989">{{cite book|author1=Gary Davis|author2=Ralph Johnes|title=The Sound Reinforcement Handbook|url=https://books.google.com/books?id=d7ft6F8ZUdcC&pg=PA232|year=1989|publisher=Hal Leonard Corporation|isbn=978-1-61774-545-4|page=232|edition=2nd}}</ref> For such devices, the peak power rating usually specifies the low frequency or pulse energy, while the average power rating limits high-frequency operation.<ref name="Whitaker2005"/> Average power calculation rating depends on some assumptions how the device is going to be used. For example, the EIA rating method for loudspeakers uses a shaped noise signal that simulates music and allows peak excursion of 6&nbsp;dB, so an EIA rating of 50 Watts corresponds to 200 Watts peak rating.<ref name="Davis1989"/>

== 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.<ref>{{cite web|url=http://www.ieso.ca/imoweb/marketdata/genDisclosure.asp|title=IESO|url-status=dead|archive-url=https://web.archive.org/web/20130903020047/http://ieso.ca/imoweb/marketdata/genDisclosure.asp|archive-date=2013-09-03}}</ref>

Within shipping, ships usually operates at the '''nominal continuous rating''' ('''NCR''') which is 85% of the 90% of MCR. The 90% MCR is usually the contractual output for which the propeller is designed. Thus, the usual output at which ships are operated is around 75% to 77% of MCR.<ref>Danish proposal to a design CO2 index for new ships to the UN’s International Maritime Organization (IMO) from the [http://www.dma.dk/sw22623.asp Danish Maritime Authority]{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

== Other definitions ==
In some fields of engineering, even a more complex set of power ratings is used. For example, [[Engineering:Helicopter#Engines|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 [[Engineering:Contingency (electrical grid)|contingency]] power (which can be sustained for two-three minutes), and emergency (half a minute) power rating.<ref name="SeddonNewman2011">{{cite book|author1=John M. Seddon|author2=Simon Newman|title=Basic Helicopter Aerodynamics|url=https://books.google.com/books?id=xUN75AXYhFgC&pg=PA231|year=2011|publisher={{wipe|John Wiley & Sons}}|isbn=978-1-119-97272-3|page=231|edition=3rd}}</ref>

{{Anchor|SERVICE-FACTOR}}
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 [[Social:Service life|service life]] is reduced to less than half for continued operation at this level.<ref name="Patel2012"/><ref name="PE2013">{{cite book|author=Michael R. Lindeburg, PE|title=Mechanical Engineering Reference Manual for the PE Exam|url=https://books.google.com/books?id=4P6jjodswwAC&pg=SA72-PA13|year=2013|publisher=www.ppi2pass.com|isbn=978-1-59126-414-9|pages=72–}}</ref> The service factor is defined in the ANSI/NEMA MG 1 standard,<ref>{{cite web |url=http://www.nema.org/Standards/ComplimentaryDocuments/Contents%20and%20Forward%20MG%201.pdf |title=Archived copy |access-date=2014-02-11 |url-status=dead |archive-url=https://web.archive.org/web/20140222101016/http://www.nema.org/Standards/ComplimentaryDocuments/Contents%20and%20Forward%20MG%201.pdf |archive-date=2014-02-22 }}</ref> and is generally used in the United States.<ref name="ToliyatKliman2004">{{cite book|author1=Hamid A. Toliyat|author2=Gerald B. Kliman|title=Handbook of Electric Motors|url=https://books.google.com/books?id=4-Kkj53fWTIC&pg=PA181|year=2004|publisher=CRC Press|isbn=978-0-8247-4105-1|page=181}}</ref> There is no [[Organization:International Electrotechnical Commission|IEC]] standard for the service factor.<ref name="Senty2012">{{cite book|author=Steve Senty|title=Motor Control Fundamentals|url=https://books.google.com/books?id=XYsJAAAAQBAJ&pg=PA81|year=2012|publisher=Cengage Learning|isbn=978-1-133-70917-6|page=81}}</ref>

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 [[Physics:Semiconductor|semiconductor]]s, 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.

== Examples ==

=== Audio amplifiers ===

For instance, the Federal Trade Commission (FTC) established an amplifier rating system in which the device is tested with both channels driven throughout its advertised frequency range, at no more than its published distortion level. The Electronic Industries Association (EIA) rating system, however, determines amplifier power by measuring a single channel at 1,000&nbsp;Hz, with a 1% distortion level—1% clipping. Using the EIA method rates an amplifier 10 to 20% higher than the FTC method.<ref name=Quilter/>

=== Photovoltaic modules ===

The nominal power of a photovoltaic module is determined by measuring current and voltage while varying resistance under defined illumination.  The conditions are specified in standards such as IEC 61215, IEC 61646 and UL 1703; specifically, the light intensity is  1000&nbsp;W/m<sup>2</sup>, with a spectrum similar to sunlight hitting the earth's surface at latitude 35°&nbsp;N in the summer (airmass&nbsp;1.5) and temperature of the cells at 25&nbsp;°C.  The power is measured while varying the resistive load on the module between open and closed circuit.

The maximum power measured is the nominal power of the module in Watts. Colloquially, this is also written as "W<sub>p</sub>"; this format is colloquial as it is outside the standard by adding suffixes to [[International System of Units|standardized units]].  The nominal power divided by the light power that falls on the module (area x 1000&nbsp;W/m<sup>2</sup>) is the ''efficiency''.

== See also ==
* [[Effective radiated power]], the regulatory analogue for VHF, UHF and FM broadcasting

== References ==
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[[Category:Electrical parameters]]
[[Category:Electric power]]

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