Power rating
Encyclopedia
In electrical and electronic 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 resistor
s, electric motor
s, and speaker
s, 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,
, 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
is the maximum safe operating temperature
of the device,
is the ambient temperature, and 
is the total thermal resistance between the device and ambient, then the maximum heat dissipation is given by
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,
. Then, the power rating is given by
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 transformer
s, or transport it from one location to another, such as transmission line
s, 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
where
is the nominal operating voltage. Note that the power rating in this case is an apparent power.
Power ratings are usually given in watt
s for real power and volt-ampere
s for apparent power, although for devices intended 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 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 are typically established by driving the device under test to the onset of clipping
, to a predetermined distortion level, variable per manufacturer or per product line. Driving an amplifier to 1% distortion levels will yield a higher rating than driving it to 0.01% distortion levels. Similarly, testing an amplifier at a single mid-range frequency, or testing just one channel of a two-channel amplifier, will yield a higher rating than if it is tested throughout its intended frequency range with both channels working. Manufacturers can use these methods to market amplifiers whose published maximum power output includes some amount of clipping in order to show higher numbers. 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 Hz, with a 1% distortion level—1% clipping. Using the EIA method rates an amplifier 10 to 20% higher than the FTC method.
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
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
Safety
Safety is the state of being "safe" , the condition of being protected against physical, social, spiritual, financial, political, emotional, occupational, psychological, educational or other types or consequences of failure, damage, error, accidents, harm or any other event which could be...
.
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 resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s, electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s, and speaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s, 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
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
, 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,
, must be calculated. This is related to the maximum safe operating temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, the ambient temperature or temperature range in which the device will be operated, and the method of cooling
Cooling
Cooling is the transfer of thermal energy via thermal radiation, heat conduction or convection. It may also refer to:-Techniques:* Air conditioning* Air cooling* Computer cooling* Cryogenics* Conduction * Infrared solar cells* Laser cooling...
. If
is the maximum safe operating temperatureOperating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
of the device,
is the ambient temperature, and is the total thermal resistance between the device and ambient, then the maximum heat dissipation is given byIf 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,
. Then, the power rating is given byNote that this is the real or effective power dissipated in the device.
In devices that primarily convert
Power converter
A power converter is an electrical or electro-mechanical device for converting electrical energy. It may be converting AC to or from DC, or the voltage or frequency, or some combination of these.Amongst the many devices that are used for this purpose are;...
between different forms of electric power, such as transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s, or transport it from one location to another, such as transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
s, 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
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
of the load whereas the heat dissipated does not. In this case, the maximum current is calculated and the power rating is given by
where
is the nominal operating voltage. Note that the power rating in this case is an apparent power.Power ratings are usually given in watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s for real power and volt-ampere
Volt-ampere
A volt-ampere is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square voltage and RMS current. In direct current circuits, this product is equal to the real power in watts...
s for apparent power, although for devices intended for use in large power systems, both may be given in a per-unit system
Per-unit system
In the power transmission field of electrical engineering, a per-unit system is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per-unit are the same regardless of the voltage level...
. 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 semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
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.
Audio amplifiers
Audio amplifierAudio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
power ratings are typically established by driving the device under test to the onset of clipping
Clipping (audio)
Clipping is a form of waveform distortion that occurs when an amplifier is overdriven and attempts to deliver an output voltage or current beyond its maximum capability...
, to a predetermined distortion level, variable per manufacturer or per product line. Driving an amplifier to 1% distortion levels will yield a higher rating than driving it to 0.01% distortion levels. Similarly, testing an amplifier at a single mid-range frequency, or testing just one channel of a two-channel amplifier, will yield a higher rating than if it is tested throughout its intended frequency range with both channels working. Manufacturers can use these methods to market amplifiers whose published maximum power output includes some amount of clipping in order to show higher numbers. For instance, the Federal Trade Commission
Federal Trade Commission
The Federal Trade Commission is an independent agency of the United States government, established in 1914 by the Federal Trade Commission Act...
(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 Hz, with a 1% distortion level—1% clipping. Using the EIA method rates an amplifier 10 to 20% higher than the FTC method.