how we can design the power rating of any electrical equipment. as per my understanding only way is is to change the resistance of coil( winding if motor, filament if lamp).if it is correct then the equipment will be burn if resistance is low for low power rating. Eg. resistance of filament of 10 w lamp is lower than the resistance of 100 w. generally life of 10 w lamp is higher than 100 w. but in electrical point of view the life of high value of resistance equipment will be higher than the low value value of resistance. means, 10w bulb will glow longer time comparatively 100 w bulb. since 10 w bulb has high resistance , so this bulb's life should be higher than 100w bulb. can anyone explain how to design the power rating of any electrical equipment in line of other electrical parameters.
The power rating of most power components and devices is limited by thermal considerations, since the life of them is a strong function of temperature. A rule of thumb is that for each 10 degrees C increase in temperature the life is cut in half.
The resistance of a heater or filament is is chosen to pass the required ampere load at the design voltage. Watts Amps x Volts.
Most equipment involves loadings on all the materials used and these materials have various load-carrying limitations. Design essentially balances these against each other whereby the materials' limitations often interact on the design. For existing design variation much empirical data and experience data will be available which will accelerate the design process but for a competely new design, the process may be iterative and toward the end may even need several prototypes to be built and tested. You can't merely change the resistance of a winding in a motor and get a higher rating. Things like core flux density and temperature-rise of insulation (affected by cooling, whatever type that may be) must all be checked. Lamp filament resistance is indeed lower for a higher rating but filament surface area (to get the radiation) and temperature will have to be decided which involves knowing and designing how the lamp radiates light and how it is cooled. The lifetime of a bulb can be quite precisely designed in, largely by the design temperature choice.
