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Unformatted text preview: Lecture 19 Energy and co-energy Consider a ferromagnetic, electromagnetic device without moving parts or stationary moving parts (device with movable parts but that are kept fixed), either linear = Li or nonlinear = f(i). By Faradays law we know d e dt = . The instantaneous power consumed by the device is ( ) ( ) ( ) ( ) ( ) d t p t e t i t i t dt = = Therefore in the time interval dt, the device absorbs ( ) dW p t dt id = = joules Since this device does not produce heat, nor does it produce mechanical energy, then dW must be stored in the device as a magnetic field. If we consider a device operating at a current i then ( ) W d W p t d t i d = = = Figure 1 - Nonlinear characteristic curve of a device The co-energy stored at ( , i ) is defined as ' i W d i = And corresponds to the area under the curve (i). One identity relating W and W is ' 0 0 W W i + = This means the sum of the areas on both sides of the curve is equal to the total area of the...
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- Spring '09