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Normally emf can be calculated by the following formula

e=N*(dф/dt) where dф/dt = rate of change of flux linkages

In actual practice it is given as e=-[N*(dф/dt) ] because as per lenzs law the voltage induced in a direction to oppose the very cause for it i.e., change in flux linkages.

vydehi

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How do you prove the existence of back EMF?

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What is Back emf of DC motor?

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only in dc motor...