Electromotive force (EMF)
The voltage induced in a coil by the change of magnetic flux.
Definition
The electromotive force (EMF), denoted E, is the voltage induced across a winding under the effect of a varying magnetic flux. It is expressed in volts and obeys Faraday’s law: E = −N × (dΦ/dt), where N is the number of turns and dΦ/dt the rate of change of the flux.
In a transformer, the secondary EMF sets the no-load output voltage. The higher the number of turns, the greater the induced EMF: this is the basis of the turns ratio. The primary EMF, in turn, almost entirely opposes the supply voltage (back-EMF), which limits the no-load current drawn.
The ABL tip
The EMF depends on frequency: a transformer designed for 50 Hz supplied at 60 Hz will see its flux — and therefore its heating — decrease, but the reverse can saturate it. Always state your network frequency in your specification.
