AUTOTRANSFORMERS

It is not necessary in a transformer for the primary and secondary to be separate and distinct windings. Figure is a schematic diagram of what is known as an AUTOTRANSFORMER. Note that a single coil of wire is "tapped" to produce what is electrically a primary and secondary winding. The voltage across the secondary winding has the same relationship to the voltage across the primary that it would have if they were two distinct windings. The movable tap in the secondary is used to select a value of output voltage, either higher or lower than E p, within the range of the transformer. That is, when the tap is at point A, Es is less than Ep; when the tap is at point B, Es is greater than E p.

Figure - Schematic diagram of an autotransformer.

Ep = N1 = a = Is

Es N2 Ip

Vp.Ip = Vs.Is

Ix = Is – Ip

MMF in both windings is equal

There fore N2.Ix = (N1 – N2).Ip

(Corrective Ix = (N1 – N2).Ip = (a -1)Ip = Is –Ip

device) N2

Ip= Is = N2.Is

a N1

a = N1 = Nc + Ns Nc = N2

N2 Nc Ns = N1- N2

Vs = Nc

Vp = Nc + Ns

Sout = VsIs

Is = Ip+ Ix = Ip+ (Is I1)

Sout = VsIp+ Vs(Is– Ip

Sout = Sc + Si

Sc – Conductively transferred Si – Inductively transferred power to load

Power to load Through (N1 – N2) winding

Si = Is– Ip= a –1 = N1 – N2 a>1

Is a N1

Sc = N2

Sout N1