2-Alternating Current and Transformers

AC Generators
Alternating Current and
Can be thought of as an electric motor in
Converts mechanical energy to electrical
Rotating a coil changes the flux and,
therefore, produces an emf.
Flux is maximum when coil is
perpendicular to B and varies as a cosine
Peak Voltage and Current
ε0 = peak voltage (EMF)
Root Mean Square (RMS)
Occurs when flux = 0
Current will vary with voltage
I0 = ε0/R
Power will also vary since P = εI, but will
always be positive while I and ε vary from
positive to negative
Slip ring rather than split ring comutator
Averages of power, voltage and current
would be helpful, but would be zero for
voltage and current
So we square, average and take square
root to get a value
Irms = I0/√2
εrms = ε0/√2
Pavg = εrms Irms
A rotating motor armature is also
changing it’s flux and, therefore, inducing
an emf.
Lenz’s law says that the emf induced will
oppose the flux that created it.
Induced current will oppose current turning
the motor
So a rotating motor has more resistance
than one that isn’t rotating.
Why lights flicker when motors cut on and
why motors burn out when not turning.
A method to change
voltage for distribution
Two coils of wire
connected by an iron
A laminated core reduces
power loss due to eddy
AC in first (primary)
creates a changing
magnetic field, which
creates a changing flux in
2nd (secondary) coil, thus
inducing a current.
The Transformer (Cont.)
Vp/Vs = Np/Ns
 Ip/Is = Ns/Np
Practice Problems:
An AC circuit provides a peak voltage of 40 V
and a peak current of 2.5 A. Calculate the rms
voltage, rms current and the average power of
the circuit.
A transformer for home use of a portable radio
reduces 120120-V ac to 9.09.0-V ac. The secondary
coil contains 30 turns and the radio draws 400
mA of current. Calculate: (a) the number of
turns in the primary coil; (b) the current in the
primary coil; and (c) the power transformed.