Tab Electronics Guide to Understanding Electricity ... - Sciences Club

The Transformer and AC Power
93
virtually zero. The inductor continually stores energy and then regenerates
this energy back into the source.
Assume that you were examining a purely inductive circuit possessing
the voltage and current waveforms shown in Fig. 3-7. Again, assume the
peak voltage amplitude to be 10 volts, and the peak current amplitude to
be 10 amps. If you used a voltmeter to measure the applied voltage, it
would read 7.07 volts; the rms value of 10 volts peak. Likewise, if you
used an ammeter to measure the current flow, you might read 7.07
amps, the rms value of 10 amps peak. If you multiplied the measured
voltage value by the measured current value, the answer should be the
rms (or effective) power value (P IE ). The answer would be 50 watts rms
(7.07 volts times 7.07 amps 50 watts). This contradicts the previous statement
regarding zero power dissipation in a purely inductive circuit. The
reason for this discrepancy is that you did not take the voltage and current
phase differential into consideration. In a purely inductive circuit,
the power calculation based on the measured voltage and current values
is called the apparent power. The actual power dissipated in an inductive
circuit when the voltage and current phase differential are taken into
consideration is called the true power.
If you wish to pursue higher mathematics, true power is calculated
by finding the apparent power, and then multiplying it by the cosine of
the differential phase angle. For any purely inductive circuit, the differential
phase angle (as stated previously) is 90 degrees. The cosine of 90
degrees is zero. Therefore, zero times any apparent power calculation will
always equal zero. (If you don’t understand this, don’t worry about it.
Depending on your interests, you might never need to perform these
calculations; but if you do, a good electronics math book will explain it
in “easy” detail.)
Another term related to inductive circuits is called the power factor.
The power factor of any circuit is simply the true power divided by the
apparent power.
true power
Power factor
apparent power
DC Resistance
Earlier in this chapter, the time constant of the circuit in Fig. 3-5 was
examined and the maximum obtainable current flow was found to be
approximately 1 amp. The analysis and calculations of this circuit were
performed by assuming the circuit to be “ideal”; that is, all components