Making sure that you can effectively answer electrical exam theory related questions is the key to successfully preparing to pass your electrical exam.
Question:
What is the true power of an 120 volt single-phase AC circuit it the circuit current is 20 amperes and the voltage and amperage have a phase angle of 45 degrees?
Solution:
True power represents the power used by a system to do useful work and is traditionally expressed in the unit of watts.
The reference to a phase angle of 45 degrees is an indication that the voltage and current are out of phase.
Step #1
Use ohm’s law to convert the voltage and amperage to volt-amps
VA = E x I
= 120 volts x 20 amps
= 2,400 volt-amps
Step #2
Find the cosine of the phase angle by entering 45, then pressing the cosine function on a scientific calculator.
The cosine of 45 degrees = 0.67 or a power factor or 67%
Multiply the volt-amps from Step #1 by the cosine of 45 degrees
= 2,400 volt-amps x .67
= 1,608 volt-amps final answer