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3.1 Introduction :
Figure 3.1: AC signal
ac signal
ac signal
AC is an electrical current whose magnitude and direction vary sinusoidally with time.
Current reverses at regular time intervals and has alternately positive and negative values. The sinusoidal waveform (sine wave) is the fundamental of AC current and AC voltage waveform.
The RMS value is the effective value of a verying voltage or current. It is equal with steady DC (constant) value which gives the same effect.
VP = VRMS × √2
The average voltage (or current) of an AC signal is zero because the positive and negative parts exactly cancel out
AC voltmeters and ammeters show the RMS value of the voltage and current.
DC meters also show the RMS value When connected to vary DC providing the DC is varying quickly.
In This Chapter
•Alternating current and voltage will be measured with the same D'Arsonval meter.
•Although D'Arsonval cannot directly measure alternating current, it will be discussed how to rectify alternating current using a diode rectifier to produce unidirectional current flow.
•Because PMMC instruments have a higher sensitivity than electrodynamometers or moving iron instruments, this method is very appealing.
•Rectifier converts AC to a unidirectional DC and then using a meter responsive to DC to indicate the value of rectified AC.
Several type of meter movements maybe used to measure AC current or voltage.
The five principal meter movements used in AC instruments are listed in the table below:
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