Electrical Engineering ⇒ Topic : Importance of R.M.S. Values
Importance of R.M.S. Values
An alternating voltage or current is always specified in terms of r.m.s. values. For example,common household appliances are rated at 230 V a.c. This is an r.m.s. value. If some other method of measurement is used, it must be specifically stated. Lacking any information to the contrary, always assume that a.c. values are rm. s. The following points will give the reader a clear concept about the r.m.s. values
(1) The domestic a.c. supply is 230 V, 50 Hz. It is the r.m.s. or effective value. It means that alternating voltage available has the same heating effect as 230 V d.c. The equation of this alternating voltage is given by
(2) When we say that alternating current in a circuit is 5 A, we are specifying the r.m.s. value.It means that the alternating current flowing in the circuit has the same heating effect as 5 A d.c.
(3) A.C. ammeters and voltmeters record r.m.s. values of alternating current and voltage respectively.
Fig. (a) summarises the various ways to measure sinusoidal voltages and the conversion constants. The relationships apply for currents as well as voltages
It is important to note that above relationship between peak, average and rm.s. quantities are applicable to only pure sine waves. In the case of other waveforms, these quantities are related by other (different) factors.
Note. R.M.S. value of an a.c. wave is always greater than the average value except in the case of rectangular and square waves when both are equal
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