Average and RMS Values of a Periodic Waveform:
(Nofziger, 2008)
Begin by defining the average value of any time-varying function over a time interval
∆t ≡ t 2 − t1 as the integral of the function over this time interval, divided by ∆t :
t2
f avg ≡
∫ f (t ) dt
t1
∆t
t
1 2
=
f (t ) dt
t 2 − t1 ∫t1
(1.1)
Numerically, this is an extension of the basic definition of the average for a discrete
N
variable, x ≡
∑x
i =1
i
, applied to a continuously-varying function.
N
Graphically, this means that the area under the function (between times t1 and t 2 ) is
equivalent to the area of a rectangle of height f avg multiplied by width (t 2 − t1 ) :
t2
f avg ⋅ (t 2 − t1 ) = ∫ f (t ) dt
(1.2)
t1
●
The average of a sine wave over one half-cycle:
Consider a sine wave of peak amplitude Ip and frequency f:
I (t ) = I p sin (2πf ⋅ t ) = I p sin (ωt )
The period of the wave, τ, is given as:
1 2π
τ≡ =
ω
f
The average value of one half-cycle in time, is then calculated as:
τ 2
2I p τ 2
1
sin (ωt )dt
I avg =
I sin (ωt )dt =
(τ 2) ∫0 p
τ ∫0
 2 I p   − cos(ωt )  τ 2  2 I p  
 ωτ
 − cos
 
= 
I avg = 
|

0
ω

 2
 ωτ  
 τ 
 2I p 
 4I p   4I p 
1  =
= 
  − cos (π ) +=
  =

 ωτ 
 ωτ   2π 
I=
avg
2I p
=
π
(.637 ) I p
(1.3)
(1.4)
(1.5)


 − (−) cos(0)


2I p
π
calculated over one half-cycle of the sine wave
(1.6)
(1.7)
●
The average of a sine wave over one full-cycle:
I avg =
1
τ
(τ ) ∫0
I p sin (ωt )dt
(1.8)
 I p   − cos(ωt )  τ  I p 
I avg =   
 |0 =  ωτ [− cos(ωτ ) − (−) cos(0)]
ω
 τ 


 Ip 
I 
[− cos(2π ) + 1] =  p [− 1 + 1] = 0
= 
 ωτ 
 ωτ 
I avg = 0
calculated over one full-cycle of the sine wave
(1.9)
While this result is mathematically correct, it doesn’t provide any physical insight into
what this sine wave can “accomplish” over one complete cycle. Consider the AC
sinusoidal voltage delivered by Tucson Electric Power at a wall outlet. Its average value
may be 0, yet we know from experience that this sine wave will light up fluorescent
bulbs, heat up tungsten filaments in light bulbs, heat wires in toasters, etc. This is
because these devices absorb energy (power) from the sine wave, whether the voltage is
positive-going or negative-going. Useful work is done during both half-cycles of the
sinusoidal waveform.
The problem, mathematically, is that we have added up negative as well as positive
values. To account for this and make sure that all parts of the waveform contribute to its
calculated effective value, first square the function f (t ) so that all values are positive.
Then add up (integrate) all values of f (t ) over one complete cycle, and finally take the
square root. This is known as the “Root Mean Square” or RMS value of any timevarying (or spatially-varying) waveform, and is defined as:
YRMS ≡
{f
2
}
(t )
AVG
=
1
τ
f
τ∫
2
(t )dt
(1.10)
0
The physical meaning of the RMS value is this—it is the constant, or “DC” value that
would cause the same physical effect as the actual time-varying waveform does, during
one complete period. This might be to deliver the same power in a circuit, to cause the
same heating effect in a toaster, to light up a bulb with the same brightness, etc.
Note that, in general, a periodic waveform may not have symmetry like sinusoidal or
triangular waveforms have. In this case, you still calculate the RMS value according to
equation (1.10), by integrating over one complete cycle. Modern-day instrumentation
(the DMMs and oscilloscopes in our lab) digitally sample a waveform, and numerically
integrate the values to calculate the RMS value, according to equation (1.10).
●
RMS Value of a Sinusoidal Waveform:
Consider a sinusoidal voltage of peak amplitude V p and frequency f:
V (t ) = V p sin (2πf ⋅ t ) = V p sin (ωt )
The RMS value is calculated as:
VRMS =
=
1
τ
τ
2
∫V (t )dt =
0
1
τ
V
τ∫
2
p
sin 2 (ωt )dt
(1.12)
0
τ
V p τ

1
(
)
1
cos
2
ω
t
dt
dt
cos(
2
ω
t
)
dt
−
=
−


∫0
τ ∫0 2
2τ  ∫0

Vp
2 τ
2
τ
2
V p  τ  sin( 2ωt )  
=
(t ) − 
 
2τ  0  2ω  0 
2
2
2
=
VRMS
=
Vp
=
2
2τ
(.707 )Vp
(1.13)
(1.14)
V p   sin( 2ωτ )
V p   sin( 2 ⋅ 2π )


− 0  =
τ −
τ −
− 0 
=


2τ   2ω
2τ  
2ω


Vp
(1.11)
[τ − 0 + 0] =
Vp
(1.15)
2
2
for a sinusoidal waveform
(1.16)
(1.17)
(Note that you get the same result whether you integrate from (0 − τ ) , (0 − τ 2) , or even
from (0 − τ 4) , because of the symmetry of this waveform. For non-symmetrical
waveforms, you have to integrate over the complete cycle.)
The quoted value of 120VAC for the voltage at the wall socket, as delivered by Tucson
Electric Power is, in fact, the RMS value of the (60Hz) sinusoidal voltage. The peak
voltage is therefore V p = 2 ⋅120 = 169 V, and the peak-to-peak value is 339V!!!
●
RMS Value of a Triangular Waveform:
Consider a triangular voltage waveform that is bi-polar, has a 50% duty-cycle
(symmetrical about V=0), and a frequency f. Let the maximum voltage be Vp, and the
minimum voltage be –Vp.
Because of the symmetry, integrate from (0 − τ 4) to calculate the RMS value:
V (t ) =
4V p
τ
(valid between t = 0 and t = τ 4 )
t
(1.18)
τ 4
VRMS = V
2
AVG
1
=
V 2 (t )dt
τ 4 − 0 ∫0
(1.19)
τ 4
τ 4
4  4V p  2
4  4V p 

 t dt =

 ⋅ ∫ t 2 dt
=
∫
τ 0 τ 
τ τ  0
2
2
64V p  t 3

=
τ3 3

VRMS
=
Vp
=
3
(.577 )Vp
τ 4
0
2
2
2

Vp
64V p  τ 3 


=
=
3
τ 3  3 ⋅ 4 3 

for a triangular waveform
(1.20)
(1.21)
(1.22)
●
RMS Value of a Rectangular Waveform:
Consider a rectangular voltage waveform that is bi-polar, has a 50% duty-cycle
(symmetrical about V=0), and a frequency f. Let the maximum voltage be Vp, and the
minimum voltage be –Vp.
Because of the symmetry, integrate from (0 − τ 2) to calculate the RMS value:
(valid between t = 0 and t = τ 2 )
V (t ) = V p
=
VRMS
=
V 2 AVG
τ 2
=
VRMS = V p
2
2
=
(Vp ) ∫ dt
τ
0
1
τ 2
τ 2
2
2
(t )dt
∫ V=
τ
0
2
2
=
(Vp ) t τ0 2
τ
for a rectangular waveform
τ 2
∫ (V ) dt
2
p
(1.23)
(1.24)
0
(V ) (τ
τ
2
2
p
2 − 0)
(1.25)
(1.26)