Form Factor; Crest Factor; Power; Active True Power - Hameg HM8015 Handbuch

Form factor

The form factor multiplied by the rectified value
equals the rms value. The form factor is derived
by:
V
rms
F = ––––– = –––––––––––––––
IuI
For a sine wave the form factor is
π
F = –––– = 1,11
2√2
HINT

Crest factor

The crest factor is derived by dividing the peak value
by the rms value of a signal. It is very important for
the correct measurement of pulse signals and a
vital specification of a measuring instrument.
û
C = –––– = –––––––––––––––
V
rms
For sinusoidal signals the crest factor is
√ √ √ √ √ 2 = 1,414
HINT
Please note that erroneous results will
show if the crest factor of a signal is
higher than that of the measuring instrument
because it will be overdriven.
STOP
Hence the accuracy of the rms value measure-
ment will depend on the crest factor of the signal,
the higher the crest factor the less the accuracy.
Please note also that the crest factor specifi-
cation relates to the full scale value, if the signal
is below full scale its crest factor may be
proportionally higher.
Form factors
C = Crest factor / F = Form factor
rms-value
rectified value
peak value
rms-value
C F
π
2 = 1,11
2 2
π
2 = 1,11
2 2
π
2 = 1,57
2
2
3 = 1,15
3
B a s i c s o f P o w e r M e a s u r e m e n t

Power

With DC power is simply derived by multiplying
voltage and current.
With AC the waveform and the phase angle resp.
time relationship between voltage and current
have also to be taken into account. For sine waves
the calculation is fairly simple, as the sine is the
only waveform without harmonics. For all other
waveforms the calculation will be more complex.
As long as the instrument specifications for
frequency and crest factor are observed the po-
wer meter will accurately measure the average
of the instantaneous power.
Active, true power (unit W, designation P)
As soon as either the source or the load or both
contain inductive or capacitive components there
will be a phase angle or time difference between
voltage and current. The active power is calcu-
lated from the rms voltage and the real compo-
nent of the current as shown in the vector dia-
gram above.
Defining: P = active power
V = rms value of voltage
rms
I = rms value of current
rms
ϕ
= phase angle
u
i
û
î
ϕ
the active power is derived as follows:
P = V
rms
cosϕ is the socalled power factor (valid for sine
waves only).
The instantaneous power is the power at
time t equal to the product of voltage and
current both at time t.
HINT
p = i · u
(t) (t) (t)
Subject to change without notice
ω
ϕ
Icos ϕ
ωt
I
· I · cosϕ
rms
U
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