An approximate expression for determining the amount of each current harmonic present for singlephase converters is given by the following equation:
where: n = harmonic order (3, 5, 7, 9, 11, 13, etc.)
A similar approximate expression for determining the amount of each current harmonic present for threephase (6pulse) converters is given by the following equation:
where: n = harmonic order (5, 7, 11, 13, 17, etc.)
In both of these expressions, the harmonic is expressed in terms of the fundamental frequency term. The results of these two expressions are shown in the table below.
Table 41. Approximate Current Harmonic Content (%) for 1Phase and 3Phase Converters
Harmonic

3

5

7

9

11

13

15

1phase converter

67

40

29

22

18

15

13

3phase converter

x

20

14

x

9

8

x

As can be seen, most singlephase converters produce all the odd harmonics at about twice the amount of their threephase converter counterparts. In addition, singlephase converters have significant amounts of third harmonic content, which the threephase converters do not have.
It should be noted that the singlephase equation underestimates the % of each harmonic and the threephase equation overestimates the harmonic content in most cases. The current total harmonic distortion (THD), which is the weighted or rms assessment of all harmonics, approaches 100% for many of these devices. The expression for THD is as follows:
where: THD_{I} = current total harmonic distortion
I_{n} = harmonic rms current (in amps or %)
I_{1} = fundamental frequency rms current (in amps or 100%)
A similar equation for voltage THD results from V being substituted for I in the above. Using the values from Table 11, the THD_{I} for a singlephase converter is around 95% versus 30% for a threephase converter, based on odd harmonics up to the 50^{th}. Up until now, because the typical system impedance up to a harmonic source is reasonably low and the typical harmonic source is a relatively small load, the resulting THD_{V} (in %) will usually be in the single digits.
The harmonic content and THD can be obtained for different levels of load for some devices. For example, a battery charger has a variable load characteristic and the harmonic content and THD varies as a function of load. It us usually sufficient to note the THD and % of individual harmonics at rated load. At lower levels of load, the resulting percentages (of individual harmonics and THD) are usually offset by the lower base current at that load. For example, with two loads having the same base current, one that produces 20% THD_{I} at 50% load is no worse than one which produces 10% THD_{I} at 100% load. They both produce the same distortion in amperes.
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