Download Practical Variable Speed Drives and Power Electronics Malcolm

Electromagnetic compatibility (EMC) 123
(ripple-free). In practice, a ripple free DC current is not feasible, so the harmonic currents
are invariably larger than the theoretical values.
I1
In =
n
where:
In the nth harmonic component of current
I1 the magnitude of the fundamental component of current
n order number of the harmonic
For example, the theoretical magnitude of the harmonic currents in the mains, generated
by a 3-phase 6-pulse power electronic converter will be:
5th
7th
11th
13th
17th
19th
23rd
25th
Harmonic (250 Hz): 20.0% of fundamental current
Harmonic (350 Hz): 14.3% of fundamental current
Harmonic (550 Hz): 9.1% of fundamental current
Harmonic (650 Hz): 7.7% of fundamental current
Harmonic (850 Hz): 5.9% of fundamental current
Harmonic (950 Hz): 5.3% of fundamental current
Harmonic (1150 Hz): 4.3% of fundamental current
Harmonic (1250 Hz): 4.0% of fundamental current
etc
etc
The total RMS current drawn by a variable speed drive is the square root of the sum of
the squares of the harmonic currents.
In a variable speed drive application, assume for example that the current drawn by the
3-phase 6-pulse rectifier at fundamental frequency (50 Hz) is 100 Amps. Using the
theoretical values listed above, the following harmonic current values will be flowing:
20 amps (20%) at the 5th harmonic frequency (250 Hz)
14.3 amps (14.3%) at the 7th harmonic frequency (350 Hz)
9.1 amps (9.1%) at the 11th harmonic frequency (550 Hz)
etc (ignoring harmonics above the 25th harmonic order)
Consequently, the magnitude of the total RMS current drawn by the VSD will be:
I RMS =
2
2
2
2
I 1 + I 5 + I 7 + ... + I 25
2
2
2
2
I RMS = 100 + 20 + 14.3 + ... + 4
I RMS = 104.1 amps
This illustrates that the total RMS current will be 4.1% greater than value of the
fundamental current. This results in extra losses in the cables and transformers that feed
the variable speed drive. It is commonly accepted practice to derate the drive cables and
transformers by 10%.
These theoretical values are based on ideal commutation and a ripple free load current
on the DC link. These ideal conditions do not exist in practice and the magnitude of the
harmonic currents depends on several factors, including: