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30 Unidrive M Regen Design Guide
Issue Number: 4
3.9.3 Switching frequency filter capacitor
This guide contains details for two types of switching frequency filter capacitors.
1. A capacitor that is designed to support lower quality power supply conditions with up to 8 % THD
v
on the grid. These capacitors are listed in
Table 3-14 to Table 3-17 and Table 10-20 to Table 10-23. The system drawings for these capacitors are found in Figure 4-4 to Figure 4-8.
2. A capacitor that is designed to support high quality, low harmonic supplies with up to 2 % THD
v
on the grid. These capacitors are listed in
Tab le 3- 18 to Table 3-21 and Table 10-24 to Table 10-27. The system drawings for these capacitors can be found in Figure 4-9 to Figure 4-13.
To determine which filter capacitors are best suited, the distortion level of the supply can be measured using a spectrum analyzer or distortion
analyzer.
3.9.4 Optional switching frequency filter capacitor fusing
The switching frequency filter capacitors are designed to function without the need for additional branch fusing provided the following conditions are
met.
• SFF capacitor cable length is less than or equal to 3 metres.
• SFF capacitor cable length is greater than 3 metres with cables sized for the main regen system supply fuses, where practical.
If the above criteria is not met then fuses must be installed.
If fuses are not installed, under fault conditions the main regen system supply fuses will clear to isolate the system from the mains supply.
The switching frequency filter capacitors are special parts which are used for filtering of the Regen drives PWM and only the recommended
parts should be used.
The 3-phase switching frequency filter (SFF) capacitors are situated on the input of the Regen system. If the harmonic distortion of the
supply is high for extended periods then this can cause a reduction of the service life of the capacitors. The effect of this is to reduce the
capacitance value. If high levels of harmonic distortion are expected then it is recommended that the capacitance values be checked
periodically, and capacitors which have fallen outside of their tolerance range be replaced.
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