I have a set of three pumps (power ~100 kW) with variable speed AC motors and I would like to know the best option(s) to reduce the cost of reactive energy.

For instance, would it make sense to install a capacitor bank in the electrical cabinet?

  • \$\begingroup\$ Condensators? You mean capacitors, don't you? What kind of motors? DC? Brushed? Brushless? AC? Three-Phase? \$\endgroup\$ – Majenko Aug 30 '14 at 10:04
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    \$\begingroup\$ I think an issue with 100kW engines justifies getting an experienced engineer on location. \$\endgroup\$ – jippie Aug 30 '14 at 10:26
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    \$\begingroup\$ This press release says: When AC variable speed drives are used, power-factor-correction capacitors should not be used, because it is usually unnecessary. Side note: it can actually reverse the power factor. \$\endgroup\$ – venny Aug 30 '14 at 12:56
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    \$\begingroup\$ @jipp: Long ago, and still in some automotive and other niche applications, a capacitor was called a condenser, but "condensator" is just babble. \$\endgroup\$ – Olin Lathrop Aug 30 '14 at 14:04
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    \$\begingroup\$ @jippie The comment system lets you just use the first three characters of someone's name so I don't think Olin was misspelling it. \$\endgroup\$ – JYelton Aug 30 '14 at 15:58

If your goal is to reduce the displacement power factor of your plant, you've already accomplished that by the use of variable frequency drives. The line will no longer see the current lag that it would see if the inductance of the motor was placed directly across the line.

However, VFDs have other issues; a standard six-pulse front-end VFD will have the usual rectifier current profile, which doesn't look at all like a sine wave. Your harmonics will likely be quite bad, which you may also be charged for. If you want to correct that, you'll need a filter of some kind. Most VFD manufacturers offer models with active front-ends, which pull sine wave currents out of the line. Many companies also offer passive harmonic filters.

Now, there are other cases where it may make sense to install capacitors in your drive cabinet. If your application is very cyclical, motoring then braking, you might use a large bank of capacitors to store the braking energy for reuse during the next motoring cycle. But that can cause issues with your VFD's precharge circuit. And regenerating that braking energy back to the power line is often more cost effective, depending on the details. Adding capacitance to the DC bus can also make your harmonics worse.

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