Control Design – May 2019

Hz. The PowerFlex 400 drives accept the

0. 00 Hz speed command and are still ac-
   tive but consume almost no power.
   I tried to do the same thing with
   customer- specified Danfoss drives on
   another job. The minimum default value
   for the motor frequency is 10 Hz, but it
   could be changed to 0 Hz if you want.
   Instead, I shut the drives run com-
   mand off and kept on the status that the
   drives had been started, so they would
   automatically come back on whenever a
   hydraulic operation was called for. It is
   not really a good idea to run a motor at
   single-digit Hz when using a VFD, but 0
   Hz is OK. I have to shut off the drive but
   keep alive my need to restart them as
   soon as a hydraulic operation is needed.
   The only problem with running at 0
   Hz is the carrier frequency is still active,
   and it would drive the super-sensitive-
   hearing plant electrical engineer I had
   on an India job crazy when he went into
   the pump room during the quiet time. I
   think he was hearing some harmonics in
   the 2 0,000 to 2 5,000 Hz range.
   Working in China was a very nice
   experience for me. We were in a small
   city called Zunyi which is about 680 km
   northeast of Kunming and about 990 km
   south of Xi’an, located in the mountains
   of central China. We flew into Beijing
   and then took Air China for about a 2 .5-
   hour flight to Zunyi. We were put into a
   new hotel less than one year old, 46 sto-
   ries tall and fitted with polished marble
   everywhere (Figure 4 ).
   Two adjacent sides of the hotel are
   outfitted with LED graphics the full
   height of the tower that is used to dis-
   play messages and graphics after dark
   (Figure 5 ).
   The TV station was always default set
   to CCTV, always full of good news, most-
   ly about what the government was up
   to, both politically and militarily. They

30 / May 2019 / ControlDesign.com

cover story

Accommodating city

Figure 4: The hotel was less than a year old. The building was 46 stories tall and fitted with polished

marble everywhere.

City of lights

Figure 5: Two adjacent sides of the hotel are outfitted with LED graphics the full length of the tower that

are used to display messages after dark.

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