We are presently doing some comparison shopping for three phase
16-20 kW uninterruptable power supplies (UPS's) for our two spectrometers
and have been presented with somewhat conflicting claims on the relative
merits of single and dual conversion technologies.
In the single conversion UPS, the input power is not rectified but
is rather filtered and passed on to the load as long as the input mains are
active. In the event of a partial or total power loss, an inverter is
turned on and battery power is converted to AC to supplement or replace the
mains current. Turning on this inverter is said to take some time, during
which time a bank of capacitors and a saturated transformer are used to
prevent a sag in voltage delivered to the load. The manufacturer who is
offering us this type of UPS emphasizes that their design does not generate
large harmonics on the input mains and that these harmonics could damage
other equipment tied to the spectrometer's feeder lines. They also
emphasize the higher efficiency of this design, which is a consequence of
the lack of a rectifier.
In the double conversion UPS, input power is rectified and then
inverted back to AC. If there is a failure in the input supply, battery
current is supplied to the inverter, which is always warmed up and in use.
While this design is claimed to give a faster response to failure and
better frequency compensation, it is less efficient and can create
harmonics as high as 20-30% in the input mains. The manufacturer offering
this UPS to us has stated that this creation of harmonics is a negligible
effect due to the small load presented by the UPS and spectrometer
relative to the size of our building's power supply.
I am very interested to hear of any experiences you may have had in
selecting between these two technologies for UPS's and in particular if you
have any information to relate regarding the input harmonic problem (or
non-problem, depending on the authority).
Regards,
Ken Fishbein
Facility Manager,
NMR Unit
NIH/NIA/GRC
Ph. (410) 558-8512
FAX (410) 558-8323 or 8173
E-mail: fishbein@vax.grc.nia.nih.gov
Address: Ken Fishbein
NIH/NIA/GRC
4940 Eastern Avenue
Baltimore, MD 21224 USA