PROPERTY OF SOLIDSTATE CONTROLS, INC. 7/1998
If it is not possible to measure the actual current, it may be necessary to use one of the
following:
2. Obtain load profile from manufacturer for the individual loads. This will probably be necessary
for new installations. Add to this any lighting, alarms and security loads which are required.
3. In existing installations, check the capacity of the step-down isolation transformer supplying the
critical load or check the size of the distribution panel feeding those loads.
4. It may be necessary to contact the electrical contractor who installed the loads for load data.
In these last three cases, be sure to determine if there are any three phase loads. Even though
there is a three phase distribution panel or isolation transformer, there need not necessarily by any
three phase loads.
Included in SCI UPS equipment is circuitry to allow the inverter to be compatible with loads
exhibiting a high crest factor. Today’s electronic computers and instrumentation equipment are
powered with high frequency switching power supplies, which draw non-linear current from the AC
source. Both SCI’s Ferroresonant and our power transistor PWM UPS systems have the designed
capability to supply these non-linear high crest factor loads. Without this capability the UPS will
either need to be over-sized or will experience unnecessary and potentially load damaging
nuisance transfers, while these high crest factor loads are operating.
For further information about non-linear loads and crest factor control, please refer to our technical
paper on the subject.
The Power source must be of sufficient capacity to handle the worst case demands for the UPS.
Primary power installations for the UPS is the responsibility of the customer. Future expansion
should be considered in determining power requirements. The customer is also responsible for the
installation of any input distribution panels, external battery disconnect switch as required by the
NEC and/or local codes, output distribution panels and wiring to the loads, unless such has been
bid by SCI as part of the customer package.
Consideration must be given to the input distribution panels for the UPS. What presently exists for
the load may not be too small, due to the losses in the UPS itself, the additional battery recharge
capability, and the overload capacity of the rectifier/charger. Some installations may have chosen a
larger rectifier/charger than that which would normally be supplied, due to some additional DC
loads or the customer’s desire to recharge the batteries at a faster rate. The maximum current
ratings provided should be consulted for this determination.
It might appear on the surface that this device is very inefficient, due to the rectifier/charger high
input current. This is not the case. Our rectifier/chargers are approximately 92-94% efficient.
The reason for the high input current is this. The output voltage regulation of the rectifier/charger is
accomplished by controlling the conduction interval of the thyristors. This is done by gating on the
thyristors at the proper time during each half cycle. This change of conduction interval allows the
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