sKVA Rating
• Generator sKVA represents the generator sets
ability to start induction motor loads
• CAT spec sheet sKVA ratings are expressed at
30% voltage drop
• Competition often expresses sKVA capability at
differing scales
– 35% voltage drop
– 90% sustained voltage
Ratings
Voltage Drop
• sKVA capability of a generator set increases as % of voltage drop
increases
• Loads are adversely affected by high levels of voltage drop
• ISO 8528-5 defines allowable voltage drop as follows:
Voltage %
Acceptance
Class G1
General purpose
applications.
Lighting and
electrical loads
Class G2
Lighting
systems,
pumps, fans
and hoists
Class G3
Telecommunications
equipment
Class G3
Dataprocessing &
Computer
equipment
-25
-20
-15
Agreed
between
manufacturer
and customer
Ratings
sKVA Capability Comparison
• sKVA ability must be compared with an
equivalent measurement technique, at equivalent
voltage drop, and with the same generator
excitation.
• Example:
CAT 3456
Kohler
Generac
Rating Spec Sheet Value 35% voltage Drop w/ PMG
500 eKW 1205 sKVA
1815 sKVA 9
500 eKW 1350 sKVA
1350 sKVA
500 eKW 1300 sKVA
1300 skVA
Ratings
“90% Rated Sustained Voltage”
• Cummins/Onan quotes sKVA at “90% Rated
Sustained Voltage”
• “Sustained Voltage” is a misnomer as a properly
designed system will see a transient voltage dip
and complete recovery as part of a motor starting
event.
Ratings
Typical Motor Starting Scenario
Motor Starting
Load Applied to
Generator
30% Voltage Dip
100% Normal
Line Voltage
Connected loads are subject to the full
sequence and magnitude of voltage dip
“90% Rated Sustained Voltage”
• Cummins Application Manual T-030 defines Sustained Voltage Dip:
“Following the relatively short (typically less than 10 cycles), steep transient voltage dip is a sustained
period of voltage recovery as shown in Figure 65. The maximum motor starting kVA on the generator
set Specification Sheet is the maximum kVA the generator can sustain and still recover to 90 percent
of rated voltage”
Level of voltage drop undefined
Timeframe to the start of 90%
sustained level undefined
Potential Negative Results
• No direct correlation can be made from 90% sustained
voltage and % voltage drop
• The two calculations are not equivalent measures of
sKVA capability
• Initial voltage drop, though short in duration, can have
adverse effect on connected loads such as relays, VFD’s
or computer equipment
• Extended timeframe to full recovery of voltage can have
adverse effect on connected loads such as motors and
under voltage relays
• Specifications should be influenced to % voltage drop