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3 Operation
Many appliances do not run continuously.
Air
conditioners and furnaces, refrigerators, sump pumps,
and other appliances cycle on and off as needed. With
the Load Control Module (LCM), less critical appliances
can be powered by the generator set when the more
important appliances are not running, allowing the use
of a smaller generator set than would be needed to run
all of the building’s electrical equipment at the same
time.
The RDC2/DC2 generator controller receives input from
current transformer (located in the ATS or generator)
and determines whether load shedding or adding shall
occur.
The LCM receives commands from the
generator controller and energizes or de-energizes the
appropriate load relays.
The LCM is activated by the ATS transferring from the
utility (normal) source to the generator. When activated,
the LCM sheds all connected loads. After transfer to the
generator set, loads are added according to their
priority.
If the ATS fails to transfer from the utility source to the
generator, the LCM will re-add all loads. When the ATS
transfers to utility, the LCM adds all loads that have been
previously shed.
For more information about the load add and load shed
timing, see Section 4, Theory of Operation.
3.1 Power Loads
Four power relays are provided for management of
non-essential secondary loads. Two (2) 120 VAC loads
(shed simultaneously) or a single 240 VAC load can be
wired to each relay. Limitations on loads selected are
40 A @ 240 VAC.
3.2 HVAC Loads
There are two (2) relays available to control two (2)
independent heating, ventilation, and air conditioning
(HVAC) loads. The relays are limited to 2 A @ 30 VAC.
A 5-minute time delay prevents HVAC loads from adding
too quickly.
Air conditioning compressors may be
damaged if they start too soon after being stopped due
to the necessity of starting the compressor against a
large residual pressure.
Five minutes is a typically
accepted time required for an AC compressor to bleed
off to a pressure level which the motor can successfully
start against.
3.3 Load Add/Shed Priority
Loads are prioritized from priority 1 to priority 6. See
Figure 8 on page 4. Priority 1 is considered the most
critical and will add first and shed last. Priority 6 is
considered the least critical and will add last and shed
first.
4 Theory of Operation
4.1 Load Add
The load control module (LCM) adds and sheds loads
based on the available capacity of the generator set.
When the generator has ample available capacity, loads
are added quickly. When the available capacity is low,
loads are added more slowly to give the generator time
to recover and to allow ample time to ensure that any
switching loads will come on before adding more load
than the generator can handle.
The load add time ranges from 15 seconds to 120
seconds depending on the loading of the generator set.
Figure 19 shows an example of the load add timing for a
20 kW generator set with the maximum capacity set to
the default setting of 70%. Figure 20 shows the HVAC
load add timing for a 20 kW generator set.
Available
Capacity
(%)
Load (%)
Load (kW) for
a 20 kW
Generator
Time
(Seconds)
70%
0%
0
15
50%
20%
4
23
37%
33%
6.6
34
30%
40%
8
40
20%
50%
10
48
5%
65%
13
60
<5%
>65%
>13 kW
Never Add
Figure 19
Example: Power Relay Load Add Timing
for a 20 kW Generator
Available
Capacity
(%)
Load (%)
Load (kW) for
a 20 kW
Generator
Time
(Seconds)
70%
0%
0
30
50%
20%
4
66
37%
33%
6.6
91
30%
40%
8
102
20%
50%
10
120
<20%
>50%
>10 kW
Never Add
Figure 20
Example: HVAC Load Add Timing for a 20
kW Generator
