TP-6322 9/044 Section 2 Operation
2.2 Automatic Operation
The ATS controller monitors the normal and emergency
power sources and determines when a power source
has failed or is acceptable and controls the system
accordingly. See Figure 4-2. Failure of a power source
occurs when its voltage on one or more phases falls
below the dropout voltage level. A power source is
acceptable when its voltage on sensed phases rise
above the pickup voltage level and stay above the
dropout voltage level. A power source is restored when
it becomes acceptable again after failure. Typical ATS
operation occurs in two separate automatic sequences.
D Failure of normal power and the resulting transfer to
emergency power.
D Restoration of normal power and the resulting
transfer back to normal power.
2.2.1 Failure of Normal Power
The main controller circuit board monitors phase A-C of
the normal power source. The normal available (NA)
LED on the main controller circuit board lights when the
normal power source is acceptable.
On single-phase switches the jumper JP3 connects
terminals GND and 3PH on the main controller circuit
board. See Figure 2-1. The controller considers the
normal source unacceptable when the normal source
voltage on phase A-C drops below the single-phase
dropout specification and acceptable when it rises
above the single-phase pickup specification.
362140
123
123456
NA
NB
NC
NSC
TEST
GND
ESC
GND
3PH
TB1
TB2
JP2 JP1 JP3
MAIN CONTROLLER CIRCUIT BOARD
Figure 2-1 Single-phase Configuration
When the normal power source fails, the normal relay
(NR) on the main controller circuit board is no longer
held, the NA LED turns off, and the controller starts a
time delay called time delay engine start (TDES). TDES
prevents unnecessary generator startup during short
utility power interruptions. The controller maintains
internal control circuits including the plant exerciser
setting for up to 90 seconds without the normal or
emergency power source. If the normal power source is
restored before TDES ends, the controller resets the
time delay. If the normal power failure persists and
TDES ends, the controller issues a signal to start the
standby (emergency) generator to produce the
emergency power source by closing the engine-start
contact between terminals ES3 and ES4 and the engine
start (ES) LED on the main controller circuit board turns
off.
After signalling the generator to start, the controller
monitors the voltage on phase A-C of the emergency
source. The emergency acceptable (EA) LED on the
main controller circuit board lights when the voltage
rises above the emergency source pickup specification.
The controller considers the emergency source
acceptable and the EA LED remains lit when the
emergency source voltage remains above the
emergency source dropout level specification. When
the emergency source becomes available, the
controller starts a time delay called time delay
normal-to-emergency (TDNE). TDNE allows
emergency power source stabilization before load
connection. When TDNE ends, the controller lights the
ER LED and energizes the emergency relay (ER). The
emergency relay signals the contactor to connect the
load to the emergency source operating the contactor
solenoid TS through a bridge rectifier BR and
coil-clearing contact SCE. When the contactor moves
away from the normal position, coil-clearing contact
SCE opens to remove power from the solenoid. The
contactor mechanism’s inertia carries it through the top
of the cycle and into the emergency position. A
contactor mechanically latches in the emergency
position until normal power source restoration and
stabilization.
2.2.2 Restoration of Normal Power
When the normal power source is restored, the
controller lights the NA LED and starts a time delay
called time delay emergency-to-normal (TDEN). If the
normal source fails before TDEN ends, the NA LED
turns off and the time delay resets. TDEN ensures
normal power source stabilization before load
reconnection.
When the controller determines that the normal power
source has maintained an acceptable level and TDEN
ends, the controller signals the contactor to reconnect
the load to the normal source. It does this by lighting the
NR LED and energizing the NR relay to operate the
contactor solenoid TS through bridge rectifier BR and
coil-clearing contact SCN. When the contactor moves
away from the emergency position, coil-clearing contact
SCN opens to remove power from the solenoid. The
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