Rockwell Automation 1902 Syncpro II - Manual (Page 12)

12 Rockwell Automation Publication 1902-IN001B-EN-E - April 2013
Chapter 1 Product Description
The SyncPro II processor scales and interprets this signal to compare it to the
power factor trip set point and to cause a trip to occur if the power factor drops
below the programmed value for more than the specified power factor trip time
delay. If the DC excitation is lost, a low voltage condition exists, or the motor is
being overloaded to a point where the motor can no longer maintain
synchronous speed, the motor power factor will react by dropping to a very
lagging value. This indicates that the motor is slipping poles and the controller
should be shut down to protect the motor.
The phase angle transducer monitors voltage across lines 1 and 2, along with the
current in line 3 to obtain a power factor reading. When the reading is below the
set points programmed, the SyncPro II will shut down the starter.
Discharge Resistor
The discharge resistor is specified by the motor manufacturer for a specific
application to obtain correct starting and pull in torques and to provide a means
of discharging the motor induced field voltage when starting and stopping the
motor. The field winding has more turns than the stator winding and when
power is applied to the stator, the field acts like the secondary windings of a
current transformer. A field winding without a discharge path will produce a
voltage greater than its insulation rating, and as such, requires a means to
discharge or limit the voltage. If the discharge resistor is not connected during a
start, the induced voltage can build to a point where the field winding insulation
can be damaged. The resistor is also used to provide reference points to the
SyncPro II synchronous motor protector (see
Chapter 4 ).
Field Contactor (FC)
The field contactor provides two normally open and one normally closed power
poles. The normally open contacts apply DC power to the motor field windings
when the contactor is energized. Prior to energization and after de-energization,
the normally closed pole makes the path to the discharge resistor to allow the
dissipation of energy induced in the field during starting. It also provides a path
to discharge the stored energy in the large inductive motor field winding on
stopping of the motor.
Resistors R F 1 and R F 2
These resistors are used to attenuate the voltage which reaches the analog/digital
pulse board. Set up of these resistors is important because if the signal voltage to
the board is too low (too much resistance) then pulses will not be produced. If
too little resistance is used, the voltage may be too high which could damage the
analog/digital pulse board (see
Figure 10 on page 33 ).