Rockwell Automation 1901 SyncPro - Manual (Page 11)

Product Description 1-7
1901-UM020C-EN-P – June 2013
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 synchronous motor protector (refer to 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. Refer to Chapter 4, page 4-2.
Analog/Digital Pulse Board
This board converts the voltage sinusoidal waveform across the
discharge resistor and, by examining the zero crossings, creates a
digital pulse train of an equal frequency to the induced slip
frequency occurring in the discharge resistor. At start (zero speed),
the frequency will be 60 Hz, at 95% speed, the frequency will be 3
Hz (for a 60 Hz system). This feedback is used by the SyncPro to
determine the speed of the motor at any time during acceleration
and when the motor has reached the desired speed set point to
synchronize.