12.
DYNAMIC BRAKING (OPEN CHASSIS CONTROL ONLY)
In a contactor –reversed or high inertial system, it may be desirable to provide
braking to cause rapid stopping of the motor and permit quicker reversing. If a
power resistor is connected across the motor armature leads, A1 and A2, after
the motor has been disconnected from the OC2 unit by contactor release, the
motor will act as a generator and brake to a stop. The braking resistor must be
removed from the circuit before reconnecting the motor to the OC2 output. In
some cases such as high inertial loads, dynamic braking is also used on non-
reversing drives to achieve faster stopping times.
13.
EXTERNAL SIGNAL PROGRAMMING (OPEN CHASIS CONTROL
ONLY)
13.1
SPEED PROGRAMMING
Instead of using the standard speed potentiometer, the OC2 control unit can
be programmed by a master reference unit, a current signal from a process
instrument controller, an external signal from a DC tachometer or other DC
voltage sources. Basically, any DC voltage which is isolated from the AC
power lines and which varies from zero to six volts, may be applied
between 1TB terminals 6 (+) and 5 on 1TB to vary the speed from zero to
100%. Speed will vary directly and linearly with the applied voltage. Also,
the control will not significantly “load” the signal source since its input
impedance at terminal 6 is high ( approximately 100K ) in respect to
common. The following general procedure applies to permit external speed
programming:
a) Disconnect the 3 speed pot wires from 1TB terminals 5, 6, and 7.
b) Connect the external signal source to terminals 5 and 6 as shown in
Figure 6.1
c) If the external signal source is not isolated from the AC lines or if this
signal is to be used by more than one drive, the AC power must be
supplied to the OC2 control via an isolation transformer, or a signal
isolator PC board must be used. When a process instrument controller
is used having an output which normally is 1-5, 4-20, or 10-50mA, a
signal isolator bard should be used in order to convert to a 0 to 6 volt
level and to set the drive minimum speed properly. See Figure 6.2
18
