MINSTER Model SCRB-68 is an electronic control unit
designed to provide controlled excitation for eddy cur-
rent coupling type variable speed drives. These con-
trols are compatible with Dynamatic, Louis-Allis
Adjusto-Spede and Torspec drives and with WER
drives that are equipped with special 90 volt D.C. cou-
pling coils. The control consists of a rugged metal hous-
ing. The SCRB-68 control may be used with drives
requiring up to 900 watts at 90 volts D.C.
To briefly describe operation, a portion of a regulated
voltage supply, selected by a speed adjusting poten-
tiometer, and a rectified feedback voltage from a
tachometer generator connected to the drive output
shaft, are compared. The error signal obtained is used
to control a silicon controlled rectifier to energize the
coupling. If the reference portion is greater than
tachometer feedback, the coupling is turned “ON.” If
the tachometer feedback is greater, the coupling is
turned “OFF” to produce less output torque and the
optional eddy current brake is fired, to allow output
shaft slowdown.
Extremely fast and accurate speed control is realized
— response time being solely a function of load and
coupling time constants.
The SCRB-68 Control consists of two circuit boards
and housing. A power switch is built-in as a convenient
means of disconnecting power. A torque limiter feature
prevents excitation of the eddy current coupling as long
as motor current exceeds the preselected torque limit
point. A speed sensitive relay drive feature is also built
into the control. This feature is used to control ener-
gization of a D.C. control relay in relation to output
speed of the drive. A precision speed reference signal
which varies in direct relation to the output speed of the
drive is provided for use with programmable controls
requiring a low voltage D.C. tachometer signal.
Three (3) neon indicator lamps are located on top of the
control housing adjacent to the power switch. The “D.C.
Power” indicator lamp will glow with a steady brilliance
whenever the unit is turned on. The other lamps are
used to indicate excitation of the eddy current coupling
or brake as noted. Since the eddy current coupling and
brake coils are constantly being pulsed “ON” and
“OFF,” it is normal for the lamps to glow with varying
brilliance.
The SCRB-68 control is usually used in conjunction
with an external control in which certain preset speeds
can be programmed. With this arrangement, press
speed will immediately and automatically change to the
rate which has been preset for a certain function when-
ever that function is selected. For variable speed drives
equipped with eddy current brakes, by adding a time
delay relay to this control, the drive can be used as a
flywheel brake to quickly slow the flywheel when the
press is shut off.
— 5 —
DESCRIPTION
OPERATION
The press drive motor and speed controller are both
energized with the “Drive Start” push button. Torque is
transmitted from the drive motor output shaft through a
set of belts to the flywheel. Once the motor is started,
the drive will continue to accelerate flywheel rotation
until it reaches the speed which has been set on the
speed control located on the operator's control panel.
The speed meter, calibrated in strokes per minute, will
indicate the approximate number of times the slide will
cycle continuously each minute. This speed will be
shown even though the press clutch is not engaged
and the slide is not moving.
The strokes per minute indicator reading may vary
slightly from the actual press speed. Accuracy of the
reading depends, to a great degree, on how closely the
meter is calibrated. A resistance network consisting of
several fixed resistors and a potentiometer are placed
in series with analog type meters to accomplish this
calibration. The digital S.P.M. meter contains an inte-
gral calibration network.
Speed of the press will be controlled from the variable
speed selector on the operator's control panel. After a
particular speed has been selected, the control will hold
that speed constant. An exception to this will exist,
however, if a torque limit control is installed and the
torque requirements are in excess of the preset torque
limit point.
Summary of Contents for SCRB-68
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