Minster SCRB-68 User Manual
Manual No. 679A
MINSTER®
8952-574
SCRB-68
EDDY CURRENT COUPLING CONTROLS
DESCRIPTION
OPERATION
MAINTENANCE
REPAIR PARTS
MINSTER SOLID STATE CONTROLS
$5.00
Manual No. 679A
MINSTER®
8952-574
SCRB-68
EDDY CURRENT COUPLING CONTROLS
DESCRIPTION
OPERATION
MAINTENANCE
REPAIR PARTS
MINSTER SOLID STATE CONTROLS
$5.00
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SERVICE
This manual has been prepared to furnish the user with
sufficient detailed information to operate and maintain
the MINSTER SCRB-68 Eddy Current Coupling Controls.
The SCR control described herein has been engineered to provide automatic speed and torque regulation of drives equipped with eddy current couplings.
This design offers a simplified method of adjustment
and includes the reliability of long life solid state components. Inherent construction provides an uncomplicated approach to troubleshooting, should the need
occur. Through the use of extra, known-good, plug-in
modules and reference to the troubleshooting section
of this manual, most technicians should have little difficulty maintaining the controls.
Individual Eddy Current Coupling Control modules, or
the complete control, may be returned to the factory for
checkout and repair. The SCR controls will be repaired
promptly at a reasonable cost.
If technical assistance is desired, please contact the
Minster Service Department.
DESIGN CHANGES
Manufacturing methods, state of the art, and availability of certain components may change. Suppliers may
discontinue production of certain devices whenever
improved types become available to replace them. As
a result of these changes, some components furnished
in MINSTER controls may vary slightly from those illustrated in the manual. Troubleshooting procedures
should not change. If questions should arise concerning the replacement of component parts, please contact the Minster Service Department.
Copyright © 2008, 2011
THE MINSTER MACHINE COMPANY
Minster, Ohio 45865
Printed in the United States of America
Reproduction or use, without express permission, of editorial
or pictorial content, in any manner, is prohibited.
REGARDING WARRANTY
TERMS OF THE WARRANTY ARE FULLY DOCUMENTED IN THE MINSTER WARRANTY.
A COPY OF THAT WARRANTY IS INCLUDED IN THE COMPOSITE SERVICE
MANUAL; ADDITIONAL COPIES ARE AVAILABLE UPON REQUEST.
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CONTENTS
Page
COMPONENT IDENTIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Explanation Of Eddy Current Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Eddy Current Brake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Function of the Speed Sensitive Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
CHASSIS AND PANEL MOUNTED COMPONENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Indicator Lights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Circuit Breaker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Control Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
ADJUSTMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Maximum Speed Potentiometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
To Adjust the Maximum Speed Potentiometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
To Adjust the Torque Limit Potentiometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
To Adjust Operating Point of Speed Sense Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
To set Relay Operating Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
To Adjust the Strokes Per Minute Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Reference Out (PLC analog Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
CONTROL REPLACEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Option A: Conversion from a SCRB 52/58 to a SCRB 68 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Option B: Conversion from a SCRB 42/48 to a SCRB 68 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Option C: Conversion from a SCR 22/28 to a SCRB 68 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
INSTALLATION NOTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
TROUBLESHOOTING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
VOLTAGE GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
FACTORY CHECKOUT AND REPAIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 1. SCRB-68 with cover removed.
COMPONENT IDENTIFICATION
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Figure 2. SCRB-68 rear view.
Terminal Strip
Power Switch
Indicators
MINSTER Model SCRB-68 is an electronic control unit
designed to provide controlled excitation for eddy current coupling type variable speed drives. These controls are compatible with Dynamatic, Louis-Allis
Adjusto-Spede and Torspec drives and with WER
drives that are equipped with special 90 volt D.C. coupling coils. The control consists of a rugged metal housing. 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 potentiometer, 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 energization 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 whenever 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.
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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 integral 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.
EXPLANATION OF
EDDY CURRENT COUPLING
(See Figure 3)
Before entering into a circuit description of the Eddy
Current Coupling Control, a few preliminary remarks
regarding the principle of eddy current coupling will be
noted. This brief description is offered merely to establish a common background for describing the control
application.
The principle of eddy current coupling makes it possible to obtain a wide range of stepless, adjustable
speeds directly from industrial A.C. power lines at standard frequencies. The drive unit consists of a constant
speed induction motor and an integral magnetic clutch,
or eddy current coupling, assembled in a common
housing. The variable speed output shaft is rotated by
the constant speed A.C. motor through the eddy current coupling.
The eddy current coupling usually consists of three
basic members: the driving member or drum assembly,
the driven member or rotor assembly, and a stationary
member which is the field coil assembly. These basic
components are illustrated schematically in Figures 3
and 4.
A high strength iron drum is connected to the driveshaft
of the constant speed motor and rotates at the speed of
the motor. This drum is the driving member and, in principle, serves as a series of conductor loops as well as
a magnetic flux carrier. The position of the drum is such
that its inner surface surrounds and partially encloses
both field coil and rotor.
The variable speed output shaft is a part of the rotor
assembly and rotates at speeds determined by the
control setting. The cylindrical portion of the rotor lies
between the drum and field coil assemblies and is
divided into magnetically isolated sections by a nonmagnetic metal ring called a magnetic barrier. Poles,
cast into each rotor section, project alternately across
the outer surface of the rotor and are isolated from the
inner surface of the drum by an air gap. As the field coil
is excited, each rotor section assumes a polarity opposite to that of the other section and becomes alternate
north and south magnetic poles.
The field coil assembly is a stationary toroidal coil located concentrically within the rotor assembly. When the
coil is excited, magnetic lines of force emanate from it,
flowing through the north poles of the rotor into the
drum, through the south poles of the rotor and back to
the field assembly. As the drum rotates, the direction of
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Figure 3. SCRB-52/58 Coupling control.
magnetic flux changes at any given point between
these eddy currents and the magnetic field produces a
drag, or relative torque, to be transmitted from the drum
to the rotor and the output shaft connected directly to it.
With no current flowing through the coupling coil, the
rotor and drum assemblies are free to rotate independently of each other. Under this no current condition,
the drum assembly will rotate at full motor speed with
little or no rotation of the output shaft. As current is
applied to the coil, the output shaft accelerates. Output
shaft speed will continue to increase, as long as the coil
remains energized, until it is rotating at a speed slightly less than the speed of the input member. Speed of
the output shaft will never exactly match that of the
motor since some slip is required to transmit torque.
Slip, in this case, refers to speed difference between
the two rotating members. With the control automatically varying the amount of coupling coil excitation, output shaft speed can be held to the preselected rate.
A tachometer generator is attached to the output shaft.
This is an alternating current device whose output frequency varies in direct proportion to the speed of the
output shaft. This voltage is fed into the SCRB-68 control where it is converted from a frequency signal to a
voltage. This voltage is compared with a reference volt-
age so that speed control can be constantly maintained. A portion of the tachometer voltage is also used
to operate the strokes per minute indicator.
EDDY CURRENT BRAKE
Eddy current drive units may be ordered complete
with eddy current brakes for the purpose of quickly
slowing speed of the output shaft. The eddy current
brake is located within the coupling housing and consists of a brake field coil assembly and the brake
rotor.
The brake field assembly is attached to the coupling
housing and includes a toroidal field coil which is surrounded by two field end rings to form an electromagnet. Poles are cast into the inner surfaces of the end
rings and machined to allow an air gap between the
rings and rotor assembly.
The brake rotor is keyed to the coupling output shaft
and rotates inside the field assembly. If no current is
applied to the brake field coil, no braking torque is
developed. However, as the field coil is excited, eddy
currents are induced in the rotating assembly which
interacts with the stationary field to develop braking
torque. Amount of braking torque may be controlled
by changing excitation of the brake coil.
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Figure 4. SCRB-42/48 coupling/brake control.
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