Woodward MSLC 9907-004 User manual

Product Manual 02022
(Revision B)
Original Instructions
MSLC
Master Synchronizer and Load Control
9907-004 (4-Wire Wye)
9907-005 (3-Wire Delta, 120 V)
9907-006 (3-Wire Delta, 240 V)
Installation and Operation Manual

General
Precautions
Read this entire manual and all other publications pertaining to the work to be
performed before installing, operating, or servicing this equipment.
Practice all plant and safety instructions and precautions.
Failure to follow instructions can cause personal injury and/or property damage.
Revisions
This publication may have been revised or updated since this copy was produced.
To verify that you have the latest revision, check manual 26311 , Revision Status &
Distribution Restrictions of Woodward Technical Publications, on the publications
page of the Woodward website:
www.woodward.com/publications
The latest version of most publications is available on the publications page. If
your publication is not there, please contact your customer service representative
to get the latest copy.
Proper Use
Any unauthorized modifications to or use of this equipment outside its specified
mechanical, electrical, or other operating limits may cause personal injury and/or
property damage, including damage to the equipment. Any such unauthorized
modifications: (i) constitute "misuse" and/or "negligence" within the meaning of
the product warranty thereby excluding warranty coverage for any resulting
damage, and (ii) invalidate product certifications or listings.
Translated
Publications
If the cover of this publication states "Translation of the Original Instructions"
please note:
The original source of this publication may have been updated since this
translation was made. Be sure to check manual 26311 , Revision Status &
Distribution Restrictions of Woodward Technical Publications, to verify whether
this translation is up to date. Out-of-date translations are marked with . Always
compare with the original for technical specifications and for proper and safe
installation and operation procedures.
Woodward reserves the right to update any portion of this publication at any time. Information provided by Woodward is
believed to be correct and reliable. However, no responsibility is assumed by Woodward unless otherwise expressly
undertaken. Copyright © Woodward 1993
All Rights Reserved

Manual 02022 MSLC Master Synchronizer and Load Control
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Contents
WARNINGS AND NOTICES........................................................................... IV
ELECTROSTATIC DISCHARGE AWARENESS ..................................................V
CHAPTER 1. GENERAL INFORMATION...........................................................1
Introduction.............................................................................................................1
Application..............................................................................................................1
Synchronizer...........................................................................................................1
Load Control...........................................................................................................2
Process Control......................................................................................................2
VAR/PF Control......................................................................................................3
Manual Organization ..............................................................................................3
CHAPTER 2. INSTALLATION........................................................................10
Introduction...........................................................................................................10
Unpacking.............................................................................................................10
Location................................................................................................................10
Installation ............................................................................................................10
Electrical Connections..........................................................................................10
Shielded Wiring ....................................................................................................11
Power Supply .......................................................................................................11
Local Bus Potential Transformers........................................................................12
Utility Potential Transformers ...............................................................................13
Utility Current Transformers .................................................................................13
Discrete Outputs...................................................................................................13
Discrete Inputs......................................................................................................15
Local Area Network..............................................................................................16
Process Control Input...........................................................................................18
Remote Load Setting Input...................................................................................18
Installation Checkout Procedure ..........................................................................18
CHAPTER 3. CALIBRATION AND ADJUSTMENTS..........................................19
Introduction...........................................................................................................19
Using the Hand Held Programmer.......................................................................19
Menu 1—Synchronizer.........................................................................................21
Menu 2—Load Control .........................................................................................22
Menu 3—Process Control ....................................................................................23
Menu 4—VAR/PF Control ....................................................................................23
Menu 5—Configuration.........................................................................................23
Menu 6—Calibration.............................................................................................24
Menu 7—Electrical Parameters............................................................................24
Menu 8—Control Status Monitor..........................................................................25
Menu 9—Discrete Inputs/Outputs........................................................................25
Menu 0—Diagnostics ...........................................................................................26
Menu (Set Point) Descriptions..............................................................................27
Prestart Setup Procedure.....................................................................................43
MSLC Adjustments...............................................................................................44
Calibration Check.................................................................................................44
Synchronizer Adjustment......................................................................................44
Phase Matching Synchronizer..............................................................................45
Voltage Matching Adjustment...............................................................................49
Load Control Adjustment......................................................................................50
Process Control Adjustment.................................................................................53
Conclusion of Setup Procedures..........................................................................54

MSLC Master Synchronizer and Load Control Manual 02022
ii Woodward
Contents
CHAPTER 4. SYNCHRONIZER DESCRIPTION ................................................55
Introduction...........................................................................................................55
Functional Description..........................................................................................55
CHAPTER 5. REAL POWER CONTROL .........................................................60
Introduction...........................................................................................................60
Power Sensor Theory of Operation......................................................................60
Load Sensor Hardware Description......................................................................61
MSLC/DSLC™ Interface ......................................................................................61
Base Load Mode...................................................................................................61
Import/Export Mode..............................................................................................61
Process Control Mode..........................................................................................62
Remote Control.....................................................................................................62
Automatic Power Transfer Control Functions.......................................................62
CHAPTER 6. VAR/PF CONTROL.................................................................64
CHAPTER 7. PROCESS CONTROL ...............................................................65
Introduction...........................................................................................................65
Description............................................................................................................65
CHAPTER 8. ECHELON®LONWORKS™NETWORK.......................................67
Introduction...........................................................................................................67
Remote Metering..................................................................................................68
Commands to DSLC Controls..............................................................................69
Control Status.......................................................................................................69
Remote Control Inputs..........................................................................................71
Specifications........................................................................................................72
CHAPTER 9. TROUBLESHOOTING ...............................................................73
General.................................................................................................................73
Bench Tests..........................................................................................................73
LonWorks Network ...............................................................................................76
CHAPTER 10. SERVICE OPTIONS................................................................81
Product Service Options.......................................................................................81
Woodward Factory Servicing Options..................................................................82
Returning Equipment for Repair...........................................................................82
Replacement Parts...............................................................................................83
Engineering Services............................................................................................83
How to Contact Woodward...................................................................................84
Technical Assistance............................................................................................84
APPENDIX A. MSLC SETUP WORK SHEET .................................................85
Synchronizer Menu 1............................................................................................85
Load Control Menu 2............................................................................................85
Process Control Menu 3.......................................................................................86
VAR/PF Control Menu 4.......................................................................................87
Configuration Menu 5...........................................................................................88
APPENDIX B. MSLC SPECIFICATIONS ........................................................89
Part Numbers........................................................................................................89
Electrical Specifications........................................................................................89
Environmental Specifications ...............................................................................90
MSLC MENU SUMMARY ............................................................................91

Manual 02022 MSLC Master Synchronizer and Load Control
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Illustrations and Tables
Figure 1-1a. MSLC (wye version shown)...............................................................4
Figure 1-1b. MSLC Dimensions (wye version shown)...........................................5
Figure 1-2. Hand Held Programmer.......................................................................6
Figure 1-3. Typical Wiring Connections .................................................................7
Figure 1-4. Typical Wiring Connections .................................................................8
Figure 1-5. Typical Wiring Connections .................................................................9
Figure 3-1. Temporary Wiring for Transformer Phase Correction........................14
Figure 3-2. MSLC in a Parallel Bus/Utility Parallel Application ............................17
Figure 4-1. Hand Held Programmer Functions ....................................................20
Figure 5-1. Synchronizer Block Diagram..............................................................56
Figure 5-2. Synchronizer Wiring Diagram............................................................57
Figure 8-1. Process Control Block Diagram.........................................................66
Figure 9-1. Typical LON Setup.............................................................................67
Table 10-1. System Troubleshooting ...................................................................80

MSLC Master Synchronizer and Load Control Manual 02022
iv Woodward
Warnings and Notices
Important Definitions
This is the safety alert symbol. It is used to alert you to potential
personal injury hazards. Obey all safety messages that follow this
symbol to avoid possible injury or death.
DANGER—Indicates a hazardous situation which, if not avoided, will result
in death or serious injury.
WARNING—Indicates a hazardous situation which, if not avoided, could
result in death or serious injury.
CAUTION—Indicates a hazardous situation which, if not avoided, could
result in minor or moderate injury.
NOTICE—Indicates a hazard that could result in property damage only
(including damage to the control).
IMPORTANT—Designates an operating tip or maintenance suggestion.
Overspeed /
Overtemperature /
Overpressure
The engine, turbine, or other type of prime mover should be
equipped with an overspeed shutdown device to protect against
runaway or damage to the prime mover with possible personal injury,
loss of life, or property damage.
The overspeed shutdown device must be totally independent of the
prime mover control system. An overtemperature or overpressure
shutdown device may also be needed for safety, as appropriate.
Personal Protective
Equipment
The products described in this publication may present risks that
could lead to personal injury, loss of life, or property damage. Always
wear the appropriate personal protective equipment (PPE) for the job
at hand. Equipment that should be considered includes but is not
limited to:
Eye Protection
Hearing Protection
Hard Hat
Gloves
Safety Boots
Respirator
Always read the proper Material Safety Data Sheet (MSDS) for any
working fluid(s) and comply with recommended safety equipment.
Start-up
Be prepared to make an emergency shutdown when starting the
engine, turbine, or other type of prime mover, to protect against
runaway or overspeed with possible personal injury, loss of life, or
property damage.
Automotive
Applications
On- and off-highway Mobile Applications: Unless Woodward's control
functions as the supervisory control, customer should install a
system totally independent of the prime mover control system that
monitors for supervisory control of engine (and takes appropriate
action if supervisory control is lost) to protect against loss of engine
control with possible personal injury, loss of life, or property damage.

Manual 02022 MSLC Master Synchronizer and Load Control
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Battery Charging
Device
To prevent damage to a control system that uses an alternator or
battery-charging device, make sure the charging device is turned off
before disconnecting the battery from the system.
Electrostatic Discharge Awareness
Electrostatic
Precautions
Electronic controls contain static-sensitive parts. Observe the
following precautions to prevent damage to these parts:
Discharge body static before handling the control (with power to
the control turned off, contact a grounded surface and maintain
contact while handling the control).
Avoid all plastic, vinyl, and Styrofoam (except antistatic versions)
around printed circuit boards.
Do not touch the components or conductors on a printed circuit
board with your hands or with conductive devices.
To prevent damage to electronic components caused by improper
handling, read and observe the precautions in Woodward manual
82715, Guide for Handling and Protection of Electronic Controls,
Printed Circuit Boards, and Modules.
Follow these precautions when working with or near the control.
1. Avoid the build-up of static electricity on your body by not wearing clothing
made of synthetic materials. Wear cotton or cotton-blend materials as much
as possible because these do not store static electric charges as much as
synthetics.
2. Do not remove the printed circuit board (PCB) from the control cabinet
unless absolutely necessary. If you must remove the PCB from the control
cabinet, follow these precautions:
Do not touch any part of the PCB except the edges.
Do not touch the electrical conductors, the connectors, or the
components with conductive devices or with your hands.
When replacing a PCB, keep the new PCB in the plastic antistatic
protective bag it comes in until you are ready to install it. Immediately
after removing the old PCB from the control cabinet, place it in the
antistatic protective bag.

MSLC Master Synchronizer and Load Control Manual 02022
vi Woodward

Manual 02022 MSLC Master Synchronizer and Load Control
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Chapter 1.
General Information
Introduction
This manual describes the Woodward MSLC Master Synchronizer and Load
Control.
Application
The MSLC is a microprocessor-based overall plant load control designed for use
in a system with Woodward DSLC™ (Digital Synchronizer and Load Control)
controls on each generator to provide utility synchronizing, paralleling, loading,
and unloading of a three-phase generating system.
MSLC functions include:
Selectable for phase matching or slip frequency synchronizing between the
utility and a local bus with voltage matching;
Automatic system loading and unloading for bumpless load transfer;
Import/Export level control capability;
Process control for cogeneration, pressure, maintenance, or other process;
Proportional loading of associated DSLC controls in isochronous load
sharing;
Adjustable Power Factor control;
Built in diagnostics with alarm relay driver output;
Multi-function adjustable high and low limit alarms and adjustable load
switches with relay driver outputs;
Digital communications network to provide loading and power factor control
of individual DSLC equipped generators;
Full setup, metering, and diagnostic capability through a hand held
programmer terminal (Figure 1-2).
Synchronizer
The MSLC uses digital signal processing techniques to derive both true RMS
voltages and relative phase angles of the fundamental frequencies of the utility
and the local bus voltage wave forms. Digital signal processing techniques offer
significantly improved measurement accuracy in the presence of wave form
distortions, particularly since the phase measurement does not depend on zero
crossings of the voltage wave forms.
Either phase matching or slip frequency synchronizing may be selected. Phase
matching provides rapid synchronizing for critical standby power applications.
Slip frequency synchronizing ensures that the initial flow of power will be either
out of the local system (export) or into the local system (import), depending on
whether a positive or negative slip is chosen. For both synchronizing methods,
the MSLC uses actual slip frequency and breaker delay values to anticipate an
adjustable minimum phase difference between the utility and the local bus.
Additional synchronizer functions include voltage matching, time delayed
automatic multi-shot reclosing, auto-resynchronizing, and a synchronizer timeout
alarm. Each of these features may be enabled or disabled during setup.

MSLC Master Synchronizer and Load Control Manual 02022
2 Woodward
Load Control
The load control uses digital signal processing techniques to provide significantly
improved accuracy and speed of response over conventional analog
measurement techniques. Accuracy is improved because the instantaneous
measurement of the voltage and current signal wave forms allows true RMS
measurement. Measuring true RMS power allows optimal load control in parallel
applications even in the presence of power line distortions. This method provides
faster response time because it eliminates the long integration times required in
analog circuits. Measurement speed is particularly important in power control
applications where rapid response to load and speed changes is essential.
Load control begins at breaker closure when the MSLC takes control of the
system load. The system load immediately prior to breaker closure is used as the
starting base load. On command, the adjustable ramp allows smooth, time-
controlled loading into a set import/export level. A ramp pause switch is provided
to stop the ramp at any point.
The import/export control is an integrating control. It adjusts the percentage of
rated load carried by the individual generators, operating in isochronous load
sharing, in order to maintain a set import/export or base load level. The MSLC
will therefore maintain a constant base load or import/export level even with
changing utility frequencies. The MSLC provides switch inputs to allow raising or
lowering the internal digital base load or import/export reference. The control also
provides a 4–20 mA (1–5 Vdc) analog input for remote reference setting, if
desired.
The MSLC is equipped with a Utility Unload switch, which provides an adjustable
time controlled ramp to lower the base load or import/export level. When the level
is below an adjustable threshold, the MSLC issues a breaker open command to
separate the utility from the local bus. Again, the ramp pause switch can be used
to stop the utility unload at any point. The maximum load that the MSLC can tell
the individual generators to carry is their rated loads. So, in the event that the
plant load is greater than the capacity of the operating generators, the utility
unload will stop when 100% rated load is reached on each of the operating
generators. This prevents accidental overloading of the local generators when a
reduced number are on line.
The MSLC also includes two adjustable load switches which can be used for
external functions or warnings when chosen system load levels are attained. The
high and low limit switches may also be activated when 100% or 0% base load
signal to the generators is reached.
Process Control
A process controller is provided for cogeneration, fluid level maintenance,
pressure control, or other applications. An adjustable bandwidth signal input filter,
flexible PID controller adjustments, an adjustable deadband, and control
selectable for direct or indirect action, allow the process control to be used in a
wide variety of applications.
A 4–20 mA (1–5 Vdc or a 4–12–20 mA) process transmitter provides the process
signal to the MSLC. The MSLC includes an internal digital process reference
which may be controlled by the raise and lower switch contact inputs or by an
external 4–20 mA (1–5 Vdc) remote process reference. The output of the
process control, like the import/export control, is the percentage of rated load
setpoint to the individual generators in isochronous load sharing.

Manual 02022 MSLC Master Synchronizer and Load Control
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An adjustable ramp allows smooth entry and exit from the process control mode.
When the process control mode is selected, the load reference is ramped in a
direction to reduce the error between the process input and the process
reference. When the error is minimized or the reference first reaches either the
high or low specified pull-in limits, the process controller is activated. When the
load reference output reaches either 100% or 0%, the control will maintain that
load reference until process control is established. The MSLC is not capable of
overloading or reverse powering in order to attempt to meet the process
reference. The high and low limit switches mentioned above can be used to
indicate that either too many or too few generators are on-line to maintain the
process within its limits.
VAR/PF Control
The VAR/PF function controls the power factor on all of the DSLC equipped
machines operating in isochronous load sharing. The PF control begins on
breaker closure. The MSLC has three modes of VAR/PF control (which are
selected in Menu 4):
Constant Generator Power Factor—sets the power factor reference on all of
the DSLC controls to the internal reference chosen in the MSLC. The power
factor can then be adjusted using the voltage raise and lower inputs. The
voltage raise command will make the power factor more lagging.
Conversely, the voltage lower command will make the power factor more
leading.
Utility Tie Power Factor Control—adjusts the power factor reference on all of
the DSLC controls in isochronous load sharing in order to maintain the
power factor across the utility tie.
Utility Tie VAR Control—adjusts the power factor reference on all of the
DSLC controls in isochronous load sharing in order to maintain the level of
reactive load being imported or exported from the utility.
Manual Organization
Page iv covers electrostatic discharge awareness. The MSLC is an
electronic device containing sensitive components. While the control is
designed for safe handling in the industrial environment, certain cautions
must be observed during handling and shipping or when removing the
cover.
Chapter 2 covers MSLC installation and wiring, including each major group
of connections, along with any special instructions.
Chapter 3 covers MSLC configuration, setup, and adjustment. The chapter
explains the purpose of each set point and recommended initial value.
Chapters 4 through 8 explain the major functions of the synchronizer,
import/export control, process control, and PF control.
Chapter 8 describes the Echelon Local Operating Network (LON) used by
the MSLC to communicate to the individual DSLC controls.
Chapter 9 provides a troubleshooting guide. Due to the extensive built-in
diagnostics, MSLC installation may be tested and verified quickly.
Chapter 10 describes how to return the control to Woodward in the event of
damage or failure of an internal component.
Appendix A contains a work sheet to aid in setup and to provide a place to
record setup values for future reference.
Appendix B lists the MSLCs physical and electric specifications.

MSLC Master Synchronizer and Load Control Manual 02022
4 Woodward
Figure 1-1a. MSLC (wye version shown)

Manual 02022 MSLC Master Synchronizer and Load Control
Woodward 5
Figure 1-1b. MSLC Dimensions (wye version shown)

MSLC Master Synchronizer and Load Control Manual 02022
6 Woodward
Figure 1-2. Hand Held Programmer

Manual 02022 MSLC Master Synchronizer and Load Control
Woodward 7
Figure 1-3. Typical Wiring Connections
(9907-004 for 120/240 V wye switchgear configuration)

MSLC Master Synchronizer and Load Control Manual 02022
8 Woodward
Figure 1-4. Typical Wiring Connections
(9907-005 for open delta, 120 V switchgear configuration)

Manual 02022 MSLC Master Synchronizer and Load Control
Woodward 9
Figure 1-5. Typical Wiring Connections
(9907-006 for open delta, 240 V switchgear configuration)

MSLC Master Synchronizer and Load Control Manual 02022
10 Woodward
Chapter 2.
Installation
Introduction
This chapter provides the general information for site selection, installation, and
wiring of the MSLC.
Unpacking
Before unpacking the control, refer to the inside front cover of this manual for
hazard alerts. Be careful when unpacking the control. Check for signs of damage
such as bent or dented panels, scratches, loose or broken parts. If any damage
is found, immediately notify the shipper.
Location
When selecting a location for mounting the MSLC, consider the following:
Protect the unit from direct exposure to water or to a condensation-prone
environment.
The continuous operating range of the MSLC is –40 to +70 °C (–40 to +158
°F).
Provide adequate ventilation for cooling. Shield the unit from radiant heat
sources.
Do not install near high-voltage, high-current devices.
Allow adequate space in front of the unit for servicing.
Do not install where objects can be dropped on the terminals.
Ground the chassis for proper safety and shielding.
The control must NOT be mounted on the engine.
Installation
Select a mounting location for the MSLC (see considerations above). Locate and
drill six (6) holes for mounting hardware (not furnished; see Figure 1-1 for
locations). Tap the holes for machine screws or bolts. Install the unit and secure
with the mounting hardware.
Electrical Connections
This section covers typical wiring connections, as shown in Figure 1-3. For other
applications, contact Woodward Governor Company for assistance.
Connect the terminals as shown in Figure 1-3. When making the connections,
observe the following:
Make all connections using insulated terminals. Use 0.5 mm2(20 AWG) or
larger stranded wire.
Make sure that all wires shown as shielded on the wiring diagram are
stranded, twisted-pair, shielded wire.
Do not place shielded wires in the same cable conduits with high-voltage or
high-current carrying cables.

Manual 02022 MSLC Master Synchronizer and Load Control
Woodward 11
Connect the cable shields to chassis ground. Shields are grounded at the
control end only for inputs and at the speed control and voltage regulator
end for outputs.
Make sure that cable shields are carried through all intermediate terminal
blocks from signal source to signal termination.
Do not subject any wiring to temperatures above 100 °C (212 °F).
Avoid kinks or sharp bends in the wiring. Make sure that all connections are
tight.
For additional wiring information, see Woodward manual 25070, Electronic
Control Installation Guide.
Install and wire the other units and actuators in your system using instructions in
applicable manuals.
Shielded Wiring
All shielded cable must be twisted conductor pairs. Do not attempt to tin the
braided shield. All signal lines should be shielded to prevent picking up stray
signals from adjacent equipment. Wire exposed beyond the shield should be as
short as possible, not exceeding 50 mm (2 inches). The other end of the shields
must be left open and insulated from any other conductor. Do NOT run shielded
signal wires along with other wires carrying large currents. See Woodward
application note 50532, Interference Control in Electronic Governing Systems for
more information.
Where shielded cable is required, cut the cable to the desired length and prepare
the cable as instructed below.
1. Strip outer insulation from BOTH ENDS, exposing the braided or spiral wrapped
shield. DO NOT CUT THE SHIELD.
2. Using a sharp, pointed tool, carefully spread the strands of the shield.
3. Pull inner conductor(s) out of the shield. If the shield is the braided type, twist
it to prevent fraying.
4. Remove 6 mm (1/4 inch) of insulation from the inner conductors.
5. Connect wiring and shield as shown in plant wiring diagram.
Installations with severe electromagnetic interference (EMI) may require
additional shielding precautions, such as wire run in conduit or double shielding.
Power Supply
The MSLC requires a nominal voltage source of 20 to 40 Vdc. Power to the
control should be maintained whenever the generator set is available for service.
Power must be applied to the MSLC at least 15 seconds prior to
expected use. The control must have time to do its power up
diagnostics and establish its location in the local area network to
become operational. Failure of the diagnostics will disable control
function and de-energize the alarm relay.

MSLC Master Synchronizer and Load Control Manual 02022
12 Woodward
Power supply output must be of a low impedance type for proper operation of the
control. DO NOT power a control from a high voltage source containing dropping
resistors and zener diodes. If batteries are used for operating power, an
alternator or other battery charging device is necessary to maintain a stable
supply voltage.
Run power supply leads directly from the power source to the control, connecting
the negative (neutral) lead to terminal 1, and the positive lead (line) to terminal 2.
DO NOT POWER OTHER DEVICES WITH LEADS COMMON TO THE
CONTROL. Avoid long wire lengths.
Do NOT apply power to the control at this time. Applying power may
damage the MSLC.
The MSLC receives its current and voltage input signals from
standard metering current transformers (CTs) and potential
transformers (PTs). The selection of these PTs and CTs will have an
effect on synchronizer voltage and phase matching and load sensing
accuracy. Identical potential transformers should be used for the
generator and bus voltages. CTs should provide 5 A RMS at full load
for best performance of the system.
The following sections describe the potential connections for the
synchronizer and power sensor functions. The MSLC real power
sensor is available in either a four-wire, three-sensor device, which
measures only line-to-neutral voltages, or in a three-wire, three-
sensor device which measures only line-to-line voltages. The voltage
sensing configuration options are described by the “WYE” and
“Delta” part number versions.
When connected to 4-wire wye systems with 120/240 Vac line-to-
neutral voltages, no potential transformers are required. Higher
voltage systems require potential transformers to reduce voltages to
the 120/240 nominal range.
Typically, 3-wire delta systems will always require potential
transformers to reduce the line-to-line voltages to the proper levels
needed at the input to the control (either 120 or 240 Vac, depending
on the control part number).
Local Bus Potential Transformers
The synchronizer potential input on the MSLC is connected to the secondary of a
120 or 240 Vac potential transformer. The local bus phase A must correspond to
phase A of the utility as wired into the MSLC load sensor. The voltage difference
between these inputs must not exceed 150 Vac RMS on inputs labeled 120V, or
300 Vac RMS on inputs labeled 240V respectively. Local bus phase rotation
must correspond to the phase rotation of the utility or tie bus for proper
synchronization.