GAE CMFD Operational manual

P.O. Box 580816 ♦Modesto, CA 95358
Product User Manual
♦♦
EPRI Commercial Microwave Fabric
Dryer
Document # 930001, Rev. 2
July 1999

Product User Manual Page 2
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
REVISION HISTORY
REV. DESCRIPTION DATE APPROVAL
1DOCUMENT WAS NUMBER ON10051 27MAR99 JFG
2ADDED CYCLE TIMER 23JUL99 JFG
WARRANTY
Products manufactured and sold by Gerling Applied Engineering, Inc. (“GAE”) are
warranted to be free of defects in materials and workmanship under normal use and
service for a period of twelve (12) months from the date of original shipment. GAE’s
obligation under this warranty is limited to repairing or replacing, at GAE’s option, all
non-consumable component parts. Consumable parts are specifically excluded from
this warranty and may include, but are not be limited to, magnetrons, fuses, lamps,
seals, o-rings, v-belts, and fluids. All warranty repairs are to be done at GAE’s facility or
as otherwise authorized by GAE. All shipping charges for warranty repair or
replacement are the purchaser’s responsibility unless otherwise agreed to by GAE.
This warranty supercedes all other warranties, expressed or implied. No warranty is
given covering the product for any particular purpose other than as covered by the
applicable product specifications. GAE assumes no liability in any event for incidental or
consequential damages, financial losses, penalties or other losses incurred in
conjunction with the use of GAE products.
DOCUMENT CONVENTIONS
NOTE:Means the reader should take note. Notes contain
helpful information, suggestions, or references to other sections,
chapters, or documents.
CAUTION: Means the reader should be careful. You are doing
something that might result in equipment damage or loss of data.
WARNING: Means danger. A situation exists that could cause
bodily injury or death. All personnel must be aware of the hazards
involved with high voltage electrical circuitry and high power
microwave devices.

Product User Manual Page 3
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
WARNING
The microwave dryer described in this manual is capable of producing a microwave field
that is potentially hazardous to operating personnel. The unit must never be connected
or operated in a manner that allows a field in excess of 10 milliwatts per square
centimeter to be generated in an area accessible to operating personnel. Contact GAE,
Inc. for technical support prior to installation and/or operation of these units if there is
any question or concern about microwave leakage.
All electrical cable connections must be secure prior to operation. Never operate the
microwave dryer without a properly rated absorbing load inside the oven cavity. To
ensure safe operation and prevent microwave leakage, the equipment must be
periodically inspected and maintained as required or recommended.

Product User Manual Page 4
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
TABLE OF CONTENTS
INTRODUCTION.............................................................................................................5
GENERAL DESCRIPTION..............................................................................................6
Electrical Specifications 7
Mechanical Specifications 7
Controls 7
Sensors 7
INSTALLATION ..............................................................................................................8
Preliminary Inspection 8
Facilities Requirements 8
Location 8
Electrical Connections 8
BASIC OPERATION.....................................................................................................10
Initial Power-Up 10
Dryer Loads 10
Drying Cycle Setting 11
Cycle Start 11
Cycle Stop 12
Sensor Lint Screen Burn-off 12
Fault Cycles 12
Lint Screen Cleaning 13
THEORY OF OPERATION ...........................................................................................14
Dielectric Heating 14
Microwave Power Generators 14
Field Strength Sensor 15
Arc Detector 16
Exhaust Humidity Sensor 16
Exhaust Gas Sensors 17
Magnetron Thermal Cutouts 17
Sensor Lint Screen Burn-off 17
Low Voltage Power Supplies 18
Drive Motors 18
USER MAINTENANCE.................................................................................................20
Lint Screen Cleaning 20
PROCESS DATA COLLECTION..................................................................................21
Terminal Emulator Data Logging 21
MS Excel Spread Sheet Data Logging 21
ATTACHMENTS:
Document Type Number Title
Drawing 910037 Outline Drawing
Drawing 910038 Block Diagram, Control System
Drawing 910048 Schematic Diagram, Comm Mw Fabric Dryer
Drawing 910049 Schematic Diagram, Microprocessor Interface
Drawing 910050 Line Power Distribution Diagram

Product User Manual Page 5
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
INTRODUCTIONThe EPRI Commercial Microwave Fabric Dryer (“CMFD”) was
designed and constructed by AGL, Inc. under contract from the
Electric Power Research Institute for the purpose of on-site
demonstration and evaluation of the microwave fabric drying
process. However, prior to completion of construction of the CMFD
the contract was terminated with AGL and funding for the dryer
development program at EPRI was temporarily halted.
During its hiatus the dryer program was transferred to GAE, Inc. to
await funding for completion of construction and general
refurbishment of the CMFD. Renewed interest in the program
focused around potential industrial applications in textiles
manufacturing. A contract was awarded to GAE to complete the
dryer’s construction and deliver it for on-site installation and
operation.
While most of the dryer’s original functionality remains intact,
certain functions and features have been altered or deleted from
the original design specification due to discrepancies or conflicts in
their operation. Some of these alterations were necessitated
because of insufficient time and/or funding in the current program to
adequately resolve the issues, while others are required in order to
address inherent deficiencies in the original design. These changes
are identified and discussed in detail within this Manual where
applicable.
NOTE:While GAE has made extensive efforts to ensure safe
and reliable operation of the CMFD, Gerling Applied Engineering,
Inc. hereby disclaims all responsibility or liability for poor reliability,
unsatisfactory performance, unsafe operation or any other problem
associated with the design and construction of the CMFD.
Operators and end-users assume all risks associated with
operation of the CMFD.
WARNING: Lethal voltages may exist inside the CMFD during
operation and non-operation. All maintenance and service must be
done by qualified technical personnel only. Under no circumstances
should untrained or unskilled personnel be allowed access inside
the CMFD.

Product User Manual Page 6
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
GENERAL DESCRIPTION
The EPRI Commercial Microwave Fabric Dryer (“CMFD”) is
designed for field trials of the microwave fabric drying process in
commercial establishments to dry fabric loads up to 12 pounds. It
combines microwave energy with conventional hot air to dry almost
any type of fabric load at faster rates and lower temperatures than
conventional dryers using hot air alone. A microprocessor control
system coordinates operation of the dryer by collecting data from a
variety of sensors to optimize performance and ensure safety.
The microwave energy is supplied by three separate 1.8 kW
generators for a total microwave power input of 5.4 kW. Hot air is
provided by three separate 1.5 kW duct heaters. Energy efficiency
is maximized by recycling the waste heat from the microwave
generators and other heat producing electronics, providing a total
hot air capacity of approximately 6.5 kW.
Selectable drying cycles include Regular and Delicate (the Dryness
selector has been disabled). The Regular drying cycle combines
microwave energy with hot air and cuts the drying time to 1/3 that of
conventional dryers. The Delicate cycle eliminates hot air altogether
and uses only microwave to provide the lowest temperature drying.
The CMFD includes state-of-the-art safety features to protect both
the dryer and operator from improper operation and use. A variety
of sensors are provided to detect the presence of improper
materials in the drying load and shut off the dryer. An indicator is
provided on the control panel to warn the operator to check the
load. Additional sensors are provided to monitor the dryer operation
and performance and shut off the dryer in the event of malfunction.
Facilities requirements for the CMFD are the same as those for
conventional commercial dryers. The input line power required is
208-220 VAC, single phase, 60 Amps, and the exhaust connection
is to a standard 4” diameter duct. No cooling or gas connections
are required.

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
Electrical Specifications
Microwave Power 5.4 kW (nominal) provided by three 1.8 kW
generators
Magnetrons Hitachi 2M130
Hot Air 6.5 kW (approx.) provided by three 1.5 kW duct
heaters plus 2 kW from recycled waste heat
Line Power Input 208-220 VAC, Single Phase, 60 Amps (2-wire
plus ground)
Mechanical Specifications
Basket Size 26” diameter x 20” deep
Basket Capacity 12 Pounds (dry weight, 100% moisture
retention)
Door Opening 16” Wide x 10” High
Exhaust Duct 4” Diameter
Free Air Displacement 175 CFM (approximate)
Overall Dimensions 30” High x 36” Deep x 62” High
Weight (approx.) 700 lb. net; 960 lb. shipping
Controls
Cycle Start Pushbutton
Cycle Stop Pushbutton
Cycle Select 3-position rotary switch for “Regular” and
“Delicate” settings (“Permanent Press” setting
has been disabled)
Dryness (Disabled)
Fault Indicators “Check Load”, “Arc Detect”
Cycle Timer Programmable timer initiates “End of cycle”
routine upon time-out
Sensors
Dryer Operation Exhaust air thermostat, exhaust air relative
humidity, exhaust air temperature (Moisture
contact sensor has been disabled)
Thermal Cut-outs Magnetrons (3), duct heaters (3), exhaust air
Safety Door interlock switches, microwave field
strength detector, arc detector, exhaust gas
sensors

Product User Manual Page 8
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
INSTALLATION
Preliminary Inspection
Upon arrival at the installation site the CMFD should be thoroughly
inspected for damage or wear caused during shipping. Any visible
damage to the shipping crate or CMFD should be noted and
reported immediately to the shipping company in accordance with
standard claims procedures. Prior to connecting line power the
outer covers should be removed and all electrical connections
inspected for loose or disconnected wires. All mechanical
components should also be inspected for any loose or broken
hardware or other wear that may have been caused during
shipping.
Facilities Requirements
•Line Power: 208-220 VAC, 60 Hz, single phase, 60 Amps, 2-
wire plus ground. Proper earth ground must be provided. Please
note that, while an internal line filter is provided, the CMFD has
not been tested for immunity to noise, spikes or other line power
conditions that may be present in typical industrial
environments. A line conditioner and/or regulator may be
required for satisfactory operation of the CMFD.
•Exhaust Duct: Standard 4” diameter dryer exhaust duct. Back
pressure must be minimized.
•Environment: Indoors only, must be clean and dry. The
allowable extremes of ambient temperature and humidity have
not been tested and are thus unknown. Ambient conditions
should be maintained as close to mid-range as possible to
ensure reliable operation.
Location The CMFD may be installed and operated on any level surface.
Adequate clearance must be provided on the sides and front near
the louvered ducts to allow sufficient intake of dryer air.
Electrical Connections
•Line Power: A 4-conductor Hubbell ‘Twist-lock’ type connector is
provided at the back of the CMFD. Connections are to two
phases plus ground with no neutral connection. A mating
connector and power cord is also provided. Pinouts are as
follows:
XPhase A (any of three phases)
Y Phase B (either of remaining two phases)

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
GGround
WNo connection (normally used for neutral)
•Line Current Monitoring: An 11-pin ‘MS’ style circular connector
(J1) located on the back of the CMFD provides three separate
analog voltage signals that are proportional (10VDC = 100A) to
the line current drawn by corresponding subsystems within the
CMFD. A mating connector is also provided. Pinouts are as
follows:
Pins A (+) and B (-) Microwave power supply current
Pins C (+) and D (-) Basket and blower drive motors current
Pins D (+) and E (-) Duct heaters current (output disabled)
Pins F and H Not used (reserved for later use)
Pins K, L and M No connection

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
BASIC OPERATION
Operation of the CMFD is very similar to that of a conventional
residential clothes dryer. A typical sequence of events is as follows:
•Load fabrics to be dried and close door
•Select drying cycle
•Press ‘Start’ button to begin drying
•Wait for buzzer signaling end of drying cycle
•Open door and remove dried fabrics
While this operation sequence appears to be relatively simple,
there are many subtle aspects of the CMFD’s operation as
explained below that should be fully understood by the operator.
Initial Power-Up After connecting line power as described in the “Installation” section
of this manual, power is applied to the CMFD by lifting the circuit
breaker handle located near the line power connector on the back
panel. This applies power to various low voltage power supplies
which in turn supply power to the microprocessor and other
controls.
NOTE:One of the subsystems to which initial power is applied is
the suite of four separate gas sensors. These sensors require a
warm-up period of approximately one hour for stable performance.
Line power can remain on indefinitely, so there is no need to turn
off and disconnect line power between daily periods of operation.
However, as a safety precaution it is recommended that line power
be disconnected during extended periods of non-operation.
Dryer Loads The CMFD is designed to accommodate most typical fabrics used
in clothing or other standard textile products. However, it should be
noted that some fabrics may contain certain materials that will
result in fault conditions during the drying process. While the CMFD
is designed to protect itself and the operator from such loads, the
operator should take necessary precautions to avoid loads
containing the following materials:
•Metallic fabrics such as woven metal screens
•Metallic or metalized threads
•Heavily studded, sequined or ornamented fabrics (a few metal
studs or metal zippers are generally OK)

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
•Non-fabric objects such as pencils, cigarette lighters, coins, hair
pins, etc.
The CMFD generally performs better and more reliably with full
loads than with lighter loads. Also, loads should be evenly wetted
as would be typical following a standard wash cycle with spin
drying.
CAUTION: Loads must not contain excessive amounts of
moisture such that water is allowed to accumulate inside the dryer
cavity and/or drip down from the dryer basket. Failure to prevent
water accumulation and dripping may result in electrical damage to
the CMFD. Spin drying of all loads prior to microwave drying is
highly recommended.
Drying Cycle Setting
Any one of three drying cycles can be selected depending on the
type of fabric load.
•Regular: This cycle combines microwave energy with
conventional hot air. During the drying stage all three microwave
power supplies are on while two of the three duct heaters are
on. The final stage of the drying cycle is a three minute cool-
down period during which the microwave supplies and duct
heaters are all turned off.
•Delicate: This cycle utilizes only microwave energy to provide
the lowest temperature drying of the three cycles. Heated air is
provided only by the waste energy that is dissipated by and
salvaged from the microwave power supplies. As with the other
cycles, the final stage is a three minute cool-down period during
which all microwave supplies and duct heaters are turned off.
Cycle Start Close the dryer door and press the ‘Start’ button to begin the drying
cycle. The basket will immediately begin to rotate and the main
blower will operate. Approximately five seconds later the microwave
power supplies will be turned on as can be verified by the ‘clack’ of
the associates contactors.
Failure to start properly will usually be due to one the following:
•An interlock is not satisfied, in which case the buzzer will
immediately sound for ten seconds (see general fault cycle
described below). Check to be sure the door is closed tightly
and all covers are in place, then try again.
•One or more gas sensors have caused a fault condition, in
which case the basket and blower will operate but the ‘clack’ of

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
the microwave power supply contactors will not be heard (see
gas sensor fault cycle described below). Try starting again after
the end of the fault sequence (ten-second buzzer turns off), but
discontinue starting and check the load if the fault cycle is
repeated.
NOTE:When starting the drying cycle for the first time following
initial power-up (see above), the gas sensor fault cycle (see below)
may be initiated due to insufficient warm-up of the gas sensors.
This should not occur during subsequent starts unless line power is
turned off and then back on again.
Cycle Stop Once started, the drying cycle will continue until one of the following
occurs:
•The cycle ends normally when moisture has been sufficiently
removed. The buzzer will sound for two seconds to indicate the
normal end of cycle.
•The ‘Stop’ button or the door switch may be pressed to stop the
drying cycle at any time and for any reason. This action initiates
the fault cycle during which the buzzer will sound for ten
seconds.
•A fault condition as explain below will stop the drying cycle
automatically.
The drying cycle can be restarted following any of the above stop
events once the buzzer has sounded.
WARNING: Fabrics may be hot due to heated moisture during
the drying cycle and can cause severe burns if improperly handled.
The operator must exercise extreme caution when opening the
cavity door and handling fabrics during the middle of a drying cycle.
Sensor Lint Screen Burn-off
The final stage of the drying cycle following the end-of-cycle buzzer
is a ten-second period during which the sensor lint screen burn-off
occurs (see related discussion in Theory of Operation section). The
operator is typically unaware that this burn-off is occurring.
However, if the cavity door is opened during this period then the
general fault cycle will be initiated (see below).
Fault Cycles Any of the following fault cycles may be initiated during the drying
cycle due to abnormal operating conditions.

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
•General Fault Cycle: This cycle occurs when any one of several
interlock devices (covers, lint screen door, cavity door, thermal
cutouts) becomes dissatisfied or the microwave field strength
exceeds specified limits. During this cycle the ‘Check Load’
lamp immediately lights up, the basket and blower both stop and
the buzzer sounds for ten seconds. The load must be checked
prior to restarting.
•Gas Sensor Fault Cycle: This cycle is similar to the general fault
cycle except that it begins with a thirty second period during
which the microwave power supplies and heaters are turned off
(as evidenced by the ‘clack’ of the contactors turning off) but the
basket and blower continue to operate. This first stage is
immediately followed by a stage identical to the general fault
cycle described above. Aside from the clack of the contactors,
the operator may be completely unaware that this fault cycle
has been initiated until the second stage begins. Checking the
load will usual provide clues to help differentiate between gas
sensor and general faults.
•Arc Detect Fault Cycle: The presence of metallic fabric or
objects can, when combined with light loads and high field
strengths, cause arcing to occur inside the dryer cavity. Arcs are
detected by a photo diode which sends a voltage signal to
sensitive electronic circuitry that initiates the fault cycle. During
an arc detect fault cycle the ‘Arc Detect’ lamp is turned on and
the drying cycle is immediately stopped. The ‘Stop’ button must
be pressed to clear the fault condition and reset the dryer for
normal operation.
NOTE:Due to unresolved difficulties associated with nuisance
tripping of the arc detect circuit, this feature has been temporarily
disabled. However, other protection devices remain in place (such
as the gas sensors) which will adequately protect the dryer and
operators from harm in the event of conditions that could cause
arcing in the dryer cavity.
Lint Screen Cleaning
The lint screen, located inside the panel beneath the cavity door,
should be inspected regularly and cleaned as necessary. Failure to
properly clean the lint screen may result in inadequate air flow and
faulty operation of the CMFD.

Product User Manual Page 14
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
THEORY OF OPERATION
This section explains in detail the various functions, features and
control sequences of the CMFD. Knowledge of these details will
assist the operator and service personnel to understand how the
functions interact, interpret process results and diagnose
operational difficulties. Refer to the attached schematic drawings
for further details in conjunction with the discussion below.
Dielectric Heating Microwave, or dielectric heating occurs when electromagnetic fields
oscillating at a specific frequency (2450 MHz) interact with certain
kinds of molecules (having a high dipole moment). These
molecules try to align themselves with the electric fields as they
rapidly reverse polarity at the oscillating frequency, causing them to
vibrate. The resulting friction between adjacent molecules
generates heat.
Materials containing highly polar molecules are said to be “lossy”
because they readily absorb microwave energy. A measure of this
lossiness is a property of the material known as its dielectric loss or
loss tangent. Water is a good example of a lossy material because
its molecules have a high dipole moment. Most fabrics are made of
materials having a relatively low dielectric loss, far lower than that
of water.
In the microwave fabric dryer microwave energy is absorbed
primarily in the water rather than the fabric, resulting in heating and
vaporizing of the moisture. The fabric tends to stay relatively cool
because it is not being heating directly. By contrast, the hot air used
in conventional dryers heats the fabric and the moisture
simultaneously, resulting in higher overall drying temperatures.
However, as moisture is vaporized and leaves the microwave dryer
the tendency to heat the fabric directly becomes greater. If
microwave energy remains on after the moisture has been removed
then damage to the fabric from burning could result.
Microwave Power Generators
The microwave power generators used in the CMFD consist of
three main elements, a magnetron oscillator, a high voltage power
supply and a filament power supply. The magnetron is a vacuum
tube oscillator that is designed operate at the desired frequency
(2450 MHz). Like other types of vacuum tubes it contains an anode,
a cathode and a filament (heater) and operates when the filament is
hot and a sufficiently high voltage is present across the anode and
cathode.

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
The high voltage, also referred to as the “plate” voltage, is
generated in the CMFD using a “full wave voltage doubler” power
supply. This power supply consists of two separate power supply
sections operating on alternate phases of the line cycle. Each
section, also referred to as a “half wave voltage doubler” and
known for its output voltage regulating characteristics, is similar to
power supplies used in conventional microwave ovens. A diode
network combines the outputs of these two sections to deliver a
fixed voltage to the magnetron which in turn delivers fixed output
microwave power to the dryer.
The filament power supplies consist simply of a high current
isolation transformer and a network of variable resistors to control
the voltage delivered to the transformer primary winding. Because
of backheating inside the magnetron, the voltage required during
operation is lower than during idle periods. A relay whose contacts
are used to change the amount of resistance in the primary line is
energized whenever line voltage is applied to the plate voltage
power supply. To provide a short warm-up period prior to
application of plate voltage, the filament power supplies are turned
on approximately five seconds before the plate voltage supplies.
Field Strength Sensor
The field strength sensor monitors the electric field strength inside
the dryer cavity. It operates by sensing the microwave field using a
small wire probe and converting the signal to a DC voltage using a
crystal diode. The microprocessor monitors the field strength signal
and controls the amount of microwave power as required to prevent
damage.
The field strength varies with the level of microwave power
delivered to the cavity and the amount of moisture (and fabric)
inside the cavity. If the field strength is too high then damage could
result to the fabrics and/or dryer.
At the beginning of the drying process moisture content is generally
high, thus the field strength is relatively low. As the drying process
progresses and moisture is removed from the dryer cavity the field
strength gradually climbs. If the field strength signal reaches or
exceeds a setpoint value roughly equal to 80% of the maximum
allowable field the microprocessor shuts off one of the magnetrons
by de-energizing a relay, thus lowering the field strength to within
acceptable limits and allow the drying process to continue. As
drying progresses and the moisture content continues to drop, field
strength will again reach a the setpoint value. At this point the
microprocessor will turn off a second magnetron to further reduce
the field strength. The drying process will then continue with only
one magnetron operating until the field strength setpoint is reached.

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
This time the microprocessor will turn off all magnetrons and initiate
the “End of Cycle” routine.
By this discussion it can be understood that full loads are preferred
to light loads so as to maintain the electric field strength at relatively
low levels throughout the drying process.
As the basket rotates and the fabrics tumble inside the dryer cavity
the electric field pattern is constantly changing. Thus, the electric
field seen by the field strength sensor is constantly changing and
results in wide variations of the output signal. The peak values
typically remain well below the setpoint values at the beginning of
the drying cycle, but towards the end of the cycle they can easily
exceed the setpoint values while the more important time averaged
value is still below the setpoint. To help compensate for these
variations the microprocessor calculates the average value of the
25 most recent field strength readings and uses the result for the
interlock function.
Arc Detector Electrical arcing can occur inside the dryer cavity under certain
conditions, typically when loose metal objects are present and the
electric field strength is high. Arcs can result in scorching of fabrics
and possibly an open flame if allowed to continue unchecked.
Arcs are detected using a photo diode that generates a small
voltage proportional to the amount of light inside the dryer cavity.
The duration of an arc is usually very short, so the detector circuitry
amplifies the signal pulse and “stretches” it sufficiently to energize a
relay and shut down the CMFD. When this occurs the ‘Arc Detect’
lamp on the front panel turns on to notify the operator of the event.
The operator must then reset the CMFD by pressing the ‘Stop’
button.
NOTE:During testing of the CMFD the arc detector was causing
numerous false shut-downs due to electrical interference from other
circuitry within the CMFD. A significant amount of time was spent
trying to isolate and correct the problem, but no solution was found
the would prevent such nuisance tripping. As a result, the arc
detect circuit was temporarily disabled. However, it was decided
that the presence of the field strength and gas sensors will
adequately protect the operator from harm and CMFD from
damage in the event that arcing occurs inside the dryer cavity.
Exhaust Humidity Sensor
The relative humidity of the exhaust air is measured using a sensor
whose output voltage is proportional to relative humidity and
calibrated in percent RH. This signal currently is not used actively

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
by the microprocessor for control of the CMFD, but rather it is
provided only for data acquisition purposes.
Exhaust Gas Sensors
A series of four sensors monitor the exhaust air for the presence of
specific compounds commonly found in smoke or other products of
combustion. Each sensor is designed to detect a different set of
compounds as defined in the following chart:
Gas Sensor Compound Sensitivity
Sensor Number Figaro Type Compounds Detected
1 822 Toluene
2 825 Hydrogen Sulfide
3 842 Methane, Propane, Butane
4 882 Alcohol
The sensors operate by passing a current through a selective
adsorbent element whose resistance varies as compounds are
adsorbed. The output voltage varies with this resistance and
increases as compounds are adsorbed. The microprocessor
monitors the output voltage and triggers the gas sensor fault cycle
(see Basic Operation section) when its slope exceeds a preset
value.
The sensor elements operate most effectively at an elevated
temperature, thus a heater voltage (+5 VDC) is supplied in addition
to an instrument voltage (+10 VDC). These voltages are supplied
whenever line power is connected to the CMFD and the main circuit
breaker is turned on. A warm-up period of up to one hour is
required for stable and reliable operation of the sensors.
Magnetron Thermal Cutouts
Each magnetron is protected from overheating by a thermal cutout
switch device mounted to its side. Each thermal cutout is connected
in series with the coil of the contactor that turns on the respective
magnetron power supply. If a magnetron overheats then its
associated cutout opens and turns off only the power supply to that
magnetron. Since the blower continues to operate, the magnetron
will cool down until the thermal cutout resets and turns the power
supply back on.
Sensor Lint Screen Burn-off
To protect the exhaust gas sensors from fouling due to lint
accumulation, a lint screen separates the sensors from the flow of
air in the exhaust duct. But if lint is allowed to accumulate on this

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EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
screen then it will gradually reduce the flow of exhaust air to the
sensors, thus reducing their sensitivity.
To prevent continual accumulation of lint on the sensor screen the
lint is burned off at the end of every drying cycle by applying an
electrical current through the screen. This current is high enough to
cause the lint to smolder but not ignite into an open flame. The
burn-off period occurs after the blower stops so that the flow of air
does not prevent the burn-off temperature from being reached. The
lint ash is then carried away by the exhaust air at the start of the
next drying cycle.
Low Voltage Power Supplies
Several low voltage power supplies provide power to various
control circuits. These power supplies are on whenever line power
is connected to the CMFD and the main circuit breaker is turned on.
Their functions are described as follows:
•24 VDC Power Supply – This power supply is the primary
source of control power to the CMFD. Several devices are
powered by this supply directly while others are powered
through additional circuitry. Devices connected to the output of
this power supply are protected by a 5 Amp fuse.
•Gas Sensor Power Supply – This power supply is dedicated to
only the gas sensors to provide +5 VDC for heater power and
+10 VDC for signal power. This dedication is necessary to
maintain stability of the supply voltages to help prevent erratic
performance of the sensors.
•Voltage Regulators – Two voltage regulators provide +15 VDC
to the humidity sensor and +5 VDC to the microprocessor and
various other devices and circuitry.
Drive Motors Two different electric motors are used to drive the blower and dryer
basket. Both motors operate simultaneously during all periods of
dryer operation except when the buzzer is on (fault or end-of-cycle)
and during the sensor lint screen burn-off.
The blower is driven by a fixed speed AC motor while the basket is
driven by a variable speed DC motor. The speed of the basket
motor, and thus the basket rotation speed, can be varied by
adjusting a trimming potentiometer (R10) located near the bottom
edge of the microprocessor interface PC board.
CAUTION: Excessive basket rotation speed can cause damage
to the internal basket support rollers and drive mechanism. The
basket speed should be such that the fabrics are allowed to fall

Product User Manual Page 19
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
freely through the center of the cavity as they tumble. In any case,
the maximum allowable basket rotation speed is 60 rpm.

Product User Manual Page 20
EPRI Commercial Microwave Fabric Dryer
Gerling Applied Engineering, Inc. Modesto, CA
USER MAINTENANCE
The CMFD is designed to relatively maintenance free. With the
exception of the lint screen, there are no operator serviceable parts
or subassemblies inside the CMFD.
WARNING: Lethal voltages may exist inside the CMFD even
during periods of non-operation. All service personnel must be
aware of the hazards involved with high voltage electrical circuitry
and high power microwave devices. In the event service becomes
necessary, all repairs should be performed by a qualified service
technician.
Lint Screen Cleaning
The lint screen, located inside the panel beneath the cavity door,
should be inspected regularly and cleaned as necessary. Failure to
properly clean the lint screen may result in inadequate air flow and
faulty operation of the CMFD.