TITLE" A method for drying of high voltage porcelain insulators with large
section thickness using pulsed microwave source".
FIELD OF INVENTION
The invention relates to the field of drying of large high voltage elector-porcelain
components in general and to a method for drying of high voltage porcelain
insulators with large section thickness using pulsed microwave source, in
particular.
BACKGROUND OF INVENTION AND PRIOR ART
Drying of large high voltage electro-porcelain components is a time-consuming
process and involves a combination of natural drying and oven drying extending
for several days and a bottle neck to achieve high productions rate. The
Microwave drying of large ceramic components such as high voltage porcelain
insulators can have a significant impact on process time as well as the quality of
dried components. The insulators have number of sheds as well as large core
thicknesses. It was observed that while establishing the Microwave drying using
(CW) microwave power of ceramic insulator components with core thicknesses in
the range of 18 cm, some ruptures due to steam escape from bulk can still occur
and this grows into a defect only during firing stage and adding to firing cost.

In pulsed mode, due to intermittent heating, the vapour does not get trapped
inside the core as it allows for sufficient time to diffuse with out increased vapour
pressure and during the Microwave OFF period, the internal temperature also
falls of rapidly and also allows the vapour escape out through the core, thus,
making the microwave heating more effective.
OBJECT OF THE INVENTION
To overcome the above drawbacks of prior art, the following remedial actions
were undertaken;-
(1) To specially address the problem of drying components with large section,
pulsed Microwave power source was propose to develop.
(2) The aim was to develop an economical pulsed microwave system, without
sacrificing the specifications for use in drying of ceramic components.
Pulsed mode of operation was achieved by switching ON and OFF the magnetron
anode supply. There are various techniques available to achieve this. But, many
of these techniques are very expensive. Since the cost of the power supply is
inversely proportional to the pulse width and the pulse width offered by off the
shelf supplies is very small (of the order of a few tens of microseconds), the
power supply cost becomes enormously high. These off the shelf supplies are

switch mode power supplies which employ a pulses transformer, which adds to
the cost of the system. So the objective was to develop an economical pulsed
microwave system, without sacrificing the specifications.
DESCRIPTION OF THE INVENTION
While the typical pulsed system required for plasma devices require very low
pulse width which will be inefficient for drying a minimum pulse width was
chosen 100ms. The circuit responses was arrived at after experimenting with the
anode supply transformer for various pulse widths starting from 10 ms. It was
observed that the no load current of the transformer increased significantly due
to switching (The transformer is designed for 50 Hz operation, and pulsing
increased the operating frequency of the transformer, thus increasing the core
losses). This led to additional heating of the transformer core. To reduce the
inrush current of the anode supply transformer, the SSR (Solid state relay) with
zero voltage turn on was chosen. To prevent the transformer core from magnetic
saturation, we had to pass equal number of positive and negative half cycles
through the transformer. The duty cycle of the source was chosen as 500ms and
which can be controlled from 10% to 90% and the total control was by using
micro controller.
The system is designed to take control signal from programmable controller.
Both temperature and humidity control can be achieved using dual out put from
the programmer. All interlocks, safety provisions and start up and stop sequence

has been integrated using the micro controller for easy integration to the
applicator. The design approach for the low cost pulsed microwave source is
discussed along with details of applicator system and some results of drying. The
3kW microwave generator is developed for a fixed frequency (at 2450 MHz
frequency) magnetron. The magnetron needs filament and anode supplies for its
operation as microwave generator. The filament supply requirement is 4 Volts
during standby and 2.2 Volts during operation. Filament current in standby
condition is 20 Amps. As per the biasing requirements of the magnetron,
filament supply is applied first and the anode supply is applied after filament
preheat time of 8 seconds. Anode supply is switched on after filament pre-heat
time. Typical anode voltage and current values are 5-2.kV DC and 840 mA. CW
microwaves are generated on application of DC Anode supply.
To obtain pulsed microwaves, the anode supply is switched on/off at the desired
duty. The duty cycle can be adjusted from 10% to 90% in 9 steps for manual
operation. The pulsing of -5.2 kV anode supply was achieved without using a
pulse transformer. For switching on/off the anode supply, solid-state relay is
used. We chose solid-state relay (SSR) over electromechanical relay to reduce
the switching on time associated with electromechanical relay (typ. 20ms). AC
mains supply to the primary of the anode supply transformer is switched by this
relay.
The control, interlock and display system (System Controller) controls all
important functions of the generator. The system controller is designed using

Analog Devices' Micro Converter ADuC841. It is a Single-cycle 20 MIPS 8051 core
microcontroller, with 8-channel, 420 kSPS, 12BIT ADC. The microcontroller
senses all control and interlock signals, and generates appropriate control
commands, generating the switching on / off sequence, displaying the status of
control and interlock parameters, handling interlock fault conditions and display
signals. All digital inputs and outputs are optically isolated. The microcontroller
also generates triggers pulses required for the solid-state relay. To synchronize
the trigger pulses with the AC mains supply, a zero crossing detector is designed.
The output of the zero crossing detector is used as interrupt signal for the
microcontroller. On receipt of the first interrupt pulse, the microcontroller
generates a trigger pulse for the solid-state relay. The set duty input decides the
pulse width of the solid-state relay trigger pulse, which is controlled by the
microcontroller by counting the number of zero crossing detector pulsed.
Provision for interfacing external signal for automatic adjustment of duty as per
external temperature profiler is made. When used in this mode, duty of
microwaves is adjusted automatically as per the input obtained from the
temperature profiler. Sufficient safety interlocks for safety of the system as well
as of the operator are provided. System Controller automatically adjusts is used
to set the duty of Anode Supply. Metal Oxide Varistor (MOV Z1 and Z2) in input
side is used to protect the controller form high voltage transients. To run the
system controller and other circuits, +5 V DC and +24 VDC supplies are
required. These are supplied by the switch mode low voltage power supply. The
entire unit was accommodated in 600 X800 X1800 mm rack.
Dated this 12th Day of AUGUST 2008