Radatherm manufactures Variable Power Microwave equipment for a wide range of laboratory and industrial applications available with the following features:
Radatherm Variable Power Microwave Furnaces and Ovens are suitable for a wide range of materials science research and development projects as well as industrial scale applications.
It enables the operator to control accurately the amount of radio frequency power being applied to the load, or to control the power applied to the load so that it follows a programmed temperature pattern. At the same time it provides feedback to the scientist about the relationship between power, temperature and the loss characteristics of the load. The cabinet can be constructed with numerous ports into the cavity so that various measurements can be made whilst microwave heating is in progress.
The microwave power output from the 2450MHz magnetron is continuously
variable from 0 to 100% of rated power. Power ranges available
include 600W, 1000W, 1500W and 2000W. It has significant advantages
over competitive systems that rely on a time slicing method to
vary the average power output.
The loss characteristics of materials vary greatly depending
on the amount of microwave power applied to them. The problem
with time slicing is that, even at low power settings, the load
is subjected to the full output of the magnetron albeit for a
short period of time. Subtle changes in the loss characteristics
can be detected using continuously variable power control where
they cannot be detected using time-slice power control systems.
The furnace has a sophisticated microprocessor control system that can not only monitor and control the microwave power applied to the load, but also monitor and control the power loss into the load. Measurements are made of the power applied to the load, and the power reflected from the load, by detectors located in the waveguide between the magnetron and the cavity. The microprocessor linearises the outputs from the detectors and converts them to RF power units (watts).
The control system has the facility to program setpoints or ramps and dwells for the applied power to follow. Alternatively, the controller can calculate the amount of power loss into the load from the difference between the forward and reflected power. It can then vary the forward power to maintain a steady or ramping loss rate.
The power control system has a serial data port for connection
to a PC type computer. This allows continuous data logging of
both the forward and reflected (and therefore loss) power so that
they can be correlated with measurements from other sensors used
with the furnace. Programs may be stored and down loaded to the
controller from the computer.
The furnace control system includes a microprocessor based programmable temperature controller that can use either an infra-red pyrometer or a thermocouple to sense the load temperature. The feedback source for the magnetron control circuit can be switched from the power control system to the temperature control system. If the temperature control system is used as the feedback source then the magnetron's output will be varied to maintain a setpoint or ramping temperature according to the program set.
The temperature control system also has a serial data port. Continuous data logging is possible as well as down loading of stored programs.
Cavity ports are provided at the top, bottom and left hand side of the cavity. The side ports can be used for sighting the pyrometer on the load or for the introduction of a thermocouple into the chamber. The bottom port can be used for continuous weighing of the load during processing. The top and bottom ports can be used together for pressing samples during processing and also for fitting work tubes so that the processing can be carried out under controlled atmospheres.
Combining the information from the two integral control systems and a weighing system the following types of measurements can be made:
Radatherm manufactures a low power temperature feedback controlled Variable Power Microwave oven for laboratory use where precise control at low temperatures is required for applications such as biological sample preparation, sample preparation by acid digestion and polymer curing.These units feature
The microwave power output from the 2450MHz magnetron is continuously variable from 0 to 600 watts. The use of continuously variable power has significant advantages over systems that rely on a time slicing method to vary the average power output.
The loss characteristics of materials vary greatly depending on the amount of microwave power applied to them. The problem with time slicing is that, even at low power settings, the load is subjected to the full output of the magnetron albeit for a short period of time. Subtle changes in the loss characteristics can be detected using continuously variable power control where they cannot be detected using time-slice power control systems.
The oven control system includes a microprocessor based programmable
temperature controller that uses a thermocouple to sense the load
temperature. If the temperature control system is used as the
feedback source then the magnetron's output will be varied to
maintain a setpoint or ramping temperature according to the program
set. The feedback source for the magnetron control circuit can
be switched from the temperature control system to the power control
system to allow manual control of heating.
The standard temperature control system allows for a single ramp and hold. Controllers with up to 16 ramps and dwells are available.
Control systems can be fitted with a serial data port. Continuous data logging is possible as well as down loading of stored programs.
|Magnetron Max. Output Power||600W|
|Power Control||Continuously Variable RF Amplitude|
|Power Distribution||Mode Stirrer|
|Temperature Control||Ramp Setpoint PID with autotuning|
There has been many a researcher who has tried to fire their ceramic component or melt a material in a domestic microwave oven. While they have probably had some degree of success, they have been limited by the simplistic control systems available on domestic and commercial units.
Domestic and commercial microwave ovens use a time-slicing method to control microwave power to the load, whether the load is anything from a cup of tea to a high precision technical ceramic article. Using this method, maximum power is applied for time intervals which are proportional the power which the user sets on the controller. For instance, if an operator selects 25% power on a microwave oven rated at 1000W, the device applies maximum power (1000W) for 25% of the time (see figure 1a).
In Variable Power Microwave Systems (VPMS), the method in which the power is applied differs in that the power is applied continuously, while the power itself, is proportional to that which the operator dials up. If we consider the same scenario as before and the operator selects 25% power, the microwave will deliver 250W of power for the entire duration of the cycle (see figure 1b).
Several problems may be encountered when people try using domestic microwaves that use the time-slicing technique. The main problem is that the pulsed power can induce uncontrollable exothermic reactions. Also, by pulsing the power, steady heatup schedules are difficult to achieve. Further, the simplistic control systems available on these units only usually allow a limited number of power steps and do not allow control of heating schedules. By now you may be wondering why this type of control system is used. The answer is price, and price alone. Using time-slicing, manufacturers of domestic and commercial microwave units can keep the price down, which is important from the point of view of the average consumer. As you have no doubt noticed through your own experience, the simple control system is easy to use for the average person and, yes, your dinner does get hot.
However, for the researcher and industrialist, who is looking to use microwave technology for drying, sintering, curing, etc., the only solution is a variable power microwave system. These systems allow accurate and continuous control of power to the load. The ultimate solution is a continuously variable microwave system that incorporates power, temperature or other process variable feedback control. This permits control not only of the power that is applied to the load, but also heatup rates etc.
Control of heatup schedules is a more complex problem than it sounds, as the rate at which the load heats up depends on the rate at which the load losses. Some of you may now ask, what does that mean. Well, the answer will be in an article in a later issue so stay tuned.
This article is to be published in
the Newsbulletin of the Australasian Ceramic Society, August/September
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Radatherm Pty Ltd
Mailing Address : P.O. Box 7188 Wetherill Park, NSW, 2164, Australia
Factory Address : 220 Woodpark Road, Smithfield NSW 2164
Telephone 02 9756 4660 (International) +61 2 9756 4660
Fax 02 9756 4552 (International) +61 2 9756 4552