Electric heating – Metal heating – Of cylinders
Patent
1988-03-01
1990-07-03
Leung, Philip H.
Electric heating
Metal heating
Of cylinders
219 1055M, 219 1055R, 315107, 328267, 323282, H05B 668
Patent
active
049393308
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and to an arrangement for controlling magnetrons in apparatus in which microwave energy is utilized for heating purposes.
Microwave heating is a technique which can be applied with great advantage in a multiple of processes which include the supply of thermal energy. One important advantage in this regard is that the heating power can be controlled in the absence of any inertia.
One drawback, however, is that microwave equipment is often more expensive than the conventional alternatives. The magnetron of such heating equipment is driven by a power unit with associated control system, which constitute the major cost of the equipment. Since the output power of a magnetron is limited, heating equipment will often require the presence of a significant number of magnetrons and associated power units and control systems to achieve a given heating requirement.
Magnetrons are used almost exclusively as microwave generators for heating purposes. Those properties decisive in this regard are the high efficiency achieved in converting d.c. power to microwave power and the compact geometry of the magnetron. One serious drawback is that the voltage required to produce a given power output varies from magnetron to magnetron. This voltage is determined by the internal geometry of the magnetron and the magnetic field strength in the cavity.
Two types of magnetrons are found, namely those in which the magnetic field is generated by a permanent magnet and those in which the magnetic field is generated by an electromagnet.
The strength of the permanent magnets varies in manufacture and during operation.
The magnetron construction includes a magnetic yoke, the permeability of which varies with temperature. The operating curve, seen as a graph in which the anode voltage is plotted against the anode current, changes together with the geometric changes that take place in the magnetron in response to changes in temperature therein. The output power is proportional to the anode current, to a high level of accuracy.
Because of these circumstances, it is not possible to drive a multiple of magnetrons directly from a common power unit. The aforementioned graph presents a knee, the so-called knee voltage, over which the power output of the magnetron is greatly increased.
When two or more magnetrons are connected to a power unit in parallel and the magnetrons have mutually different operating curves, which is the normal case, one magnetron will produce a higher power output than the other. The magnetron with the higher output power will become hotter than the other, wherewith the operating curve falls and the power unit will produce a lower output voltage. This causes the power output of the magnetron producing the lower output to fall still further, etc., until only the one magnetron produces all power, due to the failure to reach the knee voltage of the other magnetron.
The fundamental problem resides in the necessity of controlling each magnetron individually, together with the attempt to decrease the number of power units and associated control systems.
This problem is resolved by means of the present invention which enables magnetrons equipped with permanent magnets and magnetrons equipped with electromagnets to be powered by one and the same power unit.
Thus, the present invention relates to a method for controlling magnetrons with regard to their microwave power, in equipment incorporating a multiple of magnetrons, characterized by connecting two or more magnetrons in parallel with a power unit operative in producing a high voltage for operation of the magnetrons; by connecting to respective magnetrons a separate magnetron regulating circuit which includes measuring means for measuring the anode current through respective magnetrons on the high-voltage side thereof; and galvanically isolating said measuring means from a control circuit, said control circuit being constructed to control the anode current of the magnetron concerned in response to a signal produced by said measuring means.
A
REFERENCES:
patent: 3611017 (1971-10-01), Freeland
patent: 3619536 (1971-11-01), Boehm
patent: 3735237 (1973-05-01), Derby
patent: 3780252 (1973-12-01), Crapuchettes
patent: 4149057 (1979-04-01), Fritts
Berggren Benny
Gustafsson Larsgoran
Alfastar AB
Leung Philip H.
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