US3699308A - Electrical heating elements - Google Patents

Electrical heating elements Download PDF

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Publication number
US3699308A
US3699308A US80447A US3699308DA US3699308A US 3699308 A US3699308 A US 3699308A US 80447 A US80447 A US 80447A US 3699308D A US3699308D A US 3699308DA US 3699308 A US3699308 A US 3699308A
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Prior art keywords
potential
voltage
sheath
resistance element
period during
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Expired - Lifetime
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US80447A
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English (en)
Inventor
Eric Hutchinson
Gordon Hetherington
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Thermal Syndicate Ltd
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Thermal Syndicate Ltd
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B3/00—Ohmic-resistance heating
    • H05B3/40—Heating elements having the shape of rods or tubes
    • H05B3/42—Heating elements having the shape of rods or tubes non-flexible
    • H05B3/48—Heating elements having the shape of rods or tubes non-flexible heating conductor embedded in insulating material

Definitions

  • Coaxial electric heating elements of the type described above are widely used in domestic and industrial appliances (e.g. in spiral form as heating rings for domestic cookers).
  • the sheath is earthed and an AC voltage (typically between 120 and 250 volts. 50 or 60 Hz) is applied across the resistance element.
  • the refractory material surrounding the resistance element is required to transmit thermal energy as'efficiently as possible to the outer sheath, but at the same time electrically to insulate the resistance element as effectively as possible from the'earthed sheath.
  • thermal capacity of a heating element of the type described is small as possible. This allows the element to heat up (or cool down) in the shortest possible time. Small thermal capacity suggests as small a thickness of refractory material as possible.
  • leakagecurrent flowing between the resistance element and the sheath during operation of the heating element i.e. when the refractory material is at its operating temperature
  • this sug' gests as large a thickness of refractory material as possible.
  • a method for improving the performance of an electric heating element comprising an axial resistance element insulated from a conducting sheath by a layer of refractory material, comprises periodically applying a D.C. potential between the sheath and the resistance element.
  • the actual D.C. voltage applied andthe periodicity of its application can be optimized empirically for any particular refractory material.
  • a D.C. voltage of between 5 and 20 percent (conveniently around percent) of the A.C. voltage has been found to be convenient in many cases.
  • the D.C. voltage should not be less than 5 volts.
  • An on time of some 5 percent and an off time of some 95 percent has been found suitable.
  • apparatus for-improving the performance of an electric heating element comprising an axial resistance element insulated from a conducting sheath by a layer of refractory material-comprises first means for applying an A.C. voltage between the resistance element and the sheath and second means for periodically applying a D.C. voltage between the sheath and the resistance element.
  • One embodiment of apparatus in accordance with the invention would be a domestic electric cooking stove.
  • FIGS. 1 and 2 show circuit arrangements for periodi- Ycally applying a D.C. electrolysing voltage to the FIGS. 1 and 2 illustrate two simple methods of apply- I ing a periodic electrolysing D.C. voltage between the resistance element and the sheath of a heating element.
  • a switch S is operated cyclically. With the switch S closed, a heating element 1 heats up in the normal way from an A.C. supply 2, one side of which should be at, or near to earth potential.
  • D.C. current flows via a diode D, a current limiting resistor -R and the insulating material interposed between the resistance element and the outer earthed sheath of the heating element 1.
  • a radiant element on an electric domestic cooker normally employs a cyclic switch for heat control purposes and this switch can simply (and relatively cheaply) be modified to the circuit shown in FIG. 1.
  • FIG. 2 shows a modified arrangement in which the heating element 1 is energized from the A.C. supply 2 via a bi-directional thyristor T.
  • the thyristor T is asymmetrically fired by virtue of a firing delay introduced by a Zener diode D1 in circuit with a variable resistor R1.
  • the resultant wave form fed to the heating element 1 is a distorted sinusoidal wave form, the distortions being different in the positive half cycles as compared to the distortion in the negative half cycles.
  • the resultant wave form may be considered as a combination of an A.C. voltage and a D.C. bias.
  • the D.C. bias (whose magnitude can be controlled by varying the degree of asymmetry introduced into the output wave form by the delayed firing of the heating element 1) can be controlled by the variable resistor R1 since this resistor is controlling the firing angle of the thyristor T.
  • the resistor R1 by controlling the instant in each half cycle when the thyristor T is fired, controls the energy supplied to the heating element 1 and thus serves to control the temperature attained by the element.
  • the controlled circuit shown in FIG. 2 is more complicated, than that commonly employed for the heating elements of a domestic electric cooker, but it will be appreciated that the circuit shown acts as an energy regulator and does permit very smooth control over element temperature. Further, by virtue of the application of a D.C. electrolysing voltage, the electrical resistance of the insulating material in the heating element 1 has an improved performance which enables thinner layers of such material to be used and thus reduces the cost and improves the efficiency of the heating element.
  • FIG. 3 shows the variation of A.C. leakage current with the time of application of the D.C. electrolysing voltage.
  • the ordinate shown in FIG. 3 expresses the A.C. leakage current immediately after removal of the D.C. electrolysing voltage as a ratio of the A.C. leakage current existing in an unelectrolysed heating element.
  • the abscissa in FIG. 3 shows the time (in minutes) for which the D.c. electrolysing voltage is applied and it will be seen that an approximately exponential type curve is obtained indicating that only marginal improvements are obtained if the D.C. electrolysis is continued for more than about five minutes.
  • the D.C. voltage was 30 volts and the A.C. voltage was 240 volts.
  • the heating element was operated so that the sheath temperature was 850 C. and the insulating material was magnesium oxide.
  • FIG. 4 shows how the effect of the D.C. electrolysis decays with time after the electrolysing voltage has been removed.
  • the ordinate shows the ratio of A.C. leakage currents before and after application of the D.C. voltage and the abscissa shows the time (in minutes) from the termination of the electrolysis. From the graph it will be seen that a substantially linear relationship exists, the leakage current steadily returning to the value pertaining prior to the application of the electrolysing voltage.
  • the results shown in FIG. 4 were obtained using a 30 volt D.C. electrolysing voltage, 240 volts A.C. heating voltage and a heating element, with magnesium oxide insulating material, operating at a sheath temperature of 850 C.
  • FIG. 5 shows a plot of leakage currents (ordinate) in milliamps against time in hours as abscissa for a heating element which was operated normally prior to the point X and was then subjected to D.C. electrolysis in the manner described (and using a circuit as shown in FIG. 1) the A.C. voltage being applied for 97 seconds in each hundred and D.C. voltage for the last three seconds.
  • a method of effecting a significant practical reduction in the overall leakage current with time occurring across a refractory material electrically insulating an axial resistance element from a surrounding electrically conducting sheath in a heating element comprising the steps of heating said element by applying a A.C. voltage thereacross, periodically applying a unidirectional D.C. potential between the sheath and the resistance element of a magnitude and for a time sufficient to effect said reduction.
  • the D.C. potential used is between 5 and 20 percent of the A.C. voltage applied.
  • a method as claimed in claim 1 in said step the D.C. voltage applied is at least 5 volts.
  • a method as claimed in claim 9 the said ratio in said step is about 5:95.
  • An electric heating apparatus having a heating 7 element with an axial resistance element insulated from a grounded conducting sheath by a layer of a refractory material, a device for increasing the effective resistance of said refractory material comprising means applying an AC. voltage across the resistance element, second means applying between the sheath and the resistance element a unidirectional D.C. voltage of a magnitude and for a time sufficient to achieve a practical significant increase in the effective resistance of said refractory material, and third means for periodically interrupting said D.C. voltage.

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  • Control Of Resistance Heating (AREA)
  • Resistance Heating (AREA)
  • Electrostatic Separation (AREA)
US80447A 1969-10-16 1970-10-13 Electrical heating elements Expired - Lifetime US3699308A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB5088969 1969-10-16

Publications (1)

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US3699308A true US3699308A (en) 1972-10-17

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US (1) US3699308A (enrdf_load_stackoverflow)
CA (1) CA921532A (enrdf_load_stackoverflow)
DE (1) DE2050952C3 (enrdf_load_stackoverflow)
FR (1) FR2065997A5 (enrdf_load_stackoverflow)
GB (1) GB1318198A (enrdf_load_stackoverflow)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4251718A (en) * 1978-01-31 1981-02-17 Dreamland Electrical Appliances Limited Heating circuits
US5229578A (en) * 1989-09-14 1993-07-20 Canon Kabushiki Kaisha Heater activating apparatus with a switchable current controlling element

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4316080A (en) * 1980-02-29 1982-02-16 Theodore Wroblewski Temperature control devices
DE3714929A1 (de) * 1987-03-02 1988-09-15 Stiebel Eltron Gmbh & Co Kg Elektrische schutzbeschaltung eines rohrheizkoerpers

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2686250A (en) * 1951-11-02 1954-08-10 Gen Electric Electric heating apparatus
US2921174A (en) * 1959-02-13 1960-01-12 Gen Electric Electric surface heating unit system

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2686250A (en) * 1951-11-02 1954-08-10 Gen Electric Electric heating apparatus
US2921174A (en) * 1959-02-13 1960-01-12 Gen Electric Electric surface heating unit system

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4251718A (en) * 1978-01-31 1981-02-17 Dreamland Electrical Appliances Limited Heating circuits
US5229578A (en) * 1989-09-14 1993-07-20 Canon Kabushiki Kaisha Heater activating apparatus with a switchable current controlling element

Also Published As

Publication number Publication date
DE2050952A1 (de) 1971-04-29
CA921532A (en) 1973-02-20
DE2050952B2 (de) 1973-05-24
FR2065997A5 (enrdf_load_stackoverflow) 1971-08-06
DE2050952C3 (de) 1973-12-06
GB1318198A (en) 1973-05-23

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