US2381310A - Temperature control for electric heating - Google Patents

Description

Aug. 7, 1945. w. RICHTER TEMPERATURE CONTROL FOR ELECTRIC HEATING Filed June 21, 1945 Walther Richter INVENT OR.

ATTORNEY! Patented Aug. 7, 1945 TEMPERATURE CONTROL FOB ELECTRIC HEATING Walther Richter, Whitefish Bay, Wis alsignor to A. 0. Smith Corporation, Milwaukee, Win a corporation of New York Application June 21, 1943, Serial No. 491,051

9 Claims.

This invention relates to temperature controls for electric heating, and more specifically to the type of control designed to cut oil the supply of energy upon a given energy consumption as in the successive electric heating of like objects to a predetermined temperature.

In most electric heating operations the greatest variable factor encountered lies in the large fluctuations in voltage of the supply system employed, since the heating effect varies as the square 01' the voltage. A 10% rise in voltage will mean a 21% rise in heating effect for the same amount of time.

Assuming that the articles to be heated are of substantially uniform shape and mass and that a constant voltage input can be provided, it would be possible to employ a timer for cutting oil the heating operation for each article upon the elapse 01' a predetermined time to eflect a given heating each article. However, with the wide fluctuations in voltage in most factories, it is impossible to obtain a satisfactory control of temperature for the articles by merely timing the successive heating operations. Furthermore, the ordinary variations in mass of the articles being heated result in different input requirements for the same.

One of the objects of the present invention is to provide a device for cutting oil the heating current that functions to proportion the time of heating substantially inversely as the heating effect, i. e., inversely to the square of the voltage.

Another object of the invention is to provide such a timer with compensation at least partially for the variation in heat loss in the articles due to the variation in time of heating.

Another object is to more accurately control the heating operations to obtain a heating of successive articles to a predetermined like temperature.

The invention has been embodied in an apparam tus for controlling the induction heating of bomb bodies for forging the same, and the accompanying drawing illustrates this embodiment.

In the drawing, the single figure is a diagrammatic showingoi an induction furnace'with its circuit and control mechanism and an article disposed in the furnace for heating.

The furnace may be of the general type illustrated, in which a magnetic frame or core I is disposed to receive a bomb body 2 or other article to be heated. The energizing or primary coils 3 are supplied with current from a suitable power transformer 4 which in turn receives current from the power station I. The article 2 serves as the secondary oi the induction furnace and has currents induced therein 0! the article.

Since there are usually several furnaces supplied with power from the same station 5 and in addition, a large number of other power consuming devices in the same shop supplied from the same station, there is a wide fluctuation of voltage in the line. When the voltage is relatively high in any one furnace the heating is more rapid and, conversely, when the voltage is low the heating is less rapid. This rapidity of heating varies as the square of the voltage, other things remaining equal.

Likewise, the ordinary variations in size and mass of the article being heated result in different energy consumption for different articles at any given voltage.

In timing the heating of successive articles the voltage and mass is taken into account by employing a watt-hour meter 8 connected as shown in the drawing. The meter 6 is of ordinary construction having a revolving disc I rotated in response to the energy passing through the coils of t e meter. For this purpose, the two lower current coils 8 disposed beneath the dis 1 and a central voltage coil 9 disposed above the disc 1 provide the motive power for driving the disc.

The coils l are connected in series in a line it containing a relatively high constant resistance which eflect the heating II and connected across the lines I! and i3 leading to the primary coils 3 of the induction furnace,

I. A voltage divider having resistors l4 and I5 is connected across lines l2 and I3, and coil 9 is connected across the resistor is.

It the resistors l4 and ii of the voltage divider were constant or of constant ratio, the voltage applied to coil 8 would always hear a fixed ratio to the line voltage being supplied to primary coils 3 01 the furnace. Hence both the voltage applied to coil 8 and the current flowing through coils 8 will vary as the line voltage varies and will efiect driving of disc I at a speed corresponding to the square of the varying line voltage.

The rotation oi disc I eflects the opening of cutout switch I in the line l3 supplying current to the coil I or the induction furnace. This is accomplished as shown by means of the gears l8 driven by disc I and in turn tripping a pilot switch I! in an auxiliary control circuit 20 operating a solenoid II for controlling switch it. Any other suitable means may be employed for operating switch I from the rotation of disc I. A photoelectric counter responsive to the rotation of disc I has been employed.

The watt-hour meter connected as shown compensates ior variations in voltage in the line supplying the induction furnace with energy, as described above. It also compensates within limits for the different energy requirements or diflerent size workpieces. This latter compensation results from placing the current coils 8 in series with the fixed dummy resistance ll so that the energy being measured and which determines the length of the heating cycle is that being consumed in the constant resistance ll.

Should a relatively large workpiece be subjected to heating, and assuming a constant voltage, the current input to the piece would be greater thereby tending to compensate for the size or the piece. A smaller workpiece would in turn have a higher resistance and result in less energy consumption over a given period or time, thereby compensating for its lower mass. By basing the operation meter 6 upon the energy consumed by a dummy resistance, the time of heating is determined independent of the size or the workpiece. The variation in the heating time cycle results solely from the fluctuation of voltage in the line.

It the switch It were always operated to stop the supply of current to the furnace after a predetermined energy consumption registered by the meter 6, it is possible that a variable heating eflect might result from the tact that the heat losses from the article 2 are determined principally by the time involved. Where the heating time is long, there is a greater total heat loss from the article than where the heating time is short. This means that when the heating current is shut on upon the consumption or a predetermined amount of electrical energy by the dummy resistance II, the article may still have a different temperature than desired.

In order to more accurately control the temperature of the article and to compensate ior variations in the heat loss, the voltage divider is constructed with automatically varying resistances. For this purpose, resistor H may be a non-linear resistance such as an incandescent lamp with a tungsten filament which increases in resistance as its temperature rises. An increased voltage across the voltage divider will then cause the voltage across the lamp to increase proportionally more than that across the total divider.

This means that where the line voltage feeding the furnace increases so that the rate of heating 0! article 2 goes up, making for less time for heat loss, the watt-hour meter will run raster than the normal proportion of theratio oi the square of the voltages, so that less energy will be supplied to the article. Where the line voltage decreases below the voltage known to give the correct heating or the article, the voltage across resistance M will decrease in a greater proportion than the total voltage, and the watthour meter will turn at a lesser speed than that proportionate to the square of the voltage and thereby supply the predetermined greater amount of energy to the article to compensate for the larger heat loss resulting from thelonger time of heating.

The amount of compensation available can be increased by employing a non-linear resistance such as carbon filament incandescent lamps for resistor l5. Carbon filaments have a negative temperature coemcient and, therefore, would shift the voltage distribution even more to increase the proportion of voltage appearing across the resistor ll.

The amount or compensation available can be decreased by employing one or more smaller tungsten filament incandescent lamps in resistor I! to counteract in part the compensation provided by resistor ll. 4

The temperature control of the present inven tion provides a relatively accurate means of determining the heating of the article, and it compensates for differences in size of the workpieces, for the fluctuation of line voltage and for different heating time. The control can be applied in either electric induction or resistance heating.

Various embodiments may be employed within the scope of the claims.

I claim:

1. In a temperature control for electric induction and resistance heating operations, the combination with the work load and current supply lines of a heating system, a wattmeter having its voltage coil connected across the current supply lines and its current coils in series with a constant resistor also connected across the current supply lines in shunt to the work load, the connection of said voltage coil being made to a tap of a voltage divider bridging the supply line with a non-linear resistance in one element thereof to change the ratio between the voltage of said line and that applied to the coil of the meter so that the voltage will divide between the elements of the divider to give the voltage coil a higher proportionate voltage when the line voltage is high and a lower proportionate volt age when the line. voltage is low to thereby compensate for different heat losses in the work and provide for the more nearly equal heating of each successive workpiece, and means responsive to the operation of said meter for stopping the heating cycle in a predetermined time as modified by voltage changes in the supply line and substantially by varying heat losses in the workpiece.

2. In a temperature control for electric induction and resistance heating operations, the combination with the work load and current supply lines 01 a heating system, a wattmeter having its voltage coil connected across the current supply lines and its current coils also connected across the current supply lines in shunt to the work load, the connection of said voltage coil being made to a tap of a voltage divider bridging the supply line with a non-linear resistance in one element thereof to change the ratio between the voltage of said line and that applied to the coil oi the meter so that the voltage will divide between the elements of the divider to give the voltage coil a higher proportionate voltage when the line voltage is high and a lower proportionate voltage when the line voltage is low to thereby compensate for different heat losses in the work and provide for the more nearly equal heating of each successive workpiece, a constant resistor in series with the current coils of the meter to eiTect operation of the meter independent of the ing load, a tungsten filament incandescent lamp constituting at least part of the resistance of one branch of said voltage divider to modify the voltage applied to said meter so that the voltage will divide between the elements of the divider to give the voltage coil a higher proportionate voltage when the line voltage is high and a lower proportionate voltage when the line voltage is low.

4. In a device of the class described having a working load, current supply lines, and a wattmeter with its voltage coil connected across one element of a voltage divider shunting the working load, a tungsten filament incandescent lamp constituting a substantial part of the resistance of said element of said voltage divider to modify the voltage applied to said meter so that the voltage will divide between the elements of the divider to give the voltage coil a higher proportionate voltage when the line voltage is high and a lower proportionate voltage when the line voltage is low.

5. In a device of the class described having a working load, current supply lines, and a wattmeter with its voltage coil connected across one element of a voltage divider shunting the working load, a non-linear resistance constituting a substantial part of the resistance 01' one element of said voltage divider to modify the voltage applied to said meter so that the voltage will divide between the elements of the divider to give the voltage coil a higher proportionate voltage when the line voltage-is high and a lower proportionate voltage when the line voltage is low.

6. In a device of the class described having a working load, current supply lines, and a wattmeter with its voltage coil connected across one element of a voltage divider shunting the working load, a carbon filament incandescent lamp constituting at least part of the resistance of the-- other element of said voltage divider to modify the voltage applied to said meter so that the voltage will divide between the elements of the divider to give the voltage coil a higher proportionate voltage when the line voltage is high and a lower proportionate voltage when the line voltage is low.

'7. In a device of the class described having a working load, current supply lines, and a wattmeter with its voltage coil connected across one element of a voltage divider shunting the working load, a substantial part of the resistance of said element having a positive temperature coefficient of electrical conductivity to modify the voltage applied to said voltage coil during the operation of said meter so that the voltage will divide between the elements of the divider to give the voltage coil a higher proportionate voltage whenthe line voltage is high and a lower proportionate voltage when the line voltage is low.

8. In a device of the class described having a working load, current supply lines, and a wattmeter with its voltage coil connected across one element of a voltage divider shunting the working load, a substantial part of the resistance of another element of the divider having a negative temperature coeiiicient of electrical conductivity to modify the voltage applied to said voltage coil during the operation of said meter so that the voltage will divide between the elements of the divider to give the voltage coil a higher proportionate voltage when the line voltage is high and a lower proportionate voltage when the line voltage is low.

9. In a device of the class described having a working load, current supply lines, ahd a wattmeter with its voltage coil connected across one element of a voltage divider shunting the working load, the employment of resistances in the opposite elements of said voltage divider having difierent temperature coeflicients of electrical conductivity to modify the voltage applied to said voltage coil during the operation 01 said meter. so that the voltage will divide between the elements of the divider to give the voltage coil 9. higher proportionate voltage when the line voltage is high and a lower proportionate voltage when the line voltage is low.

WALTHER RICHTER.

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