US 3821456 A
Description (OCR text may contain errors)
United States Patent [191 Havas 1 POWER CONTROL MEANS FOR A SINGLE PHASE INDUCTION MELTING OR HEATING FURNACE 1 June 28, 19741  References Cited UNFTED STATES PATENTS 3,382,311 5/1968 Rydinger et al. 13/26 3,478,155 11/1969 Segsworth 13/26 Primary Examiner-Roy N. Envall, Jr.
 ABSTRACT There is disclosed herein a circuit for controlling the voltage and power delivered to a single phase induction melting or heating furnace wherein the impedance of the furnace varies. The circuit utilizes selected and controllable capacitors as control means.
' 5 Claims, 4 Drawing Figures SECOND FIRING ,rl4 R CONTROL UNIT I; I I; j;
T T T T FL FL A l Pmummz m4 382 1; 456
SHEET 1 0F 4 FIRST FlRING CONTROL UNIT A'IddfiS UV 2 I Pmmmmz H 3.821; 456
' sum 3 or 4 I I l v I I CONDUCTING RIA t I APCONDUCTING /N R, B CONDUCTING H I 1 1 l FPB H FIRST FIRING l3 CONTROL UNIT mimsnmzam 3821 45s SHEET a BF 4 V, SUPPLY My invention relates to power control means for a single phase induction melting or heating furnace operating at line frequency wherein means are provided for setting the range of voltage and power available to said furnace and for controlling the same.
It is particularly applicable to use in induction furnaces where the furnace reactance exceeds its resistance and wherein the impedance of the furnace changes substantially during the operation thereof affecting both the reactance and resistance components. A typical reactance to resistance ratio can change from :1 to 15:1 throughout a work cycle.
It is an object of my invention to achieve power control means for furnaces of the type referred to wherein the voltage across the load can be held within a range of safe and economical limits and to eliminate high and dangerous voltages.
Another object of the invention is to achieve such power control in a virtually stepless manner and to ac- FIG. 2 is a modification of the control circuit of FIG. 1 wherein a phase angle control unit actuates the first firing control unit and a furnace power control unit actuates the second firing control unit and an optional reactance is shown in the supply line;
FIG. 3 illustrates voltage V, across the load, the initial voltage e across a set of capacitors, firing pulses FPA and FPB and semiconductor current 1 at the initiation of conduction of the selected semiconductor means and subsequently the continued harmonic-free alternating current flow thereafter;
FIG. 4 illustrates the common locus of furnace voltage and series capacitor voltage for the control range at phase angles of 120, 135 and 150.
Referring now to the drawings, in all of which like parts are designated by like reference characters, in FIG. 1 the basic circuit consists of a single phase induction melting or heating furnace l0 operated from a normal line frequency alternating current power supply 11, 12. Power control means for said furnace comprise a first group of parallel connected capacitors, one set C of which is permanently connected to the induction load or furnace 10 which is connected to the supply line 11, and another set C which can be connected across the load as needed by semiconductor means, as for example, anti-parallel connected silicon controlled rectifiers R said power control means further includes a second group of parallel connected capacitors C connected in-series between the load or furnace 10 and the supply line 12 by means of anti-parallel connected silicon controlled rectifiers R as desired. FIG. 11 also illustrates control means consisting of a first (113) and second (14) firing control unit for initiating conduction of said semiconductor means R and R respectively.
In the description herein of the circuits and their operations, the semiconductor switching means are referred to as SCRs. It is to be understood that any suitable semiconductor switching means other than SCRs, such as triacs, etc., could be readily used to achieve the same ends.
The purpose of the parallel connected capacitors C and C is to set a permissible range within which the system can be operated so that, as the impedance of the load varies during a work cycle (heat or melt), the voltages V and V indicated on the drawings can be held within safe and economical limits determined by the voltage ratings of the capacitors and SCRs in use. It can be shown that the maximum value of root-meansquare (RlVlS) voltage which V, or V can attain is given by V V, E supply/sin d) where d) the angle between V, and V In other words, for a fixed supply voltage the maximum RMS voltage value V and V can attain is strictly a function of the angle between V and V Furthermore, it can be shown that for a given furnace impedance the amount of parallel capacitance needed to maintain a constant phase angle between V and V is given by where |Z I the absolute value of the uncorrected furnace impedance.
Q XL/R of the furnace,
X L furnace reactance.
R furnace resistance,
f frequency (11 the angle between V and V Once the operating range has been established by adjusting the value of the first sets of capacitors C 1 and C by means of the anti-parallel connected SCRs R until the desired value of (I) is obtained, the furnace voltage,
hence the power, can be increased or decreased by connecting or disconnecting selected sets of the second group of capacitors C in series between the furnace and the supply line 12 by rendering conductive the appropriate pair of anti-parallel connected SCRs, R
The vector diagram of FIG. 4 illustrates the common locus of V and V for (b and l50, which is a preferred operating range of phase angle between V, and V The common locus of V and V follows a circular path as the amount of capacitance connected in series between the furnace and the supply line is varied. Note that for a particular load impedance the angle (b between V and V obtained by the adjustment of the C capacitors of the first group remains constant and is independent of the value of the C capacitance connected in the circuit.
I have found that in practice, for economical and safety reasons, it is preferable to limit precisely the voltages appearing across both groups of capacitors and to regulate automatically the amount of power delivered to the load. This can be achieved readily by holding the phase angle 4) between V, and V constant for varying load impedances. In the preferred version of my invention, shown in FIG. 2, further control means are provided for the basic circuit shown in FIG. 1. The additional controls consist of a phase angle control unit 16 and a furnace power control unit 15. The phase angle control unit 16 adjusts the value of the first group of capacitors by varying the value of the C capacitors so as to maintain a constant value of phase angle 4) between the voltages V and V for a varying load impedance, thus assuring a fixed voltage range available to the furnace load. The furnace power control unit adjusts the value of the second group of capacitors C so as to maintain the desired voltage, hence power, delivered to the load.
Both the adjustment and the maintenance of the phase angle Q5 between V and V as well as the control of the power to the furnace load, is achieved by connecting the proper amount of capacitance in sets into the circuit by means of anti-parallel connected SCRs.
If desired, a reactance 17 can be connected across the supply line 11, 12, to reduce the leading current drain from the lines in a controlled mode of operation.
FIG. 3 illustrates the time relationship of the voltage V, across the load; the initial voltage e across the C capacitors; the SCR current and the firing pulses FPA and FPB beginning at the time current flow is initiated and continuing thereafter. Note that the conduction of the SCRs is always initiated at a time when the polarity of the voltage reverses across them, resulting in a virtually harmonic-free alternating current. The instant when the SCRs are gated must be controlled precisely to ensure harmonic-free operation. The gating of the SCRs at any other time results in large transient currents and harmonic distortion. The gating of the second set of SCRs is achieved in a similar manner.
From the above, it can readily be seen that with high speed response control units of the type well known in the art for initiating the conduction of appropriate sets of series connected capacitors and semi-conductor means the average effective value of the capacitance connected in the circuit by such means can be controlled in virtually stepless progression resulting in smooth and continuous power and voltage control for said furnaces or loads.
It can further be noted that the control circuits shown herein disclose a combination of variable series capacitors with partially fixed and partially variable parallel capacitors. By variable is meant the use of groups of sets of series connected capacitors and controllable semi-conductor means.
Although the invention has been disclosed in connection with certain preferred embodiments, it will be apparent that numerous and extensive departures may be made herein without, however, departing from the spirit of the invention and the scope of the appended claims.
What I claim is:
1. Power control means for a single-phase induction melting or heating furnace for operation from a normal line frequency power supply, one line of said power supply being connected to the first terminal of said furnace, said power control means comprising capacitance and a first group of sets of series connected capacitance and controllable semiconductor means all connected in parallel with said furnace and a second group of sets of series connected capacitance and controllable semiconductor means connected in parallel and interposed between another line of said power supply and the second terminal of said furnace, said semiconductor means of said first group being actuated by a first firing control means adapted to control the transition of each such semiconductor means from the nonconducting to the conducting state, the number and size of sets conducting within said first group determining the total amount of capacitance connected in parallel with said furnace and the phase angle between the furnace voltage and the voltage across said second group of sets thereby setting the range of voltage and power available to said furnace, said semiconductor means of said second group being actuated by a second firing control means adapted to control the transition of each such semiconductor means from the nonconducting to the conducting state, the number and size of sets conducting within said second group determining the total amount of capacitance effectively interposed between said other power supply line and said second furnace terminal, thereby controlling the voltage and power delivered to said furnace within the selected range as set by said phase angle.
2. Power control means as claimed in claim 1 wherein the conduction of each said semiconductor means when required is initiated substantially at the instant of polarity reversal of the voltage across said means and is maintained continuously thereafter to produce a harmonic-free alternating current in said set until deenergized.
3. Power control means as claimed in claim 1 wherein the total amount of capacitance in parallel with said furnace is such that the phase angle between the furnace voltage and the voltage across said second group of sets of capacitance and controllable semiconductor means remains between and 4. Power control means as claimed in claim 1 wherein the total amount of capacitance in parallel with said furnace is controlled so as to maintain a constant phase angle between the furnace voltage and the voltage across said second group of sets of capacitance and controllable semiconductor means while changes occur in the furnace impedance.
5. Power control means as claimed in claim 1 wherein an inductive reactance is connected across the input terminals to provide an improved line power factor.