Temperature Control System for a Salt Bath Furnace Based on a Saturable Transformer

V. P. OBRUSNIK, M. A. TYRYSHKIN

(Recommended by the scientific seminar of the Electromechanical Faculty).

The regulation and stabilization of the temperature of domestic salt furnaces and baths for the heat treatment of steel products is carried out by means of relay-contactor circuits. The stabilization process is based on the periodic switching off and switching on (partial or complete) of the voltage feeding the furnace transformer, depending on whether the controlled temperature is above or below the set value. Switching is performed directly in the power circuit by means of contactors, which is the cause of a number of shortcomings of such circuits, the most significant of which are the following:

1. Frequent repair and high depreciation of power contactors and intermediate relays. The need for constant supervision of the operation of the system.

2. Disturbance of the normal technological regime and even accidents (explosion of overheated salt) due to skewing and sticking of the contacts of electromagnetic apparatus.

3. Unsatisfactory operation of control-and-measuring instruments, caused by abrupt changes in the electrostatic and magnetic fields in the furnace.

4. Noise in the shop from the operation of contactors, periodic current surges in the network, radio interference.

The above-mentioned shortcomings are extremely undesirable in the operation of salt heating furnaces and baths, especially under production conditions where heat treatment is one of the main sections of the technological cycle for manufacturing products. They are completely eliminated when the existing relay-contactor circuits are replaced by the contactless control system shown in Fig. 1, developed and tested under production conditions on SP-2-35 furnaces by the authors of this article. Externally, the circuit differs from known ones only in that the unregulated transformer in it is replaced by a transformer controlled by biasing the shunts (TRPSh), and the electromagnetic intermediate relay is replaced by a magnetic relay (MR).

The relay principle of control in the proposed system is retained, which, with the large thermal inertia of the furnaces (8–12 min), provides-

causes their more rapid heating and cooling in the process of temperature stabilization than is possible with continuous regulation.

After ignition and preliminary heating of the furnace, the pointer U of the measuring device built into the measuring instrument of the electronic potentiometer EPD is set to the scale division corresponding to the specified operating temperature.

Fig. 1.

The photoresistor chamber FS and the illuminator OL, rigidly connected with the pointer, will assume a position determined by the location of the pointer (see Fig. 1). While the furnace temperature is below the specified value, the instrument pointer does not block the light beam directed onto the photoelement, and the resistance of FS is minimal; the current in the control winding of TRPSh is maximal, and the voltage at the electrodes of the salt furnace SP is maximal. Upon reaching

of the preset temperature, or when it is exceeded, the instrument pointer blocks the illuminator beam from the photocell; the photoresistance becomes very high, the current in the winding \(W\) of the magnetic relay drops sharply, and it removes the voltage from the control winding of the TSF. The power supplied to the furnace decreases in steps and the furnace begins to cool; the pointer again releases the beam and the regulation process is repeated.

The system in Fig. 1 makes it possible, in a convenient and simple way, during the cooling period of the furnace, to disconnect not the entire output voltage of the transformer, but only part of it, sufficient to ensure a decrease in temperature. This makes it possible to greatly extend the cooling time of the furnace during regulation, and thereby to reduce severalfold the frequency of magnetization of the TSF, the frequency of current pulses in the network, and changes in the electrostatic fields of the bath. The quality of heat treatment is thereby improved. Moreover, the process of variation of the quantities named does not occur abruptly, but smoothly. This positive factor is promoted by the electromagnetic inertia of the TSF and the MR.

The experimental temperature curves of the SP-2-35 furnace in the established regime are shown in Fig. 2. The thick curve was obtained in the system of Fig. 1, with changes in the output voltage of the TSF from 0.6 to 1 of nominal; the thin curve was obtained in a relay-contact circuit that completely disconnects the voltage for the cooling period.

Fig. 2.

A transformer regulated by magnetization of shunts and a magnetic relay, unlike contactors and relays, which are now being replaced in the circuit, under normal operation have a practically unlimited service life. Their calculation and manufacture are possible in any electrical workshop of an enterprise. The TSF may be made according to any known design: three-phase or a group unit made up of three single-phase TSF’s, see [1, 2]. For the experimental installation it was made according to the design in Fig. 3 by a simple modification of a TNT-35AO furnace transformer: a shunt with coils of the control winding was added to the magnetic core with the primary windings, and the secondary winding, with the same values of number of turns and cross-section, was made to embrace both magnetic cores.

The magnetic relay MR is conveniently made on a single-phase TSF, in which the feedback winding \(W_{\mathrm{oc}}\) is wound with a number of turns equal to \((1.1 \div 1.07) W_1\), or \((1.15 \div 1.1) W_2\), respectively for its series connection with the primary or secondary working windings.

Specific to a contactless temperature-control system for salt-bath furnaces are the calculations of the cross-section of the TSF shunt and of the magnitude of the ballast resistance \(R_{\mathrm{b}}\).

The optimum shunt cross-section \(S_{\mathrm{sh,opt}}\) determines the minimum power, found from thermal calculations or by experiment, which the TSF must deliver to the furnace during the cooling period. Theoretical and ex-

experimental analyses have shown that the value of \(S_{\text{sh}}\) lies within the range \(0.75 \div 0.55\) of the cross section of the main magnetic core \(S_0\), if the latter and the shunt are made of the same transformer steel. With a wide range of operating temperatures,

\[ S_{\text{sh}}=(0.75 \div 0.7)S_0, \]

when the furnaces operate at a single temperature,

\[ S_{\text{sh}}=(0.6 \div 0.55)S_0. \]

Fig. 3.

The parameters of the ballast resistance \(R_b\) depend entirely on three quantities: the permissible reverse voltage of the rectifier \(B\), the number of turns of the control winding of the TRPSh, and the maximum induction of the main magnetic core. The purpose of \(R_b\) is to suppress the voltage of harmonics that are multiples of three. For TRPSh designs with secondary windings encompassing the shunt and with the working windings connected according to the \(\lambda/\Delta\) scheme, the tripled emf value of these harmonics will always be in the open triangle of the bias-winding coils, and their value may be dangerous for the rectifier valves \(B\) and for the service personnel. Shunting the control winding with a resistance ensures free flow of harmonic currents that are multiples of three and sharply reduces the voltage at its terminals. It is not expedient to provide a special short-circuited winding to eliminate the emf of these harmonics, since additional costs are required for its manufacture, and the weight, dimensions, and heating of the TRPSh increase. Shunting the output terminals of the control winding with a capacitance, on the other hand, further increases the emf of the harmonics.

Neglecting the influence of all harmonics except the third—which is quite possible in view of the small specific weight of the former—when calculating \(R_b\) one may use the expressions for frequency triplers proposed in [3]. If the magnetization curves of the TRPSh magnetic cores are approximated by the formula

\[ H=a\,\operatorname{sh}\beta B, \tag{1} \]

then for the current and induction of the third harmonic of its control winding the following expression will be valid:

\[ 3\sqrt{2}\,W_y I_{y3} = al\left[ \operatorname{sh}R_3\left(\operatorname{ch}R_a+2\operatorname{ch}\frac{R_a}{2}\right) + \operatorname{ch}R_3\left(\operatorname{sh}R_a-2\operatorname{sh}\frac{R_a}{2}\right) \right], \tag{2} \]

where \(R_3=\beta B_3,\ R_a=\beta B\) are dimensionless quantities;

\[ B \text{ and } B_3 \]

are the maximum values of the induction of the fundamental and third harmonic;

\[ a,\ \beta \]

are the coefficients of the approximating equation;

\[ l \]

is the mean length of the line of force of the main magnetic core of the TRPSh;

\[ W_y,\ I_{y3} \]

are the number of turns and the current of the third harmonic of the control winding.

From expression (2), for the current of the third harmonic of a short-circuited control winding, when \(B_3=0\), we obtain

\[ I_{y3k} = \frac{al\left(\operatorname{sh}R_a-\operatorname{sh}\frac{R_a}{2}\right)} {3\sqrt{2}\,W_y}, \tag{3} \]

and for the open winding, when \(I_{y3}=0\), we find the expression for the 3rd harmonic of induction

\[ \operatorname{th} R_3 = -\frac{\operatorname{sh} R_a - 2 \operatorname{sh}\dfrac{R_a}{2}} {\operatorname{ch} R_a + 2 \operatorname{ch}\dfrac{R_a}{2}} . \tag{4} \]

Since

\[ e^{2R_2}=\frac{1+\operatorname{th} R_3}{1-\operatorname{th} R_3}=A, \tag{5} \]

then

\[ B=\frac{\lg A}{2\beta \lg e}. \tag{6} \]

Knowing the value \(B_3\), it is not difficult to find the total value of the voltage of the third harmonics at the terminals of the control winding:

\[ U_{y30}=\frac{3}{\sqrt{2}}\cdot \omega_3 \cdot B_3 \cdot S_{\mathrm{sh}} \cdot W_y, \tag{7} \]

where \(\omega_3\) is the angular frequency of the 3rd harmonic,
\(S_{\mathrm{sh}}\) is the shunt cross section.

Considering the control winding as a voltage generator of triple frequency with internal resistance

\[ X_{y3}=\frac{U_{y30}}{I_{yk3}} . \tag{8} \]

one may write the expression for the current for any value of the ballast resistance \(R_b\):

\[ I_{y3}=\frac{U_{y30}}{\sqrt{R_b^2+X_{y3}^2}} . \tag{9} \]

If it is assumed that the voltage across the resistance must not exceed some permissible value, that is, the condition

\[ U_{\mathrm{perm}}=I_{y3}\cdot R_b, \tag{10} \]

must be maintained, then from expressions (9, 10) we obtain

\[ R_b=\sqrt{\frac{U_{\mathrm{perm}}^2 X_{y3}^2}{U_{y30}^2-U_{\mathrm{perm}}^2}} . \tag{11} \]

Experimental verification showed that expressions (3–11) are quite acceptable in engineering calculations of the value \(R_b\). For example, for an experimental system for the furnace SP-2-35, having the following data for the TPRS: \(U_1=220\ \mathrm{V}\), \(W_1=58\) turns, \(W_2=4\) turns, \(W_y=450\) turns, \(R_y=1.6\ \Omega\), \(S_0=144\ \mathrm{cm}^2\), \(S_{\mathrm{sh}}=100\ \mathrm{cm}^2\), \(l=62\ \mathrm{cm}\), \(B=1.2\ \mathrm{T}\), steel E-42, at \(\alpha=0.07\ \mathrm{A/cm}\), \(\beta=4.72\ 1/\mathrm{T}\), the following calculated and experimental values were obtained:

When selecting valves for rectifier \(B\), it is necessary to take into account that, during one half-period, the alternating components of the current in the control winding will flow through the valves, bypassing \(R_b\). Therefore, they must have a current margin of not less than \(0.5 I_{uzk}\).

Operation of the described contactless control system (Fig. 1) under production conditions has shown that it can successfully replace existing relay-contactor circuits, greatly surpassing the latter in operational reliability and ease of maintenance.

References

1. A. M. Bamdas, V. A. Somov, A. O. Shmidt. Transformers and stabilizers regulated by shunt biasing. Gosenergoizdat, 1959.

2. A. M. Bamdas, S. V. Shapiro, O. L. Getmanenko. Determination of the optimal design of transformers and autotransformers regulated by biasing, and their calculation. Proceedings of the State Design Institute, vol. XVIII, issue 1, 1962.

3. L. L. Rozhanskii. Static frequency converters. Gosenergoizdat, 1962.