EXPERIMENTAL DATA ON THE POWER DISSIPATION OF ADDITIONAL RESISTORS IN ELECTRICAL MEASURING INSTRUMENTS

(UNDER FREE CONVECTION CONDITIONS)

A. S. Lyalyukov, Yu. A. Zagromov, L. S. Ershova

(Presented by Prof. Dr. G. I. Fuks)

The selection of coils according to dissipated power and enclosure characteristics [1] provides for the use of experimental values of the power dissipation of additional resistors under free convection conditions. In this connection we set ourselves the task of obtaining reliable experimental data on the power dissipation of additional resistors at various overheating temperatures of the winding surface.

The investigation was carried out on 56 specimens of additional resistors (frames according to standards MN4073-62, MN4074-62, and MN4075-62). The winding was wound in bulk up to a diameter 1.0–1.5 mm smaller than the diameter of the bead of the frame, using PEMS wire with \(d = 0.1\text{–}0.3\) mm.

A copper–constantan thermocouple with thermowire diameter of 0.1 mm was placed on the surface of the winding in its middle part; its free ends (60 cm) encircled the winding around the circumference. The thermocouple junction was glued to the winding with a thin layer of BF adhesive.

The resistances of the specimens were measured by the compensation method. A DC battery, the resistance being measured \(R_k\), and a P-331 standard coil of 1000 ohms were connected in series, and with the aid of a P-300 potentiometer the voltage drop \(U_k\) across the resistance being measured and \(U_0\) across the standard coil were measured. Owing to equality of the current,

\[ \frac{U_k}{R_k} = \frac{U_0}{R_0}, \quad \text{whence} \quad R_k = 1000 \frac{U_k}{U_0}\ [\text{ohm}]. \]

The circuit of the experimental setup on which the dependence of power on overheating of the resistance winding was determined is shown in Fig. 1.

A stabilized rectifier was used as the power source. To determine the current in the additional resistor under test, the voltage drop was measured (with a P-300 potentiometer, accuracy class 0.015) across a standard resistor, for which a P-314 resistance box of accuracy class 0.1 was used.

The e.m.f. of the thermocouple was also measured with a P-300 potentiometer; the temperature \(t_0\) in the cold-junction thermostat (a Dewar vessel filled with glycerin) and the temperature \(t_f\) of the surrounding air were measured with laboratory thermometers having a scale division of 0.1°C.

All measurements were carried out under steady-state thermal conditions, as evidenced by the stability of the readings over 3–4 observations made at intervals of 20–25 min.

To eliminate the influence of random factors (solar heating, heating by lighting lamps, etc.), the additional resistance under test was covered with a paper casing 80 cm in diameter and 120 cm high.

For each resistance, 4–5 experiments were carried out under different conditions.

As a result of processing the experimental data, dependences of the dissipated power on overheating, \(P = f(\vartheta)\), were constructed (where \(\vartheta = t - t_f\); \(t\) and \(t_f\) are, respectively, the maximum temperature of the winding surface and the temperature of the air washing over the additional resistance) for each additional resistance. These dependences in the interval

Fig. 1. 1—stabilized rectifier; 2—milliammeter; 3—standard resistance; 4—resistance under test; 5, 8—thermometers; 6—thermostat with cooling mixture; 7—vessel with thermocouples; 9—hot junction of the thermocouple; 10—null galvanometer; 11—normal element; 12—potentiometer.

\(\vartheta = 60 \div 85^\circ\text{C}\) have a character close to linear. By plotting the graphs \(P = f(\vartheta)\), the dissipated powers of the additional resistances were determined at winding-surface overheatings \(\vartheta = 60, 70, 80, 85^\circ\text{C}\), which are given in Table 1.

Calculations showed that the average error of the results obtained does not exceed \(\pm 1.73\%\).

The values of the dissipated powers of the additional resistances given in Table 1 are recommended for use in selecting coils by the method proposed in [1].

Students A. Albul, V. Andrianov, Z. Gulyaev, A. Gershkovich, P. Dudukin, T. Kostrikina, A. Kokaya, G. Kurtseva, V. Karnachuk, K. Mikhontseva, G. Popov, and L. Chuprova took an active part in carrying out the work.

Table 1

Power dissipation of coils under free-convection conditions as a function of the maximum overheating of the winding surface

Continuation of the table

REFERENCES

1. A. S. Syatkov, G. P. Fuks, G. L. Ionin. Selection of additional resistances for electrical measuring instruments according to their power and housing characteristics. In the present collection.