Experimental Determination of Limiting Characteristics of AO 2 Series Induction Motors

B. A. Itkin, E. K. Strelbitskii

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

For the proper selection of protection and of an electric motor, taking into account the full utilization of its power, it is often necessary to know that limiting thermal characteristic of the stator winding above which the motor must not be overloaded. Such a characteristic is the limiting characteristic. By the limiting characteristic of an induction motor is meant the dependence of the duration of action of the overload current in the stator winding on its value, \(t=f(I)\), at which the winding temperature will not exceed the maximum permissible value.

At present, not only are the limiting characteristics of induction motors not given in manufacturers’ catalogues or information sheets, but there is also no unified point of view regarding the temperature values for this characteristic; moreover, methods for calculating them and obtaining them experimentally are lacking.

On the basis of calculations and experiments carried out by us on the wear of the insulation of AO 2 series electric motors under emergency conditions, and of life tests at SKBEM (Tomsk), the following temperatures \(\theta\) are recommended for the limiting characteristics of general-purpose induction motors with class “E” insulation:

1. For overload ratios \(k = 1.15 \div 1.4\), \(\theta = 120^\circ\mathrm{C}\).
2. For overload ratios \(k = 1.9 \div 3\), \(\theta = 160^\circ\mathrm{C}\).
3. For overload ratios \(k = 3.5 \div 7\), \(\theta = 180^\circ\mathrm{C}\).

For induction motors of electric drives that have frequent technological overloads, the choice of points of the limiting characteristic should be made on the basis of a preliminary calculation of the possible insulation wear.

The analytical calculation of points of the limiting characteristic consists in determining the duration of action of a given overload ratio at which the stator-winding temperature reaches the adopted maximum permissible temperature.

The calculation of the temperature rise \(\Theta\) under overloads [1] or emergency conditions [2] is based, to one extent or another, on parameters and coefficients determined experimentally, usually on base machines of the given series; a considerable error is possible in calculating \(\Theta\) for other machine sizes. Therefore, for mass-produced electric motors, especially of new series, it is advisable to determine the limiting characteristics experimentally.

Experimental determination of limiting characteristics is associated with measuring the temperature of the stator winding during various overloads and short circuits.

The generally accepted schemes for measuring the winding temperature are not suitable in this case. Thus, measurement of the ohmic resistance (i.e., of the average winding temperature) by a bridge or by the ammeter–voltmeter method requires disconnecting the machine from the network. Measuring the temperature by thermocouples requires embedding them in the winding, which is quite labor-intensive, especially for enclosed induction machines; in a number of cases this reduces its reliability and gives the temperature value only at the point of installation.

In [3] a special circuit is given that makes it possible to measure the ohmic resistance of a machine winding without disconnecting it from the network. Since pointer instruments are used in this circuit, a considerable measurement error is possible at high current multiples, because the rate of temperature rise in the stator winding at \(\kappa = 3 \div 7\) is of the order of \(2 \div 6^\circ\mathrm{C}/\mathrm{sec}\). In addition, in the circuit under consideration it is necessary to insert large-capacitance blocking capacitors into the power circuit of the electric motor. The circuit of [3] was substantially supplemented by us. Thus, by installing a shunt “Sh” and two transformer–capacitor cascades \((\mathrm{Tr}_4—C_4;\ \mathrm{Tr}_5—C_5)\) in the power circuit, it was possible to avoid the use of blocking capacitors. Galvanometers \(G_1, G_2\) of the H700 oscillograph, with the corresponding shunting resistances and capacitance, were placed in the current- and voltage-measuring circuits, which made it possible to make a continuous record of the ohmic resistance of the windings and helped improve the accuracy of the measurement. Introducing into the circuit the storage battery \(B\) with an additional

Fig. 1. Circuit for measuring the temperature of the stator winding when taking the limiting characteristic.

resistance \(R_3\) with a shunting contact serves to stabilize the measuring current when changing over from measurement of the winding in the cold state to measurement at network voltage. Figure 1 shows the circuit used for taking limiting characteristics. The parameters of this circuit are given in the Appendix.

Appendix

Parameters of the circuit (Fig. 1) for measuring the temperature of the stator winding when taking the limiting characteristic:

$Tr_1$, $Tr_2$ — transformers 380/380 V; $w_1 = w_2 = 810$ turns, wire Ø 0.5 mm;

$Tr_3$ — transformer $w_1 = w_2 = 100$ turns, wire Ø 1.5 mm;

$Tr_4$ — transformer $w_1 = w_2 = 1200$ turns, wire Ø 1.2 mm;

$Tr_5$ — transformer $w_1 = w_2 = 500$ turns, wire Ø 1.2 mm;

$C_1 \div C_7$ — capacitors: $C_1 = 210$ μF, 300 V;

$C_2 = 2000$ μF, 30 V; $C_3 = 1000$ μF, 30 V;

$C_4 = 4000$ μF, 10 V; $C_5 = 3000$ μF, 10 V;

$C_6 = 1500$ μF, 10 V; $C_7 = 3000$ μF, 10 V;

$R_1 = 100$ ohms; $R_2 = 40$ ohms; $R_3 = 25$ ohms;

$G_1$, $G_2$ — galvanometers of the H-700 oscillograph;

$mV_1$ — millivoltmeter, type M 105;

$mV_2$ — millivoltmeter, type M 82;

$B$ — storage battery, 10–12 volts;

$Sh$ — manganin shunt, $R = 0.015$ ohm;

$AD$ — induction motor under test;

$Tr_6$ — current transformer;

$A$ — ammeter.

$Tr_6$ and $A$ are selected depending on the power of the motor under test.

REFERENCES

1. P. A. Suiskii. On the selection of induction motors for short-time and cyclic operating modes. Elektrichestvo, No. 10, 1964.

2. B. A. Itkin, E. K. Strelbitskii. On the rate of temperature rise under emergency conditions and on the cooling laws of AO 2 series motors, Izv. TPI, vol. 162, 1967.

3. A. M. Sharipov. A device for measuring the ohmic resistances of windings of electrical machines in the presence of alternating emf, Collection TsITEIN, No. E-61-72(21), 1961.