Proceedings of the Tomsk Polytechnic Institute
of the Order of the October Revolution
and the Order of the Red Banner of Labor
named after S. M. Kirov

Vol. 284　　　　　　　　　　　　　　　　　　　　　　　　　1974

EXPERIMENTAL STUDY OF HEATING IN INDUCTION MOTORS UNDER INTERMITTENT DUTY MODES WITH FREQUENT STARTS

V. A. ZHADAN, Yu. V. KOPYLOV, D. I. SANNIKOV

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

A considerable proportion of induction motors operate in intermittent duty modes (IDM) with frequent starts. However, the existing methods for thermal calculation of induction motors have sufficiently high accuracy only for continuous-duty operation. The methods of thermal calculation currently used for IDM are, for the most part, simplified and have a significant error, or else are complex for engineering calculations and suitable only for particular types of motors.

In order to develop a refined method for the thermal calculation of induction motors in IDM, it is necessary to carry out experimental investigations of the heating of these motors while varying, over broad limits, all influencing factors: the relative duration of energization (ED), the number of energizations per hour \(h\), the static torque on the motor shaft \(M_c\), and the inertia coefficient of the rotating masses \(K_j\), which is the ratio of the moment of inertia of all masses on the motor shaft to the moment of inertia of the motor rotor.

Usually, when conducting thermal tests of induction motors in IDM, the static torque on the motor shaft is varied with the aid of a direct-current generator. This method of varying the static torque has a number of disadvantages:

1. The inertia coefficient cannot be made less than 3, since the flywheel mass of the armature of a direct-current generator is several times greater than the flywheel mass of the rotor of an induction motor of the same power, whereas according to GOST—183—66 the regulated values of \(K_j\) are 1.2; 1.6; 2.5; 4.0. Consequently, thermal tests in IDM at \(K_j < 3.0\) can be carried out only when \(M_c = 0\).

2. The use of a direct-current generator with a continuously energized field winding as a static load may lead to excessive heating of the winding during the motor standstill period.

Therefore, for carrying out thermal investigations of induction motors with a power not exceeding 10–15 kW in IDM, a low-inertia installation is proposed, the schematic of which is shown in Fig. 1. Here the static torque on the motor shaft is produced by friction forces between brake shoes 1 and brake cylinder 2. Regulation of the static torque is accomplished by changing the pressure of the brake shoes on the cylinder by means of spring 3. In this case, the entire

the useful power of the motor is expended on friction between the shoes and the cylinder, which leads to their considerable heating. Since the shoes and the cylinder are made of steel, the permissible operating temperature of the rubbing surfaces can be fairly high. To improve the cooling of the brake cylinder, water is supplied into its internal cavity, the flow rate of which is regulated by valve 4. To remove the steam formed, an exhaust-ventilation bell is installed above the setup. After a short running-in, the static torque on the motor shaft becomes practically constant, and its regulation during the tests is almost unnecessary.

Fig. 1. Setup for studying the heating of induction motors in intermittent duty modes

The inertia coefficient is varied by changing the number of disks 5 with a known moment of inertia. The setup makes it possible to vary the inertia coefficient \(K_j\) within the range 1.2–5.0, i.e., to cover the entire range of variation of \(K_j\) provided for by GOST 183-66.

The value of the relative on-time and the number of starts per hour are set by means of a mechanical switch driven by a DC motor, which makes it possible to regulate the number of starts per hour, as well as the relative on-time, over the entire range specified by GOST 183-66 for duty \(S_4\).

To determine the thermal state, copper–constantan thermocouples, 66 in number, were embedded in the motors under study: 36 thermocouples were placed at various points along the length of the turn and around the circumference of the stator winding; 30 thermocouples were placed at various points around the circumference and along the length of the motor frame; and 6 thermocouples were placed in the rotor. The readings of the thermocouples on the rotor were measured with the rotor rotating, using a mercury slip ring RAT-12. Signals from the thermocouples were recorded using a semiautomatic potentiometer R 2/1 and an N39 recorder. The thermocouple switching circuit makes it possible to measure and record on paper the readings of any of the thermocouples embedded in the motor. The thermal regime is monitored by one

of them, located at a characteristic point of the winding, while in the established thermal regime the complete temperature field of the motor and of the frame of the self-braking motor 4A-132-M-4 was measured at duty factor \(PV = f = 40\%\), \(K = 3\), and rated static torque on the motor shaft.

Figure 2 shows curves of the change in temperature of the stator winding for different numbers of starts per hour. The curve of the change in the temperature of the stator winding during the operating cycle can be divided into three sections. When the stator winding is connected to the supply mains, its temperature increases (section 1), owing to losses from the starting current, by an amount proportional to the acceleration time of the flywheel masses.

Fig. 2. Change in heating of the stator winding and housing at different starting frequencies

On section 2, the increase in the temperature of the stator winding occurs owing to the losses in the winding corresponding to the rated static torque on the motor shaft. On section 3, cooling of the stator winding of the stationary motor occurs after it has been disconnected from the supply mains.

The curve of the change in the frame temperature in an intermittent-duty regime with frequent starts has an important feature: when the motor is rotating, the housing is cooled (section “a”), while during the pause it is heated (section “b”) owing to the influx of heat accumulated in the heat capacity of the internal parts of the machine. The change in housing temperature over the cycle is much smaller than the change in the winding temperature, which is due to its considerably greater heat capacity.

Analysis of the obtained heating curves shows that, at small numbers of starts per hour, the increase in the temperature of the stator winding over a cycle occurs owing to losses caused by the static torque on the motor shaft. As the number of starts per hour increases, the rise in the temperature of the stator winding over a cycle is determined mainly by losses in the winding caused by acceleration of the rotating masses to the specified speed. Ne-

It should also be noted that, as the number of switch-ons per hour increases, the difference between the highest and lowest winding temperatures over a cycle decreases.

On the basis of the work carried out, the following conclusions may be drawn:

1. The proposed installation for studying induction motors in the intermittent duty mode permits tests to be conducted at \(k_j = 1.2;\ 1.6;\ 2.5\), which cannot be achieved when a direct-current generator is used to set the static torque.

2. Because of the opposite nature of the temperature changes in the winding and in the frame in mode \(S_4\), when calculating the heating curve of the motor it cannot be regarded as a homogeneous body.