COMPARISON OF OVERLOAD PROTECTION SYSTEMS FOR AN INDUCTION MOTOR USING THERMISTORS AND THERMAL RELAYS WITH BIMETALLIC STRIPS

I. S. Avraamov, A. M. Paunov

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

Thermal relays with bimetallic strips, used to protect induction motors from overloads, have well-known shortcomings [3]. This work presents a comparison of thermal protection on bimetallic relays, used in the AP-50 circuit breaker, with protection based on TR-33 thermistors with a negative temperature coefficient of resistance [1, 2, 6]. Thermistors of this type exhibit a relay effect, consisting in an abrupt increase in current at a certain ambient-temperature value. The relay effect is widely used to protect induction motors from overheating [2, 3, 4, 6]. For motor protection, thermistors are selected with such temperature intervals of resistance decrease as correspond to the heat resistance of the insulating material used in the machine.

This property is well used when fixing the local temperature of the machine winding, for example the maximum temperature.

The motor under investigation, of the AO-51-6 series, with rated data
\(P_{\mathrm{n}} = 2.8\ \mathrm{kW}\), \(n_{\mathrm{n}} = 950\ \mathrm{rpm}\), \(I_{\mathrm{n}} = 6.8\ \mathrm{A}\), \(U = 380\ \mathrm{V}\), was subjected to thermal tests in order to determine the maximum overheating of the stator winding at various degrees of machine loading in continuous-duty modes (no-load; \(P = 0.5P_{\mathrm{n}}\); \(P = 0.75P_{\mathrm{n}}\); \(P = P_{\mathrm{n}}\); \(P = 1.15P_{\mathrm{n}}\); \(P = 1.25P_{\mathrm{n}}\); \(P = 1.35P_{\mathrm{n}}\), and \(P = 1.5P_{\mathrm{n}}\)), in intermittent periodic-duty modes (\(\mathrm{PV} = 40\) and \(60\%\)), and also in operation on two phases. In each of these modes, the average temperature rises of the stator winding were measured by the resistance method. Analysis of the motor’s thermal investigations makes it possible to conclude the following:

1) the most heated part of the stator winding is the end-winding portion on the drive side, where three thermistors were embedded in each phase;

2) as the motor load increases, the temperature difference between the most heated part of the winding and the less heated part (the middle of the slot) increases;

3) as the motor load increases, the temperature difference between the most heated part of the winding and the average temperature increases.

Copper-constantan thermocouples were used as temperature sensors in studying the thermal condition of the motor. In addition to the thermal analysis of the motor, investigations of thermal protection systems were carried out.

Figure 1 shows a circuit for comparing the protection systems of an induction electric-

Fig. 1. Circuit for comparing motor overload protections.

motor. The thermal protection of the AP-50-3T circuit breaker, made with bimetallic strips, is compared with an ATB-229 type device for temperature protection of motors. The temperature sensor for the motor winding is a TR-33 thermistor with a negative temperature coefficient of resistance [5, 6]. The TC thermistor is connected in series with the coil of the intermediate relay P through the rectifier V to the secondary voltage of transformer Tr. The normally closed contact of relay P is connected in the circuit of the coil of contactor K.

To stabilize the supply voltage of the thermal-sensor circuit, transformer Tr is provided with a compensation winding KO, and capacitor C is connected in the circuit of its primary winding. Such a circuit makes it possible to ensure reliable operation of the protection under significant (up to 50%) reductions in the mains voltage.

The operating temperature of the protection can be set within the range 70–120°C, in steps of 10°, by means of switch P. The error in the operating temperature does not exceed ±5°C.

The ATB-229 apparatus operates reliably with ambient-temperature fluctuations from −40 to +50°C.

The TR-33 thermistor is intended for operation in relay mode when monitoring temperatures up to +120°C in temperature-protection and signaling devices. It has a temperature coefficient of resistance from −5 to 6% per 1°C and a time constant of 30–35 sec. As the load on the motor, a separately excited direct-current generator is used.

The motor is started by switching on the circuit breaker and the contactor. In the event of overloads, the motor is disconnected both by the circuit breaker and by the thermistor protection.

Operation of the circuit

The protection-comparison circuit operates as follows. To start the motor, circuit breaker A is switched on. Then, when pushbutton P is pressed, contactor K is energized. The motor starts.

At the moment of overheating, the overload protection of the circuit breaker, made with bimetallic strips, operates, as does the thermistor protection.

The latter operates when the motor winding overheats to a temperature equal to the setting temperature. In this case, in the temperature-sensitive circuit into which relay P is connected, the current rises sharply; relay P operates, opening its make contact in the circuit of the contactor coil K. The motor is disconnected from the supply.

When investigating protection only with bimetallic strips, in order to avoid the influence of the thermistor protection, the make contact P is shunted by disconnecting switch \(K_2\). Conversely, when investigating thermistor protection and eliminating the influence of the circuit-breaker protection, the circuit breaker is shunted by disconnecting switch \(K_3\). Disconnecting switch \(K_1\) is used to create a two-phase operating mode of the motor.

Figure 2 shows the dependences of the phase current on the operating time of the motor protections when a cold motor is switched on. In the same figure, at certain points of the curves, the winding overheats at which operation occurs are indicated. The overheating is determined for the hottest parts of the winding by means of thermocouples.

Fig. 2. Dependences of the motor current on the operating time: 1—thermal protection of the circuit breaker, 2—thermal protection using thermistors, 3—thermal protection of the circuit breaker at an ambient temperature of \(60^\circ\text{C}\); the motor was started cold.

The thermal protection of the circuit breaker is adjusted in accordance with the recommendations \([2, 5]\).

The setting of the thermistor protection is selected taking into account the insulation class of the machine (class A) and the ambient-air temperature (\(35^\circ\text{C}\)) and is \(95^\circ\text{C} = 60^\circ\text{C} + 35^\circ\text{C}\).

Figure 3 shows the dependences of the phase current on the operating time of the motor protections when a motor heated by rated current is switched on up to the steady-state temperature.

From Figs. 2 and 3 it is evident that thermistor protections reflect the protective characteristic of the motor more accurately. They operate according to the protection setting, which is oriented toward the limiting temperature of the insulation. At the same time, the thermal protection of the circuit breaker disconnects the motor-

Fig. 3. Dependences of the motor current on the tripping time: 1 — thermal protection of the automatic circuit breaker, 2 — thermal protection using thermistors when the motor is put into operation already heated by the rated current to the steady-state temperature

the breaker at reduced overheating values, which leads to incomplete utilization of the motor with respect to heating. For example, at a one-and-a-half-fold overload (Fig. 2) the thermal protection of the automatic circuit breaker trips after 10 min, with winding overheating of \(28^\circ C\), whereas the thermal protection using thermistors trips after 40 min and at an overheating of \(59^\circ C\). In this example, the motor, having protection using thermistors, can operate for an additional 30 minutes. From Fig. 2 one can also see the influence of ambient temperature on the operation of the thermal protection of the automatic circuit breaker. Curve 3 was obtained by placing the circuit breaker in a thermal cabinet with a temperature of \(60^\circ C\), while the motor remained under normal temperature conditions (\(30\)—\(35^\circ C\)). The setting of the circuit breaker was taken at an ambient temperature equal to \(35^\circ C\).

Studies were carried out of thermal protection using thermistors also at other settings of the tripping temperatures. They showed clear and reliable operation of this protection. For example, with a setting for overheating of \(50^\circ C\), the protection tripped at temperatures equal to \(50 \div 52^\circ C\).

An analysis was made of the operation of the protections in the two-phase operating mode of the motor at \(1.25P_n\); \(P_n\); \(0.75P_n\); \(0.6P_n\) up to phase failure. For example, the motor was connected to the supply in the normal operating mode with rated current. After it had increased its speed to the rated value, one phase was interrupted. Fig. 4 shows the tripping curves of the protections during operation in the two-phase mode. The fastest-acting here proved to be the thermal protection of the circuit breaker. This is the only case in which the protection of the circuit breaker displayed its positive qualities.

A comparison of the two types of protection makes it possible to draw the following conclusions.

1. Reliable and clear operation of the thermal protection using thermistors is observed in the following modes of motor operation: a) in continuous operation with overloads, b) when operating on 2 phases, c) in intermittent periodic modes, d) when operating with a stalled rotor.

Fig. 4. Dependence of the motor current on the operating time when the motor operates on two phases:
1 — thermal protection of the circuit breaker, 2 — thermistor protection set for an overheating of 50°C, 3 — thermistor protection set for an overheating of 60°C

1. The operation of protection with a thermistor is not affected by the ambient environment.

2. The use of thermistors as protective devices makes it possible to utilize the electric motor more fully with respect to heating.

3. A protection system using thermistors, in comparison with the thermal protection of an AP-50 3T circuit breaker or a PME-222 magnetic starter, is several times more expensive. However, with proper operation of this protection, taking into account its increased accuracy and reliability, it is advisable to recommend thermistor protection for industrial drives, especially those operating under severe thermal conditions.

REFERENCES

1. N. P. Udalov. Semiconductor Sensors. Moscow—Leningrad, Energiya Publishing House, 1965.

2. E. N. Zimin. Protection of Asynchronous Motors up to 500 V. Moscow—Leningrad, Energiya Publishing House, 1967.

3. Yu. V. Zaitsev, A. Marchenko. Protection of electrical machines by means of semiconductor thermistors. Elektrotekhnicheskaya promyshlennost’, issue 798, 1968.

4. V. A. Kneimenov. Thermal Protection of Asynchronous Electric Motors. Moscow, VNIIEM Branch, 1966.

5. Central Institute of Scientific and Technical Information of the Electrical-Engineering Industry and Instrument Making. Automatic Circuit Breakers of the AP-50 Type.

6. Nomenclature of Products of Enterprises for 1967. “Sevkavkazelektropribor.”