Fundamentals of Electrical Engineering: An In-Depth Look at Thermal Relays

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This post was last edited by yunrun on 2020-1-2 at 15:44. It covers professional information on the working principle of thermal relays, the **standards** they follow, the principles for selecting thermal relays, as well as the relationship between thermal relays, AC contactors, and other components used for short-circuit protection. This helps electricians and instrumentation technicians to understand these common electrical components in depth and use them correctly. http://yunrun.com.cn/upload/202001/01/202001010105431715.png 1. Thermal relay: yunrun.com.cn/tech/2859.html When an electric motor is overloaded during operation, its speed decreases, the current in its windings increases, which in turn raises the temperature of the motor. If the overload current is not high and the overload duration is short, so that the temperature rise in the motor windings does not exceed the allowable value, such an overload is permissible ； If the overload duration is long or the overload current is high, the temperature rise of the motor’s windings will exceed the allowable level. This will cause aging of the insulation in the motor’s windings, reducing its service life; in severe cases, it may even lead to the destruction of the motor. Therefore, overload protection for the motor is necessary. Thermal relays provide overload protection by utilizing the principle of the thermal effect of current. When an overload that the motor cannot handle occurs, the overload current flows through the thermal element of the thermal relay, causing it to activate its protection function; this, in conjunction with the AC contactor (https://bbs.hcbbs.com/thread-2878230-1-1.html), shuts off the circuit of the motor. Thermal relays come in various types; the most common ones are those based on bimetallic strips and thermistors. Currently, bimetallic strip-type relays are the most widely used, and some models also feature phase loss protection. A bimetallic strip thermal relay is mainly composed of a main bimetallic strip, a heating element, a reset button, an operating mechanism, a contact system, a circuit adjustment knob, a reset mechanism, and temperature compensation elements. When the motor is operating normally, the heat generated by the heating element can indeed cause the main bimetallic strip to bend, but the pushing force resulting from this bending is not sufficient to activate the contacts of the thermal relay. When the motor is overloaded, the bending displacement of the bimetallic strip increases, which in turn pushes the guide plate and causes the normally closed contacts to open. This triggers the control circuit to disconnect the AC contactor, thereby cutting off the power supply to the motor and protecting it in this way. Let’s take a look at Figure 1. http://yunrun.com.cn/upload/202001/01/202001010043336458.png Figure 1: Overload inverse-time characteristic of relays. Line 1 represents the allowable overload inverse-time characteristic for three-phase squirrel-cage asynchronous motors ； Curve 2 represents the cold-state overload inverse-time operating characteristic of the thermal relay ； Curve 3 represents the thermal overload inverse-time operating characteristic of the thermal relay ； Curve 4 is the phase loss protection characteristic curve of the thermal relay. It can be seen that when using a thermal relay for overload protection of three-phase squirrel-cage asynchronous motors, it must be used in conjunction with an AC contactor; the overload protection curves 2 and 3 of the thermal relay must not intersect the allowable overload inverse-time curve 1 of the motor. The standards followed by thermal relays are IEC60947-4 and GB14048.4. Changhui Instruments presents an excerpt from these standards as follows: ① Standard number: GB 14048.4-2010 ② Standard title: Low-voltage switchgear and controlgear – Part 4-1: Contactors and motor starters; Mechanical contactors and motor starters (including motor protectors) ③ IEC standard number used equivalently: IEC 60947-4-1:2009 Ed.3.0, MOD ④ Excerpt from the standard: 5.7.3.2 Overload relays d. The maximum tripping time, in seconds, either according to the tripping levels specified in Table 2 or under the conditions stated in Column D of Table 3 in 7.2.1.5.1 ； Table 2 Tripping levels and tripping times of thermal, electromagnetic, or solid-state overload relays. Level: Tripping time Tp, in seconds, under the conditions specified in Column D of Table 3 in 7.2.1.5.1. 10A: 2 < Tp ≤ 10; 10A: 4 < Tp ≤ 10; 20A: 6 < Tp ≤ 20; 30A: 9 < Tp ≤ 10. Note 1: The tripping conditions are given in clause 7.2.1.5 depending on the type of relay. Note 2: For rotor-resistive starters, the overload relay is usually connected to the stator circuit. Therefore, the overload relay cannot effectively protect the rotor circuit, especially the resistors (typically, when the starter starts under fault conditions, the resistors are more likely to be damaged than the rotor itself and the switching devices); thus, the protection of the rotor circuit shall comply with the agreement between the manufacturer and the user (7.2.1.1.3). Note 3: For two-stage autotransformer reduced-voltage starters, the starting autotransformer is generally used only during the starting period; for example, when starting under fault conditions, the autotransformer cannot be effectively protected by the overload relay. Therefore, the protection of autotransformers shall comply with the agreement between the manufacturer and the user. Note 4: Considering the different characteristics of thermal elements and manufacturing tolerances, a lower limit value for Tp can be selected. Thermal relays with a tripping rating of 10A specified in the standards are used for lightly loaded motors, those with a tripping rating of 10A are used for ordinary motors, while thermal relays with tripping ratings of 20 and 30A can be used for motors that start under heavy loads. 2. Selection principles for thermal relays Thermal relays are primarily used for the overload protection of motors. When using them, factors such as the motor’s operating environment, starting conditions, and the nature of the load must be taken into consideration. The main considerations are as follows: ① Thermal relays are used to protect motors that operate on a continuous basis. a) Selecting a thermal relay based on the motor’s starting time There is a relationship between the return time tf and the operating time td of a thermal relay when the motor’s starting current is 6In: tp = (0.5~0.7) × td. In this formula, tf represents the time it takes for the thermal relay to return to its normal state after it starts operating, with the unit being seconds ； td is the operating time of the thermal relay, in seconds. The operating characteristics of the thermal relay with three thermal elements when the motor’s starting current is 6In are shown in Table 3. Table 3: Operating characteristics of the thermal relay with three thermal elements when the motor’s starting current is 6In. Setting current, operating time, operating conditions: 1.0In – no operation, cold condition; 1.2In – <20 min, hot condition; 1.5In – <30 min, hot condition. For a return time of tf≥3s in cold conditions, and tf≥5s or tf≥8s in cold conditions as well. The environmental conditions specified in Table 1 are an altitude of no more than 1000m and an ambient temperature of 40°C. b. Select the thermal relay based on the motor’s rated current, and set the protection parameters of the thermal relay. Generally, the setting current for the thermal relay can be determined using the formula IFR = (1.05~1.1)In, where IFR represents the set value of the thermal relay ； In is the rated current of the motor. For example, for a 30kW motor with a rated current of 56A, the setting current for the thermal relay can be calculated using the formula: IFR = (1.05~1.1) × In = (1.05~1.1) × 56 ≈ 58.8A~61.6A. Therefore, a thermal relay with a rating of 63A should be selected. For motors with relatively poor overload capacity, the rated current of the thermal relay is usually selected at 60%-80% of the motor’s rated current. c. Select the thermal relay in accordance with the requirements for phase loss protection. For motors connected in star configuration, it is recommended to use a three-pole thermal relay ； For motors with triangular connections, thermal relays equipped with phase-loss protection should be used, namely those with a tripping level of 20 or 30. The operating characteristics of thermal relays with phase loss protection are shown in Table 4. Table 4: Operating characteristics of thermal relays with phase loss protection: http://yunrun.com.cn/upload/202001/01/202001010116502016.png Note: The reset time for thermal relays should not exceed 5 minutes, while the manual reset time should not exceed 2 minutes ； Current regulation range: 66%~100%. When a motor experiences a phase loss, the currents in each of the motor’s windings, the current flowing through the thermal relay, and the protection status of the thermal relay are shown in Table 3. Table 3: Currents in various windings, current through the thermal relay, and protection status of the thermal relay when a motor has a phase loss. http://yunrun.com.cn/upload/202001/01/202001010134526886.png Figure A, Figure B, Figure C. ② Thermal relays are used to protect motors that operate in a repeated short-term duty cycle. For motors that operate in such a duty cycle, such as crane motors, the repeated starting cycles cause an increase in temperature. If the temperature rise of the bimetallic strip in the thermal relay cannot keep up with that of the motor’s windings, then reliable overload protection cannot be provided. In such cases, it is not appropriate to use bimetallic thermal relays for overload protection; instead, overcurrent relays or temperature relays that can detect the actual temperature of the windings should be used for protection. ③Select the thermal relay to be used for protecting motors during restart. When the starting inertia of the motor is high, such as in motors used in fans, winches, air compressors, ball mills and other equipment, the starting time is long, generally exceeding 5 seconds and sometimes even reaching 1 minute. To prevent the thermal relay from operating during motor startup, the following methods for using thermal relays with motors for heavy-load starting can be employed. a. Supporting Method 1: The thermal relay is connected via a saturation current transformer (Note: The heavy-load start time for electric motors is generally between 20 and 30 seconds, with a maximum of up to 40 seconds). b. Alternative method 2: During startup, use a contactor to short-circuit the wiring terminals of the thermal element of the thermal relay; then disconnect the contactor once normal operation is achieved. (Note: For long-duration startups, a time relay is required to enable repeated startup processes.) The thermal relay cannot provide overload protection when the motor starts). c. Supporting method 3: The thermal relay is connected via a current transformer; the starting time is achieved by short-circuiting the wiring terminals of the relay’s thermal element using an intermediate relay, with the intermediate relay then being disconnected during normal operation. (Note: For long-duration starting, a time relay is required, which can be used for repeated starting processes.) The thermal relay cannot provide overload protection when the motor starts). d. Use a thermal relay with a tripping level of 30 (Note: It is used for long-duration starting; a time relay is required as well, and it can be employed in repeated starting processes). The thermal relay cannot provide overload protection when the motor starts). Note: Companion Method 2 and Companion Method 3 can use ordinary thermal relays and ordinary current transformers. 4. The coordination relationship between thermal relays and AC contactors and the components that provide short-circuit protection: In the main circuit of a motor, circuit breakers or fuses are generally used for short-circuit protection as part of the main components; AC contactors are used to control the starting, stopping, and operation of the motor, while thermal relays provide overload protection for the motor. When a short circuit occurs in the motor or in the cable that connects the motor circuit to the motor, the short-circuit current will flow through the short-circuit protection devices (circuit breakers or fuses), as well as through the AC contactors and thermal relays. However, only the short-circuit protection devices are capable of cutting off the short-circuit current, while the AC contactors and thermal relays can only withstand the impact of such current. To this end, there needs to be coordinated short-circuit protection between the component that implements short-circuit protection and the AC contactor and thermal relay. Changhui Instrument Network will provide a detailed explanation on the coordination of short-circuit protection in the test titled \"Test on the Coordination Methods between Contactors and Low-Voltage Electrical Appliances for Short-Circuit Protection\". Basic electrical knowledge, principles for drawing electrical control schematics, electrical graphic symbols and electrical textual symbols