【In-depth Article】Four Methods of Reduced-Voltage Starting for Three-Phase Asynchronous Motors

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Star-delta reduced-voltage starting, autotransformer reduced-voltage starting, series resistance reduced-voltage starting, and soft starters are commonly used for the starting and operation of high-power motors. It is very helpful for electricians to have a thorough understanding of the working principles and control circuits of these four motor starting methods, as it enhances their skills in repairing and controlling electrical equipment. This post provides a detailed introduction to the voltage-reduction starting methods for three-phase asynchronous motors. For three-phase asynchronous motors with relatively high power, if direct starting is employed, the large starting current can cause a drop in voltage along the power supply lines, thereby affecting the operation of other equipment. Therefore, reduced-pressure starting should be used. However, reduced-voltage starting results in a significant decrease in starting torque; therefore, it is only suitable for starting the motor under no-load or light-load conditions. Motors with a smaller capacity can be started directly, whereas motors whose capacity (apparent power) is more than 8% of the transformer’s capacity or exceeds 30 kW must use reduced-voltage starting. In this article, Changhui Instruments will introduce several commonly used reduced-voltage starting circuits: yunrun.com.cn/tech/2175.html. 1. Control circuit for automatic switching between star and delta connections in a three-phase asynchronous motor using a time relay. In star-delta reduced-voltage starting, the motor is initially connected in a star configuration; once it reaches its normal operating speed, the connection is switched to delta configuration for operation under full voltage. ①Photos of the factory: http://yunrun.com.cn/upload/201809/04/201809040044597760.png http://yunrun.com.cn/upload/201809/03/201809032323108813.png. 1, 2, and 3 are AC contactors; 4 is a time relay; 5 is an intermediate relay; 6 is a current transformer; 7 is a thermal relay. The intermediate relay is used to compensate for the insufficient number of auxiliary contacts in the AC contactors. ②In the teaching demonstration model, 1 is the fuse for the main circuit, 2 is the fuse for the control circuit; 3, 4, and 5 are the AC contactors KM, KM△, and KMy respectively. 6 is the thermal relay, 7 is the time relay, 8 is the terminal block, 9 is the button box, 10 is the stop button, 11 is the start button, 12 are the wires of the main circuit, and 13 are the wires of the control circuit. ③ Squirrel-cage starting with star-delta voltage reduction: Circuit diagrams available at http://yunrun.com.cn/upload/201809/04/201809040052537500.png and http://yunrun.com.cn/upload/201809/04/201809040058366182.png. Diagram 1 of the star-delta voltage reduction starting main circuit is easier to construct; in this case, KM and KM△ can be of the same specification. In Diagram 2 of the main circuit, the rated current of KM should be higher than that of KM△. ④Working principle of the star-delta reduced-voltage starting control circuit: Close the circuit breaker QS. Press SB1, and the coil of the 1-time relay KT is energized, starting the timing ； 2-KM coil powered on ； From 2, it follows that the normally closed auxiliary contact of the 3-KM (located between points 7 and 8 in the diagram) opens, resulting in interlock (the circuit containing the KM△ coil is interrupted) ； 4-KM normally open auxiliary contact closes ； 5-KM main contact closed ； From 4, the 6-KM coil is powered ； As a result of 6, the two normally open auxiliary contacts of KM (located between 4 and 7, and 3 and 4 in the diagram) close, thereby achieving self-locking for KM and SB1 ； 8-KM main contact closed ； Due to the combined action of 5 and 8, motor M starts in star connection. Starting from 1, after reaching the set time, the 9-time relay KT is energized; after a delay, its contacts disconnect ； From 9, the 10-KMy coil loses power ； With a value of 10, the main contact of 11-KMy opens, and the motor loses power temporarily ； The 12-KM normally closed auxiliary contact (located between 7 and 8 in the diagram) closes, and the KM△ coil is energized ； The 13-KMy normally open auxiliary contact opens; due to the existence of Result 8, Result 13 has no effect on the circuit ； With 12 set, the 14-KM△ main contacts close, and the motor operates in delta connection ； When the 15-KM△ normally closed auxiliary contact opens, the coil of the time relay KT loses power; once KT is powered again, its contacts open after a delay and then return to their normal state (closed). Operation principle of stopping: By pressing SB2, all contacts are reset, and the motor loses power and stops. ⑤Wiring diagram: Drawing a wiring diagram in advance helps to complete the physical construction of the control circuit quickly and correctly. http://yunrun.com.cn/upload/201809/04/201809040117102291.png ⑥ Advantages and disadvantages of star-triangle reduced-voltage starting: Advantage: This control circuit has a low manufacturing cost, which is why it is widely used. Disadvantage: The pressure reduction ratio is fixed (0.58 times), and it cannot be adjusted according to needs. The starting torque is low (only 1/3 of the full-voltage starting torque), and it may not meet the requirements when the starting load is high. ⑦Required components Name Quantity Remarks Electric motor 1 Small electric motor with power below 3 kW Main circuit wires 4mm2 or 2.5mm2 aluminum-core wires Control circuit wires 2.5mm2 or 1.5mm2 aluminum-core wires Circuit breaker 1 A knife switch with a fuse can also be used Main circuit fuse 3 RL1-60/20 Control circuit fuse 2 RL1-15/5 Three-phase asynchronous motor 1 Delta connection AC contactor 3 Thermal relay 1 JR10-10 Wiring terminal block 1 16-20 pin wiring terminals Buttons 2 Electrical tools Screwdrivers, needle-nose pliers, wire strippers Multimeter 1 Time relay 1 JS14-P 2. Control circuit for reduced-voltage starting of three-phase asynchronous motors using an autotransformer Reduced-voltage starting via an autotransformer involves using such a transformer to lower the voltage applied to the stator windings during startup, thereby limiting the starting current. The capacity (apparent power) of the selected autotransformer should be comparable to that of the motor. The two AC contactors used for reduced-voltage starting operate for short periods of time; to save costs, AC contactors with lower specifications than those used in the main circuit can be chosen. ①Physical diagram: a. For the autotransformer, 1, 2, and 3 are aluminum bars (wires); 4 is a part of the autotransformer core; 5, 6, and 7 are the windings of the autotransformer; 8, 9, and 10 are copper cables (wires). 1 and 8 are the two output terminals of winding 5, 2 and 9 are the two output terminals of winding 6, and 3 and 10 are the two output terminals of winding 7. The taps of each winding are located behind that respective winding in the diagram (not visible in the diagram). b. For the current transformer and the main circuit, 1–9 are aluminum bars (1, 4, 7 correspond to L12, 2, 5, 8 correspond to L22, and 3, 6, 9 correspond to L32) ； 10, 11 Current Transformers T ； 12, 13 AC contactors (12 is KM3 in the schematic diagram, 13 is KM2 in the schematic diagram). C. Physical diagram of the components in the autotransformer reduced-voltage starting cabinet: http://yunrun.com.cn/upload/201809/04/201809040135501687.png. 1 and 2 refer to buttons SB1 and SB2 (the button covers are located outside the cabinet) ； 3 is the time relay KT (4 in total, one of which is used for reduced-voltage starting, while the others are used for protecting the circuit) ； 4 is the intermediate relay KA (3 in total, one of which is used for the reduced-voltage start control circuit, and the other 3 are used for the protection circuits) ； 5 is the thermal relay FR ； 6 is the circuit breaker QS ； 7, 8, 9 are aluminum bars ； 10 is the current transformers T1 and T2 (only one can be seen in the diagram; they are fitted around the main circuit conductors to measure the current in the main circuit), which supply power to the thermal relay ； 11, 12, 13 are AC contactors (KM3, KM2, KM1) ； 14 is the control circuit fuse FU2 ； 15, 16, 17, 18 are three-phase autotransformers ； 19 is the alarm (electric bell) ； 20 is the time accumulator (it records the total operating time of the motor). ②Schematic diagram of reduced-voltage starting using an autotransformer: http://yunrun.com.cn/upload/201809/04/201809040145476706.png. Schematic diagram of the main circuit for reduced-voltage starting with an autotransformer: http://yunrun.com.cn/upload/201809/04/201809040151364946.png. Schematic diagram of the control circuit for reduced-voltage starting using an autotransformer. Current transformers T1 and T2 are connected to the thermal relay FR through three wires a, b, and c; d is a short-circuit wire, and the ground wire can be omitted. If a 50/5 current transformer is used, the setting value of the thermal relay is 1/10 of the main circuit current ； The phase windings of the autotransformer have two sets of taps at 80% and 60%; this circuit uses the 80% tap ； The selection of specifications for the main circuit conductors depends on the current level; aluminum bars or copper cables can be used accordingly ； KM1 and KM2 main contacts close for reduced-voltage starting ； When the main contact of KM3 closes and the circuit operates at full voltage, KM1 must be disconnected; otherwise, the autotransformer cannot be separated from the power supply, resulting in unnecessary copper and iron losses in the circuit. ③ Working principle of the autotransformer reduced-voltage starting control circuit: Turn on the power switch QS and press SB2 – the coil of KM1 is energized, causing the normally closed auxiliary contact of KM1 to open, thereby interrupting the circuit associated with the KM3 coil ； 2-KM1 main contact closed ； 3-KM1 normally open auxiliary contact closes ； From 3, the 4-KM2 coil is powered ； 5- The coil of the time relay KT is energized ； From 4, it follows that the 6-KM2 main contact is closed ； The 7-KM2 normally open auxiliary contact closes, enabling self-locking ； Due to the combined action of 2 and 6, motor M starts under reduced voltage ； At time 5, after the set time has passed, the KT delay relay closes its contacts; as a result, the coil of the intermediate relay KA is energized ； Starting from 8, the normally closed contact of 9-KA opens first, while the normally open contact of 10-KA closes later ； From 9, it can be seen that the 11-KM1 coil has lost power ； As a result of 11, the main contact 12-KM1 opens, while the normally closed auxiliary contact 13-KM1 closes ； Due to the combined effect of 10 and 13, the 14-KM3 coil is energized ； As a result of 14, the normally closed auxiliary contact of 15-KM3 disconnects first; this triggers interlocking (all contacts of KM2, KT, and KA are reset, the motor loses power temporarily, and the reduced-voltage starting process comes to an end). Then, the main contact of 16-KM3 closes, and the normally open auxiliary contact of 17-KM3 closes afterward, enabling self-locking ； From 16 and 17, it can be seen that the motor operates at full voltage. ④ For testing, disconnect QS; based on the resistance of each component’s coil, and using the schematic diagram, press SB2, KM1, KM2, and KM3 either simultaneously or separately. Measure the resistance between L1 and L2, and check whether the calculated value matches the measured value. ⑤ Advantages and disadvantages of autotransformer-based reduced-voltage starting: Advantages: The reduction ratio is not fixed (80%, 75%, 60%, 50%), allowing users to choose the appropriate reduction ratio according to their needs for starting torque (by selecting different taps). Disadvantages: The cost of constructing the control circuit is relatively high. ⑥ List of required components: Name, Quantity, Remarks: Motor – 1, small motor with power below 3 kW; Main circuit wires – 4mm2 or 2.5mm2 aluminum-core wires; Control circuit wires – 2.5mm2 or 1.5mm2 aluminum-core wires; Circuit breaker – 1, a knife switch with a fuse can also be used; Main circuit fuses – 3, RL1-60/20; Control circuit fuses – 2, RL1-15/5; Time relay – 1; Current transformers – 2; Autotransformer – 1, three-phase, with an 80% tap being suitable; Three-phase asynchronous motor – 1, connected in delta configuration; AC contactors – 3; Thermal relay – 1, JR10-10; Wiring terminal block – 1, 16-20 terminals for wiring; Buttons – 2; Electrical tools – screwdrivers, needle-nose pliers, wire strippers; Multimeter – 1. 3. Control circuit for reduced-voltage starting using series resistors in a three-phase asynchronous motor: During motor startup, voltage-dividing resistors are connected in series with the three-phase stator windings to achieve reduced-voltage starting. After startup, these resistors are short-circuited so that the motor can operate at full voltage. Due to the current-limiting effect of the resistance, when a three-phase asynchronous motor starts, a series resistor is used to divide the voltage, thereby reducing the starting voltage of the windings and the starting current in the main circuit. Meanwhile, the voltage-reducing starting control circuit using a series resistor has advantages such as the stator windings not being restricted by the motor’s wiring configuration, and a simple control circuit. ①Self-made voltage-dividing resistors: Material specifications for the resistors used in self-production, as well as reference values for various electrical quantities during the reduced-voltage starting process. ② Schematic diagram of reduced-voltage starting using series resistors. ③ Working principle of the control circuit for reduced-voltage starting with series resistors. First, turn on the power switch QS. When SB1 is pressed, the coil of KM1 is energized; the 1-KM1 normally open auxiliary contact closes, enabling self-locking, and the 2-KM1 main contact closes. The motor starts with reduced voltage due to the series resistor. Meanwhile, the coil of the time relay KT is also energized ； From 3, after the set time has passed, the KT delay-closing contacts close ； From 4, the 5-KM2 coil is powered ； From 5, the normally closed auxiliary contact of KM2 disconnects first; thus interlocking is established (meaning that KM2 cannot be powered at this time) ； 7—The KM2 normally open auxiliary contact closes afterward, enabling self-locking ； As a result of 6, the 8-KM1 coil loses power, the 9-KT coil loses power, and the KT delay-close contact opens (resets) ； With 8, the main contact of 11-KM1 opens, the circuit is interrupted, and the reduced-voltage start is completed ； From 5, it follows that the 8—KM2 main contacts close, and the motor operates at full voltage. Stop process: Press SB2; the KM2 coil loses power, and all contacts return to their default state. ④Advantages and disadvantages of voltage reduction starting with series resistors: Advantage: At the moment of startup, the resistor creates a large voltage drop, resulting in significant current limitation with little impact on the entire power grid. At the end of startup, the resistance voltage division gradually decreases, and the motor torque gradually increases. Disadvantage: The resistance wire consumes energy. 4. Soft starter for three-phase asynchronous motors: A motor soft starter is a type of voltage-reducing starter; it represents a more advanced starting device following the star-delta starter and the autotransformer voltage-reducing starter. A soft starter is connected in series between the power supply and the motor to be controlled; it regulates the conduction angle of the thyristors inside the soft starter, allowing the voltage applied to the motor to increase gradually from zero according to a predefined functional relationship, until the start-up process is complete and the motor receives full voltage. During soft starting, it offers advantages such as low starting current, a stable and reliable starting speed, and minimal impact on the power grid and equipment; moreover, the starting curve can be adjusted according to the actual operating conditions on site. Soft starters can be used with cage asynchronous motors in various applications where speed control is not required. ①Main circuit connection diagram of the soft starter ② Electrical control circuit diagram of the soft starter: http://yunrun.com.cn/upload/201809/04/201809040211380124.png After the soft starter completes the starting process of the motor, the coil of the bypass electromagnetic contactor is energized and attracts, allowing the three-phase power supply to feed power to the three-phase motor through this bypass electromagnetic contactor.