Motor star-delta starting time and calculation of starting time

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Determination of the star-delta starting time: Calculation formula: (square root of capacity × 2) + 4 (seconds). Mnemonic: For motor starting using the star-delta method, the starting time can be set easily； Take the square root of the capacity, multiply by two; add four units of seconds to the result. Motor start: star-delta, overload protection: thermal element ； The setting current for the phase current is capacity multiplied by eight divided by seven. II. Capacity calculation: When the generator capacity is 200 KVA, the value in KW is calculated as follows: 200 × 0.8 = 160 KW. Here, 0.8 is the power factor (ranging from 0.7 to 0.9; 0.8 is usually used), i.e., cosφ = 0.8. (1) To calculate the motor capacity, divide the power in KW by the power factor of 0.8; for example, 37 KW ÷ 0.8 = 46.25 KVA. (2) To calculate the star-delta starting transition time, take the square root of 46.25: √2 = 6.800735, which is approximately 6.801. Then, 6.801 × 2 + 4 = 17.602, or approximately 17.6 seconds. (3) Note: Since motor loads vary widely, the start-up transition time needs to be adjusted according to the actual conditions. It is not advisable to apply formulas mechanically without consideration, as this can lead to discrepancies in practice and even cause start-up failures! III. In actual operation, the settings can be made following these methods: 1. When the motor is running under no load or with a light load, press the start button and monitor the starting current; as the motor speed increases, the current will gradually decrease until it reaches a certain value at which point it stops decreasing. This is the optimal time to switch from Y connection to Δ connection. (For safety, it’s advisable to add another 2 seconds.) 2. When the load is heavy, the torque generated by Y is only 1/3 of that produced by △; the motor’s speed stops increasing after reaching a certain level, and there is still a gap before it reaches its rated speed. In such cases, the starting current is often more than 1.2 times the rated current. Failing to switch modes in time under these conditions is harmful rather than beneficial! The motor speed may decrease, and the thermal relay may activate, resulting in a failure to start! IV. Why is star-delta starting used for motors? Star-delta starting is generally used for starting motors with higher power and under heavier loads. The purpose is to reduce the shock fluctuations generated by the motor during startup, which could otherwise affect the proper operation of other electrical devices. The current drawn when the motor starts at full voltage is 4 to 7 times the rated current, indicating that the shock is quite significant. Grid disturbances are particularly severe in power grids with low capacity and high loads. V. During the Y-△ starting process, what is the actual meaning of the Y connection? When the motor’s phase windings are connected in a Δ configuration, the voltage applied is 380 V; when connected in a Y configuration, this voltage drops to 220 V. This reduces the starting current, and the starting torque is also reduced to about 1/3 of its normal value. The electrical appliance may operate, resulting in a failure to start! VI. Conditions for using star-delta starting with three-phase asynchronous motors: 1. The star-delta starting method can be employed when the load does not have strict requirements regarding the starting torque of the motor, it is necessary to limit the motor’s starting current, and the motor meets the requirements for 380V/Δ wiring ； 2. The method is as follows: the motor is connected in star configuration at the time of startup, and once the motor starts successfully, it is switched to delta configuration (via a double-throw switch for quick switching) ； 3. Since the motor’s starting current is proportional to the supply voltage, the starting current provided by the power grid at this time is only 1/3 of the starting current that would occur at full voltage. However, the starting torque is achieved through star-delta starting, which is a type of reduced-voltage starting method; it involves sacrificing power in order to reduce the starting current. Therefore, it cannot be generalized; the decision regarding whether to use star-delta starting depends not only on the motor’s power but also on the type of load. Generally, when the load is light during startup and heavy during operation, star-delta starting can be used. Typically, the starting current of a squirrel-cage motor is 5–7 times its operating current, and the voltage requirement for the power grid is usually within a range of plus or minus 10%. To avoid excessive stress on the grid voltage, star-delta starting is employed; generally, it is required to use this method when the power of the squirrel-cage motor exceeds 10% of the transformer’s rated capacity. Only squirrel-cage motors use star-delta starting. VII. Squirrel-cage starting: primary and secondary circuit diagrams – http://www.cmiw.cn/data/attachment/forum/201903/09/114646ctqlacbu0vdrqjs3.jpg VIII. The starting current for squirrel-cage starting is calculated as follows: 1. For a motor that uses squirrel-cage starting (taking 22 KW as an example), it must operate in delta configuration in order to reach its rated performance; the rated current is given by I = 22 ÷ 0.38 ÷ 1.732 ÷ COSφ, which is approximately 44 A. The phase current flowing through each phase winding of the motor (including the external cables used to achieve a triangular connection, that is, the cables from the contactor to the motor terminals) = line current ÷ 1.732 = 25.4A. 2. When a motor operating in triangular configuration is connected in star configuration, the line current equals the phase current. Since the phase voltage applied to each phase winding of the motor is equal to the line voltage divided by 1.732, which is 220V, the line current equals the phase current, namely 25.4A. The actual starting current should be calculated by multiplying 25.4A by the starting multiplier, rather than using 44A for this calculation. 3. The cable is selected based on the actual long-term current load, rather than the starting current; therefore, for star-delta starting, the cable should be chosen to handle 25.4A. However, the cables on the power supply side, as well as the cables from the circuit breakers in the control cabinet to the contactors, must be sized for 44A, because the current flowing through these cables is line current; only the cables from behind the contactors to the motor terminals carry phase current. 4. Choose the cable based on the power supply distance, installation method, and installation environment. Generally, the rated current-carrying capacity of the cable should be greater than 25.4÷0.8=32A; therefore, cables with a cross-sectional area of 6 or 10 square millimeters can be selected. 5. When selecting contactors, it is also necessary to consider the actual conditions. For infrequent starting under no-load conditions, two 32A contactors and one 25A contactor are sufficient. In cases of starting under load, frequent starting, or when the quality of the contactors is poor, it is appropriate to choose contactors of a higher capacity. The motor operates in delta mode and starts in star mode; the starting current is 1/3 of that in direct delta start. The delta operating current can be calculated using power/3/220/power factor, and the starting current in delta mode can then be determined by multiplying that value by 1.5–2.5. Multiplying by 1/3 gives the starting current for the star connection. The result is similar to the triangular operating current; therefore, the contactors and circuit breakers are selected based on the rated current.