Questions regarding motor starting methods, star-delta starting, and soft starting
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I’m a complete beginner in electrical engineering, and I have a question for the experts here. Thank you in advance! ! ! 1. Regarding the motor starting method: Motors can start either directly or with voltage reduction. Under what conditions and circumstances can direct starting be used, and under what conditions is reduced-voltage starting necessary? What are the boundaries between these two methods? (For example, at what motor power level can direct starting be used, and at what level must reduced-voltage starting be employed?) Or what are the limitations? (Such as the type of transformer or its capacity.) 2. Reduced-voltage starting methods mainly include star-delta starting and soft starting. Are there any standards for deciding when to use star-delta starting and when to use soft starting? (For example, at what kilowatt level should star-delta starting be used, and at what level should soft starting be used?) If there are no standards, are there any constraints on choosing star-delta starting or soft starting (such as transformer type, capacity, etc.)? If the power supply conditions at the upstream end are unknown (for example, the type of transformer or the allocated capacity is not known), which approach is more appropriate in such cases?
To determine whether a water pump motor can be started directly, the following empirical formula can be used: Ist / In ≤ 3 / 4 + capacity of the power supply transformer (KVA) / 4 * motor power (KW). Here, Ist represents the full-voltage starting current of the motor, in amperes； In is the rated current of the motor, in amperes. Any case that does not meet the above conditions for direct starting must use reduced-voltage starting. The possible origin of the claim that motors over 10KW need to be started with reduced voltage: Since this notion is accepted and followed by certain professionals, there must be a reason for it; it cannot arise out of thin air from a technical standpoint. For example, a misunderstanding of certain provisions in existing standards could be one such reason. Paragraph 10.2.1.5 specifies: cases where power is supplied directly from the urban low-voltage network. (1) When powered by a public low-voltage network, units with a capacity of 11 KW or less can start at full voltage. (2). When powered by the low-voltage distribution equipment in the residential complex’s substation, devices with a capacity of 15 KW or less can be started at full voltage. These may be the basis for the aforementioned statement. The annotation for this specification clause states that: since it is powered by a public low-voltage network, the power supply transformers used are generally smaller in capacity compared to those used in residential areas; therefore, different full-voltage starting capacities are specified for each case. Obviously, the capacity of the transformer directly affects the capacity of motors that can be started at full voltage. Clause 10.2.1.2 stipulates that, when the conditions are met, the motor should be started at full voltage. The explanatory text of the standard states that starting at full voltage is simple and reliable, requires less investment, and provides high starting torque; therefore, for squirrel-cage motors, full-voltage starting should be preferred as long as it meets the requirements of clause 10.2.1.2. Voltage reduction starting results in a low starting current, low starting torque, a long starting time, an increase in the temperature of the motor windings, as well as complex starting equipment and high costs. Voltage reduction starting should only be considered when full-voltage starting is not permissible. How do the specifications define the conditions for full-voltage starting? Paragraph 10.2.1.1 stipulates that when motors start frequently, the terminal voltage of (other) motors may be reduced to 85%; when motors start infrequently, the terminal voltage of (other) motors may be reduced to 80%. Article 3.3.3 stipulates that under normal conditions, the allowable voltage deviation at the electrical equipment side is generally 5%. When a radial power supply method is used, it can be calculated that under the most unfavorable condition of a voltage deviation of -5% under normal circumstances, the voltage drop on the transformer bus side when the motor starts at full voltage should be less than 10~15%. From a power distribution perspective, full-voltage starting can be used as long as the motor capacity does not exceed 30% of the capacity of the distribution transformer.
Generally, low-power motors make use of star-delta reduced-voltage starting method, which is commonly applied to motors with a power rating of 75 KW or less. The advantage of this method is that it requires only three AC contactors and one time relay, resulting in lower costs. However, the wiring is complex, requiring wires to be led out from each of the motor’s six terminals. Soft starting relies on a microcontroller to control the conduction angle of thyristors, thereby enabling a smooth increase in voltage. This approach allows the starting current, voltage, and time to be adjusted within certain ranges, thus achieving a smooth start and reducing shock effects. Early soft starters had only one voltage start mode, but later they evolved to include various start modes to accommodate different load conditions, whether light or heavy loads. The advantages of soft starting are clear: it enables a smooth start-up, avoiding any stress on the motor and the load; it extends the service life of the equipment; and it eliminates the negative effects caused by shocks. Moreover, the wiring for soft starting is simple – there is no need to use six wires as in the star-delta connection; the motor can be connected to the load side of the soft starter using the normal delta connection. This can be seen from their wiring diagrams.