I. Regulations on voltage drop in power supply system caused by motor starting
Direct starting involves directly connecting the motor to the power grid and applying the power supply voltage to the motor stator. While this starting method is simple, the starting current of the motor is very large, typically 5-8 times the motor's rated current. Excessive starting current can cause a significant drop in grid voltage, affecting the normal operation of other equipment on the same power supply line or transformer. Therefore, practical specifications have the following explicit requirements regarding the voltage drop in the power supply system caused by motor starting:
1. For motors that start frequently, the voltage fluctuation caused during startup shall not exceed 10%;
2. For motors that are not frequently started, the voltage fluctuation caused by starting should not exceed 15%;
3. When the electric motor does not share a transformer with lighting or other voltage-sensitive loads, the voltage fluctuation caused by infrequent starting of the motor shall not exceed 20%.
4. For motors powered by a separate transformer, the allowable voltage fluctuation during startup is determined by the starting torque required by the transmission mechanism.
The table below lists the maximum power that a transformer of each capacity class can directly start a squirrel-cage motor (with a maximum starting current of 7 times the rated current) when the transformer impedance voltage Ku is 4.5%.
Currently, some domestic materials introduce the following empirical formula, which is used to determine whether a motor can be started directly in engineering applications.
However, it should be noted that this empirical formula should only be used as a reference and not as the sole basis for determining whether direct starting is possible. To determine whether direct starting is possible, one can generally consider the power supply's allowable power for direct starting, as shown in Tables 7-1 and 7-2.
The following formula can be used to roughly estimate the voltage drop across the power grid when the motor is started directly:
II. Determination of Direct Starting Power of High-Voltage Asynchronous Motors
Large drive motors are generally selected from double squirrel-cage or deep-slot high-voltage motors. These motors have good starting characteristics and are significantly more economical. However, they have a large starting peak current and a large voltage drop at startup, which can affect equipment requiring stable voltage. Therefore, proper selection is essential.
When the torque of the driven load does not exceed 0.4-0.45 times the rated torque of the motor (i.e., under light load), the following method can be used for selection.
The maximum full-voltage starting power of a high-voltage motor powered by a 35/6-10KV transformer should meet the following empirical formula:
1. Frequent starts
Based on these two empirical formulas, the selection table for the maximum direct starting power of a 6KV high-voltage asynchronous squirrel-cage motor is calculated, as shown in Table 7-3.
Selection of the maximum power for direct starting of a high-voltage motor
The selected motor should have its voltage drop verified according to formula (7-2). The voltage drop should meet the requirements for voltage drop in the power supply system caused by motor starting. If it does not meet the requirements, reduced voltage starting should be selected.
III. Comprehensive Considerations for Direct Start and Soft Start
Since ordinary squirrel-cage motors are inherently permissible to direct starting, the traditional view is that direct starting should be used as much as possible in engineering applications. However, in actual engineering, factors such as load process requirements, application environment, and economic benefits should also be considered.
The following three situations cannot be started directly.
1. The ratio of transformer capacity to motor capacity is not large enough.
2. The starting torque cannot meet the requirements.
3. The mechanical process of the load requires minimal torque impact.
For transmission systems of medium and large capacity electric motors, the large sudden torque impact during direct starting exacerbates wear and even damages bearings and gears, leading to a high failure rate of the gearbox, increased belt wear and frequent breakage. Simultaneously, excessive mechanical shock significantly reduces the lifespan of mechanical equipment, and the large inrush current causes aging of the motor winding insulation, reducing the lifespan of electrical equipment and increasing maintenance rates, thus severely impacting production. Therefore, in general industrial applications, motors of 55-90KW and above should adopt soft starting whenever economically feasible. The long-term comprehensive economic benefits of soft starting are difficult to quantify.
For example, a certain automobile group's thermal power plant has eight 22kW cooling tower fans, which use involute planetary gear transmission. Because the fan blades are made of fiberglass, they are heavy, resulting in a large starting torque and impact force, which in turn causes accelerated gear wear and frequent failures of transmission components such as bearings, gears, and splines. After switching to a soft starter, gear wear was greatly reduced.
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