Low-voltage soft starter should pay attention to the problem - Database & Sql Blog Articles

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1. Low-voltage soft start cabinet electrical wiring and component arrangement

The general low-voltage soft start cabinet electrical wiring is shown in Figure 2. The electrical components in the cabinet are arranged in the order shown in the figure, so that the main wiring is short and does not cross, which facilitates the connection of the copper bars. The soft starter is installed on the right side of the contactor and is not affected by the heat of other components. The distance between the soft starter and the side wall of the control cabinet and other components is ≥100mm, which is convenient for the soft starter to dissipate heat.

2. Soft starter selection

In addition to technical, performance, and price comparisons, specific conditions such as the grid capacity of the equipment site, the lightness of the equipment startup load, and the frequency of startup are also considered.

For the pump type, the light load device is lighter, and the soft starter can be selected with simple function, low price and convenient operation. This type of equipment can meet the needs of the motor based on the rated power of the motor, using a soft starter with the same capacity as the sample. For large wind turbines, crushers and other equipment with relatively heavy starting loads, soft starters with more start-up functions, limited flow start functions, and complete protection should be used. Especially for large power equipment (200KW or more), it is best to use the start function to compare the full performance soft starter.

3. Isolator and fuse selection

In the soft start cabinet, the isolating switch can be used as the isolation switch or the Molded Case Circuit Breaker with the isolation function. The low-power soft start cabinet should use the isolating switch fuse combination knife fuse switch. Not only does it provide protection against isolation, but it also reduces engineering costs. The rated current of the isolating switch is greater than the rated current of the motor to meet the operational requirements.

The thyristor surge Joule integral (I2t) value in the soft starter is limited. The use of a circuit breaker as a short circuit protection device cannot effectively protect the thyristor components. It is recommended to use a fast-acting fuse as a short-circuit protection device. Fast-acting fuses can be selected with aR or NGT type semiconductor protection fuses. The selection of the quick-blow fuse generally does not require the breaking capacity verification. The rated breaking capacity of the aR fuse is 50KA, and the breaking capacity of the NGT fast-acting fuse is 120KA, which can meet the needs of general power distribution engineering. In addition, the fuse also has a limited current function, and the protection of the thyristor is more reliable than the circuit breaker.

The principle of the rated current of the quick-blow fuse is that the device will not blow when it starts, and it must be reliably blown when the minimum short-circuit current occurs at the equipment installation. The specific load can be used to determine the load characteristics of the equipment and the starting current of the motor. Check the fuse manufacturer's fuse-current characteristic curve, I2t value and thyristor I2t value for calculation and selection. In the absence of the above information, it can also be calculated according to the following empirical formula: Ifn≥(1.8~2.0)*Ie(A)

Ifn—Fast fuse rated current (A)

Ie—motor rated current (A)

4. Bypass contactor selection

At the end of the soft start, the motor is running at the rated voltage. The AC Contactor can be selected according to the rated current of the motor. It should be noted that when the cabinet is wired, the soft starter and the contactor are connected in phase, and the wrong phase sequence is not connected.

5. Overload protection device selection

Soft start device overload protection device should use thermal overload relay with overload protection, phase failure protection and temperature compensation function. When the specific selection, the motor operating current thermal component should be set within the current range. It is easy to overload the equipment during work, so that the rated current of the motor is close to the lower limit of the current range of the thermal element.

6. Transformer load capacity and protection setting verification

In addition to paying attention to the above requirements when selecting the soft start device, it is also necessary to pay attention to the load capacity of the power supply transformer of the equipment. Before the transformer is almost full, the soft start device should be carefully selected. In particular, when adding a relatively large power device, it is necessary to check the load capacity and protection setting value of the verification transformer. After adding the soft start device, the transformer secondary circuit breaker short circuit short delay release setting value Ir2 is: Ir2 ≥ 1.1 (IL+1.35*K*Ie) Type A: IL—load current A during normal operation of the transformer; K—newly increase the ratio of starting current of the soft start device to the rated current of the motor (see Table 1); Ie—newly increase the rated current of the motor A. It is calculated that Ir2 should be smaller than the actual short-circuit and short-delay release setting of the secondary circuit breaker of the transformer; otherwise, when the newly added equipment is started, the secondary circuit breaker of the transformer should be tripped. The soft starter selection failed.

Application machine type selection function execution function startup current (%) startup time (seconds)

Centrifugal pump standard start to reduce impact, eliminate water hammer 3005~15

Screw compressor standard start to reduce impact and extend mechanical life 3003~20

Centrifugal compressor standard start to reduce impact and extend mechanical life 35010~40

Piston compressor standard start to reduce impact and extend mechanical life 3505-10

Conveyor conveyor standard + kick start smooth, reduce impact 3003-10

Fan standard start to reduce impact and extend mechanical life 30010~40

The standard start of the mixer reduces the starting current 3505~20

Milling machine heavy load start to reduce starting current 4505~60