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Talking about the Problems Existing in the Operation of Low-Voltage Soft Starter and Their Solutions

Jan 08, 2021

Talking about the Problems Existing in the Operation of Low-Voltage Soft Starter and Their Solutions

Low-voltage soft starters are more and more used for their small size, adjustable torque, smooth starting, small impact and soft stop function. This article briefly introduces the types of soft starters commonly used at present and their Features, and summarizes the main problems and empirical formulas that should be paid attention to when selecting soft starters in the project site.

Soft starters have been more and more used for their small size, adjustable torque, smooth start and small shock and soft stop function, and have a tendency to replace traditional auto-decompression, star-angle starters, etc. . Since the soft starter is a newly developed starting device in recent years, there is still a lack of guiding specifications and procedures in terms of design, installation, debugging and use. We also encountered some practical technical problems in the installation and debugging of the soft starter. For example: the selection of soft starters for different starting loads, the coordination of soft start impulse current and overcurrent protection settings, the relationship between soft start equipment capacity and transformer capacity, etc.  1. Control principle of soft starter Thyristor soft starter is a series of three groups of forward and reverse parallel thyristors connected between the power supply and the motor. The conduction angle is controlled by a microcomputer to achieve AC voltage regulation. The starting mode of the thyristor-type soft starter has ramp voltage type, sudden jump and ramp voltage type and current limit type to choose from. The magnetron soft starter is a soft start device made up of a three-phase saturable reactor connected in series between the power supply and the motor using the principle of magnetic amplifier. When starting, the excitation current of the magnetic amplifier control winding is adjusted through the digital control board, and the reactance value of the saturated reactor is changed to adjust the starting voltage drop to realize the soft start of the motor. Regardless of whether the thyristor soft starter or the magnetron soft starter can only adjust the output voltage when starting, it achieves the purpose of controlling the voltage drop during starting and limiting the starting current. The general soft starter cannot adjust the power frequency, so it cannot start the motor from zero frequency and zero voltage like a frequency converter to achieve no impact start. In fact, the soft starter still has to generate a certain impulse current when starting the equipment; the voltage and current change curve of the ramp voltage type control soft starter when starting is shown in Figure 1. When the thyristor soft starter is started with a ramp voltage, the soft starter should output an initial voltage (the initial voltage can be adjusted between 80~280V), so that the motor can generate an initial torque sufficient to overcome the static friction of the mechanical equipment. The moving equipment starts to rotate, and the starting current is Is. Under the control of the microcomputer, continue to increase the output voltage to accelerate the motor. When the output voltage of the soft starter is close to the rated voltage, the motor has reached the rated speed, and Is reduced to the load current In. At the end of the start time t1, the soft starter outputs the rated voltage and sends out a bypass signal to close the bypass contactor, the soft starter stops outputting voltage, and the motor enters normal operation. The initial torque of the soft start can be adjusted by a given initial voltage and starting time, and the starting current is controlled within 2 to 4.5 times the rated current of the motor. The parking modes of low-voltage soft starter mainly include free stop, soft stop and brake stop. The traditional motor parking method is usually free stop, but there are many applications where free stop will cause great problems, such as the water pump system of high-rise buildings. If free stop is used, huge water hammer effect will be produced, which will damage pipelines and pumps. Therefore, the use of soft parking can eliminate the anti-inertial impact caused by free parking. At the stop time t2, the stop command is issued, and the terminal voltage of the motor drops slowly from Un. At the moment of the voltage drop, the motor current will have a small current impact, and then the motor current will drop with the voltage drop until the motor stops. Problems in the selection of low-voltage soft starters and their solutions   2.1. Electrical wiring and component arrangement   The electrical components in the cabinet are arranged in the order shown in the figure, and the main wiring is short and does not cross, which is convenient for copper bar connection. The soft starter is installed on the right side of the contactor and is not affected by the heating of other components. The distance between the soft starter and the side wall of the control cabinet and other components is required to facilitate the heat dissipation of the soft starter.  2.2. Design and selection For pumps with lighter starting load, you can choose a soft starter with simple function, lower price and convenient operation. According to the rated power of the motor, this type of equipment can meet the needs by selecting a soft starter with the same capacity specified in the sample. For equipment with heavier starting load, such as large fans and crushers, a soft starter with more starting functions, limited flow starting function, and self-protection should be selected. Especially for equipment with relatively large power (above 200KW), it is best to choose a high-performance soft starter with a relatively complete starting function.  2.3. Isolator and fuse selection.    Molded case circuit breakers with isolation function can also be selected for the isolation switch. The low-power soft-start cabinet should use a knife-fuse switch with an isolating switch and fuse combination. It not only plays a role of isolation protection, but also reduces project cost. The rated current of the isolating switch is greater than the rated current of the motor to meet the operating requirements.   Because the surge Joule integral (I2t) value of the thyristor in the soft starter is limited, the short-circuit protection device of the circuit breaker cannot effectively protect the thyristor element. It is recommended to use fast fuse as short-circuit protection device. Fast fuse can choose aR or NGT type semiconductor protection fuse. The selection of fast fuse generally does not need to do breaking capacity verification, because the rated breaking capacity of aR fuse is 50KA, and the breaking capacity of NGT fast fuse is 120KA, which can meet the needs of general power distribution projects. In addition, the fuse also has a current limiting function, and the protection of the thyristor is more reliable than the circuit breaker.   The selection principle of the rated current of the fast fuse is that the device will not be fused when the device is started, and it must be reliably fused when the minimum short-circuit current occurs at the device installation. Specific selection can be based on the load nature of the equipment and the starting current of the motor, and consult the fuse time provided by the fuse manufacturer—current characteristic curve, I2t value and I2t value of the thyristor for calculation and selection.  2.4. Selection of the bypass contactor    The motor is already running at the rated voltage at the end of the soft start, so the AC contactor can be selected according to the rated current of the motor to meet the requirements. It should be noted that when wiring the cabinet, make sure that the soft starter and the contactor are connected in the same phase, and do not connect the wrong phase sequence.    2.5. Selection of overload protection device    The overload protection device of the soft start device should be a thermal overload relay with overload protection, phase failure protection and temperature compensation. In the specific selection, the working current of the motor should be within the setting current range of the thermal element. For equipment that is prone to overload during work, the rated current of the motor should be close to the lower limit of the setting current range of the thermal element. 2.6. Transformer load capacity and protection setting value verification    In addition to the above requirements when selecting the soft start device, attention should also be paid to the load capacity of the transformer that supplies the equipment. If the transformer is close to full load beforehand, choose soft start equipment carefully. Especially when adding equipment with relatively large power, it is necessary to check the load capacity of the transformer and the setting value of the protection.