Contact Us+8613505873345


Questions That Should Be Paid Attention To When Selecting Low-voltage Soft Starters

Sep 12, 2020

Questions that should be paid attention to when selecting low-voltage soft starters

In recent years, soft starters have received more and more applications and attention for their small size, adjustable torque, small start-up shock, and soft shutdown function. It can be said that they have begun to replace traditional auto-decompression, The trend of star-horn and other starters. 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, commissioning 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 setting, the relationship between soft start equipment capacity and transformer capacity, etc.

1. Introduction to soft starter

At present, the common soft starters on the market mainly include electronic, magnetic control, and automatic liquid resistance. The electronic type is mostly thyristor voltage regulation type. The frequency converter is also a kind of soft starter in a sense, and it is a starter that can truly realize soft start, but the cost is higher.

The thyristor soft starter is three groups of forward and reverse parallel thyristors connected in series between the power supply and the motor. The conduction angle is controlled by a microcomputer to realize 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 uses a ramp voltage to start, the soft starter should output an initial voltage at the beginning (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 starting 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 the 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 starters mainly include free parking, soft parking and brake parking. The traditional electric motor parking method is usually free stop, but there are many applications, free stop will cause great problems, such as the water pump system of high-rise buildings, if free stop is used, it will produce a huge "water hammer" effect, which will damage the pipes 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 slowly drops from Un. At the moment of 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.

2. Problems that should be paid attention to when selecting low-voltage soft starters

2.1 The electrical wiring and component arrangement of the low-voltage soft start cabinet

The electrical wiring of the general low-voltage soft-start cabinet is shown in Figure 2. The electrical components in the cabinet are arranged in the order shown in the figure, and the main wiring is short and not crossed, 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 be ≥100mm to facilitate the heat dissipation of the soft starter.

2.2 Selection of soft starter

In addition to technology, performance, and price comparisons, specific conditions such as the grid capacity of the equipment site, the starting load of the equipment, and the frequency of starting must also be considered.

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

The isolating electrical appliance in the soft start cabinet can be either an isolating switch or a molded case circuit breaker with isolation function. The low-power soft-start cabinet should use a knife-fuse switch with a combination of isolation switch and fuse. 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.

Since 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 a fast fuse as a 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 check the breaking capacity, 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 it will not fuse when the device is started, and it must be fuse reliably when the minimum short-circuit current occurs at the device installation. The specific selection can be calculated and selected according to the load nature of the equipment and the starting current of the motor, referring to the fuse time-current characteristic curve, the I2t value and the I2t value of the thyristor provided by the fuse manufacturer. In the absence of the above information, the following empirical formula can also be used for calculation:


Ifn—Rated current of fast fuse (A)

Ie—Motor rated current (A)

2.4 Selection of bypass contactor

At the end of the soft start, the motor is already running on the rated voltage, 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 starter 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 operation, the rated current value 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 starter, attention should 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;

After adding the soft-start equipment, the setting value Ir2 of the short-circuit short-delay release of the transformer secondary side circuit breaker is:

Ir2≥1.1(IL+1.35*K*Ie) A

Where: IL—load current A during normal operation of the transformer

K—The ratio of the starting current of the newly added soft-start equipment to the rated current of the motor;

Ie—The rated current A of the motor is newly added.

The calculated Ir2 should be less than the setting value of the actual short-circuit short-delay release of the transformer secondary side circuit breaker; otherwise, when the newly added equipment is started, the transformer secondary side circuit breaker will be tripped. Cause the selection of the soft starter to fail.