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Some common sense about the starting method of soft starter

Oct 30, 2020

Some common sense about the starting method of soft starter

1 Introduction 

In 2005, Handan Iron and Steel decided to expand the capacity of the 1260m3 blast furnace to 2000 m3. The supporting facilities of the blast furnace gas washing pump station should also be expanded accordingly. On the basis of the original equipment, two water pumps were added. The supporting motor is a cage asynchronous motor with a rated The power is 220kW and the rated current is 430A. Based on the principle of economy and applicability, the soft start method is selected to start the motor according to the power load and the nature of the load.

2  Choice of starting method

   Looking back at the principle and starting method of the cage asynchronous motor, we found that the starting of the motor has experienced a process from "hard start" to "soft start".

  Direct starting asynchronous motor is the most direct starting method. A large starting current will be generated when starting. This current will put pressure on the power supply, and it will overload the power line. At the same time, the torque will also increase sharply, and this sudden pull will put pressure on the running motors, machines and energy transmission equipment. Judging whether an AC motor can be directly started can be determined according to the following conditions:

  IST/IN≤(3/4+ ST/4 PN)

   where IST: motor full voltage starting current (A);

IN: motor rated current (A);

ST: power transformer capacity (kVA);

PN: motor power (kW);

   If this condition is met, it can be started under full pressure, otherwise, it should be started under reduced pressure.

   Because: IST=1350A IN=430 ST=1250 kVA PN=220 kW

  Because: IST/IN=1350/430=3.14   

3/4+ST/4 PN=0.75+1250/880=2.17

   then: IST/IN≥3/4+ST/4 PN

   Therefore, it is not possible to start the motor with full voltage. Due to cost constraints, the use of two transformers, so we start the motor by step-down starting method.

3 Selection of reduced voltage starting mode

3.1 Conventional reduced voltage starting method

Mainly include: series resistance starting, stator loop series reactor starting, autotransformer starting, star-delta starting, etc. All are stepwise starting. The starting current is 5 to 7 times the rated current during starting. The mechanical equipment to be dragged will cause great damage; the excessive voltage drop due to the excessive starting current during starting will seriously threaten the safe operation of adjacent electrical equipment; the starting torque is only 0.4~1.2 of the rated torque of the motor (The larger the motor capacity, the smaller the starting torque multiplier). For loads with larger starting torque, in order to ensure normal starting, the motor capacity must be increased, which increases the cost.

3.2 Liquid rheostat soft start

   Liquid varistor soft start has developed into a mature technology, which can fully meet the requirements of various motor soft start. During the use of this soft start, the following problems often occur:

(1) The starting current is affected by the ambient temperature;

(2) Liquid is easy to boil;

(3) Short service life;

(4) The volume is large, which increases the investment in infrastructure construction and requires regular water addition, which is troublesome to maintain.

3.3  frequency conversion soft start

The frequency conversion speed control device is also a kind of soft start device. It is an ideal one. It can continuously adjust the voltage and frequency from zero to keep the motor with a small slip rate, so it can achieve no overcurrent. , While maintaining high starting torque while limiting current, it has good starting performance, but it also has the following shortcomings:

(1) The price is higher;

(2) The circuit principle of the frequency converter is complicated, and the maintenance technical level is high. Once a fault occurs, it is difficult to resume operation in a short time.

The high-order harmonic content of the output voltage of the inverter is large, which is harmful to the motor.

3.4 Solid state soft start

   Thyristor soft start (also known as solid state soft start)

The modern solid-state soft starter is a thyristor AC voltage regulator controlled by a microcomputer. There are two thyristors connected in anti-parallel in each phase of the three-phase circuit. By changing the phase shift control angle of the thyristor, the output voltage can be adjusted, thereby To achieve the purpose of reduced voltage starting. The soft start technology is based on the mechanical characteristics of the motor, combined with the on-site technology, through setting the TVR (time, voltage, slope) function relationship, and using [industrial electrical appliances network-cnelc] to take the slope control method to implement the start S, speed n, torque M also change smoothly. As it rises smoothly, there is no inrush current to the grid. Because it is a non-contact element, it can be operated frequently.

3.5   analysis and comparison

   In the process of selecting the soft start device, the conventional step-down starting method, frequency converter, solid soft start and liquid varistor were fully demonstrated, and finally the solid soft start device with bypass was selected.

4  soft start start mode

4.1 Ramp start

  This mode is the most commonly used starting mode. By reducing the impact of starting torque, it realizes a smooth and continuous stepless acceleration of the motor.

  From the initial starting torque, during the acceleration ramp time, the output voltage of the motor slowly increases to full voltage, and the acceleration ramp time is adjustable. If the motor has reached full speed during the voltage rise process, the output voltage will automatically switch to full voltage

4.2 Soft start with pulse kick start

  Pulse kick start can provide a short-term pulse torque when starting to meet those loads that require a high starting torque at the beginning of starting. In fact, it provides a current pulse to overcome the inertia of the load.

4.3  limited current start

   Current limit starting is mainly used in occasions where the inrush current needs to be limited during acceleration. This mode provides a fixed voltage decompression starting for the motor to limit large current. Current limited starting also has pulse kick function. Selecting kick starting feature can provide a short-term pulse torque during starting to meet those loads that require a high starting torque during starting.

4.4 Double ramp start

  Dual-slope starting provides users with two independent soft-starting performances. The ramp time and initial torque can be adjusted separately. Pulse kick starting is also available in double-slope starting mode

4.5  preset slow start

  The preset slow start function can provide two slow start modes, which is convenient for process setting and calibration. These two speeds can provide forward and reverse speeds without reversible contactor. This mode is suitable for occasions where materials need to be positioned quickly. The preset low speed is in the forward direction, which can be set to 7% (low) of the rated speed or 15% (high) of the rated speed, or it can be set in the reverse direction. 10% of rated speed (low) or 20% of rated speed (high)

Figure 5  Preset slow start

4.6  Linear acceleration start

  This acceleration mode uses a closed-loop feedback system to maintain the motor's acceleration to a constant speed. The required feedback signal is realized by a DC tachometer coupled with the motor.

5 application

    In the process of selecting the soft start, through the analysis of the conventional starting mode and the starting mode of the soft start, the soft start device produced by Xi Chi Company was finally selected. It has the following characteristics: adopts full digital control, man-machine interface setting parameters, setting and adjustment is very convenient; adopts special filtering device, no harmonic pollution; small size, compact structure; thyristor is a long-life device, and it is Short-term work has a low chance of accidents.

    According to the situation of our factory, for the pump load, select the current limit starting mode to start the motor, set the current at 2.5Ie, and the measured starting time is 18s. Observe the electrical measurement parameters of the power grid for no abnormalities.

6 Conclusion

Through the long-term actual operation test, it is correct to choose the soft starter to control the start of the pump motor of the gas washing pump station. It not only avoids the impact of the motor full-voltage start on the power grid and mechanical equipment, but also saves a lot of money because the power grid does not need to be increased. cost of. The successful application of this product in our factory not only provides us with opportunities to understand new products and learn new technologies, but also enables us to gain experience in how to comprehensively consider factors such as technology, economy, operation, and rational selection of electrical equipment.