I. Overview

As we all know, three-phase AC asynchronous motors are widely used in various industries due to their low cost, high reliability and easy maintenance. However, it has great shortcomings when it is started directly: First, its starting current is as high as 5-7 times the rated current, which requires a large margin in the power grid and reduces the service life of electrical control equipment. Increasing maintenance costs, even affecting the normal operation of other electrical equipment; secondly, the starting torque can reach twice the normal torque, which will impact the load, increase the friction of the transmission components and additional maintenance. Because of the above reasons, a three-phase asynchronous motor step-down starting device appeared.

The traditional reduced voltage starting has the following methods:

1, a reactor is connected in series in the stator circuit of the motor to make a part of the voltage drop on the reactor;

2, star-delta conversion step-down starting (Y -△). The motor is connected in a star shape when starting, and after starting, it becomes a delta connection through a converter;

3. Start compensator start (start from lotus transformer).

Traditional starting equipment is bulky, costly, and has a complex structure. It is difficult to control the torque of the motor that matches the load, which means that it is difficult to obtain a suitable starting current and starting torque; and it will generate a high current spike at the moment of switching. , The resulting mechanical vibration will damage the motor rotor, shaft connector, intermediate gear, and load.

Therefore, there is a need for a starting device that can overcome the disadvantages of traditional starting. "Intelligent motor control module" adopts digital tube display and button control, and the whole starting process is automatically completed by single-chip microcomputer control. The user adjusts the parameter settings by pressing the keys, and can select different starting methods as needed, and can easily control the starting current to obtain the motor torque matching the load.

2. System configuration

As shown in Figure 1, the whole system is composed of six parts: main circuit, phase shift control, synchronous detection, voltage and current feedback, single-chip microcomputer control, display and buttons. Among them, the display and keys are part of a control panel. The other five parts are all integrated inside the module. It is connected with the control board through a 15-core parallel line.

图1

figure 2

3. Introduction of main functions

The motor soft start control module can mainly complete the following functions:

1, voltage ramp start

2, current limit start

3, soft parking

4. Energy-saving operation

5, overcurrent, overheating, phase loss protection

Now introduce separately

1, voltage ramp start

As shown in Figure 2, the system first adds a voltage Us to the motor to overcome the static friction torque, the voltage rises linearly from Us to the maximum voltage Umax. At this time, the voltage applied to the motor terminals is equal to the grid input voltage. Us is set by the user, and the voltage that can be selected by the user is 80V-300V. Ts is set by the user and can be selected from 1-90 seconds.

This starting method is characterized by a stable starting, which can reduce the impact of the starting current on the power grid, and at the same time greatly reduce the mechanical vibration caused by the starting torque to the load.

2, current limit start

As shown in Figure 3, this starting mode is set by the user a current value Ik, during the entire starting process, the actual current does not exceed the set value Ik. Ik is set by the user according to the actual load.

Current limiting starting can make the large inertia load be started and accelerated with the minimum current, and can be used to set the current upper limit to meet the use of grid capacity in limited occasions. This starting method is particularly suitable for constant torque loads.

3, soft parking

As shown in Figure 4, compared with direct parking, the voltage applied to the motor does not drop immediately, but gradually decreases from the highest voltage. After a period of time, Tp, and finally drops to 0V. Among them, the fall time Tp is set by the user, and the range is 0-90 seconds.

This kind of soft parking can greatly reduce the impact of liquid in pipeline equipment.

4. Energy-saving operation

For large friction loads, due to the large starting current required, a larger power motor is required, and the operating load torque required by the normal load is much smaller than the rated torque of the motor, which causes the motor to run at light load. For intermittent loads, the working time to maintain a large current occupies a very small part of the entire cycle, resulting in waste of light load active power loss and greatly reducing the operating power factor. The intelligent motor control module automatically adjusts the output voltage, so that the motor works in the best efficiency work area, and achieves the purpose of energy saving.

5, protection function

There are three protection functions: overcurrent protection, overheat protection, and phase loss protection.

If one of the above three faults occurs during starting or running, the module will automatically cut off the motor, and the digital tube on the control panel will flash to display the cause of the fault. After the fault is eliminated, press the reset button to return to normal.

image 3

image 4

Fourth, the experimental situation and practical application introduction

We have actually measured and recorded an intelligent motor control module in use.

The load used is a 18.5KW fan, and the actual measured voltage is about 390V.

To make a comparison, first remove the module and start directly.

After closing the air switch, the voltage immediately rises to 390V, and the current rapidly rises to 150A, lasts for a period of time, gradually decreases, and finally stabilizes at about 30A. At the same time, the sound of strong mechanical vibration generated by the fan due to the impact of large current can be clearly heard.

Then connect the motor soft start control module, set it to current limit mode start, the current limit value is 90A, and turn on energy saving operation.

Press the "Start" button, and it can be observed that the current rise speed is obviously slower, gradually rising to 90A, and after holding for 2 to 3 seconds, it gradually decreases to 30A. The voltage rises slowly from 0V to 390V. The starting time is 6 seconds. During the whole starting process, the motor starts smoothly, and no mechanical shock is heard.

After 15 seconds, the voltage gradually drops to 355V, the current does not change, and stable operation begins.

The intelligent motor control module can be widely used in various industrial fields, and its function and application fields are as follows:

1, reduce the motor starting current (generally when the AC motor is started directly, the impulse current is 5-7 times the rated current);

2. Avoid instantaneous voltage drop on the power supply line when the motor starts, which may cause malfunction of equipment and meters;

3, to prevent the moment impact when starting, causing mechanical shaft breakage or waste;

4, the motor can be started more frequently (soft starter generally allows 10 times/hour, so that the motor will not overheat);

5. It can prevent water hammer effect on pump loads and prevent pipeline rupture;

6. ​​For some process applications (such as yarn dyeing machinery), it can prevent quality problems caused by uneven dyeing due to too fast starting;

7. For some fragile container grouting production lines, it can prevent container damage;

8. The starting current needs to be controlled to reduce the impact on the machine, and it can also be adapted to the occasions of lower capacity power supply transformers (such as injection molding machines);

9. It can reduce the grid adaptation capacity and save the cost of capacity expansion;

10. It is necessary to adjust the starting characteristics easily.