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How to build a frequency converter control motor speed regulation system

Dec 12, 2020

How to build a frequency converter control motor speed regulation system

Inverter is essentially a controllable AC power supply. From a control point of view, it is relatively simple to build the so-called inverter system. The main reason is to select the correct model, and then consider the specific conditions of the process and equipment, and design the circuit and Installation, and finally parameter adjustment.

1. Selection


(1) Consideration of load type


The correct selection of the frequency converter is very critical for the normal operation of the control system. When selecting an inverter, it is necessary to fully understand the characteristics of the load driven by the inverter. People often divide production machinery into three types in practice: constant torque load, constant power load and fan and water pump load.

1. Constant torque load


The load torque TL has nothing to do with the speed n, and TL always remains constant or almost constant at any speed. For example, frictional loads such as conveyor belts, mixers, and extruders, and potential energy loads such as cranes and hoists are all constant torque loads. When the inverter drives a load with constant torque, the torque at low speed must be large enough and have enough overload capacity. If it is necessary to run at a steady speed at a low speed, the heat dissipation capacity of a standard asynchronous motor should be considered to avoid excessive temperature rise of the motor.


2. Power load


The torque required by machine tool spindles, rolling mills, paper machines, and coilers and decoilers in plastic film production lines is roughly inversely proportional to the speed, which is the so-called constant power load. The constant power nature of the load should be in terms of a certain speed range. When the speed is very low, limited by the mechanical strength, TL cannot increase indefinitely, and it becomes a constant torque property at low speed. The constant power area and constant torque area of the load have a great influence on the selection of the transmission scheme. When the motor is in constant flux speed regulation, the maximum allowable output torque does not change, which belongs to constant torque speed regulation; while in field weakening speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the constant torque and constant power speed regulation range of the motor is consistent with the constant torque and constant power range of the load, that is, in the case of "matching", the capacity of the motor and the capacity of the frequency converter are the smallest.


3. Square torque load


In various fans, water pumps, and oil pumps, with the rotation of the impeller, the resistance generated by air or liquid within a certain speed range is roughly proportional to the second power of speed n. As the speed decreases, the torque decreases according to the second power of the speed. The power required by this load is proportional to the third power of the speed. When the required air volume and flow are reduced, the frequency converter can be used to adjust the air volume and flow through speed regulation, which can greatly save power. Since the required power at high speed increases with the speed too fast, and is proportional to the third power of the speed, it is generally not allowed to make the fan and pump load exceed the power frequency.


(2) Selection principle


1. The power of the inverter is selected according to the load power it carries. Generally, the inverter of any power is selected for the motor with a large power, and the size of the larger one is also possible. Choose different inverters for different purposes. For example, there are general inverters, special inverters for fan and water pump, and special inverters for machine tool spindles. Constant power load should use vector type and so on.


1) Inverter rated power pv≥motor power pd. In a one-to-one situation, one frequency converter drags one motor.


2) When one inverter drives several motors, then pv≥pd1+pd2+pd3+..., and pd1=pd2=pd3=..., and several motors can only start and work at the same time. It can only be done under basically the same working environment and working conditions, which can save investment compared with buying multiple low-power inverters.


3) When a frequency converter is dragging several motors, when pd1≠pd2≠pd3..., and the power difference is large and cannot be started at the same time, and the working conditions are not the same, it is not advisable to use one to drag several motors. Unfavorable, at the same time pv "pd1 + pvz + pd3 +..., because the last started inverter has to withstand 5-7 times the starting current, so the power of the inverter will be very large, which is uneconomical and unreasonable and should not be used.


4) When the motor is at full load, forward and reverse rotation, and the starting torque is large, in the case that cd2 is also large, in principle, choose to amplify the pn power value of one level. 5) Normally, the rated current of the inverter is in = 1.05 id. Under normal operating conditions or in poor conditions, you can choose: in = 1.10 id.


6) Inverter rated voltage un=motor rated voltage u,.


7) The frequency of the frequency converter can be 0~240hz or 0~400hz for general frequency converters, and 0~12ohz for special frequency converters for water pump and fan.


8) The choice of inverter control mode one is mainly based on the performance of the equipment used and the process requirements, so that the amount of material is used, neither "using large materials" nor "using small materials". The former is a waste of money and the latter is Can not meet the requirements for use. The commonly used control methods of inverters are v/f control, space voltage vector control (svpwm), vector control (vc), direct torque control (dtc), and vector control without speed feedback (n-ssvc), with speed feedback vector Control (h-ssvc), modulation mode control (pwn), u-shaped characteristic curve control, fuzzy control, self-setting control (offline or online), etc. These control methods have their own characteristics and objects of use. They must be correct, reasonable and economical , Practical comprehensive consideration.


2. When selecting the inverter, the actual motor current value should be used as the basis for selecting the inverter. The rated power of the motor can only be used as a reference. In addition, it should be fully considered that the output of the frequency converter is rich in high-order harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, compared to using a power frequency grid to supply power with a frequency converter, the current of the motor will increase by 10% and the temperature rise will increase by about 20%. Therefore, when selecting a motor and a frequency converter, this situation should be considered, and a margin should be appropriately left to prevent excessive temperature rise and affect the service life of the motor.


3. If the inverter needs to run with a long cable, measures should be taken at this time to suppress the influence of the long cable to the ground coupling capacitance to avoid insufficient output of the inverter. Therefore, the inverter should be amplified for one or two gear selection or install an output reactor at the output end of the inverter.


4. When the frequency converter is used to control several motors connected in parallel, it must be considered that the total length of the cable from the frequency converter to the motor is within the allowable range of the frequency converter. If it exceeds the specified value, zoom in one or two gears to select the inverter. In addition, in this case, the control mode of the frequency converter can only be V/F control mode, and the frequency converter cannot realize the overcurrent and overload protection of the motor. At this time, a fuse must be added to each motor side to realize the protection.


5. For some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., this will cause the inverter to derate, and the inverter needs to be enlarged by one gear.


6. When using a frequency converter to control a high-speed motor, due to the low reactance of the high-speed motor, more high-order harmonics will be generated. These higher harmonics will increase the output current value of the inverter. Therefore, when selecting the frequency converter for high-speed motors, it should be slightly larger than that of ordinary motors.


7. When the frequency converter is used in a pole-changing motor, full attention should be paid to selecting the capacity of the frequency converter so that the maximum rated current is below the rated output current of the frequency converter. In addition, when changing the number of poles during operation, the motor should be stopped first, otherwise it will cause the motor to run idly and damage the inverter in severe cases.


8. When driving an explosion-proof motor, the inverter does not have an explosion-proof structure, and the inverter should be installed outside the hazardous location.


9. When using a frequency converter to drive a gear reduction motor, the range of use is restricted by the lubrication method of the gear rotating part. When lubricating oil, there is no limit in the low-speed range; in the high-speed range above the rated speed, the risk of running out of lubricant may occur. Therefore, do not exceed the maximum speed allowable value.


10. When the frequency converter drives the wound rotor asynchronous motor, because the wound motor has a smaller winding impedance compared with the ordinary squirrel cage motor, it is prone to overcurrent tripping caused by ripple current, so Choose an inverter with a capacity slightly larger than usual.


11. When the frequency converter drives a synchronous motor, compared with the industrial frequency power supply, the output capacity will be reduced by 10%-20%. The continuous output current of the frequency converter should be greater than the product of the synchronous motor rated current and the synchronous pull-in current.


12. For compressors, vibration machines and other loads with large torque fluctuations and peak loads such as hydraulic pumps, if the inverter is selected according to the rated current or power value of the motor, overcurrent protection due to peak current may occur Action phenomenon. Therefore, you should understand the power frequency operation and choose an inverter with a rated output current greater than its maximum current. 13. When the frequency converter drives the submersible pump motor, because the rated current of the submersible pump motor is larger than the rated current of the normal motor, when choosing the frequency converter, its rated current should be greater than the rated current of the submersible pump motor.


14. When the frequency converter controls the Roots blower or special blower, because its starting current is very large, it is necessary to pay attention to whether the capacity of the frequency converter is large enough when choosing the frequency converter.


15. When selecting the inverter, be sure to pay attention to whether its protection level matches the situation on site. Otherwise, dust and water vapor on site will affect the long-term operation of the inverter.


16. Single-phase motors are not suitable for inverter drive.


2. Design and installation


1. The inverter has a main circuit and a control circuit. The main circuit inlet is connected to the three-phase power supply of the grid, and the main circuit outlet is connected to the three phase wires of the asynchronous motor. Generally, the main circuit can be equipped with one more circuit breaker, and it is not suitable to use leakage devices, because high-frequency interference is prone to tripping.

2. The control circuit serves the main circuit. Generally, there are some interlocking protection of sensors, the start and stop of the inverter itself, and if it needs to be controlled by an external knob, there are also signal circuits such as potentiometers or analog quantities. Now there are some inverters controlled by bus or other communication methods, which are also summarized in the control loop.

3. As for ensuring the normal operation of the inverter, the normal operation of the inverter needs to be guaranteed: the incoming power supply voltage is within the normal range without excessive sudden rises and drops, the indoor temperature is within the normal range, the inverter has normal cooling and ventilation, and the grounding is good. Long-term overload, no condensed water, no corrosive gas, less dust, etc.;


4. If the process and equipment require multiple inverters to be linked, the distribution and feedback of linkage signals need to be considered at this time to ensure design accuracy.


5. In some occasions, additional braking function may be required, and braking resistor and braking unit can be selected to meet the requirements.


6. In some occasions, where the line is far away, you can consider adding an output reactor.


7. In some cases, the grid voltage fluctuates greatly, you can consider adding a line reactor.

8. In general, it is sufficient to use the panel to operate the start and stop, but in some cases, external terminal control or remote operation is required. At this time, it is necessary to lead the control wire to the corresponding operation box separately, and then add the display instrument, the start and stop button, and the speed control Potentiometer.


9. In some places where the environment is harsh, such as high temperature and humidity, it is necessary to separately equip the inverter electric control cabinet with a cooling device, such as installing an air conditioner.


10. Consider the control of collocation with other devices. For example, synchronous control requires a synchronous controller, logic control requires PLC and relay circuits, and tension control requires a tension controller.

Three, debugging


In most cases, the inverter is a start-stop and speed-regulating demand.

(1) Frequency setting:


1. Starting frequency: This parameter is used to set the frequency at which the motor starts to run when starting.


2. Operating frequency: Adjust the rotation frequency of the motor after operation according to the production situation.


3. Frequency upper and lower limit: This parameter avoids user misoperation to make the frequency too high and burn out the motor


(2) Frequency setting method:


1. Panel speed control: the frequency can be adjusted through the buttons on the panel.


2. Sensor control: The voltage or current change of the sensor can be used as a signal input to control the frequency.


3. Communication input: control its frequency with PLC and other upper computers.


(3) Acceleration and deceleration time:


1. Acceleration time: The acceleration time is the time from its starting frequency to its running frequency.


2. Deceleration time: You can set the time required for the motor to stop from operating frequency.


(4) Motor parameter setting:


The parameters can be set in the inverter according to the rated voltage and rated current of the motor nameplate, and correspond to it.


(5) Control method and protection


1. Direction of rotation: mainly used to set whether reverse rotation is prohibited.


2. Stop mode: used to set whether to brake or stop freely.


3. Voltage upper and lower limit: Set the limit according to the equipment motor voltage to avoid burning the motor.


(6) Other


For example, constant pressure water supply may need to set PID, some occasions need to set slave tracking, some need to set master synchronous output, etc. You can use analog and digital port functions to adjust according to the manual.