My primary school student just started to contact the inverter, and the knowledge is poor. I recently read some articles on the website. One of the websites has a network journal "Inverter World". Personal feeling is better. One of the topics is called "Frequency Conversion". The speed control lecture is very easy to understand (I am not an electrical professional).
Next, write the most basic experience, it may be superficial, ridiculous and generous, hehe!
Regarding the application of the frequency converter, it is mainly applied to the three-phase asynchronous motor in high-speed occasions. The functions include: safe and fast start-stop motor and adjustment of motor speed.
The inverter can be controlled by the control panel of the inverter, or it can be controlled by plc (this is also what I would like to know but I don't know how to start it. In fact, it is very simple, just use the switch contacts to connect the corresponding terminals of the subscriber frequency and the public. The terminals (COM) are connected together, and the corresponding operation can be performed. Because the signal of the external switch is accepted, it is called the digital input.
In the process of starting the power frequency of the motor, the synchronous speed is up to 1500r/min at the moment of power-on, the relative speed of the rotor winding and the rotating magnetic field is very high, so the rotor electromotive force and current are very high, so the stator current is very large, and the rated current can be reached. (4~7) times, from the mechanical characteristics, the dynamic torque TJ is large during the whole starting process, so the shorter the starting time, the greater the mechanical impact during the starting process; The starting current is reduced by reducing the frequency at start-up.
During the speed reduction process, the actual speed of the motor is always lower than the synchronous speed during normal operation, and is set to 1440r/min. At this time, the rotor winding cuts the rotating magnetic field in the opposite direction (in the opposite direction to the rotating magnetic field), and the rotor current and the direction of the electromagnetic force received by the rotor winding are the same as the direction of rotation of the magnetic field, thereby causing the rotor to rotate. At the moment when the frequency just drops, the rotation speed of the rotor is still 1440r/min due to inertia, but the rotation speed of the rotating magnetic field has decreased. Therefore, the rotor winding becomes a positive direction to cut the rotating magnetic field, so that the rotor electromotive force and current are opposite to the original, the motor becomes a generator, and is in a regenerative braking state. From the viewpoint of energy balance, the speed reduction process is The process of releasing the kinetic energy by the drag system converts the released kinetic energy into regenerative energy.
The electric energy emitted by the motor in the regenerative state is fully rectified by the anti-parallel diodes VD7-VD12 next to the inverter tube, and then fed back to the DC circuit to raise the DC voltage, which is called the pumping voltage. If the DC voltage is too high, the rectifier and inverter modules will be damaged. Therefore, when the DC voltage rises to a certain limit, it must be tripped. After using the inverter, you can also reduce the voltage peak by setting the deceleration time to prevent damage to the motor.
The main ways of stopping are:
Deceleration stop: It is decelerated and stopped according to the preset speed reduction time of the user;
Free braking: The inverter's inverter tube is blocked and there is no output, so that the motor is in the free braking state after the power is cut off;
Deceleration stop plus DC braking: first decelerate to a certain frequency according to the deceleration time, then perform DC braking and stop.
When the load changes:
1) Setting of U/f ratio in variable load For loads with large torque changes, it is important to correctly set the U/f ratio when using the V/F control method. There is no doubt that the first thing people care about is: Can the motor carry the heaviest load at low frequencies? Therefore, it is easy to set the U/f ratio to be large. However, the curve shown in Figure 2-27 shows that if the U/f ratio is too large, it is easy to trip at no load.
Therefore, when debugging, the U/f ratio should be gradually increased from small to small. For each additional gear, can you observe the heavy load? When it can be driven, should it be reversed to observe whether it will trip when it is empty? Until the low frequency operation, it can bring heavy load and no empty load.
2) The U/f ratio of the quadratic law load is set to the quadratic law load. At low speed, the resistance torque of the load is very small. If the U/f ratio of the inverter is set to be large for some reason. It is possible to trip because of this. Therefore, the U/f ratio should be set as small as possible to save energy.
(5) Accurate preset U/f ratio example
1) Fan fan is a quadratic load. When operating at low speed (low frequency), the resistance of the load is very small. Even without compensation, the load torque is much smaller than the effective torque of the motor. For this type of load, the inverter has specially set several "negative compensation lines";
2) Belt conveyor belt conveyors are constant torque loads. The conveyor belt for transporting coal or stone, although the load is light and heavy, although it varies slightly, in general, it can be considered that the resistance torque of the load is basically unchanged;
(1) The purpose of slip compensation is known from the natural mechanical characteristics of asynchronous motors. When the resistance torque of the load increases from light load (TL≈TM≈0) to rated value (TL≈TMN), the drag system The speed of the rotation has decreased. The purpose of the slip compensation is to make the speed of the drag system substantially constant (nM2≈n02), resulting in harder mechanical characteristics.
(2) Slip compensation method When the load increases and the speed decreases, the motor's output frequency is appropriately increased to compensate for the reduced speed of the motor due to the slip.
For example, when the load torque is TLN (≈TMN), the output frequency of the inverter is appropriately increased by preset “slip compensationâ€, so that the synchronous speed of the motor rises from n02 to n02′, and the operating point of the drag system Then rise from Q2 to Q2'. Make the speed of the drag system basically the same as the original synchronous speed n02.
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