Nov 28, 2020
Principle of sine wave drive of fish tank variable frequency water pump
The fish tank water pump adopts sine wave technology to greatly reduce the noise of the pump motor. The sine wave is a model with a relatively simple structure. In the design of electronic circuits, many waveforms are composed of sine waves. Sometimes the waveform represents the performance of the product. In some cases, the waveform needs to be filtered to get the desired sine wave type. This article will explain why you can get a sine wave after filtering.
Generally speaking, the signal wave of sinusoidal PWM in the principle of SPWM is a sine wave, that is, a sine wave is equivalent to a series of rectangular pulse waveforms of equal amplitude and unequal width, and its pulse width is generated by the natural intersection of sine wave and triangle wave. There are many methods for generating sine waves, but the typical ones are: symmetric regular sampling, asymmetric regular sampling, and average symmetric regular sampling. In terms of meaning, SPWM is called sine pulse broadband modulation.
The duty cycle of a pulse is proportional to the amplitude of the sine wave in the same period, and the average voltage of the pulse output is equal to the average amplitude of the sine wave in the time period. A (low pass) filter (or integrator) is a circuit that can play an averaging effect. Therefore, SPWM can output a sine wave after averaging.
Fish tank variable frequency water pump DC motor basics T=K*Fa*Fsinθ where K is a constant, Ff is the stator composite magnetomotive force, Fa is the rotor impulse motive force, θ is the angle between the stator magnetomotive force and the rotor magnetomotive force. Obviously The torque is maximum when θ=90°. The square wave control runs in six steps, θ varies between 60° and 120°, so it is not a constant transfer. The purpose of sine wave control is to control the direction of the stator flux linkage, and try to keep the stator flux linkage direction and the fish tank variable frequency water pump rotor flux linkage. The direction is vertical.
This is also the goal pursued by DSP vector control-a certain flux directional control. In this way, the torque is maximum and constant, and there is no torque pulsation. To achieve the above effect, you need to know the precise position of the rotor, use the photoelectric coding salt to position accurately, but the cost is high. The Hall sensor that is standard with the current fish tank variable frequency pump is used to detect the rotor position.
The rotor speed of the fish tank variable frequency water pump is considered to be constant within a 60° electrical angle, and the position of the fish tank variable frequency water pump rotor is simulated by software. The rotor of the fish tank variable frequency water pump rotates 360° electrical angle, the Hall sensor has six sum outputs, a 360° sine wave meter is made in the program, and the current value of the three halls is read by the software. In different segments, the data from the ant is sent to the duty cycle register of the PWM generator, and then a complete 360° sine wave can be reproduced. The actual test time of one Hall cycle above the interval time of the table is used as a reference for dynamic adjustment.
Advanced commutation angle processing The above scheme achieves the ideal voltage drive waveform, but it proves that the voltage vector is basically perpendicular to the direction of the rotor magnetic potential. In fact, because the electric wire is an inductive load, the fish tank variable frequency pump motor sets the current vector lag Since the voltage vector is fixed, the fixed potential also lags behind the stator voltage vector. That is to say, if the motor is driven according to the above SPWM waveform, the angle between the stator magnetic potential and the rotor magnetic potential will be less than 90°, and the motor torque is not the best for the stator. The current has a direct axis component, which produces a demagnetization effect, which causes the power factor of the controller to be low, so it is necessary to add advanced commutation processing. So that the angle between the fixed magnetic potential and the rotational magnetic potential is as close as possible to 90°.
The software implementation is very simple, as long as the initial angle is advanced when making the sine table, there is no need to change the software structure. How to adjust the speed The frequency of the sine wave is adjusted at any time according to the change of the Ha signal, which is a self-controlled passive frequency conversion. If you want to adjust the speed of the sine wave, you cannot directly modify the modulated sine wave frequency, but modify the amplitude of the modulated wave, so the sine taken out in the software The table value will be multiplied by the external speed given coefficient and then written into the duty cycle register of the PWM generator. After the modulation degree is modified, the equivalent voltage on the fish tank variable-frequency water pump will change, so the rotor speed will change, and Wang Xian modulation The frequency of the wave is passively adjusted according to the rotor Hall signal.