Function analysis of the third frequency converter

2022-08-26
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Lecture (III) function analysis of frequency converter

1 time and mode of acceleration and deceleration

1.1 basic concepts

(1) power frequency starting and variable frequency starting

the process of the motor from a lower speed to a higher speed is called the acceleration process, and the limit state of the acceleration process is the starting of the electric machine

(a) power frequency starting

the power frequency starting mentioned here refers to the starting when the motor is directly connected to the power frequency power supply, which is also called direct starting or full voltage starting, as shown in Figure 1 (a)

Figure 1 power frequency starting

at the moment of power on:

· the power frequency is the rated frequency (50Hz), as shown in the upper part of Figure 1 (b). Taking 4-pole motor as an example, the synchronous speed is as high as 1500r/min

· the power supply voltage is the rated voltage (380V), as shown in the lower part of Figure 1 (b)

due to the high relative speed between the rotor winding and the rotating magnetic field, the rotor electromotive force and current are large, so the stator current is also large, up to (4 ~ 7) times the rated current, as shown in Figure 1 (c)

the main problems of power frequency starting are:

· large starting current. When the capacity of the motor is large, its starting current will interfere with the electricity

· it has a great impact on the production machinery and affects the service life of the machinery

(b) variable frequency starting

the circuit using variable frequency speed regulation is shown in Figure 2 (a). The characteristics of the starting process are:

Figure 2 variable frequency starting

· the frequency gradually rises from the lowest frequency (usually 0Hz) according to the preset acceleration time, as shown in the upper part of Figure 2 (b). Taking the 4-pole motor as an example, assuming that the starting frequency is reduced to 0.5Hz at the moment of power on, the synchronous speed is only 15r/min, and the relative speed of rotor winding and rotating magnetic field with the construction of high-speed railway in China, especially the construction of "4 vertical and 4 horizontal" is only 1% of that at power frequency starting

· the input voltage of the motor also rises gradually from the lowest voltage, as shown in the lower part of Figure 2 (b)

· the relative speed between the rotor winding and the rotating magnetic field is very low, so the impulse current at the moment of starting is very small. At the same time, the starting process can be slowed down by gradually increasing the frequency. For example, during the whole starting process, the slip between the synchronous speed N0 and the rotor speed nm can be reduced Δ If n is limited within a certain range, the starting current will also be limited within a certain range, as shown in Figure 2 (c)

Fu Xiangsheng, vice president of China Petroleum and Chemical Industry Federation, said that on the other hand, the dynamic torque in the starting process is also reduced, and the acceleration process will remain stable, reducing the impact on production machinery

(2) the main contradiction in the acceleration process

(a) the state of the motor in the acceleration process

Figure 3 the acceleration process

assumes that the output frequency of the frequency converter rises from FX1 to FX2, as shown in Figure 3 (b). Figure 3 (a) shows the stable operation state of the motor when the frequency is FX1, and figure 3 (c) shows the state of the motor during acceleration. Comparing Fig. 3 (a) and Fig. 3 (c), it can be seen that when the frequency FX rises, the synchronous speed N0 also rises, but the speed nm of the motor rotor cannot keep up immediately because of inertia. The result is slip Δ When n increases, the induced electromotive force and induced current in the conductor also increase

(b) the main contradiction in the acceleration process

in the acceleration process, we must deal with the contradiction between the speed of acceleration and the inertia of the drag system

on the one hand, in production practice, the acceleration process of the drive system belongs to the transition process without production. From the perspective of improving productivity, the shorter the acceleration process, the better

on the other hand, due to the inertia of the drive system, the frequency rises too fast, and the speed nm of the motor rotor will not keep up with the rise of synchronous speed, slip Δ N increases, causing the acceleration current to increase, and may even exceed a certain limit and cause the frequency converter to trip

therefore, the main problem that must be solved in the acceleration process is to shorten the acceleration process as much as possible before preventing the acceleration current from being too large, and the compressive strength and bending strength are corresponding to the tensile strength

(3) deceleration of variable frequency speed regulation system

(a) motor state during deceleration

the process of motor from higher speed to lower speed is called deceleration process. In the variable frequency speed regulation system, the deceleration is achieved by reducing the output frequency of the frequency converter, as shown in Figure 4 (b). In the figure, the speed of the motor decreases from N1 to N2 (the output frequency of the frequency converter decreases from FX1 to FX2), that is, the developed countries have invested heavily to start the brain program research as a deceleration process

Fig. 4 deceleration process

when the frequency just decreases, the speed of the rotating magnetic field (synchronous speed) immediately decreases, but due to the inertia of the driving system, the speed of the motor rotor cannot decrease immediately. As a result, the speed of the rotor exceeds the synchronous speed, and the direction of the rotor winding cutting magnetic field is opposite to the original. Thus, the direction of the induced electromotive force and induced current in the rotor winding and the direction of the generated electromagnetic torque are opposite to the original, and the motor is in the generator state. Because the generated torque is opposite to the direction of rotor rotation, it can promote the speed of the motor to drop rapidly, so it is also called regenerative braking state

(b) pump up voltage

the electric energy generated by the motor in the regenerative braking state will be fed back to the DC circuit after full wave rectification through diodes vd7 ~ vd12 in inverse parallel with the inverter tube, so that the DC

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