Aug 26, 2021
And these occasions are using the ordinary three-phase asynchronous motor. The variable frequency drive (VFD). can be spread in just a few years, it’s because it can get rid of speed regulation problems with the ordinary three-phase asynchronous motor. Thus it has become the most important industrial control system.
Ordinary three-phase induction motors account for a large proportion of the industrial market. With the convenience of the VFD, many problems have been solved, three-phase induction motors may have a much higher share.
For a three-phase asynchronous motor, since it was improved and invented by Tesla, a physicist, it has been welcomed as industrial equipment for its simple and reliable performance. Besides, it has become the heart of the industrial power system.
However, the late arise of high-power electronic devices and single-chip technology makes the asynchronous motor speed regulation effect not ideal, thus the speed regulation system is basically dominated by the DC motor.
Until the end of the last century, the speed regulation system of VFDs was mature, and the price was reduced, it has completely overturned the speed regulation market, laying the foundation. All this, of course, is based on the variable frequency drive (VFD). that can be used in the ordinary three-phase induction motor.
In the 1990s, the VFD has been popular, and until today, many users still prefer an ordinary motor with a variable frequency drive (VFD).. It can save electricity, and the key is to bring the convenience of the speed change. The fact also proved that the VFD used on an ordinary asynchronous motor is very mature and stable.
Figure 2: A worker is operating on the industrial power system.
A speed control system is required to be stable and reliable, but also the speed range. The ratio of the highest speed and the lowest speed is 1. The speed range of an ordinary induction motor is definitely not high.
It’s mainly because the magnetic field of an induction motor is not like the DC motor with an independent coil, but a lot of parameters are coupled together, so the low-speed performance is not ideal, and high-speed performance is limited by the bearing and other factors.
For example, the VFD is generally used in more than 8HZ, but smaller than 75HZ, so that the speed range is less than 10:1, restricting its applications in some wide range of speed control occasions.
For instance, the gantry bed processing should be slow, and the speed fluctuation should be small, so as to ensure the processing accuracy. The speed range of the frequency converter is not enough to meet the speed regulation requirements.
But for many ordinary motors of fan and pump, there is no need to ensure such high demand for speed regulation performance, and the speed will not affect the equipment and process requirements. On the contrary, the change of speed brings a lot of benefits. If the speed is low, the bearing wear will not be severe.
Because of the square torque, the speed is reduced by 10%, the noise is reduced a lot, and in theory, the power can be saved by 30%. The VFD is successful to to solve the speed regulation of ordinary induction motors.
The VFD does not truly output the standard sine wave to an induction motor, but through the form of a PWM chopper, it outputs a series of pulse square waves. It uses the mathematical principle of calculus to achieve the effect of equivalent sine waves through the uniform area of action.
As you can imagine, when the speed of the motor is low, the output pulse density of the VFD is relatively low, so it is difficult to simulate the sinusoidal effect, which is the fundamental cause of the low-speed torque, the instability of the speed and the heating.
Therefore, an ordinary induction motor with a VFD needs to avoid working in a low-frequency state. For example, Tesla motors work at high frequencies, and of course, the bearings have to be hard first. It is then slowed down by gears to prevent the induction motor from operating at low frequencies. For some of the speed range requirements of the industrial control system, can also refer to such a control idea.
The VFD realizes on-off control of the motor coil through two states of the switch of the high-power switch tube. The coil is an inductive load, and the current cannot be abrupt. When the switch tube is forcibly opened, it will generate the DI/DT high voltage inevitably.
The instantaneous high voltage caused by these high-frequency on-off will impact the insulation system of the motor. At the same time, due to repeated on-off, it will cause some power loss, resulting in motor vibration and heating, and shortening the service life of the motor. The coil is frequently on and off and will produce electromagnetic waves, causing various types of interference.
Figure 3: Induction motors.
At a low speed and low frequency, the motor is overheated. The ordinary asynchronous motor is of a self-cooling air cooling system, which is mainly through a fan to cool the tail, and the speed of the fan and that of the motor is the same.
At low speeds, the fan speed is too low to provide cooling. So for an induction motor running at low frequencies, you can change the fan to an external drive mode, such as adding a fan to drive the cooling fan to meet the heat dissipation requirements.
When you use a VFD, it is inevitable to encounter interference problems. You can separate the strong and weak currents by isolating the power supply to avoid putting the strong and weak wires together.
You should pay attention to the grounding, which can be avoided by using some magnetic ring filter lights. For high-precision situations, you can use a variable frequency motor or even a synchronous motor to meet the requirements.
