Click on the blue word, + follow, and take you to soar into the ocean of knowledge!
With the rapid development of industrial automation, inverter, as an important electrical control component, has been widely used in various machinery and equipment. In the inverter control system, PWM technology is a commonly used adjustment method, which can control the output voltage and frequency by changing the pulse width to achieve a variety of functions such as constant torque, constant power and frequency conversion control. So, how do you use PWM technology for regulation in inverter control? Let's discuss this in detail.
The challenges of motor control
Pulse Width Modulation (PWM) is a common strategy for creating analog signals from digital sources. This is a common method when trying to drive a motor from a power supply controlled by discrete transistors, which does not provide a specific variable current output. Frequency converters operate on this principle.
Many controller outputs utilize a strategy of pulse-on and off-signal to provide a limited amount of current to the load device. This impulse voltage process allows discrete digital outputs to drive analog devices, although still cannot produce a true analog output voltage.
This method is particularly common in motor outputs, where power is required to drive the motor load. In order to drive with proper torque and speed, a certain amount of power must be delivered to the motor. This power is the product of voltage and current. If the voltage drops, the power will be directly affected, so we can't lower the voltage and still expect to have the torque needed to move the load. So as a deceleration method, it is impractical to reduce the voltage. This concept really only appears in the case of low-voltage start-up methods.
To overcome this challenge, there must be a way to provide the full voltage and current, but somehow reduce the drive speed of the motor. In DC motors, this can be achieved by reducing the average current of the magnetic field in the drive rotor, which will determine the speed at which the brushes are commutated. Pulse-width modulation (PWM) technology accomplishes this task very simply.
In a three-phase AC motor, a simple voltage pulse does not change the speed because in order to function properly, the voltage must also change polarity. To be precise, it has to do so at a very specific frequency to get the correct output speed. This is different from DC motors, where the rotation speed automatically controls the speed at which the brushes reverse, a bit like automatic feedback. On the other hand, AC motors must be used alternately by external power sources.
How does PWM work?
In order to understand how the pulsation concept works, two key concepts must be remembered. First, a constant PWM carrier frequency controls the calculation of the voltage duration. This frequency must be high enough that the output device cannot be physically turned on and off with each pulse. If you do this, it will cause the motor to pulsate, which can cause damage. The good news is that the motor reacts relatively slowly to the applied current, so the PWM carrier frequency does not have to be very high, but is usually in the range of a few hundred hertz to a few thousand hertz.
For reference, small controllers, most pins have a PWM carrier frequency of around 500Hz. There are also controllers whose carrier frequency is 4kHz by default. For many industrial drives, the frequency should match any installed line filters.
At this constant frequency, the digital DC voltage can be turned on for part of this time and then turned off for the remainder. For example, the carrier frequency is 1kHz and the time of each cycle is 1 millisecond. The DC voltage can be turned on 05ms and then turn off the remaining 05ms。The load will respond to the drive briefly, but if this constant on/off mode is repeated, the overall response will be exactly half of the motor's maximum response speed. If the "on" duration of the pulse increases, the faster the motor will rotate until it finally reaches full voltage throughout the cycle, which is the same as simply providing a constant voltage.
The ratio of the turn-on pulse duration divided by the total cycle time is called the "duty cycle" of the PWM output and is expressed as a percentage of 0%-100%. For DC motors, a constant duty cycle results in a constant but precisely adjustable speed.
How does a frequency converter control an AC motor with PWM?
For three-phase AC motors, each phase input can be considered individually. In fact, each phase is delayed from the previous one. 55ms (60 for 3 of the 3Hz cycle), but each phase is just a replica of the original PWM concept.
First, the voltages must alternate polarities to produce the desired AC waves, so the transistors driving the output are connected in a reverse configuration called an "H-bridge", allowing the controller to change the output polarity at the right moment to regenerate the frequency signal.
When the digitally created AC wave begins, the duty cycle is almost 0% (so there is no current), but it begins to increase rapidly. When the rate of change climbs to 100%, the rate of change slows down, in fact, the curve is an exact sinusoidal curve, just like the mainline voltage. The duty cycle itself corresponds to the average current supplied to the motor coil, so we expect to see the magnetic field of the coil change at a very precise rate.
For slow-rotating motors, the duty cycle increases relatively slowly. Once it reaches 100%, the duty cycle will immediately start to fall back to 0%. Then the polarity will flip and the cycle will repeat.
One of the easiest ways to consider the PWM output of a drive is to look at a standard AC voltage curve. However, instead of reading it as "voltage", it should be read as "duty cycle". Since duty cycles are created digitally, they can be created very slowly or quickly as needed.
It should not be attempted to run the motor at a frequency higher than the maximum frequency of the nameplate, the PWM carrier frequency will be many times higher. In addition, it is important to note that higher carrier frequencies will generate more interference and electrical noise, but lower frequencies will allow for an increase in the amount of current passing through each transistor and generate more heat per PWM cycle. Therefore, attention should be paid to the manufacturer's recommendations for carrier frequencies.
By continuously optimizing and improving PWM technology, we can further improve the stability and reliability of the inverter system and inject new vitality into the development of the field of industrial automation. It is believed that with the continuous progress of science and technology, the application prospect of PWM technology in inverter control will be broader.
If you have other good understandings to share, you can leave a message to communicate with me. At the same time, you can share good articles with more people, follow me, and get the latest knowledge sharing for the first time!