Views: 4 Author: Site Editor Publish Time: 2023-05-18 Origin: Site
Because it is difficult to change the frequency of an AC sine wave while in the AC mode, the first job of a frequency converter is to convert the wave to DC. As you will see a little later, it is relatively easy to manipulate DC in order to make it look like AC. The first component of all frequency converters is a device known as a rectifier or converter.
The rectifier circuit converts AC to DC and does so in much the same manner as those of a battery charger or arc welder. It uses a diode bridge to limit the travel of the AC sine wave to one direction only. The result is a fully rectified AC wave form that is interpreted by a DC circuit as a native DC wave form. Three phase frequency converters accept three separate AC input phases and convert them to a single DC output. Most three phase frequency converters can also accept single phase (230V or 460V) power but, since there are only two incoming legs, the frequency converters output (HP) must be derated because the DC current produced is reduced proportionally. On the other hand, true single phase frequency converters (those that control single phase motors) utilize a single phase input and produce a DC output that is proportional to the input.
There are two reasons why three phase motors are more popular than their single phase counter parts when it comes to variable speed operation. First they offer a much wider power range. But, equally as important is their ability to begin rotation on their own. A single phase motor, on the other hand, often requires some outside intervention to begin rotation. In this case, we will limit our discussion to three phase motors used on three phase frequency converters.
The second component, known as the DC Bus (shown in the center of the illustration) is not seen and in all frequency converters because it does not contribute directly to variable frequency operation. But, it will always be there in high quality, general purpose frequency converters (those manufactured by dedicated frequency converter manufacturers). Without getting into a lot of detail, the DC Bus uses capacitors and an inductor to filter the AC "ripple" voltage from the converted DC before it enters the inverter section. It can also include filters which impede harmonic distortion that can feed back into the power source supplying the frequency converter. Older frequency converters and some pump specific frequency converters require separate line filters to accomplish this task.
The inverter uses three sets of high speed switching transistors to create DC "pulses" that emulate all three phases of the AC sine wave. These pulses not only dictate the voltage of the wave but also its frequency. The term inverter or inversion means "reversal" and simply refers to the up and down motion of the generated wave form. The modern frequency converter inverter uses a technique known as "Pulse Width Modulation" (PWM) to regulate voltage and frequency.
Variable Frequency Drives, commonly known as VFDs, are electronic devices that control the speed and torque of electric motors by varying the frequency and voltage of the power supplied to the motor. They are widely used in industrial and commercial applications to achieve energy efficiency, precise motor control, and operational flexibility.
The primary function of a VFD is to convert fixed-frequency alternating current (AC) power from the electrical grid into variable-frequency AC power that can be supplied to the motor. By adjusting the frequency and voltage output, the VFD can control the motor's rotational speed, allowing for precise control and optimization of motor performance.
One of the key advantages of VFDs is their energy-saving capability. Traditional motor control methods such as mechanical throttling or on/off switching waste significant amounts of energy. VFDs, on the other hand, can match the motor's speed and power output to the actual load requirements, resulting in substantial energy savings. This feature is especially beneficial in applications where the motor operates at partial loads for extended periods.
VFDs also offer improved motor control and protection. With the ability to adjust the motor's speed and torque, VFDs enable smooth starting and stopping of the motor, reducing mechanical stress and wear. They can also provide advanced motor protection features, such as overload protection, short-circuit detection, and voltage/current monitoring, enhancing the motor's lifespan and reliability.
Another advantage of VFDs is their flexibility and adaptability. They allow for precise control of motor speed, acceleration, and deceleration, making them suitable for applications with varying operational requirements. Additionally, VFDs can be easily integrated into automation systems, enabling seamless integration with other control devices and enhancing overall system efficiency.
In summary, Variable Frequency Drives are essential devices that provide energy-efficient and precise control of electric motors. Their ability to vary the frequency and voltage output allows for optimal motor performance, energy savings, and enhanced motor protection. With their flexibility and adaptability, VFDs play a vital role in numerous industrial and commercial applications, contributing to improved efficiency and productivity.
