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Variable Frequency Drive

What is VFD?

Are the inverter and VFD the same?

A variable frequency drive (VFD) is a type of electric motor driven by a motor controller, which is supplied to the electric motor by changing the frequency and voltage. Other names of VFD are variable speed drive, variable speed drive, variable frequency drive, AC drive, micro drive and inverter.

Frequency (or Hertz) is directly related to the speed of the motor (RPM). In other words, the faster the frequency, the faster the RPM. If the application does not require the motor to run at full speed, the VFD can be used to reduce the frequency and voltage to meet the requirements of the motor load. As the application program changes the motor speed requirements, the VFD can simply increase or decrease the motor speed to meet the speed requirements.

How does a variable frequency drive work?

The first stage of a variable frequency AC drive or VFD is a converter. The converter consists of six diodes, similar to the check valve used in the piping system. They only allow current to flow in one direction. The direction indicated by the arrow in the diode symbol. For example, as long as the A-phase voltage (voltage similar to the pressure in a pipe system) is more positive than the B-phase or C-phase voltage, the diode will open and allow current to flow. When Phase B becomes more positive than Phase A, the Phase B diode will turn on and the Phase A diode will turn off. The same is true for the three diodes at the cathode of the bus. Therefore, when each diode turns on and off, we get six current “pulses”. This is called “six-pulse VFD”, which is the current standard configuration of variable frequency drives.

Let us assume that the drive is running on a 480V power system. The 480V rating is “rms” or root mean square. The peak value on the 480V system is 679V. As you can see, the DC voltage of the VFD DC bus has AC ripple. The voltage is approximately between 580V and 680V.

We can eliminate the AC ripple on the DC bus by adding a capacitor. Capacitors operate similarly to reservoirs or accumulators in a pipeline system. The capacitor absorbs AC ripple and provides a smooth DC voltage. The AC ripple on the DC bus is usually less than 3 volts. Therefore, the voltage on the DC bus becomes “approximately” 650VDC. The actual voltage will depend on the voltage level of the AC line supplying the drive, the voltage imbalance level on the power system, the motor load, the impedance of the power system, and any reactors or harmonic filters on the drive.

A diode bridge converter that converts alternating current to direct current is sometimes simply called a converter. The converter that converts direct current back to alternating current is also a converter, but to distinguish it from a diode converter, it is usually called an “inverter.” In the industry, it has become common to refer to any DC-AC converter as an inverter. When we turn off one of the top switches of the inverter, this connection of the motor is connected to the positive DC bus, and the voltage on this phase becomes positive.

When we close one of the bottom switches in the converter, that phase will be connected to the negative DC bus and become negative. Therefore, we can make any phase on the motor positive or negative at will, so that any frequency we need can be generated. Therefore, we can make any phase positive, negative or zero.

Please note that the output of the VFD is a “rectangular” waveform. VFD will not produce sinusoidal output. For general distribution systems, this rectangular waveform is not a good choice, but it is completely suitable for electric motors.

If you want to reduce the motor frequency to 30 Hz, you only need to switch the inverter output transistor more slowly. However, if the frequency is reduced to 30Hz, the voltage must also be reduced to 240V to maintain the V/Hz ratio (for this, please refer to the introduction of VFD motor theory). If we only have 650VDC, how can we reduce the voltage?

This is called pulse width modulation or PWM. Imagine that we can control the pressure in the water pipeline by opening and closing valves at high speeds. Although this is impractical for piping systems, it works well for VFDs. Please note that in the first half of the cycle, the voltage is on for half of the time and off for half of the time. Therefore, the average voltage is 480V or half of 240V. Through the output pulse, we can obtain any average voltage on the output of the VFD.

Why use VFD?

1- Reduce energy consumption and energy costs
If your application does not need to run at full speed, you can reduce energy costs by using a variable frequency drive to control the motor, which is one of the advantages of a variable frequency drive. VFD allows you to match the speed of electric equipment to load requirements. No other AC motor control method can achieve this goal.
Today, motor systems account for more than 65% of industrial energy consumption. By installing or upgrading to a VFD to optimize the motor control system, the energy consumption of the factory can be reduced by up to 70%. In addition, the use of VFD improves product quality and reduces production costs. Combining energy efficiency tax incentives and utility rebates, the return on investment of VFD installations can be as short as 6 months.

2- Increase production through stricter process control
By running the motor at the most efficient speed for your application, you can reduce the occurrence of errors, thereby increasing the level of production and thus earning higher revenue for the company. On conveyors and belts, you eliminate jitter at start-up, thereby achieving high throughput.

3- Extend equipment life and reduce maintenance
When the equipment is controlled by a VFD to ensure the best motor application speed, the service life of the equipment will be longer, and downtime will be reduced due to maintenance. Since the VFD can optimally control the frequency and voltage of the motor, the VFD will provide better protection for the motor from electric thermal overload, phase protection, undervoltage, overvoltage and other problems. The motor or driving load will not be subjected to the “instantaneous shock” when the entire line is started, but it can start smoothly, thereby eliminating the wear of belts, gears and bearings. This is also an excellent way to reduce and/or eliminate water hammer, because we can achieve smooth acceleration and deceleration cycles.

Which motors can the inverter control

Variable-frequency Drive, VFD is a power control device that uses variable frequency technology and microelectronics technology to control AC motors by changing the frequency of the motor’s working power supply. The frequency converter is mainly composed of rectifier (AC to DC), filtering, inverter (DC to AC), braking unit, drive unit, detection unit and micro processing unit. The inverter adjusts the voltage and frequency of the output power supply by opening and closing the internal IGBT, and provides the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation. In addition, the inverter has many protection functions. , Such as overcurrent, overvoltage, overload protection and so on. With the continuous improvement of industrial automation, frequency converters have also been widely used.
The inverter can control:

General purpose motors

This is used in occasions where torque removal, speed and accuracy are not required.
Used on the fan and water pump, it has good energy saving and automatic control effect

Vector control inverter

Generally used to control the frequency conversion special motor can also be a general-purpose motor
Vector control features have good control over large torque, load changes, and angular velocity accuracy on speed

Servo-controlled inverter

Generally used to control frequency conversion special motors. Compared with vector, it has better control function and control effect. Of course, it is more complicated.

How does the inverter change the frequency

The inverter converts direct current to alternating current through an inverter circuit Change the output frequency and voltage by triggering the frequency of the IGBT and the size of the conduction angle

How is VFD connected to PLC?

The inverter can be connected to the PLC via Rs485 or Ethernet, so you need to consult whether it supports it before purchasing.

How to save energy during the work of the inverter?

The inverter realizes energy saving by feeding back to the grid.

There are two working modes of motor work: electric and brake. When the motor is braking, it is equivalent to a generator, and the inverter realizes energy saving by feeding back the electric energy generated by the motor to the grid for utilization.

Although the energy feedback of a single motor has a small contribution to energy saving, there are often a large number of motors working in a factory, and the power is large. In this way, the electrical energy of these motors’ braking turntables is collected and fed back to the grid for further processing. Utilization can save a lot of energy.

Does the variable frequency drive damage the motor?

The damage of the inverter to the motor includes two aspects, the damage of the stator winding and the damage of the bearing. This kind of damage usually occurs within a few weeks to more than ten months. The specific time depends on the brand of the inverter, the brand of the motor, the power of the motor, the carrier frequency of the inverter, the length of the cable between the inverter and the motor, and the ambient temperature. Many factors are related. The early accidental damage of the motor brings huge economic losses to the production of the enterprise.
This loss is not only the cost of motor repair and replacement, but also the economic loss caused by unexpected shutdown. Therefore, when using a frequency converter to drive a motor, sufficient attention must be paid to the problem of motor damage.

How many motors can one frequency converter control?

If the motors are connected in parallel, please note that it depends on the power of the inverter.

If the controlled motors all work separately, only one can be controlled

How to choose the right inverter for the motor

When we select the inverter, we need to follow the type of the controlled object, speed range, static speed accuracy, starting torque, etc., so that it meets the process and production requirements while being easy to use and economical.
The general experience is that the size of the motor should be the size of the inverter, and sometimes the size can be larger.   
The power factor of the high-power inverter is lower, it is better to install an AC reactor at the input end of the inverter. One is to improve the power factor, and the other is to suppress high-frequency harmonics. If you frequently start and brake frequently, install a brake unit and a brake resistor.   

If you need to reduce noise, you can choose a water-cooled inverter;

If braking is required, a braking chopper and braking resistor are required. Or you can choose four-quadrant products, which can feed back energy to the grid and save energy;   

If there is only a DC power supply on site, you can choose a pure inverter product (using a DC power supply) to drive the motor.

The final basis for the selection of the inverter is that the current curve of the inverter includes the current curve of the mechanical load.

How to control the inverter?

There are three ways to operate the inverter without an external controller (such as PLC):
① Use the buttons on the operation panel;
②Operate the components connected to the terminal (such as buttons and potentiometers);
③Composite operation (such as setting the frequency on the operation panel, operating the button connected to the terminal block for start/stop control).
In order to facilitate operation and make full use of the inverter, PLC can also be used to control the inverter

Can the speed of the inverter exceed the maximum speed of the motor?

Install a frequency converter before the motor, and change the frequency of the motor’s input power supply through the frequency converter to change the motor speed. Because changing the power frequency changes the speed of the rotating magnetic field, it also changes the synchronous speed of the motor.

SKI780 5.5KW 380V VFD Variable Frequency Drive Inverter

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