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Inverter working principle and advantages

Nov 24, 2020

Inverter working principle and advantages

A variable frequency drive (VFD) is a type that drives a motor controller by changing the frequency and voltage supplied to the motor. Other names of VFD are variable speed drive, adjustable speed drive, frequency converter, AC frequency converter, microdrive and inverter.

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Frequency (or Hertz) is directly related to the speed (RPM) of the motor. 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 motor speed requirements of the application change, the VFD can simply increase or decrease the motor speed to meet the speed requirements.

How does the inverter work?

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


We can eliminate the AC ripple on the DC bus by adding a capacitor. Capacitors operate in a similar way to reservoirs or accumulators in piping systems. 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 depends on the voltage level fed by the inverter into the AC line, the level of voltage imbalance on the power system, the motor load, the impedance of the power system, and any reactors or harmonic filters on the inverter.

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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 often called an "inverter." In the industry, it has become common to refer to any DC-AC converter as an inverter.


When we close one of the top switches in the inverter, the phase of the motor is connected to the positive DC bus, and the voltage on that phase becomes positive. When we close one of the bottom switches in the converter, the phase is connected to the negative DC bus and becomes negative. Therefore, we can make any phase of the motor positive or negative at will, and therefore can generate any frequency we want. Therefore, we can make any stage positive, negative or zero.


The output of the VFD is a "rectangular" waveform. The VFD does not produce a sinusoidal output. This rectangular waveform is not a good choice for general power distribution systems, but it is completely sufficient for an electric motor.


If we want to reduce the motor frequency to 30 Hz, then we only need to switch the inverter output transistor more slowly. However, if we reduce the frequency to 30Hz, then we must also reduce the voltage to 240V to maintain the V/Hz ratio (for more information on this, please refer to the VFD motor theory presentation). If our only voltage is 650VDC, how will we lower the voltage?


This is called pulse width modulation or PWM. Imagine that we can control the pressure in the water pipe by switching a valve at high speed. Although this is not practical 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 half of 480V or 240V. Through pulse output, we can get any average voltage at the output of the VFD.

Why should I 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 controlling the motor through a variable frequency drive, which is an advantage of variable frequency drives. VFD allows you to match the speed of electric equipment to load requirements. No other AC motor control method allows you to achieve this.


Electric motor systems account for more than 65% of the power consumption in the industry today. By installing or upgrading the VFD to optimize the motor control system, the energy consumption of the equipment 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 may only be 6 months.


2. Improve productivity through stricter process control


By operating the motor at the most efficient speed for your application, you can reduce the chance of errors, thereby increasing the level of production, thereby bringing higher revenue to your company. On conveyor belts and conveyor belts, you can eliminate confusion at start-up to achieve high-speed transmission.


3. Extend equipment life and reduce maintenance


Your equipment will last longer and the maintenance time when controlled by the VFD is shorter, thus ensuring the best motor application speed. Since the inverter has the best control over the frequency and voltage of the motor, the inverter will provide better protection for your motor, such as electric thermal overload, phase protection, undervoltage, overvoltage, etc. When you use the inverter to start the load, the motor or the driven load will not be subjected to the "instant shock" on the starting line, but it can start smoothly, thereby eliminating the wear of belts, gears and bearings. Because we can have a smooth acceleration and deceleration cycle, it is also an excellent way to reduce and/or eliminate water hammer.