I. Structure and operating principle of frequency converter
– The inverter is made up of components that function to receive a power source with a fixed input voltage and fixed frequency, and then convert it into a power source with a three-phase variable voltage and frequency (can be changed) to control the motor speed.
Working principle:
The basic operating principle of the inverter is shown through the following two stages:
– Step 1: First, the single-phase or three-phase alternating current (AC) power source is rectified and filtered into a flat direct current (DC) source. This step is performed by a diode bridge rectifier and capacitor. The input power source can be single-phase or three-phase, but it will have a fixed voltage and frequency.
– Step 2: The above DC voltage will be converted ( inverted ) into a symmetrical 3-phase AC voltage. Initially, the generated DC voltage will be stored in the capacitor bank. This DC voltage is at a very high level at the DC bus (for a 220V inverter, the voltage at the DC bus is ………., for a 380V inverter, the voltage at the DC bus is …….…). Next, through the appropriate on/off triggering sequence, the inverter’s IGBT inverter will generate a 3-phase AC voltage using the pulse width modulation (PWM) method. Thanks to advances in microprocessor technology and current power semiconductor technology, the pulse switching frequency can reach the ultrasonic frequency range to reduce motor noise and reduce losses on the motor iron core.
II. Basic components of the inverter
Through the operation of the inverter, we can deduce the structure of the inverter including rectifier circuit, intermediate DC circuit (DC link), inverter circuit and control part. From there, we can detail into the main parts as follows:
1. Rectifier
– The first part in converting the input voltage to the desired output for the motor is the rectification process. This is achieved using a full wave diode bridge rectifier.
– A diode bridge rectifier is similar to the rectifiers commonly found in power supplies, in which single-phase alternating current (AC) is converted to direct current (DC). However, diode bridges used in inverters can also be configured with additional diodes to allow conversion from three-phase AC to DC.
– Diodes only allow current to flow in one direction, so a diode bridge directs the flow of electrons from alternating current (AC) to direct current (DC).
a. How to generate DC voltage from AC grid
– Let’s understand this principle by simply considering a single phase AC voltage source and using a resistive load. This component is used as a diode, the diode only allows current to pass in one direction and not in the other direction in the direction of voltage application.
– Using this property when AC voltage is applied to A and B in the rectifier circuit, the voltage is also applied to the load in the same direction. In other words AC voltage is converted (rectified) to DC voltage.
– Below is an example of a full rectifier diode bridge circuit:
b. Operating principle of rectifier
– For 3 phase AC input power, 6 diode junction is used to rectify the wave from AC power source and generate output voltage as shown in the diagram below.
c. Operating principle of flat rectifier circuit
– Capacitors are used to smooth the output voltage as follows:
d. Surge current limiting circuit
– The output side of the diode rectifier bridge is explained by a resistive load, but in practical applications a smoothing capacitor will be used as the load.
– The impulse current through the circuit, the instantaneous voltage is used to charge the capacitor.
– To prevent the rectifier diode from being damaged by the surge current, a resistor is inserted in the series circuit to block the surge current for a short time after the power is turned on.
– To operate for this purpose, the resistor is short-circuited across its two terminals to produce a circuit to bypass the resistor.
– This circuit is referred to as a surge current limiting circuit.
– If a surge current limiting circuit is used, the peak current value can be reduced to prevent damage to the rectifier diode.
e. Input current waveform with capacitor load
– The input current waveform in this case occurs only when the AC voltage is higher than the DC voltage. This results in a twisted waveform as shown in the diagram and not a sine wave.
As described in the above sections, the rectifier is constructed as follows:
2. One-way route
– A DC bus is a bank of capacitors that store rectified DC voltage. A single capacitor can store a large charge, but arranging them in a DC bus configuration increases the capacitance.
– The stored voltage will be used in the next stage when the IGBT generates electricity for the motor.
3. AC reactor (AC reactor)
– An AC reactor is an inductor or coil. The inductor stores energy in the magnetic field created in the coil and resists changes in current.
– The AC reactor helps reduce harmonic distortion, i.e. noise on the AC line. In addition, the AC reactor will reduce the peak level of the mains current, or in other words, reduce the overlap current on the DC line. Reducing the overlap current on the DC line will allow the capacitor to run cooler and therefore last longer.
– The AC reactor acts as a pulse delay to protect the input rectifier circuit from power supply noise and pulses caused by switching on/off other inductive loads by circuit breakers or magnetic starters.
– Disadvantages of using AC reactors are increased cost, more space required for installation and sometimes reduced efficiency.
– In some other cases, AC reactors can be used at the output side of the inverter to compensate for low motor inductance (used when the wiring distance from the inverter to the motor is 50-100 meters), but this is usually not necessary due to the good performance of IGBT technology.
4. DC reactor (DC reactor)
– The DC reactor limits the rate of instantaneous current change on the DC line. Reducing this rate of change will allow the inverter to detect potential problems that are about to occur and promptly stop/disconnect the motor.
– DC reactors are typically installed between the rectifier and capacitor on inverters 7.5 kW and larger. DC reactors can be smaller and cheaper than AC reactors.
– The DC reactor prevents harmonic distortion and superimposed currents from damaging the capacitor, however this reactor does not provide any anti-surge protection to the rectifier.
5. Inverter
a. How to convert DC voltage to AC
Learn this principle through a simple example of single-phase AC voltage as follows:
– Four switches, S1 to S4 are connected to a DC voltage source, where switches S1/S4 are paired together and switches S2/S3 are similarly paired. When the pairs of switches are turned on and off, current flows through the lamp as shown in the diagram below:
Current waveform:
– When switches S1 and S4 are turned on, current flows through the lamp in direction A.
– When switches S2 and S3 are turned on, current flows through the lamp in direction B.
If the operation of these switches repeats in a predetermined cycle, the direction of the current will change back and forth to create alternating current.
b. How is the frequency changed?
– The frequency changes as you change the ON and OFF time intervals of switches S1 and S4.
– For example, if you turn ON switches S1 and S4 for 0.5 seconds and then turn ON switches S2 and S3 for 0.5 seconds back and forth continuously, you will create an alternating current that reverses the direction of that current for 1 second, equivalent to a frequency of 1 Hz.
– In other words, the frequency is changed when the time t0 changes.
c. How is the voltage changed?
– The (average) voltage can be varied by changing the ON/OFF time ratio of the switches by changing the cycle time t0, to a shorter cycle time to turn the voltage ON/OFF.
– The frequency for these short pulses is referred to as the carrier frequency.
– For example, if the ON/OFF time ratio of switches S1 and S4 is halved, the output (average) voltage becomes an AC voltage equivalent to E/2, or half the DC voltage, E.
– To lower the (average) voltage, lower the ON time ratio and to raise the (average) voltage, raise the ON time ratio.
– The ON/OFF proportional pulse width will be controlled to change the voltage. This type of control method is referred to as pulse width modulation (PWM) and is now commonly used in inverters and other electronic components.
d. How to invert 3-phase AC voltage?
– The basic structure of a 3-phase inverter circuit and 3-phase AC voltage is shown below. If you change the order of the six switches that are turned ON/OFF, the result will change UV, VW and WU. This method is used to change the rotation direction of the motor.
– Note that in practice semiconductors are used instead of switches to transform the voltage, allowing the switches to be turned ON/OFF at very high speeds.
6. IGBT power module
– IGBT is a semiconductor power component, a type of insulated gate bipolar transistor that acts like an extremely fast on and off switch to create the output waveform for the inverter.
– IGBT devices are recognized for their high efficiency and fast switching. In the inverter, IGBTs are turned on and off in sequence to generate pulses of different widths from the DC bus voltage stored in the capacitor.
– By using pulse width modulation or PWM, the IGBT can be turned on and off in a sequence similar to the sinusoidal wave applied on the carrier.
In the figure below, the dotted triangle wave represents the carrier wave and the circle represents a portion of the sine wave.
– If the IGBT is turned on and off at each intersection between the sine wave and the carrier wave, the pulse width can be varied.
– PWM can be used to generate an output to the motor that is identical to a sine wave. This signal is used to control the speed and torque of the motor.
7. Braking resistor
– High inertia loads and vertical loads can cause the motor to accelerate when the motor is trying to slow down or stop. This motor acceleration phenomenon can cause the motor to act like a generator.
– When the motor generates voltage, this voltage will return to the DC line.
– This excess electricity needs to be dealt with somehow. Resistors are used to quickly “burn off” this excess electricity created by this phenomenon by converting the excess electricity into heat.
– Without the resistor, each time this acceleration phenomenon occurs, the drive may trip due to an Overvoltage fault on the DC Line.
III. Waveform characteristics
How to change input and output when using inverter?
· Input current: rabbit ear shaped electrical waveform (Including high slope components).
· Output current: waveform looks like a set of straight lines (rectangles). Includes high frequency components and voltage pulse components.
This waveform is generated from the ON/OFF operations of the semiconductor components in the inverter.
IV. Inverter control methods
– The only general-purpose inverter used in industrial applications in the 1980s was the V/F controlled inverter.
– Later, sensorless (speed) Vector control methods were introduced in 1990 with the aim of increasing torque in the low frequency control range more effectively than V/F control.
– Inverter capacity is increased thanks to improvements in hardware technology and control theory technology including semiconductors.
– Vector control by speed feedback (Encoder) was first applied to motors in 1990 for fields requiring high precision speed control.
– Typical inverter control methods are shown in the table below, mainly those related to speed control.
– In broad terms, remember that the power and accuracy of the inverter increases as you move to the right side of the table below the control method, but the flexibility and cost-effectiveness decrease.
– For the sensorless vector control method, below is one of the methods developed by Mitsubishi Electric and Shihlin Electric corporations.
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