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Variable frequency drives: components, types and how it works

Nov 28, 2020

Variable frequency drives: components, types and how it works

A variable frequency drive is a type of electronic controller that is designed to change the frequency and voltage supplied to the AC motor, which will result in a change in the speed of the motor as required. The VFD can simply or rotate the speed and voltage of the upper and lower motors, To meet the speed and torque requirements.


In order to control the speed of the motor, the inverter controls the frequency of the electrical energy supplied to it. Frequency (or Hertz) is directly related to motor speed (RPM). The faster the frequency, the faster the speed of the motor. As the motor speed requirements of the application change, the VFD can simply adjust the motor speed up or down to meet the speed requirements, ultimately saving energy.


Variable frequency drives can run ventilation systems, pumps, conveyors and machine tool drives on ships, used for bow thrusters, azipod systems, etc. Other names of VFD are variable speed drives, adjustable speed drives, adjustable frequency driving, etc.


What are the main components of a frequency converter?


The basic system of variable frequency drive includes AC motor, controller and operator interface. Frequency converters are usually used for three-phase induction motors because of their versatility and cost-effectiveness compared to single-phase or synchronous motors.


The variable frequency drive interface allows the user to adjust the running speed, start and stop the motor. The interface can also allow the user to switch and reverse between automatic control or manual speed adjustment.


These are the main modules of variable frequency drives:


Rectifier: Used to change the input AC power to DC power. Different designs can be used, these designs are selected according to the required performance of the VFD

Intermediate circuit: The rectified DC power supply is then regulated in the intermediate circuit through the combination of inductors and capacitors.

Inverter: Used to convert rectified and regulated DC back to an AC power source with variable frequency and voltage

Control unit: The controller monitors and controls the rectifier, intermediate circuit and inverter to provide correct output in response to external control signals. The whole process is controlled by a microprocessor, which monitors the following:

Input power supply voltage

Speed set point

DC bus voltage

The output voltage and current ensure that the motor runs within the established parameter range.


Describe the working principle of the inverter?


Initially, the input AC power is fed into the converter. The converter usually consists of six diode OR transistors, which work like a check valve. The converter converts AC power to DC power, allowing current to flow in only one direction. The output obtained from the converter is a square waveform, which contains ripple voltage, smoothed by the DC link. The rectified DC is then fed to the inverter. The inverter converts the rectified and regulated DC back to AC power at the required frequency and voltage.

In order to obtain 3-phase AC, the switches S1 to S6 are turned on/off at the same time. If you change the on/off sequence of the switches, the phase sequence is changed between UV, VW and WU, and the direction of rotation can be changed.


Note:-It is important to remember that when the output frequency of the VFD changes, the output voltage must also change; this ensures the balance of the speed-torque characteristics of the motor.


Torque Tm = K×Φ×I = K×(V / F)×I


Assuming that if the voltage is fixed and only the frequency decreases, the increased magnetic flux (Φ) causes the core to be magnetically saturated, and then the increased current causes overheating and burnout.


Therefore, change the voltage (V) and frequency (F) applied to the motor to keep their relationship constant. Even if the motor changes, the motor output torque is constant.


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



We assume that the drive is running on a 440V power supply system. The 440V rating is "root mean square" or root mean square. The peak value of the 440V system is 620V. The VFD DC bus has a DC voltage with AC ripple. VFD can eliminate AC ripple on the DC bus by adding capacitors. These capacitors absorb AC ripple and provide a smooth DC voltage. Therefore, the voltage on the DC bus becomes "approximately" 620 VDC. The actual voltage depends on the voltage level of the AC line supplying the inverter, the voltage imbalance level of the power system, the motor load, the impedance of the power system, and any reactors or harmonic filters on the inverter.


The VFD does not produce a sinusoidal output. The rectangular waveform it produces is not ideal for general power distribution systems, but it is perfectly suitable for motors.


What are the different types of inverters?


Three types of VFD are usually used according to requirements. these are:


Pulse width modulation (PWM).

Current source inverter (CSI),

Voltage source inverter (VSI),


Pulse width modulation (PWD)



The pulse width modulation VFD uses a diode bridge rectifier to convert the input AC voltage into a DC voltage. The DC output uses large capacitors to eliminate the ripple after the rectifier and generate a stable DC bus voltage.


The rectified and regulated DC voltage is then converted back to AC by the inverter. This conversion is usually achieved by using power electronics such as IGBT power transistors using a technique called pulse width modulation (PWM). The DC voltage is chopped to a variable width, but a constant level. By changing the pulse width and polarity of the DC voltage, an average sinusoidal AC output can be generated in a wide frequency range, usually 0.5-120 Hz.



Due to the smoothing effect of the motor inductance, the motor current looks almost sinusoidal. The frequency provided to the motor is determined by the number of positive to negative transitions per second. These transistors are controlled by a microprocessor or motor IC, which monitors all aspects of the driver to provide the correct sequencing.


Pulse Width Modulation (PWM) frequency converter (VFD) is one of the most commonly used controllers and has proven to be suitable for motors in the range of 0.5 HP to 500 HP. Most PWM VFDs have a rated voltage of 230V or 460V, three-phase operation, and an output frequency range of 0.5 to 120 Hz.


advantage


No motor cogging

Efficiency from 92% to 96%

Due to the fixed DC bus voltage, the input power factor is very high

Low initial cost

Can be used in applications requiring multiple motors


Disadvantage


Non-regenerative operation

High-frequency switching may cause motor heating and insulation breakdown.


Current source inverter (CSI)



Current source inverters (CSI) or AC-DC-AC synchronous inverters are usually used for AC synchronous motor drive and also for ship electric propulsion. Its converter and inverter rely on natural shutdown (line commutation) for the phase AC voltage across the thyristor 3-converter. Between the rectification and reversal phases, the current smoothing reactor coil forming the DC link.

The combination of the controllable rectifier and the DC bus is considered as the current source of the inverter, and its task is to directly block the current from entering the motor windings in sequence.

The magnitude of the direct current is set by the controlled switching of the rectifier thyristor. The input current remains constant but the input current is adjustable. The motor power supply frequency (and its speed) is set by the switching rate of the inverter. The six inverter thyristors provide 6 current pulses per cycle (called 6-pulse converters).


In layman's language, we can say that the current source (controllable rectifier stage) provides the required motor torque and the inverter stage controls the required speed. The output voltage of the inverter has nothing to do with the load. The size and nature of the load current depends on the nature of the load impedance.


Note:-In order to maintain the correct voltage and frequency (Volt/Hertz), the voltage must be adjusted through the correct SCR sequence.



Multiple runs of regenerative braking and CSI


When the motor speed is less than the synchronous speed, the machine works as a generator. Power flows from the machine to the DC link, and the DC link voltage is reversed. If a fully controlled converter is used as an inverter, the power of the DC link will be transferred to the AC power and regenerative braking will be performed. Therefore, the regenerative braking of the AC motor drive does not require additional equipment.


advantage


Renewable power capacity

Simple circuit

Reliability (current limiting operation)

Clean current waveform

high efficiency

Inherent short circuit protection


Disadvantage


Low speed motor cogging (shaft punch/motion)

The power factor decreases as the speed decreases

Requires isolation transformer on the input side

Due to internal power components, the physical size of the drive is larger

High-power harmonic generation is sent back to the power supply

Depends on motor load



Variable Voltage Inverter (VVI)




The basic variable voltage inverter uses a set of SCRs with a chopper circuit to convert the input AC voltage to DC and generate a six-step waveform. These SCRs provide a way to control the rectified DC voltage from 0 to about 620V DC. The L choke and C capacitor of the DC link section regulate the converted DC voltage. The inverter part includes six switching devices, such as thyristors, bipolar transistors, MOSFETs and IGBTs. The control logic uses a microprocessor to turn transistors on and off, thereby providing variable voltage and frequency to the motor.


This type of switching is often referred to as six steps, because six 60° steps are required to complete a 360° cycle. Although the motor prefers a smooth sine wave, the six-step output can be used satisfactorily.

The main disadvantage is the torque ripple that occurs every time a switching device such as a bipolar transistor is switched. The


When the speed of the motor changes, the pulsation will be obvious at low speeds. These speed changes are sometimes referred to as cogging. The non-sinusoidal current waveform causes additional heating in the motor, requiring motor derating.



advantage


Simple circuit

Can be used in applications requiring multiple motors

Not dependent on load


Disadvantage


High-power harmonic generation power supply

Motor cogging when PWM output is lower than 6 Hz

Non-regenerative operation

Low power factor