What is VFD(veriable frequency drive) how it works?
Working principle and block diagram of VFD.full form how variable frequency drives save energy Power factor Applications
What is VFD and Working of VFD pdf
VFD full form is variable frequency drive also called Frequency converters, have undergone extremely rapid changes, largely as a result of the development of microprocessor and semi-conductor technologies and their reduction in prices. However, the basic principles of frequency converters remains the same. Other name of VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.Ac drives(VFD) can be divided into four main components:
- Rectifier
- Intermediate circuit
- Inverter
- Control circuit
Working principle and block diagram of VFD means Variable Frequency Drive .
Veriable frequency drive(VFD) mainly has rectifier,in termed circuit and inverter to convert back dc voltage in to ac.
1. The rectifier, which is connected to a single/three-phase AC mains supply and generates a pulsating DC voltage. There are two basic types of rectifiers – controlled and uncontrolled.
2. The intermediate circuit. There are three types:
3. The inverter which generates the frequency of the motor voltage. Alternatively, some inverters may also convert the constant DC voltage into a variable AC voltage.
4. The control circuit electronics, which transmit signals to – and receive signals from – the rectifier, the intermediate circuit and the inverter. The parts that are controlled in detail depends on the design of the individual frequency converter . What all frequency converters have in common is that the control circuit uses signals to switch the inverter semi-conductors on or off. Frequency converters are divided according to the switching pattern that controls the supply voltage to the motor.
The supply voltage is a three-phase AC voltage or a single-phase AC voltage with a fixed frequency (e.g 3 × 400 V/50 Hz or 1 × 240 V/50 Hz) and their characteristic values can be illustrated as: In the illustration the three phases are displaced in time, the phase voltage constantly changes direction, and the frequency indicates the number of periods per second. A frequency of 50 Hz means that there are 50 periods per second (50 × T), i.e. one period lasts for 20 milliseconds. The rectifier of a frequency converter consists either of diodes, thyristors or a combination of both. A rectifier consisting of diodes is uncontrolled and a rectifier consisting of thyristors is controlled. If both diodes and thyristors are used, the rectifier is semi-controlled.
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Diodes allow current to flow in one direction only: from the anode (A) to the cathode (K). It is not possible – as is the case with some other semi-conductors – to control the current strength. An AC voltage over a diode is converted to a pulsating DC voltage. If a three-phase AC voltage is supplied to an uncontrolled three-phase rectifier, the DC voltage will continue to pulsate.
above Fig shows an uncontrolled three-phase rectifier, consisting of two groups of diodes. One group consists of diodes D1, D3 and D5. The other group consists of diodes D2, D4 and D6. Each diode conducts 1/3 of the periodic time (120°). In both groups, the diodes conduct in sequence. Periods in which both groups control are displaced by 1/6 of the periodic time T (60°) in relation to each other. Diodes D1,3,5 conduct when the positive voltage is applied. If the voltage of phase L1 reaches the positive peak value, terminal A assumes the value of phase L1. Above the two other diodes are reverse voltages sized UL1-2 and UL1-2. This also applies to diode group D2,4,6. Here terminal B receives the negative phase voltage. If at a given time L3 reaches the negative threshold value, diode D6 conducts. The two other diodes are subject to reverse voltages of sizes UL3-1 and UL3-2. The output voltage of the uncontrolled rectifier is the difference value of the voltages of the two diode groups. The mean value of the pulsating DC voltage is 1.35 × mains voltage.
In controlled rectifiers, the diodes are replaced by thyristors. Like the diode, the thyristor only allows the current to flow from the anode (A) to the cathode (K). However, the difference between the two devices is that the thyristor has a third terminal “Gate” (G). This gate must be controlled by a signal before the thyristor conducts. When a current flows through the thyristor, the thyristor will conduct the current until it becomes zero. The current cannot be interrupted by a signal on the Gate. Thyristors are used in rectifiers as well as in inverters. The signal to the Gate is the control signal α of the thyristor, which is a time delay, stated in degrees. The degree value indicates the delay between the voltage zero crossing and the time when the thyristor is conducting .
The controlled rectifier is basically the same as an uncontrolled rectifier except that the thyristor is controlled by α and starts conducting from the point where a normal diode begins to conduct, up to a point 30° behind the voltage zero crossing. Regulating α allows variation of the value of the rectified voltage. The controlled rectifier supplies a DC voltage with a mean value of 1.35 × mains voltage × cos α Compared to the uncontrolled rectifier, the controlled rectifier causes major losses and disturbances in the supply mains, because the rectifier draws a higher reactive current if the thyristors conduct for a short time. However the advantage of controlled rectifiers is that energy can be fed back into the mains supply.
On current-source inverters the intermediate circuit consists of a large coil and is only combined with the controlled rectifier. The coil transforms the variable voltage from the rectifier into a variable direct current. The load determines the size of the motor voltage.
On current-source inverters the intermediate circuit consists of a large coil and is only combined with the controlled rectifier. The coil transforms the variable voltage from the rectifier into a variable direct current. The load determines the size of the motor voltage.
On voltage-source inverters the intermediate circuit consists of a filter containing capacitor and can be combined with both types of rectifier. The filter smoothes the pulsating DC voltage (UZ1 ) of the rectifier.
In a controlled rectifier, the voltage is constant at a given frequency, and supplied to the inverter as pure DC voltage (UZ2) with variable amplitude.
In uncontrolled rectifiers, the voltage at the input of the inverter is a DC voltage with constant amplitude.
The intermediate circuit can also provide a number of additional functions depending on its design, such as:
• decoupling of rectifier from inverter
• reduction of harmonics
• energy storage to contain intermittent load surge
Inverter:
The frequency converter guarantees good operating conditions throughout the whole control range by adapting the output voltage to the load conditions. It is thus possible to maintain the magnetization of the motor at the optimal value.
From the intermediate circuit, the inverter either receives • a variable direct current, • a variable DC voltage, or • a constant DC voltage.
In every case, the inverter ensures that the supply to the motor becomes a variable quantity. In other words, the frequency of the motor voltage is always generated in the inverter. If the current or voltage is variable, the inverter only generates the frequency. If the voltage is constant, the inverter generates the motor frequency as well as the voltage.
Even if inverters work in different ways, their basic structure is always the same. The main components are controlled semi-conductors, placed in pairs in three branches.
The thyristors have now largely been replaced by high frequency transistors which can be switched on and off very rapidly. Although this depends on the semi conductor used, it is typically between 300 Hz to 20 kHz.
The semi-conductors in the inverter are turned on and off by signals generated by the control circuit. Signals can be controlled in a number of different ways.
In traditional inverters, dealing mainly with variable voltage intermediate current the inverter consists of six diodes, six thyristors and six capacitors.
The capacitors enable the thyristors to switch on and off, so that the current is displaced 120 degrees in the phase windings and must be adapted to the motor size. An intermittent rotational field with the required frequency is produced when the motor terminals are periodically supplied with current in turns U-V, V-W, W-U, U-V..... Even if this makes the motor current almost square, the motor voltage is almost sinusoidal. However, there are always voltage peaks when the current is switched on or off.
The diodes separate the capacitors from the load current of the motor.
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2. The intermediate circuit. There are three types:
- one, which converts the rectifier voltage into a direct current.
- one, which stabilises or smoothes the pulsating DC voltage and places it at the disposal of the inverter.
- one, which converts the constant DC voltage of the rectifier to a variable AC voltage.
3. The inverter which generates the frequency of the motor voltage. Alternatively, some inverters may also convert the constant DC voltage into a variable AC voltage.
4. The control circuit electronics, which transmit signals to – and receive signals from – the rectifier, the intermediate circuit and the inverter. The parts that are controlled in detail depends on the design of the individual frequency converter . What all frequency converters have in common is that the control circuit uses signals to switch the inverter semi-conductors on or off. Frequency converters are divided according to the switching pattern that controls the supply voltage to the motor.
Rectifier:
what is rectifier? working of rectifier :
The supply voltage is a three-phase AC voltage or a single-phase AC voltage with a fixed frequency (e.g 3 × 400 V/50 Hz or 1 × 240 V/50 Hz) and their characteristic values can be illustrated as: In the illustration the three phases are displaced in time, the phase voltage constantly changes direction, and the frequency indicates the number of periods per second. A frequency of 50 Hz means that there are 50 periods per second (50 × T), i.e. one period lasts for 20 milliseconds. The rectifier of a frequency converter consists either of diodes, thyristors or a combination of both. A rectifier consisting of diodes is uncontrolled and a rectifier consisting of thyristors is controlled. If both diodes and thyristors are used, the rectifier is semi-controlled.
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Uncontrolled rectifier:
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| diode symbol |
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| rectifier |
above Fig shows an uncontrolled three-phase rectifier, consisting of two groups of diodes. One group consists of diodes D1, D3 and D5. The other group consists of diodes D2, D4 and D6. Each diode conducts 1/3 of the periodic time (120°). In both groups, the diodes conduct in sequence. Periods in which both groups control are displaced by 1/6 of the periodic time T (60°) in relation to each other. Diodes D1,3,5 conduct when the positive voltage is applied. If the voltage of phase L1 reaches the positive peak value, terminal A assumes the value of phase L1. Above the two other diodes are reverse voltages sized UL1-2 and UL1-2. This also applies to diode group D2,4,6. Here terminal B receives the negative phase voltage. If at a given time L3 reaches the negative threshold value, diode D6 conducts. The two other diodes are subject to reverse voltages of sizes UL3-1 and UL3-2. The output voltage of the uncontrolled rectifier is the difference value of the voltages of the two diode groups. The mean value of the pulsating DC voltage is 1.35 × mains voltage.
Controlled rectifiers
In controlled rectifiers, the diodes are replaced by thyristors. Like the diode, the thyristor only allows the current to flow from the anode (A) to the cathode (K). However, the difference between the two devices is that the thyristor has a third terminal “Gate” (G). This gate must be controlled by a signal before the thyristor conducts. When a current flows through the thyristor, the thyristor will conduct the current until it becomes zero. The current cannot be interrupted by a signal on the Gate. Thyristors are used in rectifiers as well as in inverters. The signal to the Gate is the control signal α of the thyristor, which is a time delay, stated in degrees. The degree value indicates the delay between the voltage zero crossing and the time when the thyristor is conducting .
If symbol α is between 0° and 90°, the thyristor coupling is used as a rectifier, when it is between 90° and 300° the thyristor is used as an inverter.
The controlled rectifier is basically the same as an uncontrolled rectifier except that the thyristor is controlled by α and starts conducting from the point where a normal diode begins to conduct, up to a point 30° behind the voltage zero crossing. Regulating α allows variation of the value of the rectified voltage. The controlled rectifier supplies a DC voltage with a mean value of 1.35 × mains voltage × cos α Compared to the uncontrolled rectifier, the controlled rectifier causes major losses and disturbances in the supply mains, because the rectifier draws a higher reactive current if the thyristors conduct for a short time. However the advantage of controlled rectifiers is that energy can be fed back into the mains supply.
The intermediate circuit:
The intermediate circuit can be seen as a storage facility from which the motor is able to draw its energy via the inverter. It can be built according to three different principles depending on the rectifier and inverter
On current-source inverters the intermediate circuit consists of a large coil and is only combined with the controlled rectifier. The coil transforms the variable voltage from the rectifier into a variable direct current. The load determines the size of the motor voltage.
On current-source inverters the intermediate circuit consists of a large coil and is only combined with the controlled rectifier. The coil transforms the variable voltage from the rectifier into a variable direct current. The load determines the size of the motor voltage.
In a controlled rectifier, the voltage is constant at a given frequency, and supplied to the inverter as pure DC voltage (UZ2) with variable amplitude.
In uncontrolled rectifiers, the voltage at the input of the inverter is a DC voltage with constant amplitude.
The intermediate circuit can also provide a number of additional functions depending on its design, such as:
• decoupling of rectifier from inverter
• reduction of harmonics
• energy storage to contain intermittent load surge
Inverter:
Purpose and working of inverter:
The inverter is the last link in the frequency converter before the motor and the point where the final adaptation of the output voltage occurs.The frequency converter guarantees good operating conditions throughout the whole control range by adapting the output voltage to the load conditions. It is thus possible to maintain the magnetization of the motor at the optimal value.
From the intermediate circuit, the inverter either receives • a variable direct current, • a variable DC voltage, or • a constant DC voltage.
In every case, the inverter ensures that the supply to the motor becomes a variable quantity. In other words, the frequency of the motor voltage is always generated in the inverter. If the current or voltage is variable, the inverter only generates the frequency. If the voltage is constant, the inverter generates the motor frequency as well as the voltage.
Even if inverters work in different ways, their basic structure is always the same. The main components are controlled semi-conductors, placed in pairs in three branches.
The thyristors have now largely been replaced by high frequency transistors which can be switched on and off very rapidly. Although this depends on the semi conductor used, it is typically between 300 Hz to 20 kHz.
The semi-conductors in the inverter are turned on and off by signals generated by the control circuit. Signals can be controlled in a number of different ways.
In traditional inverters, dealing mainly with variable voltage intermediate current the inverter consists of six diodes, six thyristors and six capacitors.
The capacitors enable the thyristors to switch on and off, so that the current is displaced 120 degrees in the phase windings and must be adapted to the motor size. An intermittent rotational field with the required frequency is produced when the motor terminals are periodically supplied with current in turns U-V, V-W, W-U, U-V..... Even if this makes the motor current almost square, the motor voltage is almost sinusoidal. However, there are always voltage peaks when the current is switched on or off.
The diodes separate the capacitors from the load current of the motor.
Control circuit:
Control circuit of VFD:
The control circuit, or control card, is the fourth main component of the frequency converter and has four essential tasks:
- control of the frequency converter semi-conductors.
- data exchange between the frequency converter and peripherals.
- gathering and reporting fault messages.
- carrying out of protective functions for the frequency converter and motor.
Micro-processors have increased the speed of the control circuit, significantly increasing the number of applications suitable for drives and reducing the number of necessary calculations.With microprocessors the processor is integrated into the frequency converter and is always able to determine the optimum pulse pattern for each operating state.
These savings are derived from 2 components:
1.The Laws of Affinity, which shows an operating range that produces the most flow or pressure per horsepower.
2.The removal of any mechanical flow device that limits the flow of a fan or pump while the motor turns the application at a fixed speed.
Moreover, the AC drive installation continues to save energy for many years after the initial payback period.
Head (pressure) is proportional to the square of shaft speed
Power is proportional to the cube of shaft speed
Moreover, when the AC power has a high level of harmonic content called Power Factor Distortion, the displacement and distortion are multiplied by each other, which further decreases efficiency.
While power factor correction devices, such as capacitors and filters, exist on the market today there is an often overlooked method for correcting power factor displacement. An AC variable frequency drive, used with an AC motor will operate at about 0.95 power factor which is typically 5 to 10 percent better than an across-the-line motor. Furthermore, there is at least one Active Front End (AFE) AC drive available in the market today that has the ability to adjust its’ power factor from 0.8 leading to 0.8 lagging and which meets IEEE 519 harmonic standards with 4% or less harmonics. (A standard 6 pulse AC drive with a diode rectifier converts input AC voltage to DC bus voltage, with a typical harmonics level of 30 to 40 percent.)
This means the drive can improve the present power factor displacement in a facility.
generator that converts mechanical power into electricity. It all depends on whether the motor is turning a machine that requires power to turn the load or if the load will, at times, overhaul the motor. This overhauling condition may exist in several types of applications:
When a load is stopped quickly and the inertia of the load wants to keep turning such as a large drum. (In this case the cycle time, or how many times the load is stopped over time, as well as the magnitude of the stopping power required determines how much energy can be recovered)
When a load such as a decline conveyor operating under the influence of gravity will overhaul the motors speed and the drive is used to regulate the speed in a slower controlled fashion than what the natural physics of the application would produce. This would also be a general description for a hoist and crane application.
how variable frequency drives save energy
When discussing energy savings and variable frequency drives (VFD) the attention often focuses on a centrifugal fan or pump application. However, you should not overlook other applications which also have large potential energy savings and energy recovery. Applications involving regeneration, power factor correction, common bus applications or a combination of the three can also quickly achieve a significant reduction in energy use.
Centrifugal Applications
When a centrifugal fan or pump is used with mechanical flow control, converting the application to an adjustable speed AC drive can reduce energy costs by 10 to 60 percent if the fan or pump is designed to operate between 40 to 80 percent of full speed. This usually provides a return on investment which is in the 6 to 24 month time frame.These savings are derived from 2 components:
1.The Laws of Affinity, which shows an operating range that produces the most flow or pressure per horsepower.
2.The removal of any mechanical flow device that limits the flow of a fan or pump while the motor turns the application at a fixed speed.
Moreover, the AC drive installation continues to save energy for many years after the initial payback period.
The Laws of Affinity state:
Flow is proportional to shaft speedHead (pressure) is proportional to the square of shaft speed
Power is proportional to the cube of shaft speed
Power factor
AC power has two basic components: voltage and current. When these two components are not in sync (called Power Factor Displacement), AC power is wasted through inefficiency.Moreover, when the AC power has a high level of harmonic content called Power Factor Distortion, the displacement and distortion are multiplied by each other, which further decreases efficiency.
While power factor correction devices, such as capacitors and filters, exist on the market today there is an often overlooked method for correcting power factor displacement. An AC variable frequency drive, used with an AC motor will operate at about 0.95 power factor which is typically 5 to 10 percent better than an across-the-line motor. Furthermore, there is at least one Active Front End (AFE) AC drive available in the market today that has the ability to adjust its’ power factor from 0.8 leading to 0.8 lagging and which meets IEEE 519 harmonic standards with 4% or less harmonics. (A standard 6 pulse AC drive with a diode rectifier converts input AC voltage to DC bus voltage, with a typical harmonics level of 30 to 40 percent.)
This means the drive can improve the present power factor displacement in a facility.
Regeneration
An AC motor may act as either a motor that turns electrical power into mechanical power or as agenerator that converts mechanical power into electricity. It all depends on whether the motor is turning a machine that requires power to turn the load or if the load will, at times, overhaul the motor. This overhauling condition may exist in several types of applications:
1) Periodic Deceleration:
When a load is stopped quickly and the inertia of the load wants to keep turning such as a large drum. (In this case the cycle time, or how many times the load is stopped over time, as well as the magnitude of the stopping power required determines how much energy can be recovered)
2) Continuous Deceleration:
When a load such as a decline conveyor operating under the influence of gravity will overhaul the motors speed and the drive is used to regulate the speed in a slower controlled fashion than what the natural physics of the application would produce. This would also be a general description for a hoist and crane application.
3). System Tension/Holding Torque:
When two sections of a machine are used to create tension on the material between them such as on the metal strip in a strip mill. The two sections may be running at the same speed, but the process may require a certain amount of tension on the strip to run properly. This means the lead section will run in the forward direction and pull the strip and the following section will also run in the forward direction and at the same time provide the needed torque in the reverse direction of the strip thus creating the proper tension.
In each of these examples the motor and drive combination has the ability to “recover”the electrical power produced by the motor that is acting as a generator and sent by the drive to the utility company.
How much energy is saved depends on the application, but it can be significant. One such application
where significant savings can be recovered is a gearbox test stand. When the gearbox is tested, a single drive and motor combination is used to turn the gearbox while another drive and motor is used on the other end of the gearbox to simulate the load. Done correctly this application will operate using a very low amount of total energy because the amount of energy used to turn the gearbox is the same amount of energy that is recovered from the simulated load on the gearbox, minus the losses in the system.
In each of these examples the motor and drive combination has the ability to “recover”the electrical power produced by the motor that is acting as a generator and sent by the drive to the utility company.
How much energy is saved depends on the application, but it can be significant. One such application
where significant savings can be recovered is a gearbox test stand. When the gearbox is tested, a single drive and motor combination is used to turn the gearbox while another drive and motor is used on the other end of the gearbox to simulate the load. Done correctly this application will operate using a very low amount of total energy because the amount of energy used to turn the gearbox is the same amount of energy that is recovered from the simulated load on the gearbox, minus the losses in the system.
The one critical item to determine if the application is regenerative or not, is whether the load is trying to turn the motor (regenerative recovery) at any time or if the motor is being used to turn the load.
References :
Fact Worth Knowing About Frequency Converters Danfoss,
A white paper issued by Siemens. ©2010 Siemens Industry,Variable Frequency Drives and Energy Savings by: Stephen Prachy
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