variable frequency drivers ( VFD , also termed frequency converter , "variable-voltage/variable-frequency (VVVF) drives", the variable speeds, the AC drives, the micro drives or inverter drives are a type of adjustable-speed drives used in electro- mechanical to control the speed of AC motor and torque by varying the frequency and input voltage of the motor.
VFD is used in applications ranging from small appliances to large compressors. About 25% of the world's electrical energy is consumed by electric motors in industrial applications, which can be more efficient when using VFD in centrifugal load services; however, the global VFD market penetration for all applications is relatively small.
Over the past four decades, power electronics technology has reduced the cost and size of MCC and has improved performance through advances in semiconductor switching devices, driving topologies, simulation and control techniques, as well as hardware and software controls.
VFDs are made in a number of different low and medium voltage AC-AC and DC-AC topologies.
Video Variable-frequency drive
System description and operation
Variable-frequency drives are devices used in drive systems consisting of the following three sub-systems: AC motors, main drive control units, and drive/carrier interfaces.
AC Motor
The AC power motor used in the VFD system is usually a three phase induction motor. Several types of single phase motors or synchronous motors which are advantageous in some situations may be used, but a three phase induction motor is generally preferred as the most economical motor choice. Motors designed for fixed speed operation are often used. The high voltage voltage imposed on the induction motor supplied by the VFD requires that the motor be designed for a specific input feed task in accordance with requirements such as Section 31 of the NEMA Standard MG-1.
Controller
The VFD controller is a solid state power conversion system consisting of three different sub-systems: bridge rectifier, direct current (DC), and inverter converter. Voltage-source inverter (VSI) drive (see sub chapter 'generic topology' below) is the most common drive type. Most of the drives are AC-AC drives which they change the AC channel input to the AC inverter output. However, in some applications such as a DC bus or a common solar application, the drive is configured as a DC-AC drive. The most basic rectifier converter for VSI drives is configured as a three-phase, six-pulse, full-wave diode bridge. In a VSI drive, a DC link consists of a capacitor that smooths the DC output ripple of the converter and provides a rigid input to the inverter. This filtered DC voltage is converted to a quasi-sinusoidal AC output voltage using an inverter active switching element. VSI drives provide higher power factor and lower harmonic distortion than controlled current source inverters (CSI) and alternating current inverters (LCI) (see sub-generic topology section below). The drive controller can also be configured as a phase converter that has a single phase converter input and a three-phase inverter output.
Progress controllers have exploited a dramatic increase in voltage ratings and current and switching frequencies of solid-state power devices over the past six decades. Introduced in 1983, an isolated bipolar-gate transistor (IGBT) has in the past two decades come to dominate VFD as an inverter switching device.
In a suitable torque-variable application for the Volts-per-Hertz (V/Hz) drive control, the AC motor characteristics require that the inverter output voltage to the motor be adjusted to match the required load torque in a linear V/Hz relationship.. For example, for a 460 V motor, 60 Hz, this linear V/Hz relationship is 460/60 = 7.67 V/Hz. While suitable in wide applications, V/Hz control is less than optimal in high performance applications involving low or demanding speeds, dynamic speed settings, positioning, and reverse loading requirements. Some V/Hz control drives can also operate in V/Hz square mode or can even be programmed to fit a special multi-point V/Hz path.
Two other drive control platforms, vector control and direct torque control (DTC), adjust the magnitude of the motor voltage, the angle of the reference, and the frequency so as to properly control the magnetic flux and mechanical torque.
Although wide-space vector modulation (SVPWM) is becoming increasingly popular, the sinusoidal PWM (SPWM) is the simplest method used to vary the motor voltages (or currents) of drives and frequencies. With the SPWM control (see Figure 1), a quasi-sinusoidal, variable-width-pulse output is constructed from the junction of a sawtooth signal signal with a variable modulating sinusoidal signal in the operating frequency as well as the voltage (or current).
Operation of the motor above the measured name plate speed (basic speed) is possible, but limited to conditions that do not require more power than the motor name plate rating. This is sometimes called "field attenuation" and, for AC motors, means operating at a rated speed of V/Hz and above a measured lower speed. The permanent magnet synchronous motor has a very limited range of field attenuation speed due to the constant magnetic flux relationship. The rotor-synchronous sync motors and induction motors have a much wider range of speeds. For example, induction motor 100HP, 460Ã,V, 60Ã, Hz, 1775Ã, RPM (4-pole) supplied with 460Ã,V, 75Ã, Hz (6,134Ã,V/Hz), will be limited to 60/75 = 80% torque at 125% speed (2218.75Ã, RPM) = 100% power. At higher speeds, induction motor torque should be further restricted due to the decreasing torque that separates the motor. Thus, the rated power can typically be produced only up to 130-150% of the measured name plate speed. The rotor-synchronous motor can run at higher speeds. In rolling mill drives, often 200-300% of the basic speed is used. The mechanical strength of the rotor limits the maximum speed of the motor.
An embedded microprocessor governs the overall operation of the VFD controller. The basic programming of the microprocessor is provided as firmware which is not accessible to the user. User programming display parameters, variables, and function blocks are provided to control, protect, and monitor VFD, motors, and driven equipment.
The basic drive controller can be configured to selectively include optional power components and accessories like the following:
- Upstream converter connections - circuit breakers or fuses, isolation contactors, EMC filters, channel reactors, passive filters
- Connected to DC link - chopper braking, resistor braking
- The downstream section is connected to an inverter output reactor, a sine wave filter, a dV/dt filter.
Operator interface
The operator interface provides a means for the operator to start and stop the motor and adjust the speed of operation. Additional operator control functions may include flipping, and switching between manual speed adjustment and automatic control of external process control signals. The operator interface often includes an alphanumeric display or an indication lamp and meter to provide information about the drive operation. The interface keypad and operator display unit are often provided on the front of the VFD controller as shown in the photo above. The keypad display can often be connected to cables and installed within close proximity of the VFD controller. Most are also equipped with input and output (I/O) terminals to connect push buttons, switches, and other carrier interface devices or control signals. Serial communication ports are also often available to allow the MCC to be configured, customized, monitored and controlled using a computer.
Drive operation
Referring to the accompanying chart, the driver application can be categorized as single-quadrant, two-quadrant, or four-quadrant; four quarters of the graph are defined as follows:
- Quadrant I - Driving or driving, forward speeding up the quadrant with speed and positive torque
- Quadrant II - Produce or brake, forward the acceleration quadrant with positive speed and negative torque
- Quadrant III - Driving or driving, rewind quadrant with speed and negative torque
- Quadrant IV - Generates or brakes, rewinds quadrant with negative velocity and positive torque.
Most applications involve a one-quadrant load operating in quadrant I, such as variable torque (eg centrifugal pump or fan) and a certain constant torque load (eg extruder).
Certain applications involve two-quadrant loads operating in quadrants I and II where the speed is positive but torque changes the polarity as in case the fan slows down faster than the natural mechanical losses. Some sources define two-quadrant drives as loads operating in quadrants I and III where speed and torque are the same (positive or negative) polarity in both directions.
Some high-performance applications involve a four-quadrant load (Quadrants I through IV) where speed and torque can be in any direction such as hoists, elevators, and hilly conveyors. Regeneration can only occur in the DC drive link bus when the inverter voltage is smaller in magnitude than the back-EMF motor and the inverter and EMF-rear voltage are the same polarity.
In starting the motor, the VFD initially applies the frequency and low voltage, thus avoiding the high inrush associated with the direct-on-line starting. After the start of the VFD, the applied frequency and voltage increase at a controlled or enhanced level to accelerate the load. This initial method usually allows the motor to develop 150% of its rated torque while the VFD draws less than 50% of its rated current from the parent in a low speed range. VFD can be set to produce a stable initial torque of 150% from rest to full speed. However, motor cooling deteriorates and may cause excessive heat when speed decreases so that prolonged low-speed operation with significant torque is usually not possible without separate fan ventilation.
With VFD, the cessation order is the reverse as the initial sequence. The frequency and voltage applied to the motor are lowered down at the controlled rate. When the frequency is close to zero, the motor is turned off. A small amount of torque braking is available to help reduce the load slightly faster than it will stop if the motor is only switched off and allowed to shore. Additional braking torque can be obtained by adding braking circuits (resistors controlled by transistors) to remove braking energy. With a rectifier of four quadrants (active front-end), VFD can brake the load by applying a backward torque and injecting energy back into the AC line.
Maps Variable-frequency drive
Benefits
Energy savings
Many applications of fixed speed motor loads supplied directly from AC line power can save energy when operated at variable speeds through VFD. Such energy cost savings are typically pronounced in centrifugal fans and variable centrifugal pump applications, where load and power torque vary with square and cube, respectively, of speed. This change provides substantial power reduction compared to fixed speed operations for relatively small speed reductions. For example, at a speed of 63% motor load consumes only 25% of its full power velocity. This reduction corresponds to the affinity law that determines the relationship between the various variables of centrifugal loads.
In the United States, an estimated 60-65% of electrical energy is used to supply the motor, 75% of which are variable torque fans, pumps, and compressor loads. Eighteen percent of the energy used in 40 million motors in the US could be saved by energy-efficient energy technologies such as VFD.
Only about 3% of the total installed base AC motor is provided with AC drive. However, it is estimated that propulsion technology is adopted in as much as 30-40% of all newly installed motors.
Solving the energy consumption of the global population of AC motor installations is as shown in the following table:
Performance control
AC drives are used to bring process and quality improvements in acceleration, flow, monitoring, pressure, speed, temperature, tension, and torque of industrial and commercial applications.
The fixed load speed is subject to the motor to a high initial torque and to a current surge of up to eight times the full load current. AC drives instead gradually increase the motor up to speed of operation to reduce mechanical and electrical pressure, reduce maintenance and repair costs, and extend the life of motors and driven equipment.
Variable speed drives can also run motors in a special pattern to minimize further mechanical and electrical stresses. For example, the S-curve pattern can be applied to the conveyor application for smoother retardation and acceleration control, which reduces the reaction that can occur when the conveyor is accelerated or slows down.
Performance factors that tend to support the use of DC drive through AC drive include requirements such as continuous operation at low speed, four-quadrant operation with regeneration, frequent acceleration and deceleration, and the need for motors to be protected for hazardous areas. The following table compares AC and DC drives based on certain key parameters:
^ High frequency injection
Types and ratings of VFD
Generic topology
AC drives can be classified according to the following common topologies: In the VSI drive, the DC output of the bridge diode-converter stores energy in the capacitor bus to supply the rigid input voltage to the inverter. Most drives are VSI type with PWM output voltage. In the CSI drive, the DC output of the SCR bridge converter stores energy in the reactor-series connection to supply rigid current inputs to the inverter. CSI drives can be operated with PWM output or six-step waves.
Platform control
Most drives use one or more of the following control platforms:
- Scalar control PWM V/Hz
- PWM field oriented control (FOC) or vector control
- Direct torque control (DTC).
Characteristics of torque and power load
Variable-frequency drives are also categorized by the following load torque and power characteristics:
- Variable torques, such as centrifugal fans, pumps, and blower applications
- Constant torque, as in conveyor and positive displacement pump applications
- Constant power, as in machine tool applications and traction.
Power rating available
VFDs are available with voltage and current ratings that include a wide range of single phase and multi-phase AC motors. Low-voltage (LV) drives are designed to operate at an output voltage equal to or less than 690V. While LV-motor drive applications are available in rank up to 5 or 6 MW, economic considerations typically support medium voltage (MV) drives with a much lower power rating. Different MV drive topologies (see Table 2) are configured according to the voltage/current-combination ratings used in different driving controller control devices so that any given voltage rating is greater than or equal to one to the following standard nom motor voltage ratings: either 2.3/4.16 kV (60 Hz) or 3.3/6.6 kV (50 Hz), with one thyristor manufacturer rated up to 12 kV switching. In some applications a step-up transformer is placed between the LV drive and the MV motor load. MV drives are typically rated for larger motor applications from between 375 kW (500 HP) and 750 kW (1000 hp). MV Drive has historically required more application design effort than is necessary for LV drive applications. The rated MV drive power can reach 100 MW, the various hard disk topologies involved for various ratings, performance, power quality, and reliability requirements.
Drive by machine and detailed topology
Lastly useful for connecting VFD in terms of the following two classifications:
- In the case of various AC machines as shown in Table 1 below
- In the case of various AC-AC converter topologies shown in Tables 2 and 3 below.
Application Considerations
Harmonization of AC channels
Clarification note:.
While the harmonic in the PWM output can be easily filtered by the filter-related inductance of the carrier frequency to supply the near-sinusoidal current to the motor load, the VFD diode-bridge rectifier converts the AC line voltage to the output DC voltage by imposing a super-linear half-phase current pulse thereby creating harmonic current distortion, and hence voltage distortion, from the AC channel input. When the MCC burden is relatively small compared to the large and rigid power systems available from the power company, the harmonic distortion effects of VFDs from the AC network can often be within acceptable limits. Furthermore, in low voltage networks, harmonics caused by single-phase equipment such as computers and TVs are partially canceled by the three phase harmonic bridge diode due to the 5th and 7th harmonics they are on the counterphase. However, when the proportion of VFD and other non-linear loads compared with the total load or non-linear load compared with the stiffness in the AC power supply, or both, is large enough, the load may adversely affect the AC power waveform available to other power company customers in same network.
When the company's electric voltage becomes distorted due to harmonics, losses in other loads such as AC motors keep normal speeds up. This condition can lead to surgery life that is too hot and shorter. Also, substation transformers and compensation capacitors are negatively affected. In particular, the capacitor may cause a resonance condition that can increase the harmonic level. To limit the voltage distortion, the VFD load owner may be required to install the filtering equipment to reduce harmonic distortion below acceptable limits. Alternatively, the utility can adopt the solution by installing its own filtering equipment in substations affected by the vast number of VFD equipment used. In high-power installations, harmonic distortion can be reduced by supplying multi-pulse VFD-bridge rectifiers from the transformer with some phase-shifted windings.
It is also possible to replace a standard diode-bridge rectifier with a bi-directional IGBT switching bridge that reflects a standard inverter using IGBT switching device output to the motor. Such rectifiers are referred to by various designations including active infeed converters (AICs), active rectifiers, IGBT supply units (ISUs), active front end (AFE), or four quadrant operations. With PWM control and the corresponding input reactor, the ACE current line AC currents can be almost sinusoidal. AFE inherently regenerates energy in a four-quadrant mode from the DC side to the AC grid. So, no braking resistor is required, and drive efficiency is enhanced if the drive is often required to brake the motor.
Two other harmonic mitigation techniques exploit the use of passive or active filters connected to public buses with at least one VFD branch load on the bus. The passive filter involves the design of one or more low-pass filter LC filters, each trap tuned as needed to harmonic frequencies (5, 7, 11, 13,... kq/- 1, where k = integers, q = number of pulse converter).
It is a very common practice for power companies or their customers to apply harmonic distortion limits based on IEC or IEEE standards. For example, IEEE Standard 519 limits on customer connection point calls for maximum individual harmonic frequency voltage to no more than 3% of the fundamentals and calls for total distortion of harmonic voltage (THD) to no more than 5% for common AC power supply systems.
Divert frequency
Redirect foldback frequency
One drive uses a default switching frequency setting of 4 kHz. Reducing the frequency of drive switching (carrier frequency) reduces the heat generated by IGBT.
The carrier frequency is at least ten times the desired output frequency used to establish the PWM activation interval. Carrier frequency in the range of 2,000 to 16,000 Hz is common for LV [low voltage, below 600 Volt AC] VFD. Higher carrier frequency results in better estimation of sine wave but resulted in higher switching losses in IGBT, decreasing overall power conversion efficiency.
Sound alignment
Some drives have a noise smoothing feature that can be enabled to introduce random variations to the switching frequency. It distributes the acoustic sound through various frequencies to decrease the peak noise intensity.
Long-lead effect
The output voltage of the carrier pulse frequency of the PWM VFD causes a rapid rise time on this pulse, the transmission line effect to be considered. Due to the impedance of the cable transmission line and the motor is different, the pulse tends to reflect back from the motor terminal to the cable. The resulting voltage can produce a voltage equal to twice the DC bus voltage or up to 3.1 times the rated line voltage for the long-running cable, placing the high voltage on the cable and the motor windings, and finally the failure of the insulation. Standard isolation for a three phase motor is 230 V or less adequately shielded against such longer voltage. On a 460Ã ©, 465 V or inverter system with a third generation 0.1 microsecond-rising IGBT, the maximum recommended cable distance between the MCC and the motor is approximately 50 m or 150 feet. Solutions for more stresses caused by long lead lengths include minimizing cable spacing, decreasing carrier frequency, installing dV/dt filters, using rated-inverter-duty motor (rated 600 V to hold the pulse train with time rises less than or equal to 0, 1 Ã, microsecond, from a height of 1.600Ã, V), and install a low-pass filter of LCR sine filter. Regarding the decrease in carrier frequency, note that audible noise appears to increase for carrier frequencies less than about 6 kHz and is most visible at about 3 kHz. The optimal selection of PWM carrier frequencies for AC drives includes noise balancing, heat, motor insulation voltage, motor-mode induced voltage bearing damage, fine motor operation, and other factors. Furthermore, harmonic attenuation can be obtained by using low-pass filter of LCR or dV/dt filter.
Motorcycle bearing
Carrier frequencies above 5 kHz are likely to cause bearing damage unless protection measures have been performed.
PWM drives are inherently associated with common-frequency high-frequency voltages and currents that can cause problems with motor bearings. When this high-frequency voltage finds its way to earth through the bearing, transfer of metal or electrical displacement (EDM) transfer takes place between ball bearing and bearing racing. Over time, EDM-based sparks cause erosion in a bearing race that can be seen as a grooved pattern. In large motors, the capacitance of the winding provides a path for the high-frequency current passing through the end of the motor shaft, leading to a circular bearing current type. Poor grounding of motor stators can cause bearing current to the ground. Small motors with improperly grounded propulsion devices are susceptible to high-frequency bearing currents.
Prevention of current high frequency bearing damage uses three approaches: good cabling and grounding practices, current bearing disturbances, and filtering or attenuation of common mode flows eg through soft magnetic cores, called inductive dampers. Proper cabling and grounding can include the use of shielded symmetrical power geometry cables to supply motors, mounting shaft brushes, and conductive bearing lubricants. Bearing currents can be disrupted by the installation of isolated pads and specially designed electrostatic-shielded induction motors. High-frequency filtering and damping can be performed even though inserting soft magnetic cores for three phases gives high-frequency impedance to common mode or motor bearing currents. Another approach is to use instead of a standard 2 level inverter drive, using a 3-level inverter drive or a matrix converter.
Since high frequency current spikes fed by an electric current multiplier can interfere with other cabling in the facility, the flow-fed motor cable shall not only be a symmetrical shielded geometry design but must also be directed at least 50 cm from the signal cable.
Dynamic braking
The torque generated by the drive causes the induction motor to run at a synchronous speed minus the slip. If the load drives the motor faster than the synchronous speed, the motor acts as a generator, converting mechanical power back to electrical power. This power is returned to the DC drive element (capacitor or reactor). The DC-link or DC braking power switch controls the power dissipation as heat in a set of resistors. The cooling fan can be used to prevent the resistors from overheating.
Dynamic braking discards braking energy by converting it to heat. Instead, regenerative drives restore braking energy by injecting this energy into an AC line. The capital cost of the regenerative drive, however, is relatively high.
Regenerative drive
The regenerative air-conditioning drive has the capacity to recover the braking energy from loads moving faster than the motor speed determined (overhauling load ) and return it to the power system.
The Cycloconverter, Scherbius, matrix, CSI, and LCI drives inherently allow the return of energy from the load to the channel, while the voltage-source inverter requires an additional converter to restore energy to the supply.
Regeneration is useful in VFD only if the value of energy recovered is large compared to the extra cost of the regenerative system, and if the system requires braking and initiates periodically. Regenerative VFD is widely used where overhaul load speed control is required.
Some examples:
- Conveyor belt drive for manufacturing, which stops every few minutes. When stopped, the parts are assembled correctly; Once that is done, the belt moves.
- The crane, where the lifting motor stops and often retreats, and braking is required to slow the load down.
- Plug-in and hybrid electric vehicles of all types (see image and Hybrid Synergy Drive).
Historical system
Before solid-state devices become available, variable-frequency drives use rotary engines and General Electric Company acquired several patents for this in the early 20th century. One example is the US patent 949320 of 1910 which states: "Such generators find useful applications in supplying currents to induction motors for driving cars, locomotives, or other mechanisms that must be driven at variable speeds". Another is the British patent 7061 in 1911 by Brown, Boveri & amp; Cie.
See also
- The speed-adjustable drive
- AC/AC converter
- Frequency modifiers
- Variable speed air compressor
- Pump
Note
References
Source of the article : Wikipedia
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