Variable frequency drives
Exactly what is an AC drive?
The word "drive" is used loosely in the industry. It seems that people involved primarily in the world of gear boxes and pulleys refer to any collection of mechanical and electro-mechanical components, which when connected together will move a load, as a "drive". When speaking to these people, an AC
variable frequency drive, VFD may be considered by them as the variable frequency inverter and motor combination. It may even include the motor's pulley - I am not sure.
People in the electrical field and electrical suppliers usually refer to a variable frequency inverter unit alone, or an SCR power module alone (when discussing DC drives) as the "drive" and the motor as the "motor".
Manufacturers of
variable frequency drives (
VFD’s) used to refer to the drive as just that, a "variable frequency drive". More manufacturers are referring to their drive as an "adjustable speed AC drive". To make matters worse when a motor is included in the package it may be referred to as an "adjustable speed AC drive system".
A
variable frequency drive,
VFD is an adjustable speed drive. Adjustable speed drives include all types; mechanical and electrical. Now is it clear? Don't worry about it. It's not clear to anyone. As you read on, when I refer to the "drive" I am referring to the variable frequency inverter alone.
A little about AC drives
The main power components of an AC
variable frequency drive,
VFD have to be able to supply the required level of current and voltage in a form the motor can use. The controls have to be able to provide the user with necessary adjustments such as minimum and maximum speed settings, so that the drive can be adapted to the user's process. Spare parts have to be available and the repair manual has to be readable. It's nice if the drive can shut itself down when detecting either an internal or an external problem. It's also nice if the drive components are all packaged in a single enclosure to aid in installation but that's about it.
Ac drives
Fractional & Integral HPotors
Single & Three Phase
General and Specific Purpose
110 to 575V
Variable and Constant Torque
Variable and Constant Torque 110 to 460V
Dc motors
Permanent Magnet and Shunt Wound
Fractional & Integral HP
General and Specific Purpose
24 to 500V
Dc drives
Fractional & Integral HP
Single & Three Phase Inputs
Inputs to 460V AC, Outputs to 500V DC
Two & Four Quad Operations
VFD types
All variable frequency drive’s or VFD’s use their output devices (IGBTs, transistors, thyristors) only as switches, turning them only on or off. Attempting to use a linear device such as transistor in its linear mode would be impractical, since power dissipated in the output devices would be about as much as power delivered to the load.
Drives can be classified as:
In a constant voltage converter, the intermediate DC link voltage remains approximately constant during each output cycle. In constant current drives, a large inductor is placed between the input rectifier and the output bridge, so the current delivered is nearly constant. A cycloconverter has no input rectifier or DC link and instead connects each output terminal to the appropriate input phase.
The most common type of packaged variable frequency drive, VFD is the constant-voltage type, using pulse width modulation to control both the frequency and effective voltage applied to the motor load.
variable frequency drive system description VFD
variable frequency drive system
A variable frequency drive system generally consists of an AC motor, a controller and an operator interface.[6] [7]
variable frequency drive motor
The motor used in a variable frequency drive VFD system is usually a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred. Various types of synchronous motors offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice. Motors that are designed for fixed-speed mains voltage operation are often used, but certain enhancements to the standard motor designs offer higher reliability and better variable frequency drive performance.[8]
variable frequency drive controller
Variable frequency drive controllers are solid state electronic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit. The rectifier is usually a three-phase diode bridge, but controlled rectifier circuits are also used. Since incoming power is converted to DC, many units will accept single-phase as well as three-phase input power (acting as a phase converter as well as a speed controller); however the unit must be derated when using single phase input as only part of the rectifier bridge is carrying the connected load.[9]
PWM variable frequency drive Diagram
As new types of semiconductor switches have been introduced, these have promptly been applied to inverter circuits at all voltage and current ratings for which suitable devices are available. Introduced in the 1980s, the insulated-gate bipolar transistor (IGBT) became the device used in most VFD inverter circuits in the first decade of the 21st century.[10][11][12]
AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed in order to deliver the rated torque. For example, if a motor is designed to operate at 460 volts at 60 Hz, the applied voltage must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 V/Hz in this case). For optimum performance, some further voltage adjustment may be necessary, but nominally constant volts per hertz is the general rule. This ratio can be changed in order to change the torque delivered by the motor.[13] page 3.
In addition to this simple volts per hertz control more advanced control methods such as vector control and direct torque control (DTC) exist. These methods adjust the motor voltage in such a way that the magnetic flux and mechanical torque of the motor can be precisely controlled.
The usual method used to achieve variable motor voltage is pulse-width modulation (PWM). With PWM voltage control, the inverter switches are used to construct a quasi-sinusoidal output waveform by a series of narrow voltage pulses with sinusoidally varying pulse durations.[10][1] pp82-85.
Operation at above synchronous speed is possible, but is limited to conditions that do not require more power than nameplate rating of the motor. This is sometimes called "field weakening" and, for AC motors, is operating at less than rated volts/hertz and above synchronous speed. Example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole) motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power.[14]
PWM variable frequency drive Output Voltage Waveform
PWM AC variable speed drive
An embedded microprocessor governs the overall operation of the variable frequency drive, VFD controller. The main microprocessor programming is in firmware that is inaccessible to the variable frequency drive user. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the variable frequency drive controller to suit specific motor and driven equipment requirements.[10]
At 460 Volts, the maximum recommended cable distances between variable frequency drive’s, VFD’s and motors can vary by a factor of 2.5:1. The longer cables distances are allowed at the lower Carrier Switching Frequencies (CSF) of 2.5 kHz. The lower CSF can produce audible noise at the motors. The 2.5 kHz and 5 kHz CSFs cause less motor bearing problems than caused by CSFs at 20 kHz.[15] Shorter cables are recommended at the higher CSF of 20 kHz. The minimum CSF for synchronize tracking of multiple conveyors is 8 kHz.
variable frequency drive operator interface
The operator interface, also commonly known as an HMI(Human Machine Interface), provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the variable frequency drive VFD , controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the variable frequency drive controller. Most are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control signals. A serial communications port is also often available to allow the variable frequency drive to be configured, adjusted, monitored and controlled using a computer.[10][16][17]
VFD Operation
When a motor is simply switched to the grid at full voltage, it initially draws at least 300% of its rated current from the mains. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.
On the contrary, when a variable frequency drive starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Thus starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. After the start of the variable frequency drive, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while drawing less than 50% of its rated current from the mains in the low speed range. A variable frequency drive can be adjusted to produce a steady 150% starting torque from standstill right up to full speed.[18]
Thus by using a variable frequency drive it is possible to start and run a motor in a weak grid without causing excessive voltage dips and flickering of the lights. In addition to that big energy savings are often possible when the loads (for example pumps and fans) can be run at a lower speed when the maximum output is not required.
With a variable frequency drive, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit to dissipate the braking energy or return it to the power source.
Applications considerations
The output voltage of a PWM variable frequency drive consists of a train of pulses switched at the carrier frequency. Because of the rapid rise time of these pulses, transmission line effects of the cable between the drive and motor must be considered. Since the transmission-line impedance of the cable and motor are different, pulses tend to reflect back from the motor terminals into the cable. If the cable is long enough, the resulting voltages can produce up to twice the rated line voltage, putting high stress on the cable and eventual insulation failure. Because of the standard ratings of cables, this phenomenon is of little concern for 230 volt motors, may be a consideration for long runs and 480 volt motors, and frequently a concern for 600 v motors.
Available variable frequency drive power ratings
Variable frequency drives are available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. variable frequency drive controllers designed to operate at 110 volts to 690 volts are often classified as low voltage units. Low voltage units are typically designed for use with motors rated to deliver 0.2 kW or 1/4 horsepower (hp) up to at least 750 kW or 1000 hp. Medium voltage VFD controllers are designed to operate at 2400/4160 volts (60 Hz), 3000 volts (50 Hz) or up to 10 kV. In some applications a step up transformer is placed between a low voltage drive and a medium voltage load. Medium voltage units are typically designed for use with motors rated to deliver 375 kW or 500 hp and above. Medium voltage drives rated above 7 kV and 5000 or 10000 hp should probably be considered to be one-of-a-kind (one-off) designs.[19][16]
Brushless DC motor drives
Much of the same logic contained in large, powerful variable frequency drive is also embedded in small brushless DC motors such as those commonly used in computer fans. In this case, the chopper usually converts a low DC voltage (such as 12 volts) to the three-phase current used to drive the electromagnets that turn the permanent magnet rotor.
See also
AC Drives
" Adjustable speed drive (ASD) is one of the most general terms applied to equipment used to control the speed of machinery. Adjustable speed drives are also known as variable speed drives (VSD).
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Industrial machinery is often driven by electric motors that have provisions for speed adjustment. Such motors are simply larger, more powerful versions of those driving familiar appliances such as food blenders or electric drills. These motors normally operate at a fixed speed. If speed control is required that controller is called an adjustable speed drive." Click here to learn more.
This article is licensed under the GNU Free Documentation License. It uses material from the Electric Motor
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A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.[1][2] [3] A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, microdrives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.
Variable-frequency drives are widely used. For example, in ventilations systems for large buildings, variable-frequency motors on fans save energy by allowing the volume of air moved to match the system demand. Variable frequency drives are also used on pumps, conveyor and machine tool drives.
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AC Motors
"An electric motor converts electrical energy into kinetic energy. The reverse task, that of converting kinetic energy into electrical energy, is accomplished by a generator or dynamo. In many cases the two devices differ only in their application and minor construction details, and some applications use a single device to fill both roles. For example, traction motors used on locomotives often perform both tasks if the locomotive is equipped with dynamic brakes." For more information, please click here.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia Article: "ac motor".
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A Programmable Logic Controller, PLC, or Programmable Controller is a microprocessor used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-purpose computers, the PLC is designed for extended temperature ranges, dirty or dusty conditions, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are stored in battery-backed or read-only memory." Click here to learn more.
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"A linear motor is essentially an electric motor that has had its stator "unrolled" so that instead of producing a torque (rotation), it produces a linear force along its length. The most common mode of operation is as a Lorenz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field (F = i x B)." Click here to learn more.
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DC Drives/Control
"Generally, the rotational speed of a DC motor is proportional to the voltage applied to it, and the torque is proportional to the current. Speed control can be achieved by variable battery tappings, variable supply voltage, resistors or electronic controls. The direction of a wound field DC motor can be changed by reversing either the field or armature connections but not both. This is commonly done with a special set of contactors (direction contactors)." Click here to learn more.
DC Motors
"The classic DC motor has a rotating armature in the form of an electromagnet.
