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. 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.
Operating principle
The synchronous speed of an AC motor is determined by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:
where
n = Revolutions per minute (rpm)
f = AC power frequency (hertz)
p = Number of poles per phase (an even number)
The constant, 120, is 60 seconds per minute multiplied by 2 poles per pole pair. Sometimes 60 is used as the constant and p is stated as pole pairs rather than poles. By varying the frequency of the voltage applied to the motor, its speed can be changed.
For the number of poles, it is the number of poles per phase. For example a single phase motor with 4 poles will have a 60 Hz synchronous speed of 1800 RPM (see below). However, a three phase motor having 12 poles total will also have a synchronous speed of 1800 RPM, as there will be 4 poles per phase.
Synchronous motors operate at the synchronous speed determined by the above equation. The speed of an induction motor is slightly less than the synchronous speed..
Example
A 4-pole motor that is connected directly to 60 Hz utility (mains) power would have a synchronous speed of 1,800 rpm:
If the motor is an induction motor, the operating speed at full load will be about 1,750 rpm.
If the motor is connected to a speed controller that provides power at 50 Hz, the synchronous speed would be 1,500 rpm:
VFD types
All VFDs 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:
- Constant voltage
- Constant current
- Cycloconverter
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 VF drive is the constant-voltage type, using pulse width modulation to control both the frequency and effective voltage applied to the motor load.
VFD system description
A variable frequency drive system generally consists of an AC motor, a controller and an operator interface.
VFD motor
The motor used in a 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 VFD performance.
VFD 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.
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.
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 especially at low speeds, but constant volts per hertz is the general rule. This ratio can be changed in order to change the torque delivered by the motor. 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. pp82-85.
Operation of the motors above rated name plate speed (base 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, means operating at less than rated volts/hertz and above rated name plate speed. Permanent magnet synchronous motors have quite limited field weakening speed range due to the constant magnet flux linkage. Wound rotor synchronous motors and induction motors have much wider speed range. For example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole) induction 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. At higher speeds the induction motor torque has to be limited further due to the lowering of the breakaway torque of the motor. Thus rated power can be typically produced only up to 130...150 % of the rated name plate speed. Wound rotor synchronous motors can be run even higher speeds. In rolling mill drives often 200...300 % of the base speed is used. Naturally the mechanical strength of the rotor and lifetime of the bearings is also limiting the maximum speed of the motor. It is recommended to consult the motor manufacturer if more than 150 % speed is required by the application.
An embedded microprocessor governs the overall operation of the VFD controller. The main microprocessor programming is in firmware that is inaccessible to the VFD user. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the VFD controller to suit specific motor and driven equipment requirements.
VFD operator interface
The operator interface, also commonly known as an Human Machine Interface (HMI), 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 VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provide
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