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 drive 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
Variable frequency drives
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 drives
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 drive, 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.
Variable frequency drives
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 drives 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 drives 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
VFD system
A Variable frequency drives system generally consists of an AC motor, a controller and an operator interface.[6] [7]
VFD motor
The motor used in a Variable frequency drives 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 drives performance.[8]
VFD controller
Variable frequency drives 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]
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]
An embedded microprocessor governs the overall operation of the VFD controller. The main microprocessor programming is in firmware that is inaccessible to the Variable frequency drives user. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the Variable frequency drives controller to suit specific motor and driven equipment requirements
At 460 Volts, the maximum recommended cable distances between VFDs 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.Shorter cables are recommended at the higher CSF of 20 kHz. The minimum CSF for synchronize tracking of multiple conveyors is 8 kHz.
" 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).
Visit http://www.kbelectronics.com/ for an in-depth overview or Variable frequency drives
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.
Available VFD power ratings
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 drives 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 available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. VFD 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.
DC motor drives
Much of the same logic contained in large, powerful VFDs 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.
Variable frequency drives
Variable Speed Motor Drives (AC & DC)
K B electronics aims to be the most comprehensive online store for all of your industrial drives needs. We stock a wide variety of AC Drives and DC Drives from well known manufacturers like Hitachi, Polyspede and LGI in a range of configurations, combined with our excellent customer service, you will never need to look elsewhere.
We have made your choice easy by providing you with a number of ways to locate the most suitable drive for you, simply decide whether you need to search by Motor Type, Horsepower, Input Voltage, Output Current or Manufacturer and use the simple dropdown boxes to the left to find the right product for you.
K B electronics also stocks a wide variety of AC Motors and DC Motors. All products from K B electronics are backed up 12 months warranty and tech. support by a company that has been in drives business over 45 years
AC DRIVES
AC Variable Frequency Drives (VFD)
AutomationDirect carries AC variable speed drives for motor speed control and other industrial control applications.
- The high quality and low prices make our GS1 series mini AC drives and GS2 series micro variable speed drives an incredible deal.
- The DURApulse (GS3 series) sensorless vector drives can handle applications up to 100 horsepower.
- The Hitachi series of variable frequency drives (VFD) can handle applications up to 30 horsepower, with higher horsepower SJ300 series units (40/ 50/ 60 hp) available as a special order item (non-stock).
