Induction motorFrom Wikipedia, the free encyclopedia

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Three-phase induction motors

Animation of a squirrel-cage AC motor

An induction motor or asynchronous motor is a type of alternating current motorwhere power is

supplied to the rotor by means of electromagnetic induction.

An electric motor turns because of magnetic force exerted between a stationaryelectromagnet called

the stator and a rotating electromagnet called the rotor. Different types of electric motors are

distinguished by how electric current is supplied to the moving rotor. In a DC motor and a slip-ring AC

motor, current is provided to the rotor directly through sliding electrical

contacts calledcommutators and slip rings. In an induction motor, by contrast, the current is induced in

the rotor without contacts by the magnetic field of the stator, through electromagnetic induction. An

induction motor is sometimes called a rotating transformer because thestator (stationary part) is

essentially the primary side of the transformer and the rotor (rotating part) is the secondary side.

Unlike the normal transformer which changes the current by using time varying flux, induction motors

use rotating magnetic fields to transform the voltage. The current in the primary side creates

an electromagnetic fieldwhich interacts with the electromagnetic field of the secondary side to produce

a resultant torque, thereby transforming the electrical energy into mechanical energy. Induction motors

are widely used, especially polyphase induction motors, which are frequently used in industrial drives.

Induction motors are now the preferred choice for industrial motors due to their rugged

construction, absence of brushes (which are required in most DC motors) and—thanks to

modern power electronics—the ability to control the speed of the motor.

Contents

 [hide]

1   History

2   Principle of operation and comparison to synchronous motors

o 2.1   AC Induction Motor

o 2.2   Synchronous Motor

3   Construction

4   Speed control

5   Starting of induction motors

o 5.1   Single Phase

6   External links

[edit]History

The induction motor was first realized by Galileo Ferraris in 1885 in Italy. In 1888, Ferraris published

his research in a paper to the Royal Academy of Sciences in Turin (later, in the same year, Nikola

Tesla gained U.S. Patent 381,968) where he exposed the theoretical foundations for understanding

the way the motor operates. The induction motor with a cage was invented by Mikhail Dolivo-

Dobrovolskyabout a year later.

[edit]Principle of operation and comparison to synchronous motors

A 3-phase power supply provides a rotating magnetic field in an induction motor.

The basic difference between an induction motor and asynchronous AC motor is that in the latter a

current is supplied into the rotor (usually DC) which in turn creates a (circular uniform) magnetic field

around the rotor. The rotating magnetic field of the stator will impose an electromagnetic torque on the

still magnetic field of the rotor causing it to move (about a shaft) and rotation of the rotor is produced. It

is called synchronous because at steady state the speed of the rotor is the same as the speed of the

rotating magnetic field in the stator.

By way of contrast, the induction motor does not have any direct supply onto the rotor; instead, a

secondary current is induced in the rotor. To achieve this, stator windings are arranged around the

rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern

which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor

conductors. These currents interact with the rotating magnetic field created by the stator and in effect

causes a rotational motion on the rotor.

However, for these currents to be induced, the speed of the physical rotor must be less than the speed

of the rotating magnetic field in the stator or else the magnetic field will not be moving relative to the

rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically

slows slightly until a current is re-induced and then the rotor continues as before. This difference

between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is

unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor (the slip

speed) to the speed of the rotating stator field. Due to this, an induction motor is sometimes referred to

as an asynchronous machine.

[edit]AC Induction Motor

where

n = Revolutions per minute (rpm)

f = AC power frequency (hertz)

p = Number of poles per phase (an even number)

Slip is calculated using:

where s is the slip.

The rotor speed is:

[edit]Synchronous Motor

A synchronous motor always runs at synchronous speed with 0% slip. The speed of a synchronous

motor is determined by the following formula:

where v is the speed of the rotor (in rpm), f is the frequency of the AC supply (in Hz) and p is the

number of magnetic poles.

For example, a 6 pole motor operating on 60 Hz power would have a speed of:

Note on the use of p - some texts refer to number of pole pairs per phase instead of number of

polesper phase. For example a 6 pole motor, operating on 60Hz power, would have 3 pole pairs. The

equation of synchronous speed then becomes:

with P being the number of pole pairs. For P = 3 and  :

[edit]Construction

The stator consists of wound 'poles' that carry the supply current to induce a magnetic field that

penetrates the rotor. In a very simple motor, there would be a single projecting piece of the stator

(asalient pole) for each pole, with windings around it; in fact, to optimize the distribution of the

magnetic field, the windings are distributed in many slots located around the stator, but the magnetic

field still has the same number of north-south alternations. The number of 'poles' can vary between

motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.).

Induction motors are most commonly built to run on single-phase or three-phase power, but two-phase

motors also exist. In theory, two-phase and more than three phase induction motors are possible;

many single-phase motors having two windings and requiring a capacitor can actually be viewed as

two-phase motors, since the capacitor generates a second power phase 90 degrees from the single-

phase supply and feeds it to a separate motor winding. Single-phase power is more widely available in

residential buildings, but cannot produce a rotating field in the motor ( the field merely oscillates back

and forth), so single-phase induction motors must incorporate some kind of starting mechanism to

produce a rotating field. They would, using the simplified analogy of salient poles, have one salient

pole per pole number; a four-pole motor would have four salient poles. Three-phase motors have three

salient poles per pole number, so a four-pole motor would have twelve salient poles. This allows the

motor to produce a rotating field, allowing the motor to start with no extra equipment and run more

efficiently than a similar single-phase motor.

There are three types of rotor:

Squirrel-cage rotor

The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper (most

common) or aluminum that span the length of the rotor, and those solid copper or aluminium strips can

be shorted or connected by a ring or some times not, i.e. the rotor can be closed or semiclosed type.

The rotor bars in squirrel-cage induction motors are not straight, but have some skew to reduce noise

and harmonics.

Slip ring  rotor

A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to slip

rings. When these slip rings are shorted, the rotor behaves similarly to a squirrel-cage rotor; they can

also be connected to resistors to produce a high-resistance rotor circuit, which can be beneficial in

starting

Solid core rotor

A rotor can be made from a solid mild steel. The induced current causes the rotation.

[edit]Speed control

The synchronous rotational speed of the rotor (i.e. the theoretical unloaded speed with no slip) is

controlled by the number of pole pairs (number of windings in the stator) and by the frequency of the

supply voltage. Under load, the speed of the induction motor varies according to size of the load. As

the load is increased, the speed of the motor decreases, increasing the slip, which increases the field

strength of the rotor to bear the extra load. Before the development of economical semiconductor

power electronics, it was difficult to vary the frequency to the motor and induction motors were mainly

used in fixed speed applications. As an induction motor has no brushes and is easy to control, many

older DC motors are now being replaced with THR induction motors and accompanying inverters in

industrial applications.

[edit]Starting of induction motors

[edit]Single Phase

In a single phase induction motor, it is necessary to provide a starting circuit to start rotation of the

rotor. If this is not done, rotation may be commenced by manually giving a slight turn to the rotor. The

single phase induction motor may rotate in either direction and it is only the starting circuit which

determines rotational direction.

For small motors of a few watts, the start rotation is done by means of one or two single turn(s) of

heavy copper wire around one corner of the pole. The current induced in the single turn is out of phase

with the supply current and so causes an out-of-phase component in the magnetic field, which imparts

to the field sufficient rotational character to start the motor. Starting torque is very low and efficiency is

also reduced. Such shaded-pole motors are typically used in low-power applications with low or zero

starting torque requirements, such as desk fans and record players.

Larger motors are provided with a second stator winding which is fed with an out-of-phase current to

create a rotating magnetic field. The out-of-phase current may be derived by feeding the winding

through a capacitor or it may derive from the winding having different values of inductance and

resistance from the main winding.

In some designs, the second winding is disconnected once the motor is up to speed, usually either by

means of a switch operated by centrifugal force acting on weights on the motor shaft or by a positive

temperature coefficient thermistor which, after a few seconds of operation, heats up and increases its

resistance to a high value thereby reducing the current through the second winding to an insignificant

level. Other designs keep the second winding continuously energised during running, which improves

torque But we need to change the frequency as wel as voltage acordingly so that the v/f ratio should

be constant always.if we change only voltage or frequency only then the operating zone will be

increased with the increase of v or f.but the reason we always have to maintain the v/f ratio constant

because we need to maintain effeciency in good side.

Motor controllerFrom Wikipedia, the free encyclopedia

A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance

of an electric motor.[1] A motor controller might include a manual or automatic means for starting and stopping the motor,

selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting

against overloads andfaults.[2]

Contents

 [hide]

1   Applications

2   Types of motor controllers

o 2.1   Motor starters

o 2.2   Adjustable-speed drives

3   Motor control centers

4   Speed controls for AC induction motors

o 4.1   Variable frequency drives

o 4.2   Phase vector drives

o 4.3   Direct torque control drives

5   Brushed DC motor speed or torque controls

o 5.1   SCR or thyristor drive

o 5.2   PWM or chopper drives

6   Servo controllers

7   Stepper motor controllers

8   Relevant circuits to motor control

o 8.1   H-bridge

9   See also

10   References

11   External links

[edit]Applications

Every electric motor has to have some sort of controller. The motor controller will have differing features and complexity

depending on the task that the motor will be performing.

The simplest case is a switch to connect a motor to a power source, such as in small appliances or power tools. The

switch may be manually operated or may be a relay or contactor connected to some form of sensor to automatically start

and stop the motor. The switch may have several positions to select different connections of the motor. This may allow

reduced-voltage starting of the motor, reversing control or selection of multiple speeds. Overload and overcurrent

protection may be omitted in very small motor controllers, which rely on the supplying circuit to have overcurrent

protection. Small motors may have built-in overload devices to automatically open the circuit on overload. Larger motors

have a protective overload relay or temperature sensing relay included in the controller and fuses orcircuit breakers for

overcurrent protection. An automatic motor controller may also include limit switches or other devices to protect the driven

machinery.

More complex motor controllers may be used to accurately control the speed and torque of the connected motor (or

motors) and may be part of closed loop control systems for precise positioning of a driven machine. For example, a

numerically controlled lathe will accurately position the cutting tool according to a preprogrammed profile and compensate

for varying load conditions and perturbing forces to maintain tool position.

[edit]Types of motor controllers

Motor controllers can be manually, remotely or automatically operated. They may include only the means for starting and

stopping the motor or they may include other functions.[2][3][4]

An electric motor controller can be classified by the type of motor it is to drive such as permanent magnet, servo, series,

separately excited, and alternating current.

A motor controller is connected to a power source such as a battery pack or power supply, and control circuitry in the form

of analog or digital input signals.

[edit]Motor starters

Main article: Direct on line starter

Main article: Motor soft starter

A small motor can be started by simply plugging it into an electrical receptacle or by using a switch or circuit breaker. A

larger motor requires a specialized switching unit called a motor starter or motorcontactor. When energized, a direct on

line (DOL) starter immediately connects the motor terminals directly to the power supply. A motor soft starter connects the

motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps. [2][3][4]

[edit]Adjustable-speed drives

Main article: Adjustable-speed drive

An adjustable-speed drive (ASD) or variable-speed drive (VSD) is an interconnected combination of equipment that

provides a means of driving and adjusting the operating speed of a mechanical load. An electrical adjustable-speed drive

consists of an electric motor and a speed controller or power converter plus auxiliary devices and equipment. In common

usage, the term “drive” is often applied to just the controller.[3][4]

[edit]Motor control centers

right A small, early 1960's-vintage motor control center for 480 volt motors.

A motor control center (MCC) is an assembly of one or more enclosed sections having a common power bus and

principally containing motor control units.[1] Motor control centers are in modern practice a factory assembly of several

motor starters. A motor control center can include variable frequency drives, programmable controllers, and metering and

may also be the electrical service entrance for the building. Motor control centers are usually used for low voltage three-

phase alternating current motors from 230 volts to 600 volts. Medium-voltage motor control centers are made for large

motors running at 2300 V to around 15000 V, using vacuum contactors for switching and with separate compartments for

power switching and control.[5]

Motor control centers have been used since 1950 by the automobile manufacturing industry which used large numbers of

electric motors. Today they are used in many industrial and commercial applications. Where very dusty or corrosive

processes are used, the motor control center may be installed in a separate air-conditioned room, but often an MCC will

be on the factory floor adjacent to the machinery controlled.

A motor control center consists of one or more vertical metal cabinet sections with power bus and provision for plug-in

mounting of individual motor controllers. Very large controllers may be bolted in place but smaller controllers can be

unplugged from the cabinet for testing or maintenance. Each motor controller contains a contactor or a solid-state motor

controller, overload relays to protect the motor, fuses or a circuit breaker to provide short-circuit protection, and a

disconnecting switch to isolate the motor circuit. Three-phase power enters each controller through separable connectors.

The motor is wired to terminals in the controller. Motor control centers provide wire ways for field control and power

cables.

Each motor controller in an MCC can be specified with a range of options such as separate control transformers, pilot

lamps, control switches, extra control terminal blocks, various types of bi-metal and solid-state overload protection relays,

or various classes of power fuses or types of circuit breakers. A motor control center can either be supplied ready for the

customer to connect all field wiring, or can be an engineered assembly with internal control and interlocking wiring to a

central control terminal panel board or programmable controller.

Motor control centers (MCC) usually sit on floors, which are often required to have a fire-resistance rating. Firestops may

be required for cables that penetrate fire-rated floors and walls.

[edit]Speed controls for AC induction motors

Recent developments in drive electronics have allowed efficient and convenient speed control of these motors, where this

has not traditionally been the case. The newest advancements allow for torque generation down to zero speed. This

allows the polyphase AC induction motor to compete in areas where DC motors have long dominated, and presents an

advantage in robustness of design, cost, and reduced maintenance.[4]

[edit]Variable frequency drives

Main article: Variable-frequency drive

[edit]Phase vector drives

Main article: Vector control (motor)

Phase vector drives (or simply vector drives) are an improvement over variable frequency drives(VFDs) in that they

separate the calculations of magnetizing current and torque generating current. These quantities are represented by

phase vectors, and are combined to produce the driving phase vector which in turn is decomposed into the driving

components of the output stage. These calculations need a fast microprocessor, typically a DSP device.

Unlike a VFD, a vector drive is a closed loop system. It takes feedback on rotor position and phase currents. Rotor

position can be obtained through an encoder, but is often sensed by the reverse EMF generated on the motor leads.

In some configurations, a vector drive may be able to generate full rated motor torque at zero speed.

[edit]Direct torque control drives

Direct torque control has better torque control dynamics than the PI-current controller based vector control. Thus it suits

better to servo control applications. However, it has some advantage over other control methods in other applications as

well because due to the faster control it has better capabilities to damp mechanical resonances and thus extend the life of

the mechanical system.

Main article: Direct Torque Control

[edit]Brushed DC motor speed or torque controls

Main article: Torque and speed of a DC motor

An industrial grade first quadrant PWM DC-motor controller

These controls are applicable to brushed DC motors with either a wound or permanent magnet stator. A valuable

characteristic of these motors is that they are easily controlled in torque, the torque being fairly directly proportional to the

driving current. Speed control is derived by simply modulating the motor torque.

[edit]SCR or thyristor drive

Main article: Thyristor drive

SCR controls for DC motors convert AC power to direct current, with adjustable voltage. Small DC drives are common in

industry, running from line voltages, with motors rated at 90V for 120V line, and 180V for a 240V line. Larger drives, up to

thousands of horsepower, are powered by three phase supplies and are used in such applications as rolling mills, paper

machines, excavators, and ship propulsion. DC drivers are available in reversing and non-reversing models. The

waveform of the current through the motor by a single-phase drive will have strong ripple components due to the switching

at line frequency. This can be reduced by use of a polyphase supply or smoothing inductors in the motor circuit; otherwise

the ripple currents produce motor heating, excess noise, and loss of motor torque.

[edit]PWM or chopper drives

Main article: Pulse-width modulation

PWM controls use pulse width modulation to regulate the current sent to the motor. Unlike SCR controls which switch at

line frequency, PWM controls produce smoother current at higher switching frequencies, typically between 1 and 20 kHz.

At 20 kHz, the switching frequency is inaudible to humans, thereby eliminating the hum which switching at lower

frequency produces. However, some motor controllers for radio controlled models make use of the motor to produce

audible sound, most commonly simple beeps.

A PWM controller typically contains a large reservoir capacitor and an H-bridge arrangement of switching elements

(thyristors, Mosfets, solid state relays, or transistors).

[edit]Servo controllers

Main article: Servo drive

Main article: Servomechanism

Servo controllers is a wide category of motor control. Common features are:

precise closed loop position control

fast acceleration rates

precise speed control

Servo motors may be made from several motor types, the most common being

brushed DC motor

brushless DC motors

AC servo motors

Servo controllers use position feedback to close the control loop. This is commonly implemented with encoders, resolvers,

and Hall effect sensors to directly measure the rotor's position. Others measure the back EMF in the undriven coils to infer

the rotor position, and therefore are often called "sensorless" controllers.

A servo may be controlled using pulse-width modulation (PWM). How long the pulse remains high (typically between 1

and 2 milliseconds) determines where the motor will try to position itself. Another control method is pulse and direction.

[edit]Stepper motor controllers

Main article: stepping motor

A stepper, or stepping, motor is a synchronous, brushless, high pole count, polyphase motor. Control is usually, but not

exclusively, done open loop, i.e. the rotor position is assumed to follow a controlled rotating field. Because of this, precise

positioning with steppers is simpler and cheaper than closed loop controls.

Modern stepper controllers drive the motor with much higher voltages than the motor nameplate rated voltage, and limit

current through chopping. The usual setup is to have a positioning controller, known as an indexer, sending step and

direction pulses to a separate higher voltage drive circuit which is responsible for commutation and current limiting.

[edit]Relevant circuits to motor control

[edit]H-bridge

Main article: H-bridge

DC motors are typically controlled by using a transistor configuration called an "H-bridge". This consists of a minimum of

four mechanical or solid-state switches, such as two NPN and two PNP transistors. One NPN and one PNP transistor are

activated at a time. Both NPN or PNP transistors can be activated to cause a short across the motor terminals, which can

be useful for slowing down the motor from the back EMF it creates.

[edit]See also

Motor Soft Starter

Direct on line starter

Adjustable-speed drive

Electronic speed control

Variable-frequency drive

Thyristor drive

DC motor starter  section of Electric motor

Firestop

Cable

Passive fire protection

Automation

Motion control

Control system

Variable-frequency driveFrom Wikipedia, the free encyclopedia

Small variable frequency drive

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 ofadjustable-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

calledVVVF (variable voltage variable frequency) drives.

Variable-frequency drives are widely used. In ventilation systems for large buildings, variable-

frequency motors on fans save energy by allowing the volume of air moved to match the system

demand. They are also used on pumps, elevator, conveyor and machine tool drives.

Contents

 [hide]

1   VFD types

2   VFD system description

o 2.1   VFD motor

o 2.2   VFD controller

o 2.3   VFD operator interface

3   VFD Operation

4   Power line harmonics

5   Applications considerations

6   Available VFD power ratings

7   Regenerative variable-Frequency drives

8   Brushless DC motor drives

9   See also

10   References

11   External links

[edit]VFD types

All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them only on

or off. Using a linear device such as a transistor in its linear mode is impractical for a VFD drive, since

the power dissipated in the drive devices would be about as much as the 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.

[edit]VFD system description

VFD system

A variable frequency drive system generally consists of an AC motor, a controller and an operator

interface.[4][5]

[edit]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

operation are often used. Certain enhancements to the standard motor designs offer higher reliability

and better VFD performance, such as MG-31 rated motors.[6]

[edit]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 or converter bridge.

The rectifier is usually a three-phase, full-wave-diode bridge. The DC intermediate power is then

converted to quasi-sinusoidal AC power using an inverter switching circuit. The inverter circuit is

probably the most important section of the VFD, changing DC energy into three channels of AC energy

that can be used by an AC motor. These units provide improved power factor, less harmonic distortion,

and low sensitivity to the incoming phase sequencing than older phase controlled converter VFD's.

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.[7]

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.[8][9][10]

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.[11]

In addition to this simple volts per hertz control more advanced control methods such as vector

controland 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 pseudosinusoidal varying pulse durations.[12][8]

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.[13] 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.

PWM VFD Output Voltage Waveform

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.[8]

[edit]VFD operator interface

The operator 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 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

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

VFD to be configured, adjusted, monitored and controlled using a computer.[8][14][15]

[edit]VFD Operation

When an induction motor is connected to a full voltage supply, it draws several times (up to about 6

times) its rated current. 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.

By contrast, when a VFD 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 VFD, 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 the VFD is drawing less than

50% of its rated current from the mains in the low speed range. A VFD can be adjusted to produce a

steady 150% starting torque from standstill right up to full speed.[16] Note, however, that cooling of the

motor is usually not good in the low speed range. Thus running at low speeds even with rated torque

for long periods is not possible due to overheating of the motor. If continuous operation with high

torque is required in low speeds an external fan is usually needed. The manufacturer of the motor

and/or the VFD should specify the cooling requirements for this mode of operation.

In principle, the current on the motor side is in direct proportion of the torque that is generated and the

voltage on the motor is in direct proportion of the actual speed, while on the network side, the voltage

is constant, thus the current on line side is in direct proportion of the power drawn by the motor, that is

U.I or C.N where C is torque and N the speed of the motor (we shall consider losses as well, neglected

in this explanation).

(1) n stands for network (grid) and m for motor

(2) C stands for torque [Nm], U for voltage [V], I for current [A], and N for speed [rad/s]

We neglect losses for the moment :

Un.In = Um.Im (same power drawn from network and from motor)

Um.Im = Cm.Nm (motor mechanical power = motor electrical power)

Given Un is a constant (network voltage) we conclude : In = Cm.Nm/Un That is "line current (network)

is in direct proportion of motor power".

With a VFD, 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 (resistor controlled by a transistor) to

dissipate the braking energy. With 4-quadrants recifiers (active-front-end), the VFD is able to brake the

load by applying a reverse torque and reverting the energy back to the network.

[edit]Power line harmonics

While PWM allows for nearly sinusoidal currents to be applied to a motor load, the diode rectifier of the

VFD takes roughly square-wave current pulses out of the AC grid, creating harmonic distortion in the

power line voltage. When the VFD load size is small and the available utility power is large, the effects

of VFD systems slicing small chunks out of AC grid generally go unnoticed. Further, in low voltage

networks the harmonics caused by single phase equipment such as computers and TVs are such that

they are partially cancelled by three-phase diode bridge harmonics.

However, when either a large number of low-amperage VFDs, or just a few very large-load VFDs are

used, they can have a cumulative negative impact on the AC voltages available to other utility

customers in the same grid.

When the utility voltage becomes misshapen and distorted the losses in other loads such as normal

AC motors are increased. This may in the worst case lead to overheating and shorter operation life.

Also substation transformers and compensation capacitors are affected, the latter especially if

resonances are aroused by the harmonics.

In order to limit the voltage distortion the owner of the VFDs may be required to install filtering

equipment to smooth out the irregular waveform. Alternately, the utility may choose to install filtering

equipment of its own at substations affected by the large amount of VFD equipment being used. In

high power installations decrease of the harmonics can be obtained by supplying the VSDs from

transformers that have different phase shift.[17]

Further, it is possible to use instead of the diode rectifier a similar transistor circuit that is used to

control the motor. This kind of rectifier is called active infeed converter in IEC standards. However,

manufacturers call it by several names such as active rectifier, ISU (IGBT Supply Unit), AFE (Active

Front End) or four quadrant rectifier. With PWM control of the transistors and filter inductors in the

supply lines the AC current can be made nearly sinusoidal. Even better attenuation of the harmonics

can be obtained by using an LCL (inductor-capacitor-inductor) filter instead of single three-phase filter

inductor.

Additional advantage of the active infeed converter over the diode bridge is its ability to feed back the

energy from the DC side to the AC grid. Thus no braking resistor is needed and the efficiency of the

drive is improved if the drive is frequently required to brake the motor.

[edit]Applications considerations

The output voltage of a PWM VFD 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. The resulting voltages can

produce up to twice the rated line voltage for long cable runs, putting high stress on the cable and

motor winding and eventual insulation failure. Increasing the cable or motor size/type for long runs and

480v or 600v motors will help offset the stresses imposed upon the equipment due to the VFD

(modern 230v single phase motors not effected). At 460 V, 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. For applications requiring long motor cables VSD manufacturers usually

offer du/dt filters that decrease the steepness of the pulses. For very long cables or old motors with

insufficient winding insulation more efficient sinus filter is recommended. Expect the older motor's life

to shorten. Purchase VFD rated motors for the application.

Further, the rapid rise time of the pulses may cause trouble with the motor bearings. The stray

capacitance of the windings provide paths for high frequency currents that close through the bearings.

If the voltage between the shaft and the shield of the motor exceeds few volts the stored charge is

discharged as a small spark. Repeated sparking causes erosion in the bearing surface that can be

seen as fluting pattern. In order to prevent sparking the motor cable should provide a low impedance

return path from the motor frame back to the inverter. Thus it is essential to use a cable designed to be

used with VSDs.[18]

In big motors a slip ring with brush can be used to provide a bypass path for the bearing currents.

Alternatively isolated bearings can be used.

The 2.5 kHz and 5 kHz CSFs cause less motor bearing problems than caused by CSFs at 20 kHz.

[19]Shorter cables are recommended at the higher CSF of 20 kHz. The minimum CSF for synchronize

tracking of multiple conveyors is 8 kHz.

The high frequency current ripple in the motor cables may also cause interference with other cabling in

the building. This is another reason to use a motor cable designed for VSDs that has a symmetrical

three-phase structure and good shielding. Further, it is highly recommended to route the motor cables

as far away from signal cables as possible.[20]

[edit]Available VFD 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. VFD controllers designed

to operate at 111 V to 690 V 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 several megawatts.

For example, the largest ABB ACS800 single drives are rated for 5.6 MW[21] . Medium voltage VFD

controllers are designed to operate at 2,400/4,162 V (60 Hz), 3,000 V (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 5,000 or 10,000 hp should probably be

considered to be one-of-a-kind (one-off) designs.[22]

Medium voltage drives are generally rated amongst the following voltages : 2,3 KV - 3,3 Kv - 4 Kv - 6

Kv - 11 Kv

The in-between voltages are generally possible as well. The power of MV drives is generally in the

range of 0,3 to 100 MW however involving a range a several different type of drives with different

technologies.

Line regenerative variable frequency drives, showing capacitors(top cylinders)and inductors attached which filter

the regenerated power.

[edit]Regenerative variable-Frequency drives

Regenerative AC drives have the capacity to recover the braking energy of an overhauling load and

return it to the power system. [23]

Vector control (motor)From Wikipedia, the free encyclopedia

It has been suggested that Field-Oriented Control be merged into this article or section. (Discuss)

Vector control (also called Field Oriented Control, FOC) is one method used in variable frequency

drives to control the torque (and thus finally the speed) of three-phase AC electric motors by

controlling the current fed to the machine.

[edit]Method

This section may require cleanup to meet Wikipedia's quality standards. Please improve this section if you can. (September 2009)

The stator phase currents are measured and converted into a corresponding complex (space) vector.

This current vector is then transformed to a coordinate system rotating with the rotor of the machine.

For this the rotor position has to be known. Thus at least speed measurement is required, the position

can then be obtained by integrating the speed.

Then the rotor flux linkage vector is estimated by multiplying the stator current vector with magnetizing

inductance Lm and low-pass filtering the result with the rotor no-load time constant Lr/Rr, that is the ratio

of the rotor inductance to rotor resistance.

Using this rotor flux linkage vector the stator current vector is further transformed into a coordinate

system where the real x-axis is aligned with the rotor flux linkage vector.

Now the real x-axis component of the stator current vector in this rotor flux oriented coordinate system

can be used to control the rotor flux linkage and the imaginary y-axis component can be used to

control the motor torque.

Typically PI-controllers are used to control these currents to their reference values. However, bang-

bang type current control, that gives better dynamics, is also possible.

With PI-controllers the outputs of the controllers are the x-y components of the voltage reference

vector for the stator. Usually due to the cross coupling between the x- and y-axes a decoupling term is

further added to the controller output to improve control performance when big and rapid changes in

speed, current and flux linkage occur. Usually the PI-controller also needs low-pass filtering of either

the input or output of the controller to prevent the current ripple due to transistor switching from being

amplified excessively and unstabilizing the control. Unfortunately, the filtering also limits the dynamics

of the control system. Thus quite high switching frequency (typically more than 10 kHz) is required to

allow only minimum filtering for high performance drives such as servo drives.

Next the voltage references are first transformed to the stationary coordinate system (usually through

rotor d-q coordinates) and then fed into a modulator that using one of the many Pulse Width

Modulation (PWM) algorithms defines the required pulse widths of the stator phase voltages and

controls the transistors (usually IGBTs) of the inverter according to these.

This control method implies the following properties of the control:

Speed or position measurement or some sort of estimation is needed

Torque and flux can be changed reasonably fast, in less than 5-10 milliseconds, by changing

the references

The step response has some overshoot if PI control is used

The switching frequency of the transistors is usually constant and set by the modulator

The accuracy of the torque depends on the accuracy of the motor parameters used in the

control. Thus large errors due to for example rotor temperature changes often are encountered.

Reasonable processor performance is required, typically the control algorithm has to be

calculated at least every millisecond.

Although the vector control algorithm is more complicated than the Direct Torque Control (DTC), the

algorithm is not needed to be calculated as frequently as the DTC algorithm. Also the current sensors

need not be the best in the market. Thus the cost of the processor and other control hardware is lower

making it suitable for applications where the ultimate performance of DTC is not required.

[edit]History

Vector control was patented by Felix Blaschke in U.S. Patent 3,824,437 filed originally on August 14,

1969 in Germany while he worked for Siemens.

Another important contemporary publication about the same topic was

Karl Hasse: Zur Dynamik drehzahlgeregelter Antriebe mit stromrichtergespeisten Asynchron-

Kurzschlußläufermotoren. Dissertation, TH Darmstadt, 1969.

In the Blaschke's patent the rotor flux linkage was calculated from the measured air-gap magnetic

field. Thus this method is called direct rotor oriented vector control. However, to use standard induction

machines, the method to estimate the rotor flux linkage from the measured stator currents, as

proposed by Hasse, is more practical. Versions based on flux estimation instead of measuring are

called indirect rotor oriented vector controls. An early review of the possible alternatives was published

in the paper:

Blaschke, F., Böhm, K.: Verfahren der Felderfassung bei der Regelung stromrichtergespeister

Asynchronmaschinen. IFAC Symposium: Control in Power Electronics and Electrical Drives,

Düsseldorf, October 7 – 9, 1974, Proceedings Vol I, pp. 635...649.

Vector control has later been dealt with in numerous publications. Several methods have been

developed to make possible the operation without speed or position sensor. Also methods to estimate

the rotor time constant and other parameters have been presented. One good book dealing with these

issues is:

Peter Vas: Sensorless Vector and Direct Torque Control, Oxford University Press, 1998, ISBN

0-19-856465-1

In addition to induction machines, the vector control has also been applied to synchronous machines

and doubly fed machines.

After the major Siemens' patents expired in the end of 80's and beginning of 90's, many other

manufacturers begin to use this method in their products making this the de facto standard in

demanding motor control applications the only alternative being the Direct Torque Control (DTC)

developed by ABB.

[edit]See also