Variable frequency Drives.pdf

VFD

Manoj Barsaiyan

Presentation Outline

What is VFD

How it is Useful

Types of VFD

Components of VFD

Advantages & Disadvantages

Case studies

What is an Electric Drive?

Motor + Control = Drive

Speed-Torque Characteristics are adjusted

according to the requirements of the load

Feedback is necessary to take corrective

action so that the output matches the exact

requirement.

4

STARTING OF SYNCHRONOUS MOTOR

A synchronous motor has no self-starting torque i.e., a synchronous

motor cannot start by itself.

Variable Frequency Drives (VFD)

VFD substitutes the

requirement of energy

efficient motor and

gear box assembly/

scoop coupling.

The VFD schemes are

based on V/F constant.

WHAT IS VARIABLE SPEED

DRIVE

variable voltage and variable frequency drives

Supplied from the fixed voltage and frequency mains supply source

Converts the power twice, first into DC through source converter and then into a variable voltage and frequency output.

Operating principle

RPM = Revolutions per minute

f = AC power frequency (hertz)

p = Number of poles (an even number)

Rectifier is that special type of converter that converts AC to DC

Inverter is that special type of converter that converts DC to AC.

RECTIFIER I/P - CONSTANT a.c

O/P- VARIABLE d.c THYRISTOR

FIRING

1 -- 2

2 -- 3

3 -- 4

4 -- 5

5 -- 6

6 -- 1

1 -- 2

Rectifier

O/p

VD = 1.35 VRMS

Voltage and current

waveforms during

commutation

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 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.

VFD controller Insulated-gate bipolar transistor (IGBT) became the device used in

most VFD inverter circuits

AC motor characteristics require the applied voltage to be

proportionally adjusted whenever the frequency is changed in order to

deliver the rated torque. constant volts per hertz ratio is maintained.

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.

VFD controller 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.

PWM Sine Wave Synthesis

Drives & Motors Fundamentals

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER


RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

Area Under The Square-Wave Pulses

Approximates The Area Under A Sine Wave

Frequency

Volt

age


RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

How Often You Switch From Positive

Pulses To Negative Pulses Determines

The Frequency Of The Waveform

Frequency

Volt

age


Frequency = 30Hz

Frequency = 60Hz

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

Motor

PULSE WIDTH MODULATION (PWM)

INVERTER

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

Motor

RECTIFIER

Positive

DC Bus

Negative

DC Bus

+

-

INVERTER

Motor

INVERTER

i

2

1

6

5

4

3

R Ph

Y Ph

B Ph

1 2 3 4 5 6 1

FIRING SEQUENCE & PHASE CURRENTS

In power plant we require:

FLOW CONTROL-condensate, Feed

water ,Air Flow etc.

PRESSURE CONTROL-Draft, PA Header

Pressure,Seal Air Pressure Control etc.

Mechanical Capacity Control

Inlet Guide Vane

Outlet Damper Control

Blade Pitch Control

Hydraulic coupling

VFD motor

3 phase induction motor

3 phase synchronous motor

Four Quadrant Operation :

Inverter Duty Motors Inverter duty motors are specially designed to

withstand the new challenges presented by the use of

inverters. There are a number of ways to designate

motors "inverter duty,"

Class F insulation - to withstand the higher heat

generated by non-sinusoidal current from the drive.

Inverter Duty Motors

A proper inverter duty motor will have special rotor bar

construction designed to withstand variations in air gap

flux densities and rotor harmonics.

Cooling of VFD motors.

Type of Loads Constant torque loads. These loads represent 90% of all general

industrial machines (other than pumps and fans). Examples of

these load types include general machinery, hoists, conveyors,

printing presses, positive displacement pumps, some mixers and

extruders, reciprocating compressors, as well as rotary

compressors.

Constant horsepower loads. These loads are most often found

in the machine-tool industry and center driven winder

applications. Examples of constant horsepower loads include

winders, core-driven reels, wheel grinders, large driller machines,

lathes, planers, boring machines

Type of Loads Variable torque loads. Variable torque loads are most often

found in variable flow applications, such as fans and pumps.

Examples of applications include fans, centrifugal blowers,

centrifugal pumps, propeller pumps, turbine pumps, agitators, and

axial compressors.

VFDs offer the greatest opportunity for energy savings when

driving these loads because horsepower varies as the cube of

speed and torque varies as square of speed for these loads.

Type of Loads Variable torque loads.

For example, if the motor speed is reduced 20%, motor

horsepower is reduced by a cubic relationship (.8 X .8 X .8), or

51%.

As such, utilities often offer subsidies to customers investing in

VFD technology for their applications. Many VFD manufactures

have free software programs available for customers to calculate

and document potential energy savings by using VFDs.

Flow Speed

Pressure (Speed)2

Energy (Speed)3

Variable speed drives for power

generation applications

Pumps

Boiler feed-water pump

Condensate extraction pump

Cooling water pump

Ash Water pumps

Fans

Primary air fan

Secondary air fan

ID fan

Other

Raw coal feeder

Coal mill

air compressor

Gas turbine starter

Major Application Areas

Pumps

Fans

Compression type load

Efficiency v/s Different Control is as follow

Energy Saving with VFD VFD gives the highest efficiency at part load applications. So we

can say that whenever our application requires part load application

we must go for VFD.

At 80% flow rate:

Damper control absorbs 93% motor power

IGV control absorbs 70% of motor power

Eddy current coupling control absorbs 67% of motor power

VFD control absorbs 51% of motor power

Energy Saving with VFD

Energy losses Vs

Investment Cost

TYPES OF VARIABLE SPEED DRIVE

VOLTAGE SOURCE INVERTER(VSI)

A capacitor is used at the diode bridge

output to maintain voltage at input of the

Inverter

CURRENT SOURCE INVERTER(CSI)

A series high voltage reactor is used between

diode bridge and the Inverter to maintain

current during variations at supply end.

VFD controller

Voltage Source inverter control

CSI CURRENT SOURCE INVERTER

ONE SYSTEM CONTROLS ONLY ONE MOTOR

SUITABLE FOR LARGE DRIVES

PRECISE CONTROL OF MOTOR IS POSSIBLE WITH

FEEDBACK SIGNAL

Load commutated inverter fed

synchronous motor

Current Source Inverter

Comparison of CSI/VSI

CSI is more reliable than VSI

Due to presence of large inductor CSI drive

has higher cost, weight, volume and slower

dynamic response.

CSI is not suitable for multimotor drives

whereas a single VSI can feed a number of

motors connected in parallel.

Characteristic of VFD drive

V/f ratio is kept constant below the base speed and above the

base speed only V is maintained constant.

Tmax is proportional to square of V/f ratio.

WHAT IS LCI ?

LCI means Load Commutated Inverter.

Commutation is the process whereby changing

voltage cause one cell to stop conducting and

another to begin. For a very short period of time

two phases shall be short-circuited.

In Other words Control can turn ON a thyristor,

but we need the changing voltage relationship to

turn it OFF Commutation takes place.

LCI: main features

Not requires commutation circuits

size and cost of the inverter gets reduced.

Frequency of operation can be much higher.

can operate at higher power levels.

Load Commutated Inverter

(LCI) Fed Drive

Used with synchronous motor operating at

leading power factor.

At leading power factor, the inverter

thyristors can be commutated by the

armature induced voltages.

Can use either rotor position sensors or

motor terminal voltage sensors.

RECTIFIER INVERTER

REACTOR

BRUSH

LESS

Exciter

SYNC.

Motor

AC Line

DC Link

1 3 5

4 6 2

2 6 4

1 3 5

THYRISTOR FIRING

1 53

2

1

4 66 2

1 2 3 4 5 6 1 2 3

120 deg.

1 3

4 6

5

2

I

DC

Voltage

Source

Synchronous Motor Stator winding

R Ph

Y Ph

B Ph

1 3

4 6

5

2

I

DC

Voltage

Source

R Ph

Y Ph

B Ph

Synchronous Motor Stator winding

Mitigating Harmonics

Sometimes filters are used with VFD to reduce

harmonics.

VFDs Using Some VFDs are manufactured with

IGBT rectifiers. The unique attributes of IGBTs

allow the VFD to actively control the power input,

thereby lowering harmonics, increasing power

factor and making the VFD far more tolerant of

supply side disturbances.

An Active Front End Technology (AFE) drive

provides the best way to take advantage of VFD

benefits and minimize harmonics.

Multi-Pulse VFDs

There are a minimum of six rectifiers for a three

phase AC VFD. A standard six-pulse drive has six

rectifiers, a 12-pulse drive has two sets of six

rectifiers, an 18-pulse drive has three sets of six

rectifiers and so on.

If the power connected to each set of rectifiers is

phase shifted, then some of the harmonics

produced by one set of rectifiers will be opposite

in polarity from the harmonics produced by the

other set of rectifiers. The two wave forms

effectively cancel each other out.

Single Channel (6 Pulse)

Duel Channel (12 Pulse)

Duel Channel has inherent advantage of lower torque pulsation,as well as reduction in harmonics after converter.

VFD Driven ID Fans

Block diagram of 12 pulse VFD

LCI

DRIVE

Thyristor

comutation

2-channel VFD

Maintenance of VFD

VFD maintenance requirements fall into three

basic categories:

keep it clean;

keep it dry;

keep the connections tight.

Photo 1, Fan Injecting Dust into Drive Enclosure

Photo 2, Corrosion on Board Traces Caused by Moisture

Photo 3, Arcing Caused by Loose Input Contacts

Photo 4, Arcing Caused by Loose Output Contacts

ADVANTAGE OF VFD OVER OTHER SPEED/FLOW CONTORLLLERS

1. Very high efficiency at lower loads.

2. Very smooth starting of Drives. There is no inrush of huge starting current which in turn increases motor insulation life.

3. As the starting current is low, it eliminates the disturbance in Electric System.

4. No limitation on number of Start/Stop of Drives.

5. Reduced Vibration & wear and noise on mechanical system. Increased life of mechanical system.

6. Precise speed control possible.

7. Regeneration of stored energy in form of inertia is also possible.

Drawback of VFD System Increased number of Switchgear Equipments. This

multiplies its cost many fold.

Use of complex Electro-Electronic System makes it more complicated. There has been many starting problem in the past.

It requires Air Conditioning System which in addition to adding cost also consumes substantial Aux. Power

Load current has ample quantity of Harmonics which in addition to contributing to the loss also make torque pulsating. This is more pronounced during starting.

For supporting VFD and a.c. system, additional civil structure is also required which further add the cost.

EOT CRANE APPLICATION USED FOR MAIN HOIST,

CROSS TRAVEL & LONG TRAVEL

ADVANTAGES

IMPROVED RELIABILITY

PRECISE SPEED & TORQUE CONTROL

SAFELY LOWERS LOAD IN THE EVENT OF A BRAKE FAILURE (BRAKE SLIP DETECTION CAPABILITY)

ENERGY SAVING IN OPERATION AS WELL IN REGENERATIVE BREAKING.

Agenda

Agenda

Cooling Tower Application

Fan Motors Used to remove heat from the water

By evaporation of sprayed water

By Forced draft

Traditional approach of Forced Draft

ON/OFF of Fan motors

Dual speed Fan motors

Variable pitch axial fans

Agenda

Agenda

VFD Application for Cooling Tower Fan

TemperatureSensor

Cooling TowerFan Drive

CoolingTower Cells Chiller

CondenserWater Pump

Energy Savings on load profile but could be as high as 40% also with extreme climate variations

Optimum Return Water Temperature for optimum Chiller Efficiency

Reduced Water Loss

Other advantages of Drive usage like pf improvement, no starter, only three(3) motor cables etc.

CONCLUSION

Considerations while selection of VFD:

Proveness of the system

Requirement of Inverter Duty motor

Distance of the Equipment from the Inverter

Redundancies in Power and Control circuit hardware

Logics build in the system to avoid tripping of drive

unit in case of any one channel failure.

Harmonic generated in the system

Noise level limitations in operation

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