Variable frequency Drives.pdf

Manoj Barsaiyan

Presentation Presentati on Outline 

What is VFD



How it is Useful



Types of VFD



Components of VFD



Advantages & Disadvantages



Case studies

Presentation Presentati on 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.

STARTING OF SYNCHRONOUS MOTOR 

A synchronous motor has no self-starting torque i.e., a synchronous motor cannot start by itself.

4

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.

WH AT I S VARI ABL E 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

Voltage and current waveforms during commutation

V D =

1.35 × V RMS

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 pulsewidth 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

Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER


Positive DC Bus

Negative DC Bus

+

-

RECTIFIER

INVERTER

Drives & Motors Fundamentals Area Under The Square-Wave Pulses Approximates The Area Under A Sine Wave

Positive DC Bus

+ e g a t l o V

Negative DC Bus

-

RECTIFIER

INVERTER

Frequency


Drives & Motors Fundamentals How Often You Switch From Positive Pulses To Negative Pulses Determines The Frequency Of The Waveform Positive DC Bus

+ e g a t l o V

Negative DC Bus

-

RECTIFIER

INVERTER

Frequency


Frequency = 30Hz

Frequency = 60Hz

PULSE WIDTH MODULATION (PWM) RECTIFIER Positive DC Bus

Negative DC Bus

INVERTER

+

-

Motor

INVERTER RECTIFIER Positive DC Bus

Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor


Negative DC Bus

INVERTER

+

-

Motor

2

6

4

5

3

1

1

2

3

4

5

6

1

R Ph

Y Ph

B Ph

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 machines (other than pumps and fans). Examples of these load types include general machinery, hoists, conveyors, conveyors , printing presses, positive displacement pumps, 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. Flow Pressure Energy

Speed (Speed) 2 (Speed) 3



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.

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 VA RI AB L E SPEE D DRI VE 

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.

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.



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.

•

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 DC Link

1

3

5

2

6

4

6

2

5

3

4

SYNC. Motor

AC Line

REACTOR

1

BRUSH LESS Exciter

T HYRI YRIST ST OR FIRIN FIRING

1

2

3

5

3

1

6

4

2

120 de .

6

1

5

4

2

3

1

6

2

I

DC Voltage Source

1

3

5

4

6

2

R Ph Y Ph B Ph

Synchronous Motor Stator winding

I

DC Voltage Source

1

3

5

4

6

2

R Ph Y Ph B Ph


I

DC Voltage Source

1

3

5

4

6

2

R Ph Y Ph B Ph


I

DC Voltage Source

1

3

5

4

6

2

R Ph Y Ph B Ph


I

DC Voltage Source

1

3

5

4

6

2

R Ph Y Ph B Ph


I

DC Voltage Source

1

3

5

4

6

2

R Ph Y Ph B Ph


I

DC Voltage Source

1

3

5

4

6

2

R Ph Y Ph B Ph




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.

VFD Driven ID Fans 

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.

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 Enclosure

Photo 2, Corrosion on Board Board Traces Traces Caused by Moisture

Photo 3, Arcing Caused by Loose Input Contacts

ADVANTAGE OF VFD OVER OTHER SPEED/FLOW CONTORLLLERS 1. 2.

3.

4. 5.

6. 7.

Very high efficiency at lower loads. Very smooth starting of Drives. There is no inrush of huge starting current which in turn increases motor insulation life. As the starting current is low, it eliminates the disturbance in Electric System. No limitation on number of Start/Stop of Drives. Reduced Vibration & wear and noise on mechanical system. Increased life of mechanical system. Precise speed control possible. 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.

Cooling Tower Application Fan Motors

Used to remove heat from the water • By evaporation of sprayed Agenda

water • By

Forced draft

Traditional approach of Forced Draft ON/OFF of Fan motors Dual speed Fan motors Variable pitch axial fans

VFD Application for Cooling Tower Fan •Energy

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

Cooling Tower Cells

Chiller

variation’s’ •Optimum

Return Water Temperature for optimum Chiller Efficiency •Reduced •Other

Agenda

Water Loss

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

Temperature Condenser Sensor Water Pump

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

Variable frequency Drives.pdf

Recommend Documents