VFD

CERTIFICATE

This is to certify that the final term appraisal project entitled “Variable Frequency Drives” submitted by Harsh Tiwari in the fulfillment of probation period requirement, for the award of Direct on Job training completion, to the NCTPP,Dadri is the bonafide work of the ET carried out under my supervision and guidance.

(B.K. Singh)

Sr. Manager – C&I

Variable Frequency Drive Page 1

Acknowledgement

It is a matter of privilege and honour for me to express my sincere thanks and gratitude to Mr.

B.K. Singh (project guide) for his unfailing guidance and inspiration throughout the project. His

abundant enthusiasm and zeal, with which he solved my difficulties whenever I approached him,

will be remembered with gratefulness.

I would also like to express my sincere thanks to Mr. S.K. Samal (AGM- C&I) for continuous

encouragement.

Also I am grateful to Mr. Pradeep Mourya, Mr. Bhaskar J. Kalita and Mr. Amit Sharma for their

suggestions and timely feedback on my work.

At last I wish to thank all the persons who helped me out during this project and gave support

directly or indirectly during the completion of the project.

Harsh Tiwari


Introduction

Many electric motor-driven devices operate at full speed even when the loads they are serving

are less than their capacity. To match the output of the device to the load, some sort of part load

control is in use for the majority of their life. Examples include pumps, fans, conveyors, injection

molding machines, air compressors and chillers.

Many part load control strategies waste energy. One of the most efficient method of part load

control, resulting in minimal wasted energy, is the variable frequency drive (VFD).

VFDs accomplish part load control by varying electric motor speed. Energy savings of 50

percent or more over other part load control strategies are common. In addition to this, VFDs

prove advantageous for many other reasons such as soft start, low maintenance, high reliability,

regenerative breaking etc. that help in quick cost recovery.


What is VFD?

The Synchronous Speed of an AC motor is determined by the following equation:

RPM = 120 x f

P

Where RPM = Revolutions Per Minute

f = AC power Frequency (hertz)

p= Number of Poles (an even Number)

By varying frequency of Supply voltage , Speed can be changed.,

Since we know that V∞øf, ie. ø ∞V

f

Where V =supply voltage,, ø=working flux

If we change the speed of AC motor by varying the supply frequency, working flux will vary ,

but it is always required to maintain the working flux constant. So we have to vary the supply

voltage in proportion to supply frequency for constant flux. This is called V/F control of ac

motors. This v/f control of AC motor can be done by VFD(variable frequency drive

VFD means Variable Frequency Drive, i.e., adjustable speed AC motor drive system to control

and/or optimize processes. Also known as Adjustable Frequency Drive (AFD), Variable Speed

Drive (VSD), AC Drives, or VVVF (Variable Voltage 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. ).


The primary functions of a variable speed AC drive, is to convert electrical power to the usable

form for controlling speed, torque and direction of rotation of AC motor. AC Line frequency

power is converted to DC & again inverted to AC power of required frequency to be applied to

the motor stator to get speed as per process requirement. AC to DC conversion & again DC to

AC inversion are done by semiconductor devices (this case it is Thyristors) & electronic control.

A line reactor may also be used for current smoothening.

LCI type VFD

Thyristors simply require a gate pulse to turn on. This unique property allows them to conduct

even if the voltage across the gate is removed provided they are forward biased and current is

non zero. Commutation is the process that can force the thyristor to turn off when the next

thyristor begins to share load. This prevents short circuiting.

Load Commutated Inverter (LCI) is a static, adjustable frequency drive system which controls a

synchronous motor from near zero to rated speed. Basic system consists of a line commutated

thyristor convertor (rectifier) which feeds a load commutated thyristor convertor (inverter)

through a dc link reactor. Line commutation is done by source supply while the load

commutation is facilitated by back emf of the synchronous motor.

This system can control the speed of synchronous motor because the synchronous motor has the

ability to deliver the leading VARs which commutate inverter bridge . This system is not used in

induction motor. LCI type VFD can be either 6-pulse single channel type or 12-pulse dual

channel type . Each channel consists of an isolating transformer, source converter, DC link

inductor and load converter. In a single channel type VFD, the synchronous motor will have one

winding whereas in dual channel type VFD, the synchronous motor will have two windings, one

for each channel.

Since rectifier and inverter are in phase opposition, leading current for inverter will be lagging

for the rectifier. That’s why we prefer synchronous motors that can work in leading power factor

that is necessary for rectifier operation. Lagging current in rectifier ensures that when the current

is zero through them, voltage across the thyristor is negative. This is very important concept of


the LCI type VFD operation. This is also the reason for why induction motor is not used since it

is not possible to control the excitation or indirectly the power factor.

Line Diagram of VFD


Control System

The speed reference from the process controller is a compared to a speed feedback

derived from the integrated motor voltage & error is fed to a speed regulator.

The output of speed regulator is a torque command.

This torque command is sent to the field controller and to a current controller as a current

command.

By proper control of stator current & field excitation , the machine has a fairly linear

torque-per-ampere characteristic.

The most straight forward approach is a speed controller with a V/f controller for field

excitation.

There are three controllable element which can be used to control the power to the

motor:-

-The source side converter

- The load side converter

- the field exciter

All three of these use phase control of thyristor gating angle to control output.

Major function of load side converter control is to maintain a minimum margin angle

which will keep the power to the motor at the highest possible power factor.

A current feedback signal is derived from the AC current into the source side power

converter. The output of the current controller is used to control the gate firing angle


α of the source side converter to adjust the DC link voltage necessary to produce the

command.

System Description

Main parts of the system are as follows:

HT breaker : This is a 6.6 kV breaker which isolates whole system from HT side. It is

located in 6.6 KV switchgear at ‘0’ meter.

Transformer : It is a 6.6/1.2 kV transformer which provides isolation from the ac system

bus, provides correct voltage at the terminals of the rectifier and limits fault magnitude

due to its internal impedance.

Rectifier bridge : It is also called source bridge. It is a thyristor bridge circuit that converts

incoming ac power to dc power of controlled voltage. The time of each thyristor’s firing

is controlled to determine what portion of the sine wave is allowed to conduct, thus

controlling the voltage on dc circuit.

Dc link reactor : The output of the rectifier bridge is fed to the dc link reactor, whose

function is to smooth the current and keep it continuous over the operating range of the

system. It is a 2.55 h and 9.2 ohm reactor.

Inverter bridge : It is also called load bridge. It is again a thyristor bridge circuit. It

converts dc back to ac power. By controlling sequence and timing of thyristor firing,

variable frequency three phase power is obtained at motor terminals.

Microprocessor - Microprocessor based electronic hardware controls firing of thyristor

bridges


Synchronous motor - A synchronous motor excited by a brushless exciter coupled to the

motor shaft. The brushless exciter is a wound rotor induction machine whose rotor

voltage is rectified to supply field current to the synchronous motor. The exciter stator

phase rotation is connected such that its rotor voltage and thus the synchronous motor

field excitation will vary with motor speed.


Line Diagram for 12-pulse VFD


Twelve Pulse Operation

Twelve pulse operation is like two identical six-pulse drives operating from a common

source at the same current and firing angle with the firing angles shifted 30 degrees by

the isolating transformers.

But the two motors are combined into one frame . This reduces the motor cost including

installation, and reduce the torque pulsation amplitude while raising the torque pulsation

frequency. This is achieved by separating the stator winding into two identical windings ,

but isolated and phase-shifted 30 degrees.

The construction includes a common magnetic frame, including a common field, causes

the load side converter voltage to be equal in amplitude and frequency between the two

drive channels. The motor windings are shifted to obtain smoother torque for equal

current. The transformer windings are shifted to reduce the harmonic distortion on power

system and to raise the harmonic frequencies.

Inter channel communication allows one channel to be master and the other to be slave

which takes its speed reference from its master. Thus the channels deliver equal power,

take equal current, and fire at the same relative firing angle.

A 12 pulse system also presents the capability of shutting down one channel for

maintenance while the motor continues to run on the other channel. When the out of

service channel is ready for operation, it may be returned to service without interrupting

the drive system.


Small Electronics Module :-

It consists of following Cards:

1A : HRDA (RELAY DRIVER CARD)

1B : HISA (INPUT SIGNAL CONDITIONING CARD)

1C : NLIB (SORCE) (FLUX INTEGRATOR CARD )

1D : NLIB ( LOAD) (FLUX INTEGRATOR CARD )

1E : NLCB (ANALOG CARD PROVIDING MISCELLANEOUS SIGNALS )

2A : NSFE ( FIELD EXCITATION CARD)

2B : NSFC ( LOAD) (SYSTEM FIRING CARD )

2C : NSFC ( SOURCE) (SYSTEM FIRING CARD )

2D : HAIA (ANALOG/DIGITAL INTERFACE CARD)

2E : HLCB (PROCESSOR EXTENDER BOARD )

2F : HMPG & DMPG ( PROCESSOR AND MEMORY CARD)


Some important tests :-

LT Test:

All incoming voltages, power supply cards output voltage measurement.

DC offset setting

Checking communication between input cards and relays

Self test

Healthiness of Cards, jumper connections etc

Gate test

Availability of firing pulses to each Thyristor.

HT Test:

No load full Voltage Test (Source Side)

Cards calibration for trip setting (OV,UV ,OSP etc)

No load full Voltage Test (Load Side)

Cards calibration for trip setting (OV,UV ,OSP etc)

Field excitation Test

Actual charging of exciter and checking of associated voltage waveforms.

Crowbar Test

Current characteristic of each Thyristor.


Applications

Due to the decline in costs over the past five to ten years, and the potential for significant energy

savings, variable frequency drives can be cost effective on a very large range of applications.

Variable air volume systems

Variable air volume systems should always have a variable frequency drive installed to control

volume. A variable frequency drive serving a variable air volume system operating for a typical

3,000 hours per year will pay for itself in two to five years for a return on investment of roughly

20-50 percent. Variable frequency drives on larger motors will offer a higher return on

investment.

For “custom” applications associated with process fans or pumps, almost any scenario where the

flow is being reduced to 90 percent or less of full volume for 4,000 hours per year or more is a

good candidate for a variable frequency drive. Variable frequency drives installed to reduce flow

significantly can easily pay for themselves in under a year for a return on investment of over 100

percent.

Circulating pumps for hot water heating systems

Circulating pumps for hot water heating systems are a good application for variable frequency

drives, provided the hot water temperature is not reset according to building heating loads or

outdoor air temperature.

Hot water temperature reset is an automatic lowering of water temperature when the heating

loads on the building decrease. When this happens more flow is required to meet the heating

demands of the zones in the building. This keeps the required flow higher, which reduces

variable frequency drive savings.

It should be noted though that disabling hot water temperature reset is not advised to achieve

pumping savings. Hot water reset results in lower heating costs and allows the temperature

controls to work considerably better which provides a more comfortable building.


Chilled water circulating pumps

Chilled water circulating pumps provide good opportunity for savings for variable frequency

drives. Although the cooling season in the Midwest is fairly short, in most cases the pumps run

continuously, including during light cooling loads outside normal business hours.

When retrofitting an existing system, there is often an added cost for achieving savings for a

chilled water pump application. Typical chilled water systems have three-way control valves at

the cooling coils which maintains a constant flow through the pump and chiller.

To achieve variable flow savings, the valves will need to be converted to two-way valves so that

reduced pumping volume can result during periods of low cooling loads.

Additionally, a primary/secondary chilled water system may have to be established to maintain

the required minimum flow through the chiller. Check with the chiller manufacturer.

Geothermal heat pump systems

Almost without exception, variable frequency drives should be installed on circulating loop

pumps for geothermal heat pump systems. The long hours of annual operation, particularly at

low heating and cooling loads, provide lots of savings potential even for small pumps less than

Injection-molding equipment

Retrofitting a variable speed drive on a hydraulic plastics injection-molding machine requires a

controls package that is customized for the application.

Savings are typically in the 33-50 percent range, which over the typical long operating hours of

injection molders can quickly generate savings for a high rate of return.


Cooling towers

Cooling towers can be a good application for variable speed drives. The savings for cooling

towers are generated by operating the fan(s) at lower speeds for longer periods of time as

opposed to cycling the fans on and off at full speed. This reduces the energy consumption and in

some periods may reduce billed demand.

Some chilled water applications that use a cooling tower may have condenser water temperature

reset where the condenser water temperature is lowered during periods of low wet bulb

temperature (or dewpoint). This saves chiller energy and although the tower fan will have to run

faster to achieve lower condenser water temperature, the chiller savings more than offset the

extra tower fan energy.

Applications with little energy saving potential

Variable frequency drives offer a number of solutions that may save little energy. In some cases

a variable frequency drive is used to provide a soft start for an electric motor to help prevent

equipment damage.

Although this will limit a momentary inrush current associated with bringing a motor to full

speed almost instantly there is negligible energy savings and demand savings unless the motor

continues to operate at lower speed during typical operation.


Comment on efficiency

Comparison of efficiency of various control devices


VFD (LCI) VS Hydraulic Couplings

Advantages of Variable Frequency Drives

Feature Benefits

Soft starting Reduced impact on electrical network means

no penalties from utility

Reduced stress on motor, coupling and load,

giving extended life time

Unlimited number of starts per hour

Precise speed and torque control Better product quality

improved cost of ownership

variations and sudden load changes

Low audible noise Improved working environment for operators


Capability for speed reversal / regenerative

braking

Desired torque during braking, therefore better

product quality

improved braking characteristics

Higher efficiency

Power loss ride through Reduced number of drive trips

Better process availability

Energy saving AC drives can be retroffied to standard

induction motors, to provide substantial energy

savings


Conclusion

In the project I have gone through the entire system of control of ID fan speed

using variable frequency drive. The project involved study of the LCI system and

the overall electronics involved. Various HT and LT tests for VFD were studied and

observed in due course of time. The documents containing details of faults,

alarms and related logics were studied and compared with the system in place at

Dadri Stage II unit 5 and 6. Also I have included the comparison of VFD system

with other traditional methods of ID fan control like hydraulic coupling, etc. The

other application areas of VFD were also studied in brief.


Documents

VFD