VFD BASICS

Industrial Pumps Division

BASICS OF KIRLOSKAR VARIABLE SPEED DRIVES AND INTRODUCTION TO ELECTRICAL DEVICES


WHAT IS VFD VARIABLE FREQUENCY DRIVE AN ELECTRONIC POWER DEVICE FOR CHANGING SPEED OF 3 PHASE AC INDUCTION MOTORS SPEED CHANGE METHOD - BY FREQUENCY VARIATION,PULSE WIDTH MODULATION. RANGE OF FREQUENCY - 0 - 400 HZ(DEPENDS HZ(DEPENDS ON MODEL MODEL OF DRIVE)



TYPICAL VFD RECTIFIER AND FILTER THE RECTIFIER CONVERTS AC INPUT INTO DC AND FILTER FILTERS IT FURTHER .

PFC The PFC(POWER FACTOR CORRECTION) CIRCUIT HELPS TO MAINTAIN AN APPROXIMATE UNITY PF.BY EXECUTING COMPLEX ALGORITHEM IN REALTIME USING THE MICROCONTROLLER,THE USER CAN EASILY LIMIT FLOW OF HARMONICS FROM LINE TO MOTOR AND HENCE,NEAR UNITY PF POWER CAN BE DRAWN BY LINE.

SMPS THE SMPS(SWITCH MODE POWER SUPPLY) PROVIDES DC POWER TO ALL CONTOL CIRCUITS AND DISPLAY UNIT.


TYPICAL VFD INVERTER SECTION INVERTER CONVERTS DC INPUT TO AC AGAIN AND FED TO MOTOR. IT IS IN THE FORM OF PWM(PULSE WIDTH MODULATED )


TYPICAL VFD


TYPICAL VFD INVERTER PRICIPLE INVERTER CIRCUITARY GENERATES AN ALTERNATING CURRENT(AC) BY SEQUENTIALLY SWITCHING A DIRECT CURRENT(DC) IN ALTERNATE DIRECTIONS. THE ILLUSTRATION ABOVE SHOWS THE GENERATION OF POSITIVE PULSE AND A SINGLE NEGATIVE PULSE (GREEN) WHICH OCCURES 180 ELECTRICAL DEGREES LATER. TRANSISTORS 1A AND 2B ARE TURNED ON & OFF BY MICROCONTROLLER & CURRENT FLOWS FROM THE DC BUS POSITIVE THROUGH THE MOTOR WINDINGS AS SHOWN BY RED ARROWS. TO GENERATE NEXT HALF CYCLE TRANSISTORS 1B & 2A WILL BE TURNED ON & OFF & THE CURRENT FLOW WILL REVERSE THROUGH MOTOR WINDINGS SHOWN BY GREEN ARROWS.


TYPICAL VFD DRIVER CIRCUIT DRIVER CIRCUIT WITH MICROCONTROLLER BASICALLY CONTROLS SWITCHING ON-OFF OF THE TRANSISTORS HENCE SWITCHING PATTERN OF PWM

MICROCONTROLLER MICROCONTROLLER IS A BRAIN OF VFD.ALL DECISIONS ARE TAKEN BY MICOCONTROLLER CONSIDERING USER PARAMETERS & VARIOUS FEEDBACKS FROM DEVICES. ALL CONTROL TERMINALS ON TERMINAL STRIPS (DIGITAL INPUTS,DIGITAL OUTPUTS,ANALOG INPUTS,ANALOG OUTPUTS) ARE CONNECTED TO MICROCONTROLLER. ACCORDING TO INPUT STATUS & USER SETTINGS MICOCONTROLLER DRIVES MOTOR AS WELL AS OUTPUTS. ADDITIONAL FEATURES ARE PROVIDED LIKE THE DC BUS VOLTAGE SENSING,OV AND UV TRIP,OVERCURRENT PROTECTION,ACCURATE SPEED/POSITION CONTROL,TEMPERATURE CONTROL,EASY DISPLAY SETTING FOR MONITORING PROCESS PARAMETERS.


TYPICAL VFD DISPLAY AND CONTROL PANEL THIS UNIT IS A COMBINE UNIT INCLUDES LCD DISPLAY & SETTING KEYS. THE DISPLAY UNT CAN BE USED FOR MONITORING PROCESS PARAMETERS LIKE SPEED,CURRENT,VOLTAGE,KW AS WELL AS IT HELPS INITIALLY FOR SETTING UP THE DRIVE & ALSO USEFUL FOR TROUBLE SHOOTING. NORMALLY KEYS ARE PROVIDED FOR START/STOP,RESET,SET,SPEED UP THIS UNIT IS A COMBINE UNIT INCLUDES LCD DISPLAY & SETTING KEYS. THE DISPLAY UNT CAN BE USED FOR MONITORING PROCESS PARAMETERS LIKE SPEED,CURRENT,VOLTAGE,KW AS WELL AS IT HELPS INITIALLY FOR SETTING UP THE DRIVE & ALSO USEFUL FOR TROUBLE SHOOTING. NORMALLY KEYS ARE PROVIDED FOR START/STOP,RESET,SET, SPEED UP/DN OPERATIONS.


TYPICAL VFD RS 232 LINK MOST OF THE DRIVES PROVIDE THIS PORT WHICH PROVIDES COMPUTER CONNECTIVITY SO THAT IMPORTANT PARAMETERS CAN BE LOGGED IN PC . THIS FUNCTION IS VERY USEFUL FOR ANALYSIS IN CASE OF TROUBLE AS DATA IS STORED IN COMPUTER DATE & TIMEWISE OR FOR EVEN FOR KNOWING SYSTEM PERFORMANCE .


V/F CURVE


V/F CURVE AS SEEN IN FIG.21 THE VOLTAGE AND FREQUENCY ARE VARIED AT CONSTANT RATIO UP TO THE BASE SPEED. THE FLUX AND TORQUE REMAINS ALMOST CONSTANT UP TO THE BASE SPEED.BEYOND THE BASE SPEED THE SUPPLY VOLTAGE CANNOT BE INCREASED AND INCREASING FREQUENCY BEYOND THE BASE SPEED RESULTS IN FIELD WEAKENING AND THE TORQUE REDUCES. ABOVE THE BASE SPEED THE TORQUE GOVERNING FACTORS BECOME MORE NONLINEAR AS THE FRICTION LOSS INCREASES. THIS CONTROL IS MORE POPULAR IN INDUSTRY AND CALLED AS V/F CONTROL. WHILE OPERATING MOTOR AT VERY LOW SPEED RISTRICTION COMES MAINLY DUETO INHERENTLOSSES IN THE MOTOR LIKE FRICTION,IRON ETC.THESE LOSSES ARE ALMOST CONSTANT OVER THE ENTIRE SPEED.THEREFORE TO START THE MOTOR ,SUFFICIENT POWER MUST BE SUPPLIED TO OVERCOME THESE LOSSES AND THE MINIMUM TORQUE HAS TO DEVELOPED TO OVERCOME THE LOAD INERTIA.


SPEED OF MOTOR SPEED OF AC INDUCTION MOTOR IS DEPEND ON NO OF POLES FREQUENCY AC MOTOR SPEED = 120F/P F = FREQUENCY & P = POLE FOR EXAMPLE SPEED OF 2 POLE MOTOR AT 120*50/2 = 3000 RPM

50 HZ =


SPEED AND POWER RELATION AC MOTOR POWER IS DEPENDS ON TORQUE SPEED THE FORMULA IS HP = T x N/5252 HP = HORSEPOWER T TORQUE IN lbft N = SPEED IN RPM


WHAT HAPPENS WHEN MOTOR CONNECTED TO VFD SPEED = 120F/P HP = T x N/5252 T = V/F VFD VARIES FREQUENCY SO SPEED VARIES VFD TRIES TO KEEP TORQUE CONSTANT BY MAINTAINING V/F RATIO AS A RESULT WHEN SPEED DECREASES AS TORQUE IS CONSTANT HP REDUCES


ENERGY SAVING FOR PUMPS IF WE REDUCE SPEED HP GETS REDUCE AND HENCE WE CAN SAVE ENERGY WITH THE FELP OF VFD AS PUMPS NOT NEEDED TO RUN AT FULL SPEED ALL THE TIME.


ADVANTAGES OF VFD FOR PUMPING UP TO 50% ENERGY SAVING COMPARED TO CONVENTIONAL METHOD MAINTAINS ACUURATE PRESSURE WITH THE HELP OF BUILTIN PID CONTROL FLEXIBILITY TO FINETUNE FLOW AS PER REQUIREMENT SO NO NEED OF MECHANICAL ALTERATIONS


ADVANTAGES OF VFD FOR PUMPING SOFT STARTING AND STOPINGS AVOIDS WATER HAMMERING IN PIPINGS SO INCREASES LIFE AVOIDS RESONANCE OF THE SYSTEM TAKES CARE OF ALL NECESSARY INTERLOCKS LIKE EM STOP,LEVEL SWITCHES ETC.


ADVANTAGES OF VFD FOR PUMPING USER FRIENDLY DISPLAY USED TO MONITOR SYSTEM STATUS,FAULTS & PROCESS PARAMETERS LIKE ENERGY,FLOW PRESSURE DATA LOGGING OF ENERGY,WORKING HRS ETC. KEYPAD ENABLES AUTO/MAN MODE,PROGRAMMING OF PUMP PARAMETERS & CURVES OF OPTIMISATION


MOTOR AND VFD EFFICIENCY FREQUENCY CONVERTERS WITH NON-SINUSOIDAL CURRENT CAN CAUSE HARMONIC LOSSES IN THE MOTOR AND INCREASE MOTOR LOSSES BY UP TO 10 PERCENT WHICH TRANSLATES INTO OVERALL REDUCTION IN MOTOR EFFICIENCY OF UP TO 1 PERCENT ELECTRICAL SWITCHING WITH TRANSISTORS IS VERY EFFICIENT. SO THE EFFICIENCY OF A FREQUENCY CONVERTER IS ALSO VERY HIGH FROM 0.97 TO 0.99 FOLLOWING FIG CAN APPROXIMATELY TELLS YOU THE RELATION BETWEEN EFFICIENCY SPEED & LOAD.


MOTOR AND VFD EFFICIENCY


Motor Contactor


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Motor Contactor Three Phase Switching Device used for Starting/Stopping Motor Elements of Contactor –

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–

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Power Contacts- For switching power Electromagnetic Coil- For command On/Off Terminals - For Wiring Auxilary contacts - For logic interlocks •

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NO - Normally Open NC - Normally Closed

When Coil gets energize contacts change their state.No to NC & Vice Versa Coil

NO

NC


Relay •

Switching Device used for Control logic

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Elements of Relay –

Change over Contacts- For switching controls

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Electromagnetic Coil- For command On/Off

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Terminals - For Wiring

Pole

When Coil gets energize contacts change their state.No to NC & Vice Versa

Coil

NO

NC


Over Load Relay


Over Load Relay •

Motor protection Device - Protection against Overload(over current)

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Principle - Bimetallic Strips.

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Elements –

Change over Contacts- For switching control

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Three phase Studs

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Reset button & current setting dial Pole

When excess current passes,temp rises & Strips get separated & contacts change their state.No to NC & Vice Versa NO

NC

Change-over


Miniature Circuit Breaker


Miniature Circuit Breaker •

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•

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Motor protection Device - Protection against Overload(over current) & Short Circuit Principle - Bimetallic Strips & Magnetic Rating - Up to 63 Amp, 1 - 4 poles Elements On / Off Lever Input & Output Terminals –

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When excess current passes,temp rises & Strips get separated & contacts gets open. In case of short circuit current rise is Rapid & sensed it & tripped by magnetic coil


Electronic Timer


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Electronic Timer Used for Timed delayed logic Elements of Timer –

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Change over Contacts- For switching controls Source - For Start command Terminals - For Wiring & setting dial

Pole

Types –

On Delay Off Delay -

When source gets energize After preset Delay contacts change their state.No to NC & Vice Versa When source gets De-energize After preset Delay contacts change their state.No to NC & Vice Versa

Source

NO

NC

Change-Over


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•

•

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Voltage Monitoring Relay Used for Mains voltage monitoring, Protection Device If supply variations is more than specified/preset limits, relay operates Also monitors phase sequence & phase failure Starts Automatically once supply voltage comes in limit If phase sequence gets change we have to change it manually


Liquid Level Controller Relay •

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Used for controlling liquid levels in tanks Operates between two levels - Pump Shut off below Min Level or Above Max Level & Pump Start Level Includes Three Probes –

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Reference Level Start Level Stop Level

Principal - Conductivity


PID Controller •

Controller used to control process by taking feedback of process by sensor. From Sensor

Set Value by Keys

PV

O UT

SV

Compares Process Value coming from Sensor with Set Value & accordingly output will be varied or made on / off

VFD BASICS

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