what is variable frequency drive & how it works
TRANSCRIPT
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Understanding “VFD
Variable Frequency Drives
Xubair khan19092(BE)-Electronics
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Contents
• Introduction
• Block Diagram
• Building blocks
• Modes of operation
• VFD Parameters
• Braking Technique
• Start/Stop Command methods
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Introduction
– A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor.
Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, or an AC drive.
– Frequency (or hertz) is directly related to the motor’s speed (RPMs)
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Energy Usage
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Electric Motor Facts
• Electric motors account for more than two-thirds of all energy used in industry
• 63% of the energy consumed is linked to air & liquid flow
• A typical 50HP industrial motor, with an efficiency of 90%, costs over $25,000 to operate continuously over a year.
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– Standard motors are constant speed and when they are energized they run at a 100% speed no matter the load.
– What if the speed of the driven machine (Fan, Pump) is to be changed?
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What is a VFD?
– Variable Frequency Drive (VFD)
– Equation of motor speed
Speed= 120 x f /P
• P=No. of poles
• F=Line Frequency
– How to change line frequency?
Constant =50Hz
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Block Diagram
VFD Fundamentals
50 Hz Power
Electrical Energy
ABB
Variable Frequency
To
Motor
VFD
RECTIFIER
(AC - DC)
INVERTER
(DC - AC)
AC DC AC
VFD
Variable Frequency50 Hz
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VFD Explored
First, the Converter (usually a diode rectifier) converts three-phase AC power to DC power.
Next, the DC Bus stores and filters the DC power in a large bank of capacitors. Last, the Inverter (usually a set of six IGBTs) switches or inverts the DC power in a
Pulse Width Modulated (PWM) AC waveform to the motor.
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Rectifier
• Basic Building blocks of rectification
– Diodes (Uncontrolled)
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Thyristors
• Controlled
• Output voltage can be controlled
Gate Pulse
Conduction after Gate
Pulse
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Three Phase Rectifiers
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Output Voltage
• Output Voltage (dc) = 3× 3×𝑉𝑚
∏x cos𝐴
–Vm=Peak Value of voltage
–A= Firing Angle
• Firing at zero gives maximum output dc voltage
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Is it a perfect Direct Current?
• Conversion of AC into DC a perfect process?
– Ripples
• How to eliminate the ripples?
– Filters
DC bus in VFD
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Inverter Action
• Switching DC voltage ON and OFF will make it AC
• Filtered output from DC bus is sent to inverter in VFD
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RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
How switching can convert DC into AC?
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RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
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RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
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RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
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RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
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Pulse width modulation
• Such a waveform is not acceptable
– Nowhere near Sine wave
• Contains harmonics
– Multiples of fundamentals
• Solution
– Pulse width modulation
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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
Vo
ltag
e
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Area Under the Curve
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VFD Control Mode
• Scalar mode• Vector mode
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V/F Control Mode
• Scalar mode
• Drive is unaware of what is happening in the motor
Example:
• A 400V scalar drive is told to run a 400V, 50 Hz motor at 50% speed Following V/F pattern, Voltage applied by the drive will also be half.
• Perfect when at no load.
• After loading, motor will run at less than 50% speed
• Drive is unaware of it
SolutionVector Control
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MotorDrive
Speed setting=50% Speed at no load=50%
Load increase
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MotorDrive
Speed setting=50% Speed=40%
No idea what is happening
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Vector Control Mode
• Vector control with sensor
– Feedback through encoder
– Better speed regulations up to 0.01%
– Faster response to load variations
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MotorDrive
Feedback
Enco
der
sen
sor
Drive INC/DEC
Speed AccdFb ..
Vector Control Mode
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VFD input Parameters
• Max./Base frequency setting
• Motor rated output
• Motor rated voltage
• Motor rated current
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VFD input Parameters
• Frequency Reference setting methods
• Stop Command method
• Start frequency
• Stop frequency (DC Braking starts)
• Torque Boost
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Frequency Reference Setting Methods
• Potentiometer
• 0-10V input voltage
• 4-20(mA)
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Frequency Reference Setting through Potentiometer
Min
Max
Min Max
Freq
ue
ncy
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Main Control Panel
Potentiometer
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VFD drives ……….
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Stop Command methods
• Coast to stop
• Ramp to stop
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Coast to stop
Coast to Stop
Frequency
Run Command
Motor speed
Command is removed
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Ramp to stop
Frequency
Run Command Command is removed
DC Brake
Ramp to Stop
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Auto Tuning
• Drive familiarizing itself with motor
VFD here I am M
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Auto Tuning
MotorDrive
Start of Auto tuning
Primary resistanceLeakage reactanceDC Brake VoltageTorque Boost
VoltageSlip compensation
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DC Injection Braking
• No mechanical Contact
• DC is applied at the stator winding
• DC causes stator to be become a magnet with constant field
• A voltage is induced inside the rotor causing current to flow
• According to Lens’s law, this current will cause rotor to stop
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Resistive Braking
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Dynamic Braking
Electrical Energy
DiscardUtilize
ResistiveElevators
RegenerativeElectric grid
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Braking resistor
VFD
Braking Unit
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Thank you…….
Engineering garage v=ir