Date post: | 15-May-2015 |
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Contents
• Introduction• Block Diagram• Building blocks• Modes of operation• VFD Parameters• Some Potential Problems• Harmonics and THD• Recent Improvements in the FFCL system
Variable Frequency Drives
– 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?
What is a VFD?
– Variable Frequency Drive (VFD)
– Governing Equation of motor speedSpeed= 120 x f /P
• P=No. of poles
• F=Line Frequency
– How to change line frequency?
Constant =50Hz
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
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.
Output Voltage
• Output Voltage (dc) = x – Vm=Peak Value of voltage– A= Firing Angle
• Firing at zero gives maximum output dc voltage
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
Inverter Action
• Switching DC voltage ON and OFF will make it AC
• Filtered output from DC bus is sent to inverter in VFD
Pulse width modulation
• Such a waveform is not acceptable– Nowhere near Sine wave
• Contains harmonics– Multiples of fundamentals
• Solution– Pulse width modulation
RECTIFIER
PositiveDC Bus
NegativeDC Bus
+
-
INVERTER
How Often You Switch From PositivePulses To Negative Pulses DeterminesThe Frequency Of The Waveform
Frequency
Vo
lta
ge
Basic Purpose achieved
• Speed of the motor now can be controlled
• Is changing motor frequency alone enough?
Φ=
V/F Control Mode
• Flux = V/F
0
230
400
Volts
Hertz
25 50
400 V
50 Hz= 8
V
Hz
230 V
50 Hz= 4.6
V
Hz
If 230 VAC Power Line:
230 V Motor
400
V Moto
r
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
Vector Control Mode
• Sensor less vector control mode– No feedback through speed sensor– Feedback is derived through motor terminals– Drive need to go through “Auto tuning”
• Vector control with sensor– Feedback through encoder– Better speed regulations up to 0.01%– Faster response to load variations
VFD input Parameters
• Max./Base frequency setting
• Motor rated output
• Motor rated voltage
• Motor rated current
• Carrier frequency
VFD input Parameters
• Frequency Reference setting methods
• Stop Command method
• Start frequency
• Stop frequency (DC Braking starts)
• Torque Boost
• Frequency Skip
Start/Stop Frequency/Acceleration/Deceleration
Frequency
Start Frequency
Acceleration Time
DC Brake
Stop Frequency
Deceleration Time
Time
Start/Stop Frequency/Acceleration/Deceleration
Frequency
Start Frequency
Acceleration Time
DC Brake
Stop Frequency
Deceleration Time
Time
Auto Tuning
MotorDrive
Start of Auto tuning
Primary resistanceLeakage reactanceDC Brake VoltageTorque Boost VoltageSlip compensation
Auto Tuning Procedure
Auto tuning procedure PreparationTurn Power ONStart VAT 300
Select the control mode
Motor ratings
Can motor rotate?
yes
NoInput 1: Simple adjustment
mode
Input 2: High adjustment
mode
Input 1: Simple adjustment
mode
Input 2: High adjustment
mode
LED flickers
Start Auto-TuningPress Fwd Revor
RUN LED ON
Auto-tuning End
10s for V/F mode
1min. for vector mode
Auto Tuning Procedure Contd.
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
Dynamic Braking
• Concept of Braking– Kinetic energy keeping the object moving– Energy cannot be destroyed but can be converted
Kinetic Energy Heat Energy
Mechanical Brakes
Wear and Tear
Dynamic Braking
• Some other form of energy– Electrical
Kinetic Energy Electrical Energy
DiscardUtilize
ResistiveElevators
RegenerativeElectric railcars
Total Harmonic Distortion
• Harmonic Content– Deviation of waveform from pure sinusoidal
shape• Present due to non-linear devices (switching)
– Power electronics• THD = x 100
519-1992 - IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
• IEEE Std. 519 (1981) – Revision (1992)
• Deals with harmonics introduced by the static power converters
• Overall THD < 5%
• Any single harmonic < 3%
AREVA report on THD in MCC (VFD) at NP plant
Voltage (V) R-N Y-N B-N
RMS Voltage 237 237 237
Peak Voltage 362 364 364
THD (%) 4.7 5 5
Harmonics Voltage 11 12 12
AREVA report on THD in MCC (VFD) at NP plant
Harmonic # R-N Y-N B-N
1
3
5 3.2 3.5 3.4
7 2.9 3.1 3.2
11 1.0 1.0 1.0
13 0.9 1.0 1.1
Line Reactors
• Installed ahead of the drive• Protect the drive from sudden disturbances• Reduces the harmonics content introduced by VFD
VFD
Bearing Damage
• Pulse width modulated voltage induces bursts of shaft currents
• Grounded through bearings• Eventually bearing failure
Solution for shaft currents
• Shaft grounding through carbon brushes– Wear and corrode– Need maintenance
• Insulated bearings are used– Partial solution– May flow through driven equipment– Insulation may become a capacitor
• Shaft grounding rings– A combination of both