VFDs, Basic operation, modes of operation and parameterization

2. Variable Frequency Drives
Saqib Saeed
Graduate Trainee Engineer
(E&I) - Electrical

3. Contents
• Introduction
• Block Diagram
• Building blocks
• Modes of operation
• VFD Parameters
• Some Potential Problems
• Harmonics and THD
• Recent Improvements in the FFCL system

4. 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?

5. What is a VFD?
– Variable Frequency Drive (VFD)
– Governing Equation of motor speed
Speed= 120 x f /P
• P=No. of poles
• F=Line Frequency
– How to change line frequency?
Constant
=50Hz

6. 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 Frequency
50 Hz

7. 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.

8. Rectifier
• Basic Building blocks of rectification
– Diodes (Uncontrolled)

9. Thyristors
• Controlled
• Output voltage can be controlled
Gate Pulse
Conduction
after Gate
Pulse

10. Three Phase Rectifiers

11. Output Voltage

12. 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

13. Inverter Action
• Switching DC voltage ON and OFF will make it
AC
• Filtered output from DC bus is sent to inverter
in VFD

14. RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER
How switching can convert DC into AC?

15. RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER

16. RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER

17. RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER

18. RECTIFIER
Positive
DC Bus
Negative
DC Bus
+
-
INVERTER

19. Pulse width modulation
• Such a waveform is not acceptable
– Nowhere near Sine wave
• Contains harmonics
– Multiples of fundamentals
• Solution
– Pulse width modulation

20. 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
Voltage

21. Area Under the Curve

22. Basic Purpose achieved
• Speed of the motor now can be controlled
• Is changing motor frequency alone enough?

23. 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:

24. 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
Solution
Vector
Control

25. MotorDrive
Speed setting=50% Speed at no load=50%
Load increase

26. MotorDrive
Speed setting=50% Speed=40%
No idea what
is happening

27. 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

28. VFD input Parameters
• Max./Base frequency setting
• Motor rated output
• Motor rated voltage
• Motor rated current
• Carrier frequency

29. VFD input Parameters
• Frequency Reference setting methods
• Stop Command method
• Start frequency
• Stop frequency (DC Braking starts)
• Torque Boost
• Frequency Skip

30. VFD input Parameters
• Max./Base frequency setting

31. VFD input Parameters
• Motor Rated Output
• Motor Rated Voltage

32. VFD input Parameters
• Motor rated Current
• Carrier Frequency

33. Frequency Reference Setting Methods
• Potentiometer
• 0-10V input voltage
• 4-20(mA)

34. Frequency Reference Setting through Potentiometer
Min
Max
Min Max
Frequency

35. Centrifuge Main Control Panel
Potentiometer

36. Start/Stop Frequency/Acceleration/Deceleration
Frequency
Start Frequency
Acceleration
Time
DC Brake
Stop Frequency
Deceleration
Time
Time

37. Start/Stop Frequency/Acceleration/Deceleration
Frequency
Start Frequency
Acceleration
Time
DC Brake
Stop Frequency
Deceleration
Time
Time

38. Stop Command methods
• Coast to stop
• Ramp to stop

39. Coast to stop
Coast to
Stop
Frequency
Run Command
Motor speed
Command
is removed

40. Ramp to stop
Frequency
Run Command Command
is removed
DC Brake
Ramp to
Stop

41. Torque Boost
Voltage
Frequency
Rated
Frequency
Rated
Voltage
Voltage
Boost

42. Frequency Skip

43. Auto Tuning
• Drive familiarizing itself with motor
VFD here I am IM

44. Auto Tuning
MotorDrive
Start of Auto tuning
Primary resistanceLeakage reactanceDC Brake Voltage
Torque Boost
Voltage
Slip compensation

45. Auto Tuning Procedure
Auto tuning procedure Preparation
Turn Power ON
Start VAT 300
Select the
control mode
Motor ratings
Can motor
rotate?
yes
NoInput 1: Simple
adjustment
mode
Input 2: High
adjustment
mode

46. Input 1: Simple
adjustment
mode
Input 2: High
adjustment
mode
LED flickers
Start Auto-Tuning
Press Fwd Revor
RUN LED ON
Auto-tuning End
10s for V/F mode
1min. for vector mode
Auto Tuning Procedure Contd.

47. 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

48. 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

49. Dynamic Braking
• Some other form of energy
– Electrical
Kinetic Energy Electrical Energy
DiscardUtilize
Resistive
Elevators
Regenerative
Electric railcars

50. Resistive Dynamic Braking in Elevators

51. Braking resistor for CAN Elevator
VFD
Braking
Unit

52. Resistive Dynamic Braking
• A built in dynamic braking resistor (<15Kw)
• An external DB unit

53. Potential Problems
• Harmonic Distortion
• Bearing Damage

54. Total Harmonic Distortion

55. THD Percentage
VFD BUS Non-VFD BUS

56. Harmonics Spectrum
VFD BUS Non-VFD BUS

57. Current Waveform Comparison
VFD BUS Non-VFD BUS

58. 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%

59. IEEE standard for THD

60. 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

61. 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

62. Line Reactors
• Installed ahead of the drive
• Protect the drive from sudden disturbances
• Reduces the harmonics content introduced by VFD
VFD

63. High peaks
without reactor
Reduction Up to
35% after
reactor

64. Line reactors installed at NP SS
14 Line reactors are installed, 7 on each side of MCC-VFD

65. VFD and Line Reactor
Line reactor
VFD
Main Breaker

66. THD after installation of Line Reactors

67. Bearing Damage
• Pulse width modulated voltage induces
bursts of shaft currents
• Grounded through bearings
• Eventually bearing failure

68. Bearing Damage

70. Pitting of bearing due to Shaft Currents

71. 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

72. Shaft Grounding Ring

73. Questions