Download Variable Frequency Drives

2010
ASHRAE Rocky Mountain Chapter
VFD Fundamentals
April 16, 2010
Jeff Miller - ABB
© ABB
Month DD, YYYY | Slide 1
What is a Drive / VFD/ AFD?
460 V
460
60 Hz
= 7.67
V
Hz
Volts
230
If 230 VAC Power Line:
230 V
60 Hz
0
30
60
Hertz
= 3.83
V
Hz
What is a Drive?
L
+
+
L1
+
+
C
Motor
L2
L3
_
_
_
_
+
_
Input Converter
(Diode Bridge)
DC Bus
(Filter)
+
_
Output Inverter
(IGBT’s)
What is a Drive?
VFD Fundamentals
A variable frequency drive converts incoming 60 Hz utility
power into DC, then converts to a simulated variable voltage,
variable frequency output
AC
DC
RECTIFIER
(AC - DC)
60 Hz Power
INVERTER
(DC - AC)
Zero - 120 Hz
60 Hz
VFD
ABB
To
Motor
Zero - 120 Hz
Electrical Energy
VFD
AC
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Positive
DC Bus
Negative
DC Bus
+
-
RECTIFIER
INVERTER
Area Under The Square-Wave Pulses
Approximates The Area Under A Sine Wave
+
Voltage
Positive
DC Bus
Negative
DC Bus
-
RECTIFIER
INVERTER
Frequency
How Often You Switch From Positive
Pulses To Negative Pulses Determines
The Frequency Of The Waveform
+
Voltage
Positive
DC Bus
Negative
DC Bus
-
RECTIFIER
INVERTER
Frequency
Frequency = 30Hz
Frequency = 60Hz
RECTIFIER
Positive
DC Bus
Negative
DC Bus
INVERTER
+
-
Motor
RECTIFIER
Positive
DC Bus
Negative
DC Bus
INVERTER
+
-
Motor
RECTIFIER
Positive
DC Bus
Negative
DC Bus
INVERTER
+
-
Motor
RECTIFIER
Positive
DC Bus
Negative
DC Bus
INVERTER
+
-
Motor
RECTIFIER
Positive
DC Bus
Negative
DC Bus
INVERTER
+
-
Motor
RECTIFIER
Positive
DC Bus
Negative
DC Bus
INVERTER
+
-
Motor
RECTIFIER
Positive
DC Bus
Negative
DC Bus
INVERTER
+
-
Motor
Non-Linear Loads?
• Loads which draw non-sinusoidal
current from the line:
–
–
–
–
–
–
–
–
Non-incandescent lighting
Computers
Uninterruptible power supplies
Telecommunications equipment
Copy machines
Battery chargers
Electronic variable speed drives
Any load with a solid state AC to DC
power converter
Typical AC Drive Configuration
460VAC
3-phase
M
Simulated AC
650VDC
(PWM)
All AC Drives rectify AC to DC, then convert to simulated AC
(PWM) to provide the motor Variable voltage and Frequency. The
AC to DC conversion generates harmonics.
Harmonics — Definitions
• Non-linear loads draw current in a non-sinusoidal or
distorted manner
• Harmonics or harmonic content is a mathematical
concept implemented to allow quantification and
simplified analysis of non-linear waveforms
• Harmonics are typically present in both network
currents and network voltages
• Non-linear current draw creates non-linear voltage as it
flows through the electrical network
– Current harmonics  Voltage harmonics
Harmonic Frequencies
Fundamental
5th Harmonic
7th harmonic
11th Harmonic
13th Harmonic
17th Harmonic
19th Harmonic
60 Hz
300 Hz
420 Hz
660 Hz
780 Hz
1020 Hz
1140 Hz
= 2p × f
i (Fundamental,
t) = 0.32 × cos ( w × t - p )
Thew Theory:
5th and 7th
= 420
w = 2p × f
i (Harmonics
t) = 0.09 × cos ( w × t - p )
Fundamental
f5 = 300
5
5
5
5
f7
7
7
7
7
1
5th
0.5
i 1 ( t)
i 5 ( t)
Components
0
i 7 ( t)
7th
0.5
iT ( t) = i1 (1
t) + i5 ( t) + i7 ( t)
0
0.005
0.01
0.015
1.5
0.02
0.025
0.03
t
1
0.5
i T( t)
0
Summation
0.5
1
1.5
0
0.005
0.01
0.015
0.02
t
0.025
0.03
Harmonic Content, 6- Pulse Drive
PWM Drive Harmonic Input Spectrum
Fundamental
5th
7th
11th
13th
Harmonics — Why worry?
– Harmonic Current Distortion —
• Added heating in transformers and
cables, reduces available capacity
• May stimulate a resonance condition
with Power Factor Correction
Capacitors
–Excessive voltage
–Overheating of PF correction capacitors
–Tripping of PF protection equipment
Voltage Distortion interfering w/ sensitive
equipment. Largest Concern!
Harmonics — A System Issue!
– Harmonics produced by an individual load are only
important to the extent that they represent a significant
portion of the total connected load
– Linear loads help reduce system harmonic levels
– TDD equals the THD of the nonlinear load multiplied by
the ratio of nonlinear load to the demand load:
TDD = THD NL ×
Where
TDD
THDNL
NL
DL
=
=
=
=
NL
DL
TDD of the system
THD of the nonlinear loads
kVA of nonlinear load
kVA of demand load
(nonlinear + linear)
Harmonics — By the Numbers
IEEE 519 - 1992
Table 10.2
Low-Voltage System Classification and Distortion Limits
Special
Applications
General
System
Dedicated
System
Notch Depth
10%
20%
50%
THD (Voltage)
3%
5%
10%
16,400
22,800
36,500
Notch Area, mVs
Note: Notch area for other than 480 V systems should be multiplied by V / 480.
Harmonics — By the Numbers
(cont.)
IEEE 519 - 1992
Table 10.3
Current Distortion Limits for General Distribution Systems
ISC / IL
<11
11£h<17
17£h<23
23£h<35
35£h
TDD
<20
4.0
2.0
1.5
0.6
0.3
5.0
20<50
7.0
3.5
2.5
1.0
0.5
8.0
50<100
10.0
4.5
4.0
1.5
0.7
12.0
100<1000
12.0
5.5
5.0
2.0
1.0
15.0
Note: All harmonic current levels are in percent with fundamental current IL as the base.
Harmonics — Attenuation Options
• Reactors (Chokes)
• Passive Filters
– Harmonic Trap
– Hybrid
• High Pulse Count
Rectification
• Active Filters
– Drive Front End
– Stand Alone
Reactors (Chokes)
• Simplest and least
expensive harmonic
reduction technique
• May be included in base
drive package
• Often meet harmonic
needs provided drive
load is a small portion
of total connected load
• May be implemented
with AC line reactors or
with DC link reactors
– AC line reactors provide
better input protection
– DC link reactors provide
load insensitive drive
output voltage
– Both types provide
similar harmonic benefits
• “Swinging” choke
design provides
enhanced light load
harmonic performance
Reactors, AC Line or DC Link
AC Line
Reactor
M
DC Link
Reactor
M
• Different design
techniques
• Equal harmonic
reduction for
same normalized
% reactance
• Typical full load
THD (current) at
drive input
terminals
28%  46%
Hybrid Filter
• Installs in series with
drive input
• May feed multiple
drives
• Improves power factor
(may go leading)
• Typical full load THD
(current) at filter input
terminals
5%  8%
• Relatively unaffected
by line imbalance
High Pulse Count Rectification
• Typical configurations are either 12 pulse or 18
pulse
• Phase shifting transformer is required
• Additional drive input bridge(s) is needed
• Typical full load THD (current) at transformer
primary 8%  12% (12 pulse), 4%  6% (18
pulse)
• Performance severely reduced by line
imbalance (voltage or phase)
• Excellent choice if step-down transformer is
already required
High Pulse Count Rectification
(cont.)
• 6 pulse
rectifier
• 18 pulse
rectifier
DC/AC
Transformer and
cabling simple
Current very distorted
Ithd typically 45% with
3% reactor
DC/AC
• 12 pulse
rectifier
DC/AC
Transformer and cabling
complicated
Current distorted
Ithd 8% to 12% (depending
on network impedance)
Transformer and cabling
complicated
Current wave form good
Ithd 4% to 6% (depending on
network impedance)
Active Filter Front End with LCL Filter
L
L
M
Line
inverter
(rectifier)
C
Motor
inverter
Motor
LCL filter
 LCL Filter (Sine Filter) removes high frequencies >1 kHz.
(Current and voltage)
 Full output voltage is available with 80% input voltage
(400VIn = 480VOut)
 Full regenerative capability
 No transformer required
 Not affected by line imbalance
Harmonic Reduction Summary
Effectiveness of Harmonic Mitigation Techniques
(Assuming 100% Nonlinear Loading, ISC / IL = 60)
THD
(Current)
Harmonic
Reduction
No mitigation (reference level)
72%

3% line reactors (or equivalent DC link reactor)
39%
45.8%
5% line reactors (or equivalent DC link reactor)
33%
54.2%
5% line reactors + 5th harmonic trap filter
12%
83.3%
12 pulse input rectifier with 5% impedance transformer
10%
86.1%
Hybrid filter
7%
90.3%
18 pulse input rectifier with 5% impedance transformer
5%
93.1%
12 pulse input rectifier with 5% impedance transformer
+ 11th harmonic trap filter
4%
94.4%
3.5%
95.1%
Technique
Active harmonic filter
Remember!
Even an 80% THD nonlinear load with a will result in only 8%
TDD if the nonlinear load is 10% and the linear load is 90%.
(80%•(10%/(10%+90%))=8%)
Summary – Practical Advice
• With a main distribution transformer, 20-30% of its
load on non-linear loads will typically comply with
IEEE 519-1992
• Voltage distortion causes interference with sensitive
equipment, not current distortion!
• 5% reactors address 90+% of typical applications.
They also provide protection against line transients
and keep input currents low to avoid oversizing power
wiring to comply with NEC.
• Make VFD vendor perform a harmonic distortion
calculation with the submittals.
 PEAK: 1,040 volts
Peak Voltage all at 50’ of cable
Peak Voltage has
many Contributing
Factors
Inverter Rated
Motors Help
Minimize the Issue
Less dV/dT
minimizes; problems
with RFI/EMI Motor
Insulation & Bearing
Current
Drive
Peak Voltage
1
1040
2
1110
3
1180
4
1290
5
1350
6
2454
Recommendations
• Keep cable length short as possible
• Use a NEMA MG1, Part 31 motor (not
“inverter duty” or “inverter ready”
• Ensure that grounding is sound