Technical Paper
filters usually contain an input line reactor and a shunt a
tuning circuit (tuning reactor and capacitors) as seen in
Figure 1 [2]. When only the VFD is bypassed, it does not
produce any harmonics, however, the displacement power
factor (DPF) of the filter load current reduces from almost
unity to the motor power factor of 0.85 DPF. Depending on
the passive harmonic filter design, there can be a severe
voltage drop across the line reactor in this condition. If the
voltage drop is too high, it may cause the motor driven fans
and pumps to be non-operational when the VFD is bypassed.
Eliminating Filter Bypass
Contactor in Offline Drive
Applications
Written By:
Josh Haase, Neil Wood
Abstract
A
filter bypass contactor is not necessary to maintain
motor voltage with TCI’s HarmonicGuard® Passive
(HGP) filter when the variable frequency drive (VFD) is
in bypass. A reliable topology is available which in most
cases replaces the need for a fully rated filter bypass
contactor. This filter supported VFD bypass approach uses a
lower cost, auxiliary contactor to engage the filter tuning
circuit while the VFD is in bypass to mitigate the inductive
drop across the filter line reactor and maintain voltage
support.
Figure 1: Simplified one-line diagram of a passive harmonic filter
Due to these concerns, some filter manufacturers
recommend a filter bypass option, as shown in the schematic
in Figure 2. This option requires an additional fully rated
contactor which certainly increases the cost and potentially
reduces the reliability of the system.
VFD Bypass
VFDs offer many benefits such as reducing energy costs,
enhancing process control and extending life of mechanical
equipment. Using VFDs, motors can be operated at variable
speeds, allowing the process output to match demand
optimally. Sometimes it is necessary to remove a VFD
electrically from the system for reprogramming, servicing or
routine maintenance. In many applications such as HVAC,
this is accomplished with a VFD bypass contactor so the
motor operates across the line. A VFD bypass is defined as
a means of taking the VFD out of an electrical power loop
while keeping the system up and running. [1].
Passive Filter Applications
Six pulse VFDs have diode rectifier inputs that present a nonlinear load. The VFD draws harmonic currents from the AC
voltage source. Passive harmonic filters are commonly used
to mitigate harmonics from the AC source. Passive harmonic
Figure 2: Simplified one-line diagram of filter bypass contactor
TCI, LLC | Bypass Disconnect in Offline Drive Applications
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TCI’s HGP filter design offers an alternative approach that
eliminates the need to bypass the filter when the VFD is
bypassed, while maintaining nominal rms voltage to the
motor.
HGP with VFD in bypass
Motor Voltage (V)
Due to the HGP’s tuning circuit design, there is only a 3.5%
max voltage deviation from nominal 480V rms systems as
seen in Figure 3. The capacitors in the HGP tuning circuit
provide voltage support.
505
495
485
475
Many passive filters have trap contactors in series with the
trap circuit to disengage the tuning circuit at light loads and
when the drive is off. In this case, it is critical to engage the
trap contactor when the VFD is in bypass to maintain voltage
support. This can be achieved using a small auxiliary contact
on the VFD bypass contactor that pulls in the trap contactor
engaged when the VFD is in bypass.
Some passive filter designs rely on oversized line reactor
impedance to minimize tuning circuit capacitance cost or
accommodate some other design concession. For those
products the voltage drop in bypass will be so great the only
alternative is to add a fully rated contactor to bypass the filter
instead of the lower cost filter supported VFD bypass
approach outlined above. All TCI passive filter designs
support the lower cost filter supported VFD bypass approach
outlined in this whitepaper.
465
455
20%
40%
60%
80%
100%
Load
VFD bypass without HGP filter bypass
480V Reference
+5% Motor Supply Voltage
-5% Motor Supply Voltage
Figure 3: Motor Voltage under different VFD bypass configurations
Motors are designed to operate with a supply voltage within
±5% of nominal voltage. The voltage support provided by the
HGP capacitors limits the drop in output voltage to 3.5%.
Additionally, the capacitors provide power factor correction
while the drive is bypassed, improving the source power
factor as seen in Figure 4.
1.1
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Conclusion
Bypass disconnect contactors are an added expense when it
comes to taking a passive filter and drive out of a power
system. TCI’s HGP allows a customer to leave the filter
online while disconnecting the VFD. It is a cost effective
option and provides better input power factor correction from
the capacitors.
The motor input voltage is still fully
operational providing the customer protection from harmful
voltage drops from added impedance in the power system.
References
[1] “VFD Electronic Bypass Benefits”, Siemens Building
Technologies.
http://www.achrnews.com/articles/91703-vfd-electronicbypass-benefits
[2] “Generator Compatibility of Passive Filters”, Wallace.
http://www.transcoil.com/Public/Documents/TechnicalPaper
s/Generator-Compatibility-Passive-Filters.pdf
PF
0.9
0.8
0.7
0.6
0.5
20%
40%
60%
80%
100%
Load
VFD bypass without HGP filter bypass
VFD bypass with HGP filter bypass
Figure 4: Input PF under different VFD bypass configurations
TCI, LLC | Bypass Disconnect in Offline Drive Applications
TCI, LLC
W132 N10611 Grant Drive
Germantown, WI 53022
(414) 357-4480
FAX (414) 357-4484
PQ Helpline (800) TCI-8282
www.transcoil.com
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