Specifications

38 Catalog WMC-4
Power Factor
The full load power factor exceeds 0.90 for all
capacity selections.
VFD Line Harmonics
Despite their many benefits, care must be
taken when applying VFDs due to the effect
of line harmonics on the building electric
system. VFDs cause distortion of the AC
line because they are nonlinear loads, that
is, they don't draw sinusoidal current from
the line. They draw their current from only
the peaks of the AC line, thereby flattening
the top of the voltage waveform. Some
other nonlinear loads are electronic ballasts
and uninterruptible power supplies.
Line harmonics and their associated
distortion can be critical to ac-drives for
three reasons:
1. Current harmonics can cause
additional heating to transformers,
conductors, and switchgear.
2. Voltage harmonics upset the smooth
voltage sinusoidal waveform.
3. High-frequency components of
voltage distortion can interfere with
signals transmitted on the AC line
for some control systems.
The harmonics of concern are the 5
th
, 7
th
,
11
th
, and 13
th
. Even harmonics, harmonics
divisible by three, and high magnitude
harmonics are usually not a problem.
Current Harmonics
An increase in reactive impedance in front
of the VFD helps reduce the harmonic
currents. Reactive impedance can be added
in the following ways:
1. Mount the drive far from the source
transformer.
2. Add line reactors. They are
standard equipment on WMC
chillers.
3. Use an isolation transformer.
4. Use a harmonic filter.
Voltage Harmonics
Voltage distortion is caused by the flow of
harmonic currents through a source
impedance. A reduction in source
impedance to the point of common coupling
(PCC) will result in a reduction in voltage
harmonics. This can be done in the
following ways:
1. Keep the PCC as far from the
drives (close to the power source)
as possible.
2. Increase the size (decrease the
impedance) of the source
transformer.
3. Increase the capacity (decrease the
impedance) of the busway or
cables from the source to the PCC.
4. Make sure that added reactance is
"downstream" (closer to the VFD
than the source) from the PCC.
Line Reactors
Five-percent line reactors are standard equipment
on WMC chillers and located in each compressors
power panel. They are employed to improve the
power factor by reducing the effects of harmonics.
Harmonic Filter
The harmonic filter is an option for field mounting
and wiring outside of the power panel. It works in
conjunction with the line reactor to further
minimize harmonic distortion. It is wired between
the line reactor and compressor. IEEE 519-1991
Standard defines acceptable limits.
See the WMC certified drawings for harmonic
filter dimensions and wiring information.
EMI (Electro Magnetic Interference) and
RFI (Radio Frequency Interference) Filter
This filter is a factory-installed option. The terms
EMI and RFI are often used interchangeably. EMI
is actually any frequency of electrical noise,
whereas RFI is a specific subset of electrical noise
on the EMI spectrum. There are two types of EMI.
Conducted EMI is unwanted high frequencies that
ride on the AC wave form.
EMI-Radiated EMI is similar to an unwanted
radio broadcast being emitted from the power
lines. There are many pieces of equipment that can
generate EMI, variable frequency drives included.
In the case of variable frequency drives, the
electrical noise produced is primarily contained in
the switching edges of the pulse width modulation
(PWM) controller.
As the technology of drives evolves, switching
frequencies increase. These increases also increase
the effective edge frequencies produced, thereby
increasing the amount of electrical noise.