VTdrive Variable Frequency Drive(VFD) load filters0 pages

VFD load filters or VFD load reactors?
At one time reactors were used to add load impedance in cases where the motor had
a “low leakage inductance”. Over time, the higher carrier frequencies of IGBT drives
all but eliminated the need to add inductance to the load. So when does it make
sense to add a load reactor? Based on some of the potential issues, it may not make
any sense at all.
Side effects of VFD load reactors
The reflected wave (or standing wave) problem associated with fast switching IGBT
drives is often cited as the reason to apply a load reactor. Standing waves can
create voltage spikes in excess of 1600 volts (480 circuit) at the motor terminals.
This is particularly true when long motor lengths are involved (at as little as 200’).
Once the reactor is installed it will slow the rise time (change in voltage) of the spike,
extending it over time (change in time). As a side note – a term often used to
describe change in voltage over time is dv-dt. Getting back to the reactors, they
cannot actively limit the voltage spike to a safe threshold (1000 volts). In cases of
very long leads load reactors may have little to no impact. Load reactors can
actually create problems, such as resonance from the inductive reactor and the
capacitive cable.
VFD load filters
As stated before, the reactor can decrease the rise time of spikes, but cannot
adequately limit the amplitude of spikes. Filters, on the other hand, add additional
components (resistors and capacitors) that act to limit the actual amplitude, and
therefore can have a much greater impact on motor health.
Dv/dt filters versus “sine wave” filters
VFD output filters begin with a reactor and are built out with additional resistive and
capacitive components, forming a low pass filter. They are designed to limit the
peak voltage to 1,000 volts. These filters are typically applied to applications lead
lengths are up to about 1,000 feet.
A “sine wave” output filter goes another step beyond the dv/dt filter. These filters
are “tuned” to the characteristics of the typical VFD output modulation, and
effectively eliminate or greatly reduce the carrier frequency effects of modulation.
These filters have been successful at lead lengths of up to 15,000 feet.
Benefits of VFD load filters

Slow down the pulse dv/dt.