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When the load on an
electric motor exerts pressure on the motor, as in braking
conditions, the motor becomes a generator. Rather than consuming
electric power, it generates it. This causes current to flow in
reverse, back to the utility lines.
When a phase
converter is inserted between the motor and the utility line, it
must be able to handle regenerative power without damaging the
converter or the load.Rotary and static
converters handle regenerative power fairly well because two of the
three-phase leads are connected directly to the single-phase line.
Regenerative current from these two legs flows back to the utility
unimpeded. Because the third leg is loosely
coupled through
these converters, the power generated on this leg will likely cause
a voltage rise, impeding the smooth flow of power back to the
utility. In most cases, regeneration is intermittent and this
unbalanced flow of power back through the converter will not create
a problem.
VFDs are usually not
designed to handle regenerative power. The rectifiers on input of
the
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VFD only allow power to flow into the device. Regenerative
power will increase the voltage on the DC bus of the drive which can
result in damage to the drive. VFDs exposed to regenerative power
must be fitted with braking resistors on the output side which
dissipate the power as heat. There are regenerative
VFDs which
utilize IGBTs in series with an inductor as the input rectifier
instead of a diode or SCR rectifier. This allows the drive pass the
regenerative power back through to the utility. Regenerative drives
are relatively uncommon because of their high cost.
A
digital phase converter handles regenerative power well.
Two of the three-phase leads are connected directly to the
single-phase line, allowing this power to pass through unimpeded.
Even though the third leg utilizes a double conversion system, the
rectifier is an IGBT in series with an inductor. This allows
well-regulated regenerative power to pass through to the utility on
the third leg.
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