Dakota Supply Group - Index

Dakota Supply Group - dakotaoctober - Index

motor power production. Standard product with
three-phase voltage input has an allowable range
of +10% to –15%. Therefore, a stricter input
voltage tolerance of +10 to –5% applies when
using the drive with a single-phase supply.
The average bus voltage with single-phase input
is lower than the equivalent of a three-phase input.
Therefore, the maximum output voltage (motor
voltage) will be lower with a single-phase input.
The minimum input voltage must be no less than
228Vac for 240 volt models and 456Vac for 480
volt models to ensure motor voltage production
of 207Vac and 415Vac, respectively. Thus, if full
motor torque must be developed near base speed
(full power) it will be necessary to maintain a rigid
incoming line voltage so that adequate motor
voltage can be produced. Increasing the incoming
voltage by tapping-up the supply transformer
will be advantageous, should rated motor HP
be required. However, caution should be used
to ensure the input voltage does not increase to
damaging levels during periods of low demand,
such as off-peak and/or weekend hours.
Operating a motor at reduced speed (reduced
power), or using a motor with a base voltage that
is lower than the incoming AC supply rating (ex.
208Vac motor with a 240Vac supply), will also
minimize the effect of voltage deprivation.
DC Link Choke, Input
Wiring And Branch
Circuit Protection
It is important that input wiring and branch
circuit protection be selected based on the drive’s
single-phase input current rating. The single-phase
input current after derating differs from the threephase
input indicated on the VFD nameplate.
Connect the AC single-phase supply to VFD
terminals L1(R) and L2(S) to accommodate
models that have cooling fans and soft charge
circuits that utilize the AC line supply. The
drive’s internal AC fan(s) and soft charge contactor
are wired to terminals L1(R) and L2(S) from
the factory.
As previously stated, external DC link chokes
are required on ALL models that do not have
them built in as standard to limit the DC bus
ripple and converter currents, and to improve
input power factor. For this same reason, the
use of three-phase line reactors is undesirable.
The voltage drop associated with three-phase line
reactors is significantly larger compared to that
of the DC link choke. A large reduction in motor
power generation may result when using a threephase
line reactor due to excessive drop in line/
bus voltage. If power quality concerns, such as
line transients and notching, that dictate a threephase
line reactor must be used, consider using a
reduced voltage motor (i.e., a 208V or 400V motor)
to reduce the effect of voltage deprivation.
VFD Overload Capacity
Under single-phase use, the DC bus ripple will
increase and the minimum bus level will decrease
below what is normally generated with three-phase
input. For this reason, any overload requirements
must be fully identified and understood. The
VFD can supply 120% of the drive rated output
current for starting purposes (maximum one
minute duration under non-repeating conditions).
The VFD will not generate overload in excess of
120%, except at low speeds where motor power
production remains low. The DC bus voltage may
drop beneath critical levels, when load in excess of
120% occurs at higher speeds. Therefore, be sure
to select the drive anticipating no more than 120%
of motor rated current for starting purposes only.
Please consult the factory for additional guidance
concerning repeating overloads (cyclic loads) or
when an overload in excess of 120% is required.
Generator Applications
When used on generator sets, because of their
non-linear load characteristics, many VFDs can
induce distortion into the generator output
voltage feeding the VFD. Typically, over sizing the
generator will be required to prevent the generator
from overheating due to increased harmonic
currents and lower power factor induced by the
VFD. When applying any VFD to a generator, it is
recommended that the generator manufacturer
be consulted to review the application and system
loads to prevent any power issues.
Single-Phase Drive Selection
Before you begin your selection, make sure
you know the motor nameplate data, including
motor horsepower (HP) and the full load amps
(FLA). The chosen drive must meet or exceed
continued on page 44
AUTOMATION
Fall 2008 CONNECTIONS 43