14-25 Trip Delay at Torque Limit
works with torque limit.
This parameter selects the length of time the adjustable
frequency drive operates in torque limit before a trip. The
factory default value is off. This means that the adjustable
frequency drive will not trip on torque limit, but it does
not mean it will never trip from an overload condition.
Built into the adjustable frequency drive is an internal
inverter thermal protection circuit. This circuit monitors the
output load on the inverter. If the load exceeds 100% of
the continuous rating of the adjustable frequency drive, a
timer is activated. When the load remains excessive long
enough, the adjustable frequency drive trips on inverter
time. Adjustments cannot be made to alter this circuit.
Improper parameter settings affecting load current can
result in premature trips of this type. The timer can be
displayed.
5.1.1 Overvoltage Trips
This trip occurs when the DC bus voltage reaches its DC
bus alarm voltage high (see
). Before tripping, the adjustable frequency
drive displays a high voltage warning. Most times, an
overvoltage condition is due to fast deceleration ramps
regarding the inertia of the load. During deceleration of
the load, inertia of the system acts to sustain the running
speed. Once the motor frequency drops below the running
speed, the load begins overhauling the motor. The motor
becomes a generator and starts returning energy to the
adjustable frequency drive. This is called regenerative
energy. Regeneration occurs when the speed of the load is
greater than the commanded speed. The diodes in the
IGBT modules rectify this return voltage, which raises the
DC bus. If the amount of returned voltage is too high, the
adjustable frequency drive trips.
One method is to reduce the deceleration rate so it takes
longer for the adjustable frequency drive to decelerate.
One method is to reduce the deceleration rate so it takes
longer for the adjustable frequency drive to decelerate.
The adjustable frequency drive can only decelerate the
load slightly faster than it would take for the load to
naturally coast to a stop. A second method is to allow the
overvoltage control circuit to take care of the deceleration
ramp. When enabled, the overvoltage control circuit
regulates deceleration at a rate that maintains the DC bus
voltage at a level that keeps the unit from tripping.
Overvoltage control corrects minor, but not major discrep-
ancies between ramp rates. For example, if a deceleration
ramp of 100 seconds is required due to the inertia, and the
ramp rate is set at 70 seconds, the overvoltage control
corrects it. However, with the same inertia, if the ramp is
set at a larger difference, such as three seconds,
overvoltage control engages initially and then disengages
allowing the adjustable frequency drive to trip. This trip is
done deliberately to avoid confusion about the operation
of the adjustable frequency drive. A third method in
controlling regenerated energy is with a dynamic brake.
The adjustable frequency drive monitors the level of the
DC bus. If the level becomes too high, the adjustable
frequency drive switches the resistor across the DC bus,
and dissipates the unwanted energy into the external
resistor bank mounted outside of the adjustable frequency
drive. This increases deceleration rate.
Less often, the load causes an overvoltage condition while
running at speed. When this condition occurs, the dynamic
brake option or the overvoltage control circuit can be
used. It works with the load in this way. As stated earlier,
regeneration occurs when the speed of the load is greater
than the commanded speed. If the load becomes
regenerative while the adjustable frequency drive is
running at a steady state speed, the overvoltage circuit
increases the frequency to match the speed of the load.
The same restriction on the amount of influence applies.
The adjustable frequency drive adds about 10% to the
base speed before a trip occurs. Otherwise, the speed
could continue to rise to potentially unsafe levels.
5.1.2 Line Phase Loss Trips
The adjustable frequency drive actually monitors phase
loss by monitoring the amount of ripple voltage on the DC
bus. Ripple voltage on the DC bus is a product of a phase
loss. The main concern is that ripple voltage causes
overheating in the DC bus capacitors and the DC coil. If
the ripple voltage on the DC bus is left unchecked, the
lifetime of the capacitors and DC coil is drastically reduced.
When the input voltage becomes unbalanced or a phase
disappears completely, the ripple voltage increases. This
causes the adjustable frequency drive to trip and issue
Alarm 4. A line disturbance or imbalance can also cause
increased bus ripple. Loads affecting the form factor of the
AC waveform, such as notching or defective transformers,
cause line disturbances. Line imbalances that exceed 3%
cause sufficient DC bus ripple to initiate a trip.
Adjustable Frequency Drive ...
Service Manual
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Danfoss A/S © Rev. 2014-02-10 All rights reserved.
MG94A222
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Summary of Contents for VLT FC 103
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