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Frequency Converter and Mot...
5.1.1 Overvoltage Trips
This trip occurs when the DC bus voltage reaches its DC
bus alarm voltage high (see ratings tables in introductory
section). Prior to the trip, the frequency converter will
display a high voltage warning. Most times an over voltage
condition is due to fast deceleration ramps with respect to
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. At this point the
motor becomes a generator and starts returning energy to
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the frequency converter. This is called regenerative energy.
Regeneration occurs when the speed of the load is greater
than the commanded speed. This return voltage is rectified
by the diodes in the IGBT modules and raises the DC bus.
If the amount of returned voltage is too high, the
frequency converter will trip.
There are a few ways to overcome this situation. One
method is to reduce the deceleration rate so it takes
longer for the frequency converter to decelerate. A general
rule of thumb is that the frequency converter 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 an acceptable level. One caution with
overvoltage control is that it will not make corrections to
unrealistic ramp rates. For example, if the deceleration
ramp needs to be 100 seconds due to the inertia, and the
ramp rate is set at 3 seconds, overvoltage control will
initially engage and then disengage and allow the
frequency converter to trip. This is purposely done so the
units operation is not misinterpreted. A third method in
controlling regenerated energy is with a dynamic brake.
The frequency converter monitors the level of the DC bus.
Should the level become too high, the frequency converter
switches the resistor across the DC bus and dissipates the
unwanted energy into the external resistor bank mounted
outside of the frequency converter. This will actually
increase the rate of deceleration.
Less often is the case that the overvoltage condition is
caused by the load while it is running at speed. In this
case the dynamic brake option can be used or the
overvoltage control circuit. 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. Should
the load become regenerative while the frequency
converter is running at a steady state speed, the
overvoltage circuit will increase the frequency to match
the speed of the load. The same restriction on the amount
of influence applies. The frequency converter will add
about 10% to the base speed before a trip occurs.
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High Power Service Manual for Modular F Frame Drives
Otherwise, the speed could continue to rise to potentially
unsafe levels.
5.1.2 Mains Phase Loss Trips
The frequency converter 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 would be drastically reduced.
When the input voltage becomes unbalanced or a phase
disappears completely, the ripple voltage increases. This
causes the frequency converter to trip and issue the Alarm
4. In addition to missing phase voltage, increased bus
ripple can be caused by a line disturbance or imbalance.
Line disturbances may be caused by line notching,
defective transformers or other loads that may be effecting
the form factor of the AC waveform. Mains imbalances
which exceed 3% cause sufficient DC bus ripple to initiate
a trip.
Output disturbances can have the same effect of increased
ripple voltage on the DC bus. A missing or lower than
normal output voltage on one phase can cause increased
ripple on the DC bus. When a mains imbalance trip occurs,
it is necessary to check both the input and output voltage
of the frequency converter.
Severe imbalance of supply voltage or phase loss can
easily be detected with a voltmeter. Line disturbances
most likely need to be viewed through an oscilloscope.
Conduct tests for input imbalance of supply voltage, input
waveform, and output imbalance of supply voltage as
described in the chapter Troubleshooting.
5.1.3 Control Logic Problems
Problems with control logic can often be difficult to
diagnose, since there is usually no associated fault
indication. The typical complaint is simply that the
frequency converter does not respond to a given
command. There are two basic commands that must be
given to any frequency converter in order to obtain an
output. First, the frequency converter must be told to run
(start command). Second, the frequency converter must be
told how fast to run (reference or speed command).
The frequency converters are designed to accept a variety
of signals. First determine what types of signals the
frequency converter is receiving. There are six digital
inputs (terminals 18, 19, 27, 29, 32, 33), two analog inputs
(53 and 54), and the fieldbus (68, 69). The presence of a
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