Danfoss VLT HVAC FC 100, Руководство по обслуживанию, Страница: 82 / 220

seconds, overvoltage control will initially engage and then disengage and allow the adjustable
frequency drive 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 adjustable frequency drive
monitors the level of the DC bus. Should the level become 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 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 adjustable
frequency drive is running at a steady state speed, the overvoltage circuit will increase the fre-
quency to match the speed of the load. The same restriction on the amount of influence applies.
The adjustable frequency drive will add about 10% to the base speed before a trip occurs. Oth-
erwise, 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. 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 causing the adjustable frequency drive to trip and issue an Alarm 4. In addition to miss-
ing 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. Line 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.
Should a line imbalance trip occur, it is necessary to check both the input and output voltage of
the adjustable frequency drive.
Severe phase imbalance or phase loss can easily be detected with a voltmeter. Line disturbances
most likely need to be viewed on an oscilloscope. Conduct tests for input imbalance of supply
voltage, input waveform, and output phase imbalance as described in the chapter
Troubleshoot-
ing
.
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 adjustable frequency drive does not re-
spond to a given command. There are two basic commands that must be given to any adjustable
frequency drive in order to obtain an output. First, the adjustable frequency drive must be told to
run (start command). Second, the adjustable frequency drive must be told how fast to run (ref-
erence or speed command).
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
5 Adjustable Frequency Drive and Motor Ap-
plications
MG.90.L1.22 - VLT ® is a registered Danfoss trademark
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