A Variable Frequency Drive with Nusiance Overload Trips
Copyright © 2005 Francis J. Martino
I am currently attempting to find a problem with a VFD running the blower fan on two
identical boilers. The drives trip on overload at least twice a week. There is a problem with
harmonics, and the boilers are located only about 40m from the main distribution. We are
currently looking at line reactors, but according to the manufacturer's manuals for
drives, they have an AC choke built-in. My questions are--would putting a line reactor
in front of the drive have any effect? Is the choke in the drive of good quality?
All drives will generate harmonics, a six pulse drive being the
type that will generate the highest amplitude in the most troublesome
of harmonics: the fifth harmonic.
Ask the manufacturer what the impedance rating of the chokes are.
You need three to five percent of impedance. Add a reactor if necessary.
With a high system operating pressure and an operating speed near ninety per cent, the drive
will be overloaded if it is programmed for variable torque. Almost all variable torque applications
need the drive to be programmed for constant torque. Measure the actual current and compare
it to the current that the motor should be drawing based on its per cent loading.
See the second paragraph of footnote three:
A input voltage sag in one of the three phases will cause two of the six input diodes to be
reversed biased and stop conducting. The drive will then "see" three phases (because all three
phases are present) but operate on single phase because the voltage sag has shut down two of
the diodes. That condition will generate a third harmonic which is typical of a single
phase non-linear power supply.
The result will be a high rms ripple on the DC bus which will be interpreted by the drive as a
high AC current component. The drive will then trip on overcurrent protection. A reactor will
help only sags of very short durations. Long duration sags will not be mitigated with reactors.
The type of drive which creates an imbalance through a "lead phase" will be found to have an
input bridge of three diodes and three SCRs. It is an active front end which allows a lower
charging inrush and eliminates the need for a charging resistor and charging relay.
The drive may also be designed to operate continuously under light load with the third
phase conducting but supplying low current. Thus, when lightly loaded, the drive input
presents a predominately single phase load and, with it, increased harmonic generation that is
inherent with an imbalanced load. I have seen no harmonic studies of the imbalance and probably
Too bad the manufacturers have, in eliminating the charging resistor, chosen to lower the cost
of their drives by a hundred dollars rather than use an active front end to reduce harmonic generation.
The marketing people are firmly in control and would have their customers spend a thousand
dollars on harmonic filters so the drive could be a lousy hundred hundred bucks cheaper.
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