How to Maintain a Variable Frequency Drive?
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retro-fitting existing equipment with variable frequency drives. Contact your
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How To Maintain a VFD
By: Dave Polka
Do you know how to maintain Variable Frequency Drives (VFDs)?
Doing so is easier than you might think. By integrating some simple, logical
steps into your preventative maintenance program, you can ensure your drives
provide many years of trouble-free service. Before looking at those steps,
let's quickly review what a VFD is and
how it works.
A Quick Overview
A VFD controls the speed, torque and direction of an AC
Induction motor. It takes fixed voltage and frequency AC input and converts it
to a variable voltage and frequency AC output. See Training Note "What
is a VFD?" for a more detailed description of VFD concepts
and operating principles. In very small VFDs, a single power pack unit
may contain the converter and
inverter.
Fairly involved control circuitry coordinates the switching
of power devices, typically through a control board that dictates the
firing of
power components in the proper sequence. A microprocessor or Digital Signal
Processor (DSP) meets all the internal logic and decision requirements.
From this description, you can see a VFD is basically a
computer and power supply. And the same safety and equipment precautions
you'd apply to a computer and to a power supply apply here. VFD maintenance
requirements fall into three basic categories:
- keep it clean;
- keep it dry; and
- keep the connections tight.
Let's look at each of these.
Keep it Clean
Most VFDs fall into the NEMA 1 category (side vents for
cooling airflow) or NEMA 12 category (sealed, dust-tight enclosure). Drives
that fall in the NEMA 1 category are susceptible to dust contamination. Dust
on VFD hardware can cause a lack of airflow, resulting in diminished
performance from heat sinks and circulating fans (Photo 1).
Photo 1, Fan Injecting Dust into Drive Enclosure
Dust on an electronic device can cause malfunction or
even failure. Dust absorbs moisture, which also contributes to failure.
Periodically spraying air through the heat sink fan is a good PM measure.
Discharging compressed air into a VFD is a viable option in some environments,
but typical plant air contains oil and water. To use compressed air for
cooling, you must use air that is oil-free and dry or you are likely
to do more harm than good. That requires a specialized, dedicated, and
expensive
air supply. And you still run the risk of generating electrostatic charges
(ESD).
A non-static generating spray or a reverse-operated ESD
vacuum will reduce static build-up. Common plastics are prime generators
of static
electricity. The material in ESD vacuum cases and fans is a special,
non-static generating plastic. These vacuums, and cans of non-static generating
compressed air, are available through companies that specialize in static
control equipment.
Keep it Dry
In Photo 2 you can see what happened to a control board
periodically subjected to a moist environment. Initially, this VFD was
wall-mounted in a clean, dry area of a mechanical room and moisture was
not a problem. However, as is often the case, a well-meaning modification
led to
problems.
In this example, an area of the building required a
dehumidifier close to the mechanical room. Since wall space was
available above the VFD, this is where the dehumidifier went. Unfortunately,
the VFD was a NEMA 1 enclosure style (side vents and no seal around the
cover). The obvious result was water dripping from the dehumidifier into
the drive. In six months, the VFD accumulated enough water to produce circuit
board corrosion.
Photo 2, Corrosion on Board Traces Caused by Moisture
What about condensation? Some VFD manufacturers included
a type of "condensation protection" on earlier product versions.
When the mercury dipped below 32 degrees Fahrenheit, the software logic
would not
allow the drive to start. VFDs seldom offer this protection today. If you
operate the VFD all day every day, the normal radiant heat from the heatsink
should prevent condensation. Unless the unit is in continuous operation,
use a NEMA 12 enclosure and thermostatically controlled space heater
if you locate
it where condensation is likely.
Keep Connections Tight
While this sounds basic, checking connections is a step
many people miss or do incorrectly - and the requirement applies even
in clean
rooms. Heat cycles and mechanical vibration can lead to sub-standard
connections, as can standard PM practices. Retorquing screws is not a good
idea, and further tightening an already tight connection can ruin the
connection (see Sidebar).
Bad connections eventually lead to arcing. Arcing at
the VFD input could result in nuisance over voltage faults, clearing
of input fuses,
or damage to protective components. Arcing at the VFD output could result
in over-current faults, or even damage to the power components. Photos
3 and 4
show what can happen.
Loose control wiring connections can cause erratic operation.
For example, a loose START/STOP signal wire can cause uncontrollable VFD
stops. A loose speed reference wire can cause the drive speed to fluctuate,
resulting in scrap, machine damage, or personnel injury.
Photo 3, Arcing Caused by Loose Input Contacts
Photo 4, Arcing Caused by Loose Output Contacts
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Re-torquing - A Screwy Practice
Although "re-torquing" as a way of checking tightness is
common in many PM procedures, it violates basic mechanical
principles and does more harm than good. A screw has maximum
clamping power at a torque value specific to its size, shape, and
composition. Exceeding that torque value permanently reduces the
clamping power of that screw by reducing its elasticity and
deforming it. Loosening and then re-torquing still reduces
elasticity, which still means a loss of clamping power. Doing this
to a lock washer results in a permanent 50% loss. What should you
do? Use an infrared thermometer to note hot connections. Check their
torque. If they have merely worked loose, you can try retightening
them. Note which screws were loose, and be sure to give them an IR
check at the next PM cycle. If they are loose again, replace them.
Finally, don't forget the "tug test." This checks
crimps, as well as screw connections. Don't do this with the drive
online with the process, though, or you may cause some very
expensive process disturbances.
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Additional Steps
- As part of a mechanical inspection procedure, don't overlook internal
VFD components. Check circulating fans for signs of bearing failure or
foreign objects - usually indicated by unusual noise or shafts that appear
wobbly.
- Inspect DC bus capacitors for bulging and leakage. Either could be a
sign of component stress or electrical misuse. Photos 5 and 6 show fan
and capacitor stress problems.
Photo 5, Foreign Object in Fan
- Take voltage measurements while the VFD is in operation.
Fluctuations in DC bus voltage measurements can indicate degradation of
DC bus capacitors. One function of the capacitor bank is to act as a
filter section
(smoothing out any AC ripple voltage on the Bus). Abnormal AC voltage on
the DC bus indicates the capacitors are headed for trouble.
Most VFD manufacturers have a special terminal block for this type of
measurement and also for connection of the dynamic braking resistors.
Measurements more than 4VAC may indicate a capacitor filtering problem
or a possible problem with the diode bridge converter section (ahead
of the bus). If you have such voltage levels, consult the VFD manufacturer
before taking further action.
With the VFD in START and at zero speed, you should read output voltage
of 40VAC phase-to-phase or less. If you read more than this, you may
have transistor leakage. At zero speed, the power components should not
be operating. If your readings are 60VAC or more, you can expect power
component failure.
- What about spare VFDs? Store them in a clean, dry environment, with no
condensation allowed. Place this unit in your PM system so you know to
power it up every 6 months to keep the DC bus capacitors at their peak
performance capability. Otherwise, their charging ability will significantly
diminish. A capacitor is much like a battery-it needs to go into service
soon after purchase or suffer a loss of usable life.
Photo 6, Capacitor Failure
- Regularly monitor heat sink temperatures. Most VFD manufacturers make
this task easy by including a direct temperature readout on the Keypad
or display. Verify where this readout is, and make checking it part of
a weekly or monthly review of VFD operation. You wouldn't place your laptop
computer outside, on the roof of a building or in direct sunlight, where
temperatures could reach 115 degrees Fahrenheit or as low as -10 degrees
Fahrenheit. A VFD, which is basically a computer with a power supply, needs
the same consideration. Some VFD manufacturers advertise 200,000 hours-almost
23 years-of Mean Time Between Failures (MTBF). Such impressive performance
is easy to obtain, if you follow these simple procedures.
This information has been
provided by: ABB Inc. - Drives and Power Electronics
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1724 Tomich Court