vfd maintence

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Dave PolkaABBs Drives & PowerProducts Group

Do you know how to maintain a variable frequency drive (VFD)? Its easier than you

might think. By integrating some simple, logical steps into your preventive main-

tenance (PM) program, you can ensure your drives provide many years of trouble-free

service. Before looking at those steps, however, lets quickly review what a VFD is and

how it works.

A VFD controls the speed, torque, and

direction of an AC induction motor.

It takes fixed-voltage, fixed-

frequency AC input and con-

verts it to a variable-voltage,

variable-frequency AC output.

In very small VFDs, a single

power-pack unit might contain

the converter and inverter.

Intricate control circuitry coordinates

the switching of power devices, typicallythrough a control board that dictates fir-

ing power components in the proper

sequence. A microprocessor or digital sig-

nal processor meets all the internal logic

and decision requirements.

From this description, you can see a

VFD is essentially little more than a com-

puter and a power supply. For that rea-

son, the same safety and equipment

precautions youd apply to a computer/

power supply combination apply here.

VFD maintenance requirements fall into

three basic categories:

Keep it clean.

Keep it dry.

Keep the connections tight.

Lets look at each of these.

Keep It Clean

Most VFDs fall into either the NEMA 1

(side vents for cooling airflow) or the

NEMA 12 (sealed, dust-tight enclosure)

category. Drives categorized as NEMA 1

are susceptible to dust contamination (seeFigure 1). Dust on an electronic device

(such as VFD hardware) can cause a mal-

function or even failure. It contributes to a

lack of airflow, resulting in diminished

performance from heat sinks and circu-

lating fans.

Dust absorbs moisture, which also

plays a role in failure. Periodically spray-

ing air through the heat sink fan is a good

PM measure. However, although dis-

charging compressed air into a VFD is a

viable option in some environments, typ-ical plant air contains oil and water. To

use compressed air for cooling, you must

BasicVFDMaintenance

Its not hard to

keep a drive running,

if you follow a few

simple steps.

34 June 2001 Motion Control

This VFD shows evidence of fan-injected dust particles.1


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use dry, oil-free air or youre likely to do more

harm than good. This, in turn, requires a spe-

cialized, dedicated, and expensive air supply.

Moreover, you still run the risk of generating

electrostatic discharges (ESDs). A non-static-

generating spray or a reverse-operated ESD vac-

uum reduces static buildup. Common plastics

are prime generators of static electricity. Thematerial in ESD vacuum cases and fans is a spe-

cial, non-static-generating plastic. These vacuums,

and cans of non-static-generating compressed

air, are available through companies that spe-

cialize in static control equipment.

Keep It Dry

Figure 2 shows a control board that was period-

ically subjected to a moist environment. Initially,

this VFD was wall mounted in a clean, dry area

of a mechanical room, and moisture wasnt a

problem. However, as is often the case, a well-

meaning modification led to problems. An areaof the building required a dehumidifier close to

the mechanical room. Because wall space was

available above the VFD, this is where the dehu-

midifier went. Unfortunately, this was a NEMA

1style VFD. The obvious result: Water dripped

from the dehumidifier into the drive. In six

months, the VFD accumulated enough water to

produce circuit board corrosion.

What about condensation? Some VFD manu-

facturers included a type of condensation pro-

tection on earlier product versions. When the

mercury dipped below 32F, the software logicwouldnt 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 heat

sink should prevent condensation.

However, unless the unit is in con-

tinuous operation, use a NEMA 12

enclosure and thermostatically con-

trolled space heater if you locate it

where condensation is likely.

Keep Connections Tight

While this sounds simple, checkingconnections is a step many people

miss entirely or do incorrectlyand

the requirement applies even in clean

rooms. Heat cycles and mechanical

vibration can lead to substandard con-

nections, as can standard PM prac-

tices. Retorquing screws isnt a good

idea, and further tightening an

already tight connection can ruin it

(see sidebar, page 36).

Bad connections eventually lead

to arcing. Arcing at the VFD inputcould result in nuisance overvoltage

faults, clearing of input fuses, or

damage to protective components. Arcing at

the VFD output could result in overcurrent

faults or even damage to the power compo-

nents. Figures 3 and 4 demonstrate potential

results.

Loose control wiring connections can cause

erratic operation. For example, a loose START/

STOP signal wire can cause uncontrollable VFDstops. A loose speed reference wire can cause

the drive speed to fluctuate, resulting in scrap,

machine damage, or personnel injury.

Control

June 2001 Motion Control 35

Loose output power connections.4

Loose input powerconnections.3

Corrosion caused byexcessive moisture.2


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Additional Steps

1) As part of a mechanical in-

spection procedure, dont over-

look internal VFD components.

Check circulating fans for signs

of bearing failure or foreign

objectsusually indicated by

unusual noise or shafts thatappear wobbly (Figure 5).

2) Inspect DC bus capacitors for

bulging and leaking (Figure 6).

Either could be a sign of compo-

nent stress or electrical misuse.

3) Take voltage measurements

while the VFD is in operation.

Fluctuations in DC bus voltage

measurements can indicate deg-

radation 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 indi-

cates the capacitors are headed

for trouble. Most VFD manufac-

turers have a special terminal

block for this type of measure-

ment and for connecting the

dynamic braking resistors. Mea-

surements more than 4 VAC

may indicate a capacitor filter-

ing problem or a possible prob-

lem with the diode bridge

converter section (ahead of the

bus). If you have such voltagelevels, consult the VFD manu-

facturer before taking further

action.

With the VFD in START and

at zero speed, you should read

output voltage of 40 VAC phase

to phase or less. If you read

more than this, you may have

transistor leakage. At zero

speed, the power components

shouldnt be operating. If your

readings are 60 VAC or more,

you can expect power compo-

nent failure.

4) What about spare VFDs? Store

them in a clean, dry environ-

ment, with no condensation

allowed. Place this unit in your

PM system so you know to

power it up every six months to

keep the DC bus capacitors at

their peak performance capabil-

ity. Otherwise, their charging

ability will significantly dimin-

ish. A capacitor is much like abattery: It needs to go into ser-

vice soon after purchase or suf-

fer a loss of usable life.

Control

36 June 2001 Motion Control

An obvious capacitor stress problem.6

A fan with a foreign object in it.5

5) 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 read-

out is, and make checking it part of a weekly

or monthly review of VFD operation.

You wouldnt place your laptop computeroutside, on the roof of a building, or in direct

sunlight, where temperatures could reach as

high as +115F or as low as 10F. A VFD, which

is essentially a computer with a power supply,

needs the same consideration. Some VFD man-

ufacturers advertise 200,000 hoursalmost

23 yearsof mean time between failures. Such

impressive performance is easy to obtain if you

follow these simple procedures. MC

Make Contact!

Dave Polka is the drives training manager for ABBs

Drives & Power Products Group. Contact him at

16250 West Glendale Drive, New Berlin, WI 53151;

tel: (414) 785-3517; fax: (414) 789-8608; dave.polka@

us.abb.com; www.abb.com/usa.

RetorquingA Screwy Practice

Although retorquing 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 perma-

nently reduces the screws clamp-

ing power by reducing its elasticity

and deforming it. Loosening and

then retorquing still reduces elas-

ticity, which still means a loss of

clamping power. Doing this to a

lock washer results in a permanent50% loss. What should you do?

Use an infrared (IR) thermometer

to note hot connections. Check

their torque. If theyve merely

worked loose, try retightening

them. Note which screws were

loose, and be sure to give them an

IR check at the next PM cycle. If

theyre loose again, replace them.

Finally, dont forget the tug test.

This checks crimps, as well as

screw connections. Dont do this

while the drives online running the

process, though, or you may causesome very expensive disturbances.

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