How to Maintain a Variable Frequency Drive

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Home > Services > How to Maintain a VFDWednesday, September 11

2013

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Training Notes

How to Maintain a Variable Frequency

Drive?

Many utilities are now offering rebates for the installation of VFD's or retro-fitting existing equipment with variable frequency drives. Contact your localutility or search VFD rebate or go to the DSIREwebsite, a comprehensivesource of information on state, local, utility and federal incentives andpolicies that promote renewable energy and energy efficiency.

Joliet Technologies has partnered with

Electrical Solutions Network (ESN), inour effects to promote energyefficiency through the use of variablespeed drives.

Nationwide energy reduction incentives.

How To Maintain a VFDBy: Dave Polka

Do you know how to maintain Variable Frequency Drives(VFDs)? Doing so is easier than you might think. Byintegrating some simple, logical steps into your preventativemaintenance program, you can ensure your drives providemany years of trouble-free service. Before looking at thosesteps, 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 ACInduction motor. It takes fixed voltage and frequency ACinput and converts it to a variable voltage and frequency ACoutput. See Training Note "What is a VFD?"for a moredetailed description of VFD concepts and operating principles.In very small VFDs, a single power pack unit may contain theconverter and inverter.

Fairly involved control circuitry coordinates the switching ofpower devices, typically through a control board that dictatesthe firing of power components in the proper sequence. Amicroprocessor or Digital Signal Processor (DSP) meets all

the internal logic and decision requirements.

From this description, you can see a VFD is basically acomputer and power supply. And the same safety andequipment precautions you'd apply to a computer and to apower supply apply here. VFD maintenance requirements fallinto three basic categories:

keep it clean;keep it dry; andkeep the connections tight.

Let's look at each of these.

Keep it Clean

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Motor Bearing Currents Under

PWM Variable Speed Drive

Operation

posted on May 29, 201301:19:40 pm

Readers? Questions ? Part III

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Readers? Questions ? Part II

posted on April 16, 2013

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This information provided by:

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Technologies to get the answers.

Most VFDs fall into the NEMA 1 category (side vents forcooling airflow) or NEMA 12 category (sealed, dust-tightenclosure). Drives that fall in the NEMA 1 category aresusceptible to dust contamination. Dust on VFD hardware cancause a lack of airflow, resulting in diminished performancefrom heat sinks and circulating fans (Photo 1).

Photo 1, Fan Injecting Dust into Drive Enclosure

Dust on an electronic device can cause malfunction or evenfailure. Dust absorbs moisture, which also contributes tofailure. Periodically spraying air through the heat sink fan is agood PM measure. Discharging compressed air into a VFD is aviable option in some environments, but typical plant aircontains oil and water. To use compressed air for cooling, youmust use air that is oil-free and dry or you are likely to domore harm than good. That requires a specialized, dedicated,and expensive air supply. And you still run the risk ofgenerating electrostatic charges (ESD).

A non-static generating spray or a reverse-operated ESDvacuum will reduce static build-up. Common plastics areprime generators of static electricity. The material in ESD

vacuum cases and fans is a special, non-static generatingplastic. These vacuums, and cans of non-static generatingcompressed air, are available through companies thatspecialize in static control equipment.

Keep it Dry

In Photo 2 you can see what happened to a control boardperiodically subjected to a moist environment. Initially, thisVFD was wall-mounted in a clean, dry area of a mechanicalroom and moisture was not a problem. However, as is oftenthe case, a well-meaning modification led to problems.

In this example, an area of the building required adehumidifier close to the mechanical room. Since wall spacewas available above the VFD, this is where the dehumidifierwent. Unfortunately, the VFD was a NEMA 1 enclosure style(side vents and no seal around the cover). The obvious resultwas water dripping from the dehumidifier into the drive. In sixmonths, the VFD accumulated enough water to produce circuitboard corrosion.
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Photo 2, Corrosion on Board Traces Caused by Moisture

What about condensation? Some VFD manufacturers includeda type of "condensation protection" on earlier productversions. When the mercury dipped below 32 degreesFahrenheit, the software logic would not allow the drive tostart. VFDs seldom offer this protection today. If you operatethe VFD all day every day, the normal radiant heat from the

heatsink should prevent condensation. Unless the unit is incontinuous operation, use a NEMA 12 enclosure andthermostatically controlled space heater if you locate it wherecondensation is likely.

Keep Connections Tight

While this sounds basic, checking connections is a step manypeople miss or do incorrectly - and the requirement applieseven in clean rooms. Heat cycles and mechanical vibrationcan lead to sub-standard connections, as can standard PMpractices. Retorquing screws is not a good idea, and furthertightening an already tight connection can ruin the connection(see Sidebar).

Bad connections eventually lead to arcing. Arcing at the VFDinput could result in nuisance over voltage faults, clearing ofinput fuses, or damage to protective components. Arcing atthe VFD output could result in over-current faults, or evendamage to the power components. Photos 3 and 4 show whatcan happen.

Loose control wiring connections can cause erratic operation.For example, a loose START/STOP signal wire can causeuncontrollable VFD stops. A loose speed reference wire cancause the drive speed to fluctuate, resulting in scrap, machinedamage, or personnel injury.

Photo 3, Arcing Caused by Loose Input Contacts


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Photo 4, Arcing Caused by Loose Output Contacts

Re-torquing - A Screwy PracticeAlthough "re-torquing" as a way of checking tightness is commonin many PM procedures, it violates basic mechanical principlesand does more harm than good. A screw has maximum clampingpower at a torque value specific to its size, shape, andcomposition. Exceeding that torque value permanently reduces

the clamping power of that screw by reducing its elasticity anddeforming it. Loosening and then re-torquing still reduceselasticity, which still means a loss of clamping power. Doing thisto a lock washer results in a permanent 50% loss. What shouldyou do? Use an infrared thermometer to note hot connections.Check their torque. If they have merely worked loose, you cantry retightening them. Note which screws were loose, and besure to give them an IR check at the next PM cycle. If they areloose again, replace them. Finally, don't forget the "tug test."This checks crimps, as well as screw connections. Don't do thiswith the drive online with the process, though, or you may causesome very expensive process disturbances.

Additional Steps

1. As part of a mechanical inspection procedure, don't overlook internalVFD components. Check circulating fans for signs of bearing failureor foreign objects - usually indicated by unusual noise or shafts thatappear wobbly.

2. Inspect DC bus capacitors for bulging and leakage. Either could be asign of component stress or electrical misuse. Photos 5 and 6 showfan and capacitor stress problems.

Photo 5, Foreign Object in Fan

3. Take voltage measurements while the VFD is in operation.


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Fluctuations in DC bus voltage measurements can indicatedegradation of DC bus capacitors. One function of the capacitor bankis to act as a filter section (smoothing out any AC ripple voltage onthe Bus). Abnormal AC voltage on the DC bus indicates thecapacitors are headed for trouble.

Most VFD manufacturers have a special terminal block for this typeof measurement and also for connection of the dynamic brakingresistors. Measurements more than 4VAC may indicate a capacitorfiltering problem or a possible problem with the diode bridgeconverter 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 outputvoltage of 40VAC phase-to-phase or less. If you read more than this,you may have transistor leakage. At zero speed, the powercomponents should not be operating. If your readings are 60VAC ormore, you can expect power component failure.

4. What about spare VFDs? Store them in a clean, dry environment,with no condensation allowed. Place this unit in your PM system soyou know to power it up every 6 months to keep the DC buscapacitors at their peak performance capability. Otherwise, theircharging ability will significantly diminish. A capacitor is much like abattery-it needs to go into service soon after purchase or suffer aloss of usable life.

Photo 6, Capacitor Failure

5. Regularly monitor heat sink temperatures. Most VFD manufacturersmake this task easy by including a direct temperature readout on theKeypad or display. Verify where this readout is, and make checkingit part of a weekly or monthly review of VFD operation. You wouldn'tplace your laptop computer outside, on the roof of a building or indirect sunlight, where temperatures could reach 115 degreesFahrenheit or as low as -10 degrees Fahrenheit. A VFD, which isbasically a computer with a power supply, needs the sameconsideration. 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 simpleprocedures.

This information has been provided by: ABB Inc. - Drives and PowerElectronics

Joliet Technologies, L.L.C.Toll Free (866) 492-9888 1724 Tomich Court Crest Hill, IL 60403

Phone: (815) 725-9696 Fax: (815) 725-9393 E-mail: [email protected]

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How to Maintain a Variable Frequency Drive