Motor Drive Systems: Maintenance/Troubleshooting

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Whether it be AC or DC drive systems, maintenance of the electronic equipment is rather easy if some basic steps are followed. In this section, the focus will be on the preventative maintenance of drive equipment. If a drive should malfunction, some practical tips will be presented on how to find the source of the problem.

First and foremost, the focus will be on how to avoid drive problems in the first place. By integrating some simple, logical steps into a preventative maintenance program, drives can provide many years of trouble-free ser vice. Before looking at those steps, a quick review of variable-frequency drives (VFDs) is in order.

A VFD controls the speed, torque, and direction of an AC induction motor.

It takes fixed-voltage and fixed-frequency AC input and converts it to a variable-voltage and variable-frequency AC output. 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.

As noted earlier (Drive Types) a VFD is basically a computer and power supply. The same safety and equipment precautions you would apply to a computer or a power supply apply here.

Routine Drive Maintenance

VFD maintenance requirements fall into three basic categories:

  • 1. Keep it clean.
  • 2. Keep it dry.
  • 3. Keep the connections tight.

Keep It Clean

Most VFDs fall into the NEMA 1 category (side vents for cooling air flow) 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.

---Fan injecting dust into drive enclosure (ABB Inc.)

--- Corrosion on board traces caused by moisture.

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 preventative-maintenance mea sure. Discharging compressed air into a VFD is a viable option in some environments, but typical industrial 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 buildup. 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 com pressed air are available through companies that specialize in static-control equipment.

Keep It Dry

Notice what happened to a control board periodically subjected to a moist environment.

Initially, the 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.

What about condensation? Some VFD manufacturers included a type of condensation protection on earlier product versions. When the temperature dropped below 32ºF, 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 a thermostatically controlled space heater if you locate it where condensation is likely.

---Arcing caused by loose input contacts

---Arcing caused by loose output contacts

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 substandard connections, as can standard preventative-maintenance practices. Re-torquing screws is not a good idea, and further tightening an already tight connection can ruin the connection.

Here's why:

Although re-torquing as a way of checking tightness is common in many preventative-maintenance 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 also reduce 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. (A "hot connection" results when poor contact is made between two conductors, which causes increased resistance and an elevated temperature in the connection.) Check their torque (using a torque wrench, and matching the value against the manufacturer's installation recommendations). If they have merely worked loose, you can try re-tightening them.

Note which screws were loose and be sure to give them an infrared check at the next preventative maintenance cycle. If they are loose again, replace them.

---Foreign object in fan

---Capacitor failure

Finally, don't forget the "tug test." To do the "tug test" simply pull, with moderate pressure, on each of the wires inserted into terminal blocks. This should be done to both power and control terminal connections. This checks crimps, as well as screw connections. Don’t do this with the drive online with the process, or you may cause some very expensive process disturbances.

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-cur rent faults or even damage to the power components. 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 personal injury.

Additional Steps Beyond Routine

1. 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.

2. Inspect DC bus capacitors for bulging and leakage. Either could be a sign of component stress or electrical misuse. Fan and capacitor stress problems.

3. 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 4 VAC 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 40 VAC 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 60 VAC or more, you can expect power component failure.

4. What should be done with spare VFDs? Store them in a clean, dry environment, with no condensation allowed. Place this unit in your preventative-maintenance system so you will remember to power it up every 6 months or so 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 it will suffer a loss of usable life.

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 readout is and make checking it part of a weekly or monthly review of VFD operation.

You would not place your laptop computer outside under the roof of a building or in direct sunlight where temperatures could reach 115ºF or as low as -10ºF. 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.

General Troubleshooting

Even if the steps outlined above are followed, there is still a possibility of a drive malfunction. If that does occur, the obvious questions about where you should start would be, "Check at the motor? Check at the drive input? Check at the drive display?" The answers are-yes.

There have been many guides, articles, and pamphlets written about the proper troubleshooting techniques. Drives would certainly be no different than any other device, mechanical or electronic. However, the following simple tips may assist in troubleshooting a specific electronic drive problem. Some technicians identify a starting point at the source of torque development-the motor. Once the motor is cleared of any wrongdoing, then move backward to the drive, and then to the input to the drive.

Therefore, the logical place to start is at the motor. A simple but important question to ask is, "Should it be turning?" This may seem like a ridiculous statement, but the question does bear asking. Maybe the motor should not be turning because of a mechanical brake that didn't release. Maybe there is some other mechanical reason the motor is at a standstill. If the motor is supposed to be turning, then it’s time to move backward and check the drive.

Standard drive questions are as follows: "Does the drive have power? Is the circuit breaker closed? Are the fuses OK? Is the disconnect operational? Is the display operational?" If the drive appears to have no power, don’t assume that the drive is dead, with no power. A drive panel or display could be malfunctioning; connections to the panel could be broken.

Always verify power to the drive by checking the input power terminals or DC bus terminals.

If the display is operational, then it’s time to check the two items that satisfy a drive-a start command and speed reference. The easiest place to check it would be on the drive's I/O status section of parameters. This is a read-only section that some drive manufacturers include in the list of parameters. By viewing the display with 1's and 0's, it’s easy to see if the drive has a start command. It’s also easy to see if the drive has a run enable signal. Another place to look for drive status is in the operating data section. Some manufacturers include this section in parameters and include a multitude of items such as DC bus voltage, output volts and amps, heat-sink temperature, analog input values, reference input values, etc.

If the drive truly is receiving a start command and speed reference from an external source, then the problem is with the drive. If, through viewing the I/O status or operating data, it’s determined that the drive is not receiving a start command and speed reference, the problem is with the external devices (e.g., operator station, PLC, auto-control system, etc.). The easiest way to tell if the drive is the problem is to operate the drive from keypad mode (no external connections). If the drive runs the motor up and down in speed from keypad mode, then the problem is external to the drive (wiring, sensors, and automated system). If the drive does not run the motor up and down in speed, then the problem lies with the drive.

Further investigation into the drive is required.

The latest versions of drives on the market offer varying degrees of troubleshooting through software. Some manufacturers present a display with hints on where to check (e.g., "comm loss," "control bd error," etc.). Some manufacturers go one step further and offer a computer software support tool that identifies the source of the software or hardware problem. It then sends the user to a page that lists possible problems and the effective remedies.

In all cases, the drive manufacturer's user manual or installation and troubleshooting manual should be consulted for specific recommendations.

The manufacturer is the best judge as to how far to go with repairs. Given the cost and simplicity of some fractional and low horsepower drives, one option may be to purchase a new drive rather than spend the time trying to repair a component on a very dense circuit board.

REVIEW

To avoid having to troubleshoot a drive, there are several steps in maintaining a healthy drive:

Keep it clean and dry and keep the connections tight.

The heat sink should be routinely sprayed with compressed air to reduce any buildup of dust and particles on the chassis. Avoid inducing com pressed air into the drive electronics unless it’s ionized air or compresses air, specially packaged to reduce ESD contamination.

Ensure that the drive is not located in an area where moisture could be of concern. Less than 95% humidity with no condensation allowed is the standard rating for all drives. Moisture is a major enemy of electronics, as is excessive heat.

Check tightness of connections as part of a routine preventative maintenance procedure. Don't over-tighten power connections. Mechanically overstressing bolts or nuts could actually reduce their clamping power.

Routine inspection of the drive's electronic and mechanical parts is helpful. Periodic voltage measurements will indicate if the drive is headed for trouble in the near future. Be aware that even in start, with zero speed, the drive will have output voltage phase to ground-maybe as high as 40 V. If the reading exceeds 60V, power-component failure could occur in the near future. Power up spare VFDs once every 6 months or so. The DC bus capacitors ability diminishes if it’s not electrically stressed periodically.

In general troubleshooting, start at the motor and work backward. It must be determined first if the motor should be turning. Once that is deter mined, then move to the drive and verify the status. Verify that the drive operates the motor in keypad (drive-control mode). If the answer is "yes," then the drive is not the problem. The problem exists with equipment external to the drive. Check all interface wiring and components connected to the drive.

QUIZ

1. What are the three main procedures in maintaining a drive?

2. Why is accumulated dust on circuit boards a potential problem?

3. What is the procedure for removing accumulated dust on boards?

4. Why is it not a good idea to continually retighten a screw or nut?

5. What are specifics on checking capacitors?

6. What is the easiest method of determining if the drive has a problem?

7. Where would an operator look in a drive to obtain specific information on status or fault conditions?

-- Answers --

1. Keep it clean, keep it dry, and keep the connections tight.

2. Dust attracts moisture. Moisture can cause the microelectronic circuits to misfire or develop a short. In addition, moisture assists in the development of corrosion. Corrosion on circuit board traces will eventually cause the electrical contacts to disintegrate.

3. Special non-ESD generating air should be used. Cans of ionized air are available for this purpose. Static electricity (ESD-electrostatic discharge) is the silent killer of small microcircuits that cannot handle several hundred volts of static charge.

4. Retightening (re-torquing) of screws and nuts causes a loss of clamping power-in some cases, as much as 50%. A tendency exists to over-tighten the device, which causes a reduction of compression strength. Replace devices that work loose, rather than keep retightening them.

5. Check capacitors periodically for leaks and bulges. Capacitors should be completely dry and not be deformed in any way. DC bus capacitor measurements should also be periodically done. AC ripple voltage of more than 4 VAC would indicate a possible capacitor malfunction. Spare drives should be powered up every 6 months or so. This keeps the capacitors at their maximum charging capability.

6. Isolate the drive from the external control signals. If the drive satisfactorily operates the motor when in keypad mode, then the drive is not the problem and something external to the drive is causing the problem. If the drive does not operate the motor satisfactorily, then check all inputs to the drive. The drive must have two conditions to operate: start command and speed reference.

7. The operator would look at the I/O status location for the conditions of the digital inputs, analog inputs, etc. A section called operating data would also be helpful in determining drive "healthiness" as well as the last several faults that occurred.

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