Plant & Works Engineering
What causes variable-speed drive trips and failures?
Published:  04 February, 2016

Alan Jones, sales director for APDS, and one of ABB’s authorised value providers based in Bristol, gives an insight into why drives fail and how to prevent such occurrences.

A variable-speed drive (VSD) could have a mean time between failure of over half-a-million hours (533,200 hours actually or nearly 62 years). Today’s drives are exceptionally reliable, but they’re not indestructible. A drive will get nowhere near these figures if it is not correctly maintained.

The maintenance requirements for VSDs fall into four basic categories:

Check motors - Check your electric motors before installing a VSD. This ensures that the motor is the right size and operating efficiently. Some old motor models may not be compatible with VSDs.

Keep it clean - Dust on VSD hardware can cause a lack of airflow, resulting in diminished performance from heat sinks and circulating fans. Dust absorbs moisture and on an electronic device this can cause malfunction or even failure. Periodically spraying air through the heat sink fan is a good preventive maintenance measure. Discharging compressed air into a VSD is a viable option in some environments, but typical plant air contains oil and water. So when using compressed air for cooling, you must ensure that it is oil-free and dry or you are likely to do more harm than good. 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.

Keep it dry - Corrosion caused by moisture on a circuit board can happen at any time. In one instance a VSD was wall-mounted in a clean, dry area of a mechanical room and moisture was not a problem. However, the building required a dehumidifier close to the mechanical room, which was then installed on a wall space above the VSD. The VSD then had the wrong enclosure style (side vents and no seal around the cover). Water dripped from dehumidifier into the drive and within six months the VSD had accumulated enough water to produce circuit board corrosion.

Keep connections tight - Checking connections is a step many miss or do incorrectly. Heat cycles and mechanical vibration can lead to sub-standard connections. Over torqueing screws is not a good idea, and connections should only be re-torqued to the manufacturer’s recommendations after the connection is proven to be clean and free from burning. Further tightening an already tight connection can ruin the connection. Bad connections lead to arcing. Arcing at the VSD input could result in nuisance overvoltage faults, clearing of input fuses, or damage to protective components.

Other considerations - Check circulating fans for signs of bearing failure or foreign objects. This is usually indicated by an unusual noise or shafts with irregular movement. Inspect DC bus capacitors for bulging and leakage when the drive is isolated and proven to be dead. Also, monitor DC bus voltage readings via the keypad display while the VSD is in operation as fluctuations can indicate degradation of DC bus capacitors. If you suspect problems with your DC link capacitors please contact ABB or your local Authorised Value Provider who will be able to carry out more advanced tests.

Furthermore, store spare VSDs in a clean, dry environment, with no condensation. Place the unit in your preventive maintenance system so you know to power it up every six 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.

Regularly monitor heat sink temperatures. Locate the direct temperature readout on the keypad or display and verify where this readout is, and make checking it part of a weekly or monthly review of VSD operation.

ABB and its Authorised Value Providers offer a range of maintenance services to suit your needs, from annual inspections all the way up to remote monitoring services that can tell ABB that something’s amiss in your drive long before it displays any signs of wear.

Life Cycle Assessment

Although drives are not normally the most expensive capital equipment, they often perform critical duties and have a high in-service value. A drive failure can result in loss of production and revenues, as well as having safety and environmental consequences. To reduce the risk and consequences of failure, the drive must be properly maintained at the right times during its life cycle.

All electrical and electronic components degrade over time, some quicker than others. Looking after a drive, in line with a detailed preventive maintenance schedule, is essential if the drive is to retain its ability to save energy, as well as avoiding unexpected failures. Preventive maintenance avoids premature replacement of parts and drives, reducing the real cost of maintenance.

With such a service the drive is subjected to regular visual inspections and measurements and components included within the drive care contract or the preventive maintenance schedule are immediately ordered and replaced with genuine, factory-certified parts. The drive undergoes complete functional testing.

Drives users should also consider using a service that repairs or replaces a failed drive. ABB, for instance, offers such as service which removes any risk of extended downtime from the customer. It also means that local maintenance teams can be engaged in other areas, thereby saving the company unnecessary and unplanned costs.

A drive care contract is ideal for small- and medium-sized organisations – including commercial buildings - that have less in-house maintenance resource or that simply want to avoid the costly replacement of a drive that stops working.

To help keep track of where drives are installed, ABB introduced a drives registration scheme. The scheme ensures that a drive’s installation and parameters are remotely stored on an off-site database. It provides immediate back-up should the data be lost or setting reconfirmation required. This allows efficient management and service of customers’ assets.

For more information about ABB drives, take a look at some of the other videos in our “Ask the experts” series on the ABB Authorised Value Provider UK YouTube channel.