Repair or Replace? A Complex Decision for Electric Motors

06 August 2008

ENERGY EFFICIENCY is a key attribute of today’s electric motors and the efficiency of a motor will decline over its design life depending on usage conditions. As electric motors are at the heart of most industrial processes, careful monitoring of condition and energy consumption should be a routine part in these days of high energy costs.

Optimum operating conditions including running within the specified temperature and load / torque range, gentle starting procedures, regular maintenance will ensure the natural aging of the insulation system and consequently a long motor lifetime.

On the other hand, low, high speed or uncontrolled variable speed operation, temporary load peaks, high ambient temperatures, wide voltage variations and other operating conditions outside the specified envelope can rapidly degrade a motor’s insulation system and reliability. While these factors are particularly relevant to the motor winding, bearing condition can also be affected.

Andrew Glover, Product Manager for LV Machines at WEG commented: “Control of energy costs is becoming an increasingly top priority, as lifetime energy cost savings start to outweigh capital costs for equipment. At some point, therefore, the decision must be made to rectify the situation, preferably before it is forced on the engineer by motor failure and plant downtime.

“Apart from failure, there are several variables which need to be considered and their importance weighted. For example, what is the difference between the efficiency rating of the rewound motor and of a new high efficiency electric motor? A rewound motor is rarely as efficient as a new motor, and frequently less reliable as well. Perhaps the original efficiency of the old motor was never as high as a new motor would be anyway.”

In looking at the new versus rewind, current operating conditions should be carefully considered, including running hours, load / torque expected and whether the old motor was actually sufficient for the task expected of it.

Rewinds cost money, of course, but there is the cost of the new motor to weigh against this, plus the lifetime running costs of a new high efficiency motor versus the likely life and reliability of a refurbished motor. Would it be necessary to replace bearings and other items to obtain a worthwhile life and reliability?

When calculating the balance of costs and comparing the return on investment likely from the rewind and the new motor, the availability of grants and other financial assistance offered by Governments to promote the investment in energy efficient equipment must be kept in mind. These can range from simple non-refundable grants to zero sales tax rate or offsetting income tax against the purchase price. The contribution these make can be significant.

However, the price of electricity is unlikely to reduce in the next few years so the difference in efficiency between the rewound and the new motor may prove to be the most significant cost.

The energy cost per annum of a motor can be calculated by a simple formula to arrive at a typical price. By applying this to the rewound and the new motor, the annual cost can be compared.

The energy cost is the motor operating hours multiplied by the cost of electricity per kilowatt-hour by the operating point power, divided by the % efficiency.

For example, a typical 4000 hours X 7.5p per kW hour X 90kW power = £27,000.

For a high efficiency (EFF1) motor at 95.3%, the cost would be £28,330 annually

For a rewound motor (EFF2, originally 94.2%) at 93.2%, the cost would be £28,970 annually.

The difference in energy cost is therefore £640 annually.

Even if the rewound motor offered a projected life of 10 years, its increased energy usage (if the tariff remains constant for 10 years) would therefore cost £6,400.

However, this does not take into account the one-off cost of a rewind, which could easily add £1000 to the first year’s cost using the 90kw example above.

In addition, we have taken a best possible scenario for the efficiency loss resulting from a rewind. A loss of only 1% would be achievable under near perfect factory conditions with high specification insulation and highly trained mechanics. Unfortunately, less than perfect conditions could result in a drop of efficiency of 1.5% or 2% very easily. Every extra % is another £640 annually, and doubles the increase in running costs.

Even if a new motor was bought, the contribution to reduced costs and improved profit margins would be considerable, and does not take into account any energy savings grants or tax relief, so the difference could well be more. Reliability and maintenance costs on a rewound motor could also be higher.

Andrew added: “So the conclusion must be that, in most cases, a new high efficiency motor is the best long term solution, both for reducing overall energy consumption and for its contribution to the profitability of a company. A short term solution like rewinding the motor is only a temporary answer where capital expenditure is very constrained; it will be expensive in the medium and long term.”

See also the article "Billions in energy could be lost due to poor motor sizing"

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