ENG07
Motor repair & replacement policy
The decisions that are made when a motor requires repair can have a significant impact to your energy consumption.
In larger buildings electric motors are ubiquitous, with three-phase squirrel-cage induction motors comprising a considerable percentage of a countries electrical load.
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Induction motors typically range in full load efficiency from 87% through to 93%. This efficiency is very difficult to measure accurately in the field, requiring specialized equipment. Fortunately, energy saving projects associated with electric motors do not require actual efficiency of a given motor to be established.
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Rewinding a damaged induction motor is a common practice in industry, but studies have proven that rewinding an induction motor drops its efficiency by a 1 or 2%, and often higher than this. Multiple rewinds can further reduce the efficiency of the rewound motor.
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While a drop in efficiency seems insignificant, a quick estimate reveals that this reduction can be costly. A standard efficiency 15kW motor operating 8000 hours annually, for example, costs £12000 to operate at an average electricity rate of £0.1 per kWh. Once this motor fails, for many organisations the default option for returning it to service is typically rewinding.
The annual energy savings associated with replacing an older failed motor with an new high-efficiency model, with an efficiency at the top end of the spectrum, would give a simple payback of less than one year. Add to this the reliability benefits of having a new motor installed and the case for replacement rather than repair becomes compelling.
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Understanding the causes that lead to decisions to rewind rather than renew is important to enabling a replace first strategy, which is counter-intuitive to the broader sustainability philosophy. Often times downtime is the critical factor in determining the best solution to a motor failure and measures that can reduce leadtime for motor replacement should be proactively examined. Proactively determining the appropriate specification for a replacement motor, and perhaps even holding one as a critical spare, goes a significant way in reducing downtime.
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Ultimately, defining and implementing a motor repair policy that defines replacement with the highest efficiency equivalent, should always be the preference before rewinding is considered.
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