Electric motor loading is key to energy efficiency, warns Weg20 September 2011

Determining that electric motors are properly loaded enables users to make informed decisions about when to replace them and which to prioritise.

That is the advice from Marek Lukaszczyk, of Weg Electric Motors, who adds that measuring motor loads is relatively quick and easy, with the right equipment, and every plant with a significant motor stock should be looking to perform tests as part of their preventative maintenance and energy conservation programmes.

"Analysis is necessary because there is not much point replacing existing standard ac electric motors with energy efficient types, if the motors are mismatched or oversized for the loads they are intended to serve," he states.

"Too often, motors are oversized or have been rewound several times, leading to gross inefficiencies in their operation," he explains.

For him, the point is that most electric motors are designed to run at 50% to 100% of their rated load, with maximum efficiency around 75%. Hence, a 10kW motor has an acceptable load range of 5kW to 10kW, with peak efficiency at 7.5 kW.

"As a general rule, high efficiency motors garner the maximum savings when they are loaded in excess of 75% of full load, and are operated more than 4,000 hours a year," he assets.

And that matters, because the problem with motor efficiency is that it tends to decrease dramatically below about 50% load – which is bad news when as few as 20% of electric motors in the UK are running at their full rated input, precisely because of the oversizing problem.

"It is also bad news in terms of energy costs, as it has been calculated that a single percentage point increase in efficiency will save lifetime energy costs generally equivalent to the purchase price of the motor," states Lukaszczyk.

And he adds that while oversizing is the most common problem, under-sizing can be just as damaging, because such motors are likely to be overloaded – causing them to overheat, lose efficiency, and probably fail prematurely, with al the usual cost ramifications for production.

Lukaszczyk believes that one of the problems for plant engineers concerns the term 'motor service factors', which, he believes are interpreted "too liberally".

"A service factor is a multiplier that indicates how much a motor can be overloaded under ideal ambient conditions," he explains. "For example, a 10kW motor with a 1.15 service factor can handle an 11.5kW load for short periods of time, without incurring significant damage," explains Lukaszczyk.

"Although many motors have service factors of 1.15, running the motor continuously above rated load reduces efficiency and motor life. In addition, the motor must never be operated in an overloaded state when the voltage is below nominal, or when cooling is impaired by altitude, high ambient temperature, or dirty motor surfaces," he adds.

He goes on to remind plant engineers that efficiency is also lost when motors are run either above or below their design voltages. "The result of over-voltage is a lower power factor, which reduces overall motor effectiveness.

"The same is true of motors that are operated at less than 95% of their design voltage. They typically lose two to four points of efficiency, and also suffer service temperature increases of up to 7C, greatly reducing motor insulation life and impairing reliability," he says.

Which is why Lukaszczyk is recommending that motor users should survey and test all of their motors that operate for more than 1,000 hours per year.

"Then, using the analysis results, they should divide the motors into the categories:

Motors that are significantly oversized and underloaded: replace these with more efficient, properly sized models at the next opportunity, such as scheduled plant downtime.

Motors that are moderately oversized and underloaded—replace with more efficient, properly sized models but wait until they fail.

Motors that are properly sized but standard efficiency—replace most of these with energy-efficient models, but again, wait until they fail.

"One problem with this strategy is that it is often difficult to determine the characteristics of motors that have been in service for some time," he warns.

Why? "Because it is not uncommon for the nameplate on the motor to be lost or painted over. Further, if the motor has been rewound, there is a probability that its efficiency has been reduced."

The solution: "When the nameplate data is missing or unreadable, efficiency values must be determined at the operating load point for the motor. This involves using power, amperage or slip measurements to identify the load imposed, then obtaining a motor part-load efficiency value. Finally, if direct-read power measurements are available, derive a revised load estimate, using both the power measurement at the motor terminals and the part-load efficiency value."

Meanwhile, for rewound motors, Lukaszczyk advises that an adjustment to efficiency values is required to reflect that efficiency will be less than that of the original motor.

"To reflect typical rewind losses, two points should be subtracted from a standard motor efficiency on smaller motors [less than 30kW], and one point for larger motors. However, it must be stated that rewind companies with the best quality-control practices can often rewind with no significant efficiency degradation at all."

Brian Tinham

Related Companies
WEG Electrical Motors (UK) Ltd

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