Steer Clear of Energy Draining Motors

01 May 2005Archived News Energetics in the News

PUBLISHED: WME Magazine.

Electric motors use masses of industrial power, so selecting an efficient model can pay off quickly, writes Garth Lamb.

Almost half the total energy used in industry is for powering electric motors. By themselves motors in Australia cost $3 billion a year to run and account for as much greenhouse gas emissions as all the vehicles on our roads. With such a massive consumption of energy, small increases in motor efficiency can quickly add up to large savings in the system.

But they also make sense at a company level. Andrew Krumm, senior consultant with Energetics (an Australian energy and environmental consultancy), says the most important thing when considering motor efficiency is to do the lifecycle maths. They typically last 15 to 40 years, so the purchase price often accounts for between 2-5 per cent of a motor's total lifetime cost. The rest comes in the shape of electricity bills.

"Over the long life expectancy of a motor, small efficiency gains can lead to definite cost savings," he says. "With dated motors, some 30-40 years old, there are great efficiency gains to be made... On the whole, motors now are 3-4 per cent more efficient than 20 years ago," said Krumm.

There are plenty of opportunities in Australian industry, where many plants are using equipment older than the 20 year mark. Energetics recommended the replacement of motors on two 40-year-old, 110KW boiler feed water pumps during a recent project and expects efficiency gains to pay back the premium for the high efficiency motors in 1.2 years.

Companies should also consider high efficiency motors. Jason Desouza, an engineer with motor manufacturer Baldor, says the company's top of the line 7.5KW has an efficiency of 90.3 per cent compared to 85.5 per cent for its standard motor.

There energy savings vary according to the applications, however, with optimal savings gained during long periods of continuous operation at load levels above 75 per cent. In stop-start applications, for example, the heavier materials of the high efficiency model reduce efficiency and it may be worth considering the standard make.

Efficiency

Motor efficiency is a measure of how well the machine transforms electrical energy into mechanical energy. It can never be 100 per cent as around five to 10 per cent of total energy is absorbed by the motor itself, with losses broken down into five factors (see table).

Manufacturers increase motor efficiency a number of ways. Thicker copper wire can reduce resistance losses in the stator, the stationary part in which the rotor turns. Construction from high quality and low magnetic loss steel also increases efficiency, while stray loss due to magnetic flux leakage can be combated by optimising motor design and manufacturing. Desouza says design has improved in recent years, with good bearing, fan and airflow design, for example, reducing friction and windage losses.

Another factor that can dramatically affect efficiency is correct sizing during motor selection. "Efficiency depends on the load placed on a motor. They are most efficient when they're at full load... sizing the right motor for the job is very important because over-sizing reduces efficiency," says Desouza.

There is another factor to consider with size. While intuition suggests the greatest gains are likely to come from larger motors which use more energy, the biggest efficiency savings actually come from smaller motors in the 5.5-75KW range. Large motors are typically quite efficient because losses do not increase in ratio with motor size, helping the 90KW Baldor HEM for example to achieve 95 per cent energy efficiency.

Costs

High efficiency motors cost 10-25 per cent more than the common variety, although the price differential is shrinking in most cases. Whether the higher purchase price is warranted will depend on applications and size.

Energetics conducts energy audits by compiling an asset list of all the motors at a plant and looking at the name plate or tags on each one. Plate information includes the rated kilowatt of the motor, its supply voltage and amps at full load (some newer motors also provide efficiency data). The information is used to determine the amount of power the motor uses at full load.

"By contrasting this with similar high efficiency motors for the tasks the motor performs, like 100 per cent loading or switching on and off, we come up with a payback period or return on investment," says Krumm.

The focus for replacing motors is usually on older models and those in the target size of 5.5-75KW. As a very rough rule of thumb, he says payback time for upgrading common small motors is around five years.
"A number of organisations will go for the idea of high efficiency motors (HEM), but not necessarily with the simultaneous replacement of all targeted motors. If you put in place a HEM purchasing policy, saying when you do have to replace a motor you will go for a HEM, the purchasing guys can just apply that rule," he told WME.

The Australian Greenhouse Office introduced standards in 2001 stipulating minimum energy performance standards for motors sized between 0.73 and 185 kW (Australian Standard AS/NZS 1359.5-2000, updated to 1359.5-2004 last year). Electric motor manufacturers are aware of true lifecycle costs and regulations like MEPS are improving industry standards. Some manufacturers even offer on-line tools that calculate the lifecycle cost of their products.

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