Electrifying motors

Jan. 1, 2006
Premium efficient motors were introduced in the early 1980s and employed better lamination, lower-loss cooling fans, and more active material than previous

Premium efficient motors were introduced in the early 1980s and employed better lamination, lower-loss cooling fans, and more active material than previous models. These improvements helped in many ways. For instance, better lamination reduces iron losses, which creates less heat. Therefore, a smaller cooling fan can be used, producing fewer winding losses. And, adding more material (lamination stack length and copper windings) than necessary to achieve a certain horsepower leads to greater motor efficiency.

The National Electrical Manufacturers Association (NEMA) created guidelines constituting a “high efficiency” motor and first defined standard and energy efficient motors in 1990; these definitions later became standards for the Energy Policy Act of 1994 (EPAct). The Consortium for Energy Efficiency (CEE) then established “premium” efficiency guidelines in 1996. In 2001, both groups harmonized their efficiency standards, establishing NEMA premium efficiency standards for open drip-proof (ODP) and totally enclosed fan cooled (TEFC) one to 500 hp two, four, and six-pole motors for low and medium voltage.

Abiding by these standards increases throughput and reduces personnel and maintenance expenses. Following these standards for an appropriate motor enclosure and adequate environmental protection also promote uptime.

Measuring efficiency

The U.S. standard test for motor efficiency — IEEE Standard 112, Method B — compares measured input and output watts (rather than assigning stray load losses as IEC tests do). IEEE segregates motor losses into five categories:

Iron core: Magnetic losses in laminations, inductance, and eddy currents

Stator resistance: Current losses in the windings

Rotor resistance: Current losses in rotor bars and end rings

Windage and friction: Mechanical drag in bearings and cooling fans

Stray load: Magnetic transfer loss in the airgap between stator and rotor

Some losses stay consistent while others decrease, resulting in improved machine efficiency. While designers debate ways to distribute individual losses and maximize performance, the total of these losses most impacts efficiency. For example, reducing certain losses can increase the difficulty of starting a motor across the line.

When designers balance performance parameters, general-purpose motors often contain design compromises. Some common parameters follow.

Lamination steel development

Lamination coatings evolved from basic organic (C3) to various inorganic/combined configurations (C4/C5/C6) and recently to oxide coatings. Actual losses in the steel declined from four to five W/lb to less than two W/lb — due to material advancements, such as better electrical-grade steel.

Other “premium” motor benefits

Additional active material — such as laminations and copper wire — increases efficiency. Severe-duty and IEEE 841 motors specify cast-iron housings that are usually finned to amplify heat dissipation. Laminations are fully round on their outer diameter for better thermal conductivity to the housing. Smaller internal and external fans decrease winding losses.

Gaining efficiency

For low motor losses, smaller cooling fans with less friction and windage can be used, as well as smaller bearings. However, one drawback — especially with belted loads — is limited shaft loading. To simplify maintenance, some operators prefer installing the same size bearing on both ends of the motor instead of a larger bearing on one end of the drive. However, same size bearings increase friction and reduce motor efficiency. The opposite drive-end bearing can be lightly loaded and therefore, doesn't require a large bearing.

Corrosion and moisture protection

After bearings, corrosion remains the most frequent cause of premature motor failure. Highly corrosive liquids and gases are the most likely offenders. Specifically, new washdown practices that prevent bacterial growth require stronger cleaning solutions, hotter water, and higher pressures than in the past. This can damage the motor finish or contaminate bearing greases. Further, small scratches on the finish can cause corrosion under the paint, surface bubbling, and then paint to flake off. To prevent corrosion in nonfood processing applications, end users should specify cast-iron motor housings and TEFC motors (IP54 - IP55). Open (IP23) motors should not be specified where corrosion or moisture is problematic. In food processing areas, washdown-duty motors are recommended.

Most severe-duty motors are built with cast-iron housings, endplates, fan covers, and conduit boxes. These standard construction features comply with IEEE 841 and test to ASTM B117-97. A corrosion resistant primer and finish paint coat the motor's inside and outside; a similar coating also protects the rotors.

Besides these corrosion precautions, joints between the housing and endplate must be protected from rust, and gaskets should seal conduit boxes. Often, plated hardware on severe duty and washdown motors prevents rust formation and stainless hardware can add extra corrosion resistance. Today, many motors utilize lead separators as part of the gasket system between conduit box and motor frame. To upgrade the motor's varnish system, end users should choose either a double-dip process or vacuum pressure impregnation (VPI).

Despite sealing, moisture from condensation can still form inside motors. At worst, this moisture may contain chlorine or other corrosive liquids, which should exit through breather drains at the lowest point in the motor. Commonly, stainless steel or composite materials form the drains and may be molded into a “T” configuration for two points of exit.

Most washdown motors contain a large condensation hole in all quadrants rather than a T-drain, which plugs unused holes. Upon mounting, the open drain hole must reside at the bottom. When resealing the motor, remove plugs on the bottom and replace at the top. For severe condensation, space heaters can keep the motor from cooling and drawing in outside air because they elevate the temperature above ambient.

Starting methods, adjustable speed drives

At one time, a simple starter initiated many motors across a line. Part winding or wye-delta started larger motors. Today solid-state starters (sometimes used with a bypass) can ramp up motor voltage to a peak pulse that moves heavy loads and then tapers off for light loads.

Commonly, end users add adjustable frequency drives to existing motors. Motors combined with inverter drives, however, may not start across the line because they were optimized for adjustable frequency use. If the motor must run directly from the line or with a bypass and a drive, end users should specify this upon ordering.

Most low-voltage motors (less than 600 V) can operate with an adjustable speed drive (ASD) for variable torque applications such as pumps and fans. In fact, many premium efficiency and severe-duty motors include an insulation system with magnet wire that withstands voltage spikes from PWM drives.

Most ASD applications present the motor with a variable torque load that decreases proportionally with speed. For constant torque loads such as conveyors, most four and six-pole premium efficiency motors can meet a 10:1 constant torque-speed range (CTSR). Used over a wider speed range, totally enclosed blower cooled (TEBC) models can provide full torque to zero speed with a flux vector drive. Operators should evaluate motor manufacturer performance data to view the speed-torque operating envelope.

Motors that connect to adjustable-speed drives may be susceptible to bearing currents caused by common-mode voltages. Preventing bearing damage, such as fluting, calls for proper wiring and cable type. By eliminating common-mode voltages, such measures prevent circulating currents that might otherwise pass through the bearings on their way to ground. Other ways to combat bearing currents include using hybrid (ceramic ball) or isolated bearings on both ends of the motor, while also isolating the motor shaft from the load. This eliminates the current path from rotor to ground altogether. Another method is to apply a shaft grounding brush, which provides an alternate current path rather than the bearings.

Bearing down

According to EASA figures, about 60% of premature motor failures involve bearings. Most IEEE 841 and a few washdown-duty motors utilize noncontact labyrinth seals to minimize bearing contamination. Some manufacturers supply these seals on both the drive and fan-end of the motor. On the other hand, contact seals cause friction losses, and wear reduces their sealing capabilities.

Bearing manufacturers are developing noncontact and lower friction bearing seals. Another alternative is ceramic balls for antifriction bearings, which decrease motor losses, reduce lubrication intervals, and provide a “self-healing” feature when contaminated.

On many washdown-duty motors, contact bearings contain a lip that requires lubrication and continuously contacts the rotating shaft. This prevents contaminants from entering, and ultimately, premature failure.

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