Ac motors

In our very last Productivity Forum, we concentrate on one of Nikola Tesla's most significant inventions — the ac motor. In this report, Motion System Design editors polled ac motor experts for advice on how to maximize productivity in industrial applications with these essential machines. Here are the responses that we believe you'll find most helpful.

HOW THEY'RE MADE

What design/construction-related features in ac motors lead to longer service life and higher productivity?

Lee/Danaher: Choosing an incorrect motor can reduce productivity. When suppliers offer limited motor choices, inertia, the torque constant, or size can be compromised. Specifically, excessive inertia wastes energy, a high torque constant (KT) limits speed, and an incorrect envelope can negatively affect machine size.

Tim/SEW-Eurodrive: Motors overcome temperature variations by different housings that the stator is pressed into, as well as wire size, the varnish coating that is applied to the winding and even the bearing type and lubrication used.

Motors are also designed for protection against the environment. Specifically, water causes stator insulation to break down until it shorts phase-to-phase or phase-to-ground. Seals, bearings, sealants of the end bells, gaskets of the conduit boxes … all impact how well a motor shields against the elements.

John/Baldor: Engineers must select a proper motor that meets rms torque requirements. Oversized motors add inertia that must be started and stopped frequently. This additional inertia and regenerated energy to the drive must both be addressed. Also, incremental encoders are fine for most applications, but one without optics (a magnetoresistive model, for example) is suitable for dusty or wet applications.

Tim/Bodine: A properly selected winding increases the life of all other components. Motors designed for long life use bearing lubricants applicable to the machine's operating temperature class. Similarly, insulation materials — such as the magnet wire insulation — are chosen for their operating temperature and dielectric properties.

In addition, mechanical vibration can result from poor design or improper rotor fabrication or assembly. Rotors with inherent imbalance experience premature bearing failure. And while there are industry standards for allowable rotating imbalance, ultimately the end user's application dictates the acceptable criteria.

Rick/Reliance Electric: Motor design and construction features are closely related to application environment. The two basic motor enclosures are open and enclosed. Open motors let air pass directly over the stator windings; several levels of protection against entering water droplets or solid particles. The lowest level permits unrestricted ventilation; the highest level incorporates screens over ventilation openings, provides for internal baffling, and specifies guidelines for correct opening placement. A fan motor operating inside an HVAC unit on a rooftop is a typical example of an open motor application.

Enclosed motors prevent the free exchange of air between motor windings and housing. Although an enclosed motor is not air tight, dust entering it will not inhibit normal motor operation. A fan motor operating outside an HVAC unit on a rooftop and fully exposed to the elements is an example of an enclosed motor application.

Ray/Baumüller: Conventional ac induction motors are inherently durable. The absence of moving, contacting parts (such as brush and commutator in dc motors) immediately gives them the advantage of durability over their dc kin. Also, the largest heat-generating component of the motor (namely the winding) is located on the outside stator of an ac induction motor as opposed to the inside rotor of a dc motor. As a result, removing heat from ac motors is much easier (even with a totally enclosed design) eliminating the costly maintenance of open designs.

HOW THEY'RE APPLIED

How can designers optimize service life and productivity when selecting and applying ac motors?

Tim/SEW-Eurodrive: A motor's service life depends on many variables surrounding the application. If an application demands several starts and stops (duty cycle) then the peak (acceleration) current must be considered, due to heat generated in the windings. This heat must be removed from the motor through its surface, but the amount dissipated depends on ambient surroundings. For example, it is critical that head be removed in an oven application, but not so important in a deep freezer.

Elevated temperatures generated in motors that frequently start and stop can also be a reflection of the load-to-rotor inertia. The higher the load inertia, the more current it takes to accelerate the load, which creates higher internal motor temperatures. One way to reduce this inertia mismatch is by adding ratio via a gearbox.

Tim/Bodine: The very popular permanent split-capacitor motor inherently operates at its hottest level when lighter loads are applied. Therefore, it is designed for operating at the nameplate horsepower rating and for high-starting torque, but not for applications that require frequent cycling. This is because these motors utilize either a mechanical or solid state starting switch that have a finite life. Three-phase machines are among the best for output power per size, high starting torque, and smooth rotational operation. The drawback to this type of machine is the three-phase power source, which isn't available in most consumer and light industrial environments.

Rick/Reliance Electric: Successful motor operation depends on defining application speed, torque, reflected inertia, and time. These combined parameters are commonly referred to as duty cycle. Their accurate prediction results in a cost-effective motor that delivers the defined duty cycle.

Motor control techniques applied to challenging duty-cycle applications include Y-start/Delta run, soft start, solid-state devices, and variable frequency drives (VFDs). Motor operation on a VFD presents several motor insulation challenges due to harmonic-rich content of the power source. Always review product specifications to ensure the motor can operate with a VFD.

John/Baldor: Like a servo application, rms torque must be calculated for correct motor and drive sizing. One must remember that most ac drives only offer 115% peak current, so oversizing the drive is probably necessary for aggressive acceleration or deceleration. But again, more motor (added inertia) is not good when starting and stopping frequently.

Lee/Danaher: Resonance is often overlooked. Mounting a motor is critical to good performance. When operating the load, any deflection is countered by the feedback system. If compliance happens at a sympathetic frequency, instability can result in resonance — the bane of control engineers.

HOW THEY'RE OPTIMIZED

How can end users optimize service life and productivity from the ac motors on their machines?

John/Baldor: Users must pay attention to their motor bearings to prevent premature failure. This involves regreasing as defined by the manufacturer;the grease applied should be compatible with what was used.

If a motor fails from entering water, end users should switch to a severe duty or washdown design. Bearing failures can be analyzed as well, and adding V-ring or rotating labyrinth seals further protect the motor. Motor repair shops and manufacturers can help select the right motor for each application.

Lee/Danaher: Misunderstanding the larger system can contribute to reduced performance. Heat from an undersized motor reduces life, increases maintenance cycles, and leads to accuracy problems if there is poor thermal management. An oversized motor wastes power during acceleration. All of these factors must be considered when sizing an application.

Ray/Baumüller: Newer versions of the dc motor eliminate many of its problems by moving the winding to the stator rather than the rotor. While these don't technically qualify as ac induction motors, they are ac motors — of the synchronous variety. However, these synchronous-type ac motors lack the durability of true ac induction motors.

HOW THEY FAIL

What are some of the design/construction-related causes of failure in ac motors, leading to lower productivity?

John/Baldor: Failure can be narrowed down to two causes: electrical and mechanical. Electrical issues are basically due to selecting an incorrect motor for an application. The speed and torque requirements do not match the capability of the motor. Often in a motion application, an oversized motor is selected and its high inertia causes overheating. Many times duty cycle (rms) is not clearly defined by the end user. The drive can also be the culprit, not large enough (115% peak current limit) and the motor is not able to do its job.

Mechanical issues are many, from wrong enclosures to levels of environmental protection (needing severe duty or washdown levels of protection). There could be bearing issues with coupled or belted loads. One must remember that bearing failures are responsible for over 60% of motor failures. All of this could be a topic for much more than space allows here.

Ray/Baumüller: Failure of ac induction motors generally occurs in one of two places: Bearings or windings.

HOW THEY'RE APPLIED

What are some of the common mistakes that designers make when selecting and applying ac motors, leading to lower productivity?

John/Baldor: Oversized motors often present a problem. It is too common for the engineer to build in a safety factor, but in motion applications, the added inertia could increase torque requirements, leading to extra heating. Often the true duty cycle and rms torque requirements are not understood or correctly calculated. This results in selection of the wrong motor.

Tim/Bodine: Deciding on the proper ac motor takes more than looking in a catalog at the horsepower rating. There are many attributes such as starting torque, breakdown torque, temperature class, design type, and operating speed (also expressed as percent slip). Once you get past that, mounting and control technique must be selected.

APPLICATION PITFALLS

What are some of the common mistakes end users make with ac motors, leading to lower productivity? How can end users optimize service life and productivity from the ac motors on their machines?

Rick/Reliance Electric: End users can maximize motor service life by recognizing the application and environmental factors that accelerate motor failure if not properly addressed. Important environmental factors include ambient operating temperature range, humidity range, duty cycle, airborne particulates, and corrosive atmospheres. Strict adherence to shaft alignment and motor mounting procedures reduces shaft breakage, vibration, and bearing failure. Motors are robust machines that operate for decades when routine inspection and maintenance is performed at the manufacturers recommended interval.

Tim/Bodine: Ac machines used in the proper environment and at the intended ratings can run indefinitely. However, when the environment around the machine is more severe than intended, life of the product begins to degrade. Bearings are probably the number one cause of motor failure and usually occur when the lubricant is purged from the bearing, becomes thermally degraded, or contaminated. Lubricant that purges from the bearing is typically related to excessive operating temperature or (though much less likely) a pressure differential. When bearings become contaminated with foreign debris, premature wear occurs and operating temperature rises at the journal surfaces. These two conditions dramatically reduce the operating life of ac machines.

Routine maintenance every six months ensures the surrounding area has not degraded. Make sure all protected guards, gaskets, and other protective means are in good working condition. Overconfining the machine can cause elevated thermal build-up so keep ventilated passages from becoming blocked or restricted.

Tim/SEW-Eurodrive: Today's fractional hp motors are generally maintenance free, meaning the bearings are lubricated for the life of the motor. Motors usually fail in the windings or have bearing failure. Most manufacturers can provide data on the bearing life of the motor, and based on the operating conditions can recommend when they should be replaced. Motors that have brakes and are frequently cycled, the brake may need to be adjusted after a period of time. This is load and cycle dependent. As long as the equipment designer selects an appropriate motor, the service life should be many years. If there is a coupling between the output shaft of the motor and the load, the couple can become worn and cause misalignment. The misalignment leads to additional loads on the output bearing and can lead to bearing failure of the motor. This is also true when replacing a failed motor; the new motor must be properly aligned or the same failure can occur.