Understanding modern motors and controllers.

March 1, 1995
Included are the latest developments, fundamentals, NEC rules, standards, special features, and other important details on motors and controllers.Motors and controllers are better than ever. They perform better, are more versatile, have wider application, are more dependable, and have longer life. To use this equipment effectively, you should have a thorough knowledge of the many advances, features,

Included are the latest developments, fundamentals, NEC rules, standards, special features, and other important details on motors and controllers.

Motors and controllers are better than ever. They perform better, are more versatile, have wider application, are more dependable, and have longer life. To use this equipment effectively, you should have a thorough knowledge of the many advances, features, and variations of the latest motors and controls. For example, some modern motors use more efficient cooling techniques, better bearings, and similar design refinements. Specialized motors, such as the variable-frequency geared elevator motor, are now available. Also, the design ramifications of the metric motor must be considered.

Likewise, improvements in motor controls have been significant, primarily due to the rapid application and success of solid-state design. Practically, every motor controller available comes in a solid-state version, or at least has some components such as overload relays that are solid state. Adjustable-speed drives and soft-start units are typical of modern controls that have become highly successful.


In addition, it's important that you have an in-depth knowledge of motor and controller basics to assure proper selection, application, installation, and maintenance.

Motor selection

Available motors include single- and 3-phase induction types, wound-rotor motors, synchronous machines, DC motors, and a variety of special designs.

Squirrel-cage induction motors are the most widely used because of their simple design, low cost, and minimal maintenance requirements. Because of the widespread use of integral 3-phase induction motors, we'll focus on these motors. Because induction motors vary considerably in design and performance, their selection and installation demands care on your part.


The fundamental first step in selecting an induction motor that will drive a certain load is to determine its horsepower rating. Sometimes, this is as simple as obtaining the specifications from the nameplate on the driven load. Possibly the rated load can be obtained from the supplier or from analyzing other similar loads. The horsepower requirements can also be calculated from known data, or possibly the actual load can be tested and the required power measured. Ideally, the motor should be sized so that the load is 75 to 95% of the motor's full-load rating. This assures high efficiency.

As a final resort, try driving the load at rated load and voltage with a motor that appears to have about the right rating. Measure the input current and temperature rise of the motor to determine if it's too small or too large, thus, selecting the proper size motor.

Torque and speed

There's more to selecting a motor than determining its horsepower. The horsepower rating is determined from the motor rated-load output characteristics of torque and speed. For a particular application, the motor must have a rated-load torque to drive the machine at the required speed. However, there are three other torque characteristics that must be considered, as shown in Fig. 1. These include locked-rotor or starting torque, pull-up torque, and breakdown torque.

A motor must have sufficient starting and pull-up torque to bring the driven machine to operating speeds, and it must be able to overcome peak loads (breakdown torque) without stalling. The National Electrical Manufacturers Association (NEMA) has available standard curves to which all NEMA design motors must adhere. These curves permit effective selection of the motor for the job at hand.

The NEMA "design type" determines the electrical characteristics of induction motors. NEMA designates standard design types as A, B, C, and D. You can obtain the performance of each type from the appropriate NEMA curves, which show available torque at various speeds. [ILLUSTRATION FOR FIGURE 2 OMITTED] These characteristic curves are used to select the proper design letter to match the load. For example, if you use a Design B motor to drive a load that needs a high starting torque, the motor may over-heat during starting and trip out prior to reaching operating speed. When this happens, you may decide to defeat the motor protection, causing the motor to burn out. Or, you may install a larger motor that will not only cost more, but because it's oversized, will operate inefficiently.

The differences shown by the curves are due primarily to the differences in the rotor resistance and reactance introduced during design. The curves for any specific design also vary according to motor size. Output torque values drop as rated horsepower increases at any given synchronous speed.

Design B is the industry's "workhorse" for general-purpose, across-the-line starting duty. It has a "normal" or relatively high starting torque for accelerating high-inertia loads and handles short-duration overloads to 200% full-load torque or more before reaching the breakdown point.

Where the load duty-cycle has a peak in excess of the Design B breakdown torque, a Design A motor may be used. This type has a starting torque very close to the Design B motor but develops a higher breakdown torque and will have a higher starting current.

The Design C motor is characterized by a low starting current and high starting torque. It's suitable for loads requiring a high starting torque and rather rapid acceleration, such as conveyors and compressors.

For extremely heavy starting conditions, the Design D motor is available and is well suited for such high-inertia loads as punch presses, cranes, and elevators, although its running efficiency is low. Characteristics of NEMA design motors and their appropriate applications are summarized in Table 1.

Load variations. Where the load varies with time, a horsepower-versus-time curve will permit the determination of required peak horsepower. Calculation of the root-mean-square horsepower will indicate the proper motor rating from a heating standpoint. In case of extremely large variations in load, or where shutdown accelerating or decelerating periods are a large portion of the cycle, the horsepower may not give a true indication of the equivalent continuous load. Thus, the motor manufacturer should be consulted.


By selecting the right motor and speed, you can sometimes avoid the necessity of using a speed-control device. Constant-speed motors operate at practically uniform speed during normal operations. Induction motors are available from 514 to 3600 rpm in the smaller sizes. Synchronous speed ratings of integral-horsepower motors are given in Table 2.

Multispeed motors are available for application to loads most effectively operated at two or more specific speeds. A 2-speed motor can be of the single-winding type with two fixed speeds or a special 2-speed, single-winding type with flexible ratio of low to high speed. Multispeed motors can be selected as either variable torque for fans and centrifugal pumps; constant torque for conveyers, compressors, and positive-displacement pumps; and constant horsepower for winches and machine tools.

Where the application requires speed adjustment over a range, the DC drive, variable-speed AC motor drive, or mechanical speed changer can be provided.

Code letter

The NEC requires that the motor nameplate contain a code letter indicating the locked-rotor (starting) kVA per horsepower. (See Table 3.) These code letters are important since they determine the starting current of the motor, thus aiding in the selection of the proper type of motor starting control. This data is also used to obtain the maximum setting of motor branch circuit protective devices (NEC Table 430-152).

Service factor

Service factor is defined as the permissible amount of overload a motor will handle within defined temperature limits without overheating. When voltage and frequency are maintained at nameplate rated values, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate. However, locked-rotor torque, locked-rotor current, and breakdown torque remain unchanged. NEMA has defined service factor values for standard polyphase dripproof, 60 Hz motors in its standards in accordance with horsepower, speed, and NEMA design. These values range from 1.00 to 1.15 for standard motors and for open motors, up to 1.25.

Insulation and temperature rise

The insulation of a motor's windings is subject to thermal aging. Degradation of the insulation's dielectric capability allows shorting to occur between conductors, causing failure. There is a specific temperature rise permitted by standards based on insulating material capabilities. A rule-of-thumb says that for every 10 [degrees] C rise above the limit, insulation life is halved. The total allowable temperatures for the different insulation classes, including ambient temperature and temperature rise, are as follows.

* Class A, 105 [degrees] C.

* Class B, 130 [degrees] C.

* Class F, 155 [degrees] C.

* Class H, 180 [degrees] C.

Depending on the method of measurement, size of motor, ambient temperature, etc., the permitted temperature rise will vary; nevertheless, the maximum temperature must not be exceeded.

Normally, it's not necessary to indicate on a specification the type of insulation required. Class B insulation is considered standard and will most often be supplied. Requirements such as a 1.15 service factor for a totally enclosed motor will usually be met by the manufacturer by supplying a higher grade of insulation. There are cases, however, when selecting a higher insulation class is justified as a safety factor or to provide for some particular condition that may not be adequately covered by the ambient temperature chosen. An encapsulated motor includes more material over the windings, leading to higher-than-normal temperatures. The increased temperature of an open dripproof motor with a 1.15 service factor can be compensated for by reducing the service factor or by supplying a higher-rated insulation.

Permitted temperature rise of different insulations is based on operation of the motor at altitudes of 3300 ft or less. When this elevation must be exceeded, there are several alternatives. If the motor has 1.15 service factor, it can then be operated at unity factor at altitudes up to 9000 ft in a 40 [degrees] C ambient.

Cycling of the load also affects the temperature of the windings. Standard motors are rated for continuous duty; that is, the load is relatively constant for long periods of time. If the application requires that the motor be started and stopped often, or if the load is cyclical, duty-cycle information should be included in the specifications. Larger frame sizes or higher-rated insulations may be required.

Efficiency considerations

The operating efficiency of a motor has become a major factor because of ever-increasing energy costs. More than half of the typical industrial user's power costs are for energy consumed by motors. This makes it essential that operating costs vs initial costs be considered when selecting a motor.

High-efficiency motors are available that have substantially lower losses than standard lines. In recent years, most major manufacturers have standardized on the term "premium efficiency" (PE) to define their most efficient motors. These newer motors have improved steel, better laminations and insulations, more copper, and rotor fin designs that provides more cooling.

Efficiency of a motor is determined by a standard test called for by the NEMA MG-112.53 standard. The test technique, called IEEE 112A-Method B, provides a consistent efficiency-measurement standard for those who use it. The testing provides an efficiency number, which is stamped on the motor nameplate, that indicates the motor' s efficiency. (See Table 4.) This number represents the nominal or average efficiency of the motor. In addition, the value of the motor's minimum efficiency must be determined; this value is available from the manufacturers. You should use the minimum efficiency value in investment payback calculations so that a more conservative estimate is obtained.

Responsible motor manufacturers feel that the IEEE 112A-Method B test procedure produces more accurate results and consequently have accepted it as their test method. However, some manufacturers use other procedures that tend to result in higher efficiency numbers. These numbers may mislead users into thinking they are getting more than they really are.

The cost of a PE motor is usually greater than a standard motor, depending on the quality of its design. If the motor runs continuously or at least 16 hrs/day or more, this extra cost is usually well justified and will be returned in 1 to 2 years. In some instances, even an 8-hr operation may result in reduced total costs that will justify the initial premium paid for a PE motor. For best results, be sure the motor selected is of the correct size and design for the job, and that it's installed properly and tested for efficient operation.

Electrical connections

After you've made sure that supply voltage requirements are correct, you can make the motor terminal connections. Stator winding connections should be made as shown on the nameplate connection diagram or in accordance with the wiring diagram attached to the inside of the conduit box cover.

Terminal connection problems are usually caused by the branch circuit conductor being a size that is different from the motor leads. Branch circuit wire size is normally determined in accordance with the NEC, based on the motor full-load current, increased where required to limit voltage drop. The motor leads, on the other hand, are permitted a higher current-carrying capacity for a given AWG size than equivalent conductors used in branch circuit wiring because they are exposed to circulating air within the motor.

When connecting the motor terminals to the line leads, you should use connectors sized to the conductor. There may be difficulties in matching the two sets of connectors so that the full surface area of the small terminal can contact that of the larger. Washers should never be used between motor lugs and the branch circuit lugs in an attempt to promote better contact.

A higher degree of reliability is possible if connections are tightened according to torque specifications as shown in Table 5. Recommended terminations can be made up with a torque wrench.

Motor leads should be clamped at or near the point where they enter the box. A neoprene sheet "lead separator gasket" is helpful. This limits movement of cable under starting currents. Extra sleeving around leads in this area eliminates chafing or cutting of cable jackets. Holes in steel plates through which leads pass should be chamfered or at least deburred.

Terminal box

Be sure the motor terminal box is of sufficient size to permit good, reliable connections. Over the years, the most frequent single complaint of those who work with motors is that "the terminal box is too small!" In the 250- to 1000-hp range, the type and size of incoming cable determine how much room is needed.

When mounting conditions permit, the motor terminal box may be turned so that entrance can be made upward, downward, or from either side. For oversize conduit boxes, such as those required for stress cones or surge protection equipment, the mounted height of the motor may have to be increased for accessibility.

When starting up large drives, watch the light reflect from flat terminal box surfaces. A "shimmer" indicates box vibration of fairly high amplitude, even when motor frame vibration is quite low. Such shaking may eventually crack the box, break bolts, or damage the leads. When in doubt, brace it.

Hazardous locations

Motors suitable for use in hazardous locations are tested and listed by UL in its Hazardous Location Equipment Directory. These motors, which are supplied with a testing lab Class I, Groups C and D, or Class II, Groups E, F, or G labels, have been designed and manufactured in accordance with standards established for explosion proof (Class I) and dust-ignition proof (Class II) machines. Parts are machined to very close tolerances, including conduit box and/or collector ring access covers. Extreme care must be taken during disassembly and reassembly, since any nicks or burrs may destroy the explosion proof or dust-ignition proof features of the machine. If these features are altered in any way, the machine will no longer comply with the provisions of the inspection and label service manual and will no longer be properly classified as a labeled motor. The label should therefore be removed and the motor considered unsafe for use in hazardous locations. Always consult the manufacturer or a listed apparatus repair firm to assure safe and proper assembly.


Motors for use in hazardous locations must be marked with an identification number that indicates the operating temperature range.

To date, there are no motors listed for Groups A and B; hence, where such conditions are encountered, motors must be located outside the hazardous area. Motors suitable for other Class I locations are designated as explosion proof.

Likewise, motors for use in Class III locations are not listed. Class III locations are those where ignitable fibers or flyings are present, such as in textile mills and woodworking plants. However, totally enclosed nonventilated motors and the so-called lint-free or self-cleaning textile squirrel-cage motors are commonly used. The latter may be acceptable to the local inspecting authority if only moderate amounts of flyings are likely to accumulate on or near the motor, which must be readily accessible for routine cleaning and maintenance.

Motor controls

Selecting the correct starting equipment for a motor is dependent on the system and load characteristics. Be sure the controller enclosure is appropriate for the location and environment. Normally, one of the following types of starters will be appropriate.

Across-the-line versus reduced-voltage starting is the principal decision where induction motors are involved. The distribution system must be capable of supplying starting current without excessive voltage drop at the motor. Torque sufficient to accelerate the load is dependent on voltage. Full-voltage (across-the-line) starting should be specified unless there are limitations on inrush currents, the load is of the high-breakaway type, or the local utility restricts such starting.

Reduced-voltage starting provides a cushioned start and has the effect of stabilizing the line voltage. There are several methods used to obtain reduced-voltage starting: part-winding, reactor or resistor, auto-transformer, and wye-delta are the most common. Solid-state starters also have become an effective choice in reduced-voltage applications. Aside from not having moving power-circuit parts, their chief advantage is tailoring the output closer to the needs of the load, since many elements are field-adjustable.

There are many additional ways that induction motors are controlled, including the use of reversing, 2-speed, and other specialized starters. Well pumps and fire pumps require equipment packaged to meet their special requirements.

Solid-state starters

Modern solid-state starters offer numerous advantages where reduced-voltage starting is required. In addition to lower starting currents and reducing mechanical shock to equipment upon starting, they are frequently furnished with a wide variety of functions that boost performance and increase reliability. These features include precise overcurrent protection, current-limiting, single-phasing protection, transient protection, shunt trip, precise starting and stopping time, power-factor control, and others.

A particularly lost-cost but valuable option is a "power-factor corrector" that permits significant energy savings when motors are operated at very low loadings, up to 50 to 69% of full load. Solid-state starters are available in ranges up to several hundred horsepower.

Variable-speed drives

The most commonly used variable-speed drives include the eddy-current clutch or magnetic couplings, variable-pitch sheave drives, and DC motors. In the early 1960s, SCR (thyristor)-controlled DC drives came on the scene to provide an effective and efficient means of speed control of DC motors. However, application of the DC motor, particularly in a harsh environment, still presents maintenance problems.

Adjustable-speed drives, which are essentially solid-state, have proven to be highly effective in recent years and are being applied successfully in conjunction with AC induction motors. Of course, AC squirrel-cage motors are constant-speed machines. Use of these adjustable-speed drives permits these motors to run at selected speeds as required for a process and/or to conserve energy.

Presently, three basic adjustable-speed techniques are being used: variable voltage input (VVI), current source inverter (CSI), and pulse width modulation drives (PWM). A variation of the VVI is termed a chopper variable voltage input (CVVI). Each type offers advantages and disadvantages, as shown in Table 6 (see page 32). All types use controlled rectifiers to convert incoming power from AC to DC voltage. Thyristors or high-power transistors are used at the output to provide the desired frequency and voltage for the speed desired or required, resulting also in more efficient operation of the motor. Final choice depends on the application, performance desired, and characteristics of each motor-driven load arrangement.

Solid-state protective relays

Solid-state protective relays have been in use since the early '80s and provide excellent protection for motors that are expensive or used in critical applications. These relays are microprocessor-based and offer a wide variety of protection and operating functions such as overload, jam, (locked rotor), ground fault, motor overtemperature, beating over-temperature, phase unbalance or reversal, plus many others.

New developments

Similar to the rest of the electrical industry, motor controller development is rapidly changing the way motors are started, protected, and controlled.

* Self-protected motor controllers provide for motor disconnect, branch circuit, short-circuit, and ground-fault protection and overload protection within one package.

* Thermal memory circuitry permits the protective relay to "remember" selected heat buildup levels and prevents the motor from being damaged.

* New types of current-transformer-based overload relays directly sense current flowing in the motor leads from the load side of an electromechanical starter. This provides greater accuracy, single-phase protection, no wasting of energy by eliminating overload heaters, and eliminates stocking quantities of overload heaters.

* Adjustable-speed drives, programmable logic controllers, annunciators, sensing and logic devices all can be integrated and prewired along with the electromechanical starters typically found in an MCC.

* IEC-type controllers have permitted the downsizing of motor starting equipment. For instance, more starters can be mounted in an MCC vertical section than was previously practical.

* New panelboard-type enclosures provide flexibility in locating IEC-type motor controllers near equipment where space is at a premium.

Installation considerations

Proper installation of an electrical motor is essential to obtain top-quality operation, efficient performance, and maximum reliability. For a totally cost-effective installation, procedures should consider all aspects of engineering, design, selection, application, and maintenance as well as the details of assembly, hardware, and interrelationship of components and materials. The work demands close coordination, planning, and teamwork on the part of the engineers, installers, and maintainers on the project. And the completion of a successful motor installation requires that the latest and best construction techniques be employed.

Receiving and handling. When a motor is received, it should be thoroughly inspected for dents or other signs of damage. This inspection should be done before the motor is moved from the shippers truck or vehicle. Examine all literature provided with the motor. Do not remove tags pertaining to assembly, storage, lubrication, and operation. File all literature with specifications and drawings pertaining to the motor for reference during installation and for guidance during startup and operation.

On smaller motors, turn the shaft to be sure that it spins freely. If equipped with antifriction bearings, they will normally be prelubricated and ready for operation. However, motors having sleeve bearings are usually shipped without lubricating oil in the bearing; often they are filled with an antirust fluid. These are usually large motors with large bearings that should be inspected through the sight glass and bearing drain openings. Check for any accumulation of moisture and remove the fluid. Then, fill the bearing reservoirs to normal level with a high-grade industrial lubricating oil. Remove any dirt, metal filings, or other contamination that might appear in the oil, or replace the oil.

Always check motor nameplate for proper voltage, phase, frequency, horsepower, etc. Large motors are sometimes shipped disassembled. When assembling, be sure all mating parts are clean. Cleaning can be done with a magnet, vacuum cleaner, or dry compressed air (air pressure less than 60 psi).

When handling motors by hoist, be sure to use lifting bolts if they are provided. Always disconnect any coupling between motor and load before lifting, unless the base is strong enough to assure that shaft or bearings will not be damaged.

Handling of large, heavy motors should be supervised by experienced and qualified personnel. Safety of workers and avoidance of damage to the motor are primary considerations. Details concerning types of cranes, hoists, jacks, rollers, wire ropes, cables, hooks, slings, and the many other aspects of moving heavy apparatus are extensive.

Safety is of paramount importance during the installation, startup, and operation of motors. Safety begins with proper design, application, and selection of the motors and associated components. Be sure that the motor has been well matched to handle the type of load to be driven. Be certain that the enclosure is suited to the surrounding environment and that there is adequate ventilation to assure operation at or below motor design temperature. Check that the motor, gears, belts, driven machinery, etc. are guarded so that anyone near the installation will not be harmed by accidental contact.

All personnel involved with the installation should be familiar with NEMA MG2, Safety Standards for Construction and Guide for Selection, Installation and Use of Electric Motors and Generators. Pertinent NEC rules, especially Art. 430, and all local safety rules must be observed. In addition, OSHA rules must also be followed during the installation of motors and controls. These regulations are included in Part 1010 of the Occupational Safety and Health Standards. Obtain a copy of this document from any local OSHA office.

Foundation. A rigid foundation is essential for minimum vibration and proper alignment between motor and load [ILLUSTRATION FOR FIGURE 3 OMITTED]. Concrete, reinforced as required, makes the best foundation, particularly for large motors and driven loads. In sufficient mass, it provides rigid support that minimizes deflection and vibration. It may be located on soil, structural steel, or building floors, provided the total weight (motor, driven unit, foundation) does not exceed the allowable bearing load of the support. Allowable bearing loads of structural steel and floors can be obtained from engineering handbooks. Building codes of local communities give the recommended allowable bearing loads for different types of soil. For rough calculations, the sub-foundation should be approximately 2 1/2 times the total unit weight.

Whether the motor base is concrete or steel, it must be level. If concrete, be sure it is not too high. A motor can always be raised with shims; but reduction of height is difficult.

In the event that the motor must be mounted on steel, all supports must be of adequate size and strength and braced to assure maximum rigidity. The requirement for a level base is critical [ILLUSTRATION FOR FIGURE 4 OMITTED]. Usually, for a motor installation, there will be four points of mounting- one at each corner of the mounting base. Then there will be mounting requirements for the driven load. All mounting points must be on the exact same plane or the equipment will not be level. This is why a thick, rigid steel baseplate is preferred over an assembly of steel, or at least a steel baseplate should be used in conjunction with the steel assembly.

Before pouring the concrete foundation, locate foundation bolts by use of a template and provide secure anchorage (not rigid). It is recommended that a fabricated steel base be used between motor feet and foundation. See certified drawings of motor, base, and driven unit for exact location of foundation bolts.

Enclosures. Always try to locate the motor in the best possible environment: a clean, dry, cool location. The type of environment in which the motor will operate, in turn, will determine the type of enclosure. Available enclosures have been standardized by NEMA, simplifying selection.

An open-type motor is usually the best choice for installation in locations reasonably free of moisture, dust, or lint. Be sure space is available for maintenance or replacement. Open motors having commutators or collector rings must be located or protected so that sparks cannot reach adjacent combustible material. This does not preclude the mounting of such motors on wooden platforms or floors.

Dripproof motors are intended for use where the atmosphere is relatively clean, dry, and noncorrosive. Keep windings clean with a soft brush, cloth, or vacuum. Totally enclosed motors may be installed where dirt, moisture, and corrosion are present, or in outdoor locations. If a drain plug is provided in the end bracket or bell, it should be removed periodically to drain any accumulated condensation.

Problem locations. When extreme environments or unusual conditions exist (high temperatures, excessive vibration, etc.), special enclosures or arrangements must be incorporated into the installation.

Moisture problems require special consideration. Suitable guards or enclosures must be provided to protect exposed current-carrying parts of motors and the insulation of motor leads where dripping or spraying oil, water, or other injurious liquid may be present, unless the motor is specially designed for the existing conditions.

For standby service or for damp-location operation, a low single-phase voltage (on the order of 5 to 10% of rated voltage) is sometimes applied to the motor windings to combat moisture. Some larger motors are available with built-in strip heaters or tubular-type space heaters for preventing condensation.

Installation techniques

Mounting. A motor can be mounted in many ways, depending on the size, weight, and use for which it is intended. Small motors may incorporate a rigid mount; the frame being welded directly to a steel plate formed to match the shape of the frame and incorporating mounting holes. Most commonly, medium- and large-size motors have mounting feet cast integrally with the frame. Vertical motors require an end bell specially machined to receive a mounting flange. Where it is important to isolate vibration and noise or reduce the shock of starting and stopping, various types of resilient mounts and cushion bases are available.

After alignment with the load, bolt the motor in place with maximum size bolts. It is advisable to provide some variation in the location of the foundation bolts. This can be done by locating the bolts in steel pipe embedded in the foundation [ILLUSTRATION FOR FIGURE 5 OMITTED]. It is recommended that a competent engineer, familiar with motor foundation designs, be called on to design and supervise foundations and support assemblies for large motors.

Sliding bases and adapters are available for use with T-frame motors when they replace an old U-frame motor. Also, check whether other components or equipment, such as gears, special couplings, and pumps, are to be mounted on the motor. If so, be sure space is available.

After the motor base is in place, but before it is fastened, shim as required to level. Use spirit level (check two directions at 90 [degrees]) to ensure that motor feet will be in one plane (base not warped) when base bolts are tightened. Set motor on the base, install nuts, and tighten. Do not make a final tightening until after alignment. NEMA standards give dimensions for foot mountings and some flange mountings.

Couplings. Direct coupling (rigid, flexible, or fluid) of the motor shaft to the driven load results in a 1:1 speed ratio. Where application demands other than standard available speeds, gears or pulleys/belts may be used. Variable speeds are possible by making available several gear ratios or pulleys with variable diameters. Chain drives may also be used where shaft center-to-center distance is too great for gears or too short for belting.

Direct-connected motors with ball or roller bearings may be coupled to the load through flexible couplings. A coupling half should not be installed by hammering or pressing. Always heat the coupling to install it on the shaft. Accurate mechanical lineup is essential for successful operation. Mechanical vibration and roughness during the operation of the motor may be indications of poor alignment. In general, using a straight edge across and a feeler gauge between coupling halves is not sufficient. It is recommended that the lineup be checked with a dial indicator and checking bars connected to the motor and loaded machine shafts.

With few exceptions, a flexible coupling is used to connect the motor to the load. This type of coupling is designed to tolerate some misalignment; however, this misalignment can cause vibration and/or stress on the motor bearings. Consequently, the shafts in all coupled applications should be lined up with the same high degree of accuracy, regardless of the type of coupling or type of bearing used.

Alignment. The following steps in attaining correcting alignment of direct-connected drives should be implemented.

* The foundation for the motor and driven load should provide a permanent fixed relationship of the motor with respect to the driven load. The foundation should provide a solid anchor that will maintain this fixed relationship after alignment is completed.

* Position the motor on its foundation to obtain the correct spacing between the motor shaft and the driven shaft. This distance is specified by the coupling manufacturers, but is usually in the range of 1/8 to 3/8 in.

* In the case of sleeve-bearing motors, positioning should limit the axial movement of the coupling to keep the motor bearing from floating off the thrust shoulders. These bearings will not take continuous thrust. When positioning the shaft of motors with end play, the shaft should be placed on the midpoint of the end play. (Ignore the magnetic center indication.)

* Adjust the position of the motor with jackscrews, shimming, etc., until misalignment between the two shafts is within the following limits as measured with a dial indicator with the motor bolted down.

When adjusting the position of the motor, care should be taken to assure that each foot of the motor is shimmed before the motor is bolted down so that no more than a .002-in. feeler gauge can be inserted in the shim pack.

Belt drives require that the motor be mounted on slide rails or a bedplate with provisions for adjusting the belt tension. Align the pulleys so that the belts run true (perpendicular to the shaft) and with uniform tension on all belts. The slide rails should be located so that the motor is near the end of the slide rail closest to the driven machine. This permits maximum adjustment (travel) for belt tensioning and readjustment later to compensate for belt wear or stretch.

Tighten the belts to prevent slippage at the rated horsepower. Excessive belt tension causes unnecessary loads on the shaft and bearings. On high-inertia loads or equipment that could jam or stall causing belts to squeal or slip during acceleration, or where torque approaches pull-out torque during overload or stall, tightening to prevent this squeal or slippage will result in overloading the bearings or shaft. Belt speeds are normally limited to 5000 ft/min for "E-Section" belts and 6500 ft/min for "8-V Section" belts. Speeds in excess of these limits should not be used without consulting the belt manufacturer.

Gear drives require accurate alignment and rigid mounting for a satisfactory drive. Pitch diameter and width should not be outside recommended gear manufacturer's limits. Check the factory for bearing thrust capacity before installing helical gears. In all cases, gear teeth must be centered with each other, correct shaft center distance must be obtained, and gear faces must be parallel. Gear teeth must fully engage to a depth giving approximately 0.002 in. minimum backlash through one complete revolution. Test backlash and face parallelism again after tightening mounting bolts.

Sleeve bearings are supplied with a babbitted face to restrain axial rotor movement during stamp or while running disconnected from the load. The babbitted faces are not intended to withstand continuous thrust loads and care must be exercised in the lineup to prevent this from occurring during operation. Lineup should provide operation in approximately the mechanical center between the extremes of the end play; this is very close to the magnetic center location. Standard motors are supplied with more end play. It is necessary that a limited end-float flexible coupling be used on sleeve bearing motors to limit the total axial movement to less than that shown in the motor outline drawings. As noted in NEMA Standards MG1-14.38, sufficient thrust to damage bearings may be transmitted to the motor bearing through a flexible coupling.

Ball bearings. Unlike motors with sleeve bearings, motors with ball bearings should be coupled so that there is more end play in the coupling than in the motor. This is because ball bearings will take enough thrust, without damage, to slip the coupling axially to accommodate thermal expansion in the system. The end play of these motors may be as much as 50 to 150 mils, and the coupling should have at least this much float. Correct axial positioning can be obtained by tilting the motor toward the outboard end to move the rotor as far as it will go in that direction (the rotor will not be easy to move axially since bearings must slide in the housing); or by barring the rotor over to the outboard end, and then positioning the motor to give at least 150 mils between the coupling halves or shaft ends. Or, the motor may be positioned without regard to rotor position so that coupling will allow 100 mils travel in either direction.

Mechanical alignment

Experience has shown that any base-mounted assemblies of motor and driven load, no matter how rugged or deep in section, may twist out of alignment during shipping or any moving, and that alignment by eye is ineffective. Proper alignment of direct-coupled drives can be accomplished by a dial-indicator ([ILLUSTRATION FOR FIGURE 6 OMITTED], see page 39) or computerized instrumentation.

Angular misalignment is the amount by which the faces of the two coupling halves are out of parallel. Angular maize-alignment may be determined by mounting a dial indicator on one coupling half with the indicator probe on the face of the other half, then rotating both shafts together through 360 [degrees] to determine any variation in reading.

It is important, during this check, to keep the shaft of a motor with end play against its thrust shoulder, and the shaft of a driven bad with end play against its thrust shoulder to prevent false readings due to shaft movements in the axial direction.

Parallel misalignment is the offset between the centerline of the two shafts. Checking this is done by mounting a dial indicator on one coupling half with the indicator probe bearing radially on the other coupling half, then rotating both shafts together through 360 [degrees].

The motor and load must be correctly aligned under actual operating temperatures and conditions. Machines that are correctly aligned at room temperature may become badly misaligned, due to deformation or different thermal growth as they increase in temperature. The alignment must be checked, and corrected if necessary, after the motor and driven machine have reached their maximum temperature under load.

It is recommended that "floating shaft couplings" or "spacer couplings" be used on motors where the coupling alignment cannot be accurately checked and/or maintained. Misalignments of several thousandths of an inch will result when there are relatively small changes in the temperature differences in larger motors and the equipment driven.

After the alignment procedure is completed, the equipment should be given a test run to verify that the lineup gives satisfactory performance. Once satisfactory performance has been verified, the machines should be dowelled to their bed-plates. Recommended dowelling is two dowels per machines, one in each of the diagonally opposite feet, with the size of the dowels approximately 1/2 the diameter of the hold-down bolts.

Machines that are correctly aligned when they are first installed may subsequently become misaligned due to wear, vibration, shifting of the base, settling of the foundations, thermal expansion and contraction, corrosion, etc. Therefore, it is advisable to recheck the alignment periodically to correct for any changes.

About the Author

Robert J. Lawrie

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