Selection of Electric Motors for the Job

Selection of Electric Motors for the Job

For an electric motor to perform efficiently, the proper type of motor and control for the application must be selected. To make such a selection, a knowledge of the characteristics of ac and de motors and controls is essential. It is also essential to have a knowledge of the general operating characteristics of the load the motor will operate, such as power and speed requirements, special operating conditions, control features·, and similar items.

TYPES OF DRIVES

In general, two methods of drive have been traditional: using one motor to drive two or more machines as a group or by having an individual motor for each machine. While the former was the popular method de­ cades ago, the latter method is the most popular today. In the group drive system, the various machines are connected together by shafting and belts as shown in Fig. 12-1, although chains, gears, or other mechan­ ical devices may be used.

Seldom will a group drive system be seen in any modern industrial or commercial application. Practically all systems are now of the individ­ ual drive type (Fig. 12-2). The reasons are many, but flexibility of location or arrangement of the machine with individual motor drive makes it possible to place each machine in the best position to suit the flow of material and save handling and trucking. Also, as manufacturing conditions change, each machine-complete with its drive and control as a unit­ may be readily moved to some other location. The elimination of over­ head belting and shafting greatly improves the lighting on the machines and also facilitates the use of material-handling equipment, such as con­ veyor systems, small cranes, and similar devices. The most important factor, however, is probably the ease with which machines using individ­ ual motors may be started and stopped. Also, with this system, the exact speed required may be obtained and maintained indefinitely, being inde­ pendent of the load on other machines and of temperatures and atmo­ spheric conditions, which may affect belt transmission.

SELECTING THE MOTOR FOR THE JOB

When selecting motors for a particular application, no set rules will fit each and every application, but there are some important machine and load characteristics that should be known for practically any and all re­ quirements:

1. The speed requirement of the driven shaft, in rpm

2. The range of speed required, if the machine is adjustable

3. The horsepower required at maximum speed or loading

4. If the load is not constant, the cycle duty, including variable items, such as load, time, speed, weights, and other factors If the speed varies, the variation of the torque with the speed

5. Torque: starting, pull-in, pull-out, or maximum-all in percent of full-load values

7. Mechanical connection: belt, chain, coupling, or gear.

DETERMINING TORQUE. It is sometimes desirable to check the manufacturer’s data giving the motor characteristics. This is done by installing a temporary motor and taking power readings under various operating conditions.

The starting torque required can be determined by wrapping a rope around the driven pulley and then measuring, with a spring scale, the pull which will start the machine and turn it over. The starting torque, in pound-feet, is the product of the reading on the scale, in pounds, and the radius of the pulley, in feet. For example, assume that it requires a pull of 75 lb on a driven pulley of 6-in. radius to start the motor. The starting torque required is

In most cases, however, the manufacturer’s data can be relied upon and will suffice for most applications when selecting a motor for a par­ ticular job.

POWER SUPPLY

If the power supply is alternating current, it is necessary to know the fre­ quency, voltage, and number of phases. However, if the voltage is direct current, only the voltage need be known.

The characteristics of the electric service and its limitations must be considered in every instance when selecting motors for a given applica­tion.

LOAD FACTOR

The load factor is the ratio of the average load to the maximum load over a certain period. The time may be either the normal number of operating hours per day or may be 24 h. The average load is equal to the kilowatt­ hours used in the specified time as measured by watt-hour meters divided by the number of hours. The maximum load is the highest load at any one time as measured by some form of maximum-demand or curve-drawing watt-hour meter.

When comparing industrial loads, the maximum load is taken as the load which would be obtained if all motors were operating continuously at the full-rated load for the same period. This input to the motors is ob­ tained by using the equation

If the load factor is based on the number of working hours per day, the 24-h load factor may be obtained by multiplying the given load factor by the number of hours and dividing by 24.

TYPES OF MOUNTING AND ENCLOSURES

Two important mechanical characteristics must be considered when se­ lecting motors for a particular application. One is the type of mounting to be used, and the other is the type of enclosure and ventilation.

The types of motor mounting to consider are the horizontal, vertical,flange, gear motor, and others. Common types of enclosures include open drip-proof, totally enclosed fan-cooled, totally enclosed explosion-proof, and separately vented-pipe vent or blower.

OPEN DRIP-PROOF. This design draws outside cooling air into the motor for ventilation. It is primarily for clean, dry areas indoors. Con­ taminated cooling air will normally reduce the life of the motor (winding and bearing grease). This motor is good for general-purpose use but not ideal where little maintenance can be performed.

TOTALLY ENCLOSED FAN-COOLED. Outside cooling air is directed over the motor by an exterior shaft-mounted fan. It is ideal for dusty atmospheres and many hostile areas. It is primarily used in areas where the motor may not be accessible for maintenance. Cooling is not as efficient as an open-type motor.

TOTALLY ENCLOSED EXPLOSION-PROOF. Ventilation here is the same as the totally enclosed fan-cooled. The motors are specifically designed for installation in hazardous areas as defined by the NE Code. The motor is built to contain and withstand an explosion within its own enclosure. Because highly flammable gases and dust require special design requirements (thermal, mechanical, and thermal protective devices), attention is given to machined fits and mating surfaces to en­ sure adequate flame paths to contain and extinguish flames or sparks before they reach the outside air.

SEPARATELY VENTED-PIPE OR BLOWER. These motor  types are the least common. Blower cooling is used where high heat­ generating duty cycles are found and/or motor size is at a premium. Blower-forced air cooling can remove heat quickly. This design approach can be expensive compared to conventional enclosures and is dependent on heat removal as the key design factor rather than torq-ue limitations. Pipe ventilation is similar except here the motor is generally ducted to a supply of clean, fresh air. It may or may not have a blower. Often the motor may require oversizing here where the duct is long. Maintenance is critical to each of these to ensure that ventilation passages are clear. The application considerations that follow can be applied to the basic enclosures outlined.

HIGH-SPEED BELTING

An ever-increasing number of processes require higher speed for greater output or simply to properly blend special materials being mixed. This, coupled with the fact that higher-capability fiberglass belts are often used, can reduce bearing and shaft life.

High-speed belting usually employs a 3600-rpm motor. Conven­ tional bearings operating at high speed and high load factors begin to ap­ proach their design point in limiting speed. Lubrication breakdown is also prevalent.

Drive considerations as a minimum should utilize ductile iron, dynamically balanced sheaves, and matched belts. Transmittal of this in­ formation to the motor supplier with the belt drive details (center dis­ tance, sheave pitch diameters, etc.) is critical if a marriage of the motor drive is to be successful.

The bearing should be selected to maximize life, and this can mean a standard ABEC-1 commercial-motor-quality standard bearing, an ABEC-3 for truer tolerances on the ball and shaft bore, or a bronze or phenolic retainer for the balls to increase speed capability. It is critical not to neglect the limiting bearing speed recommended by the bearing manufacturers.

Lubrication is generally grease of a No.2 grade. Oil lubrication may be a consideration as well as oil mist. Grease is a more economical ap­ proach and is the first choice.

Shaft deflection imposed by heavy belt tensions or overtensioning can mean oversize shafting will be required as well as increased diametri­ cal clearance between the shaft and bearing housing to prevent rubbing. A common misunderstood fact is that changing to higher-level psi shaft material will not provide a stiffer, lesser deflecting assembly:

Diameter changes have the greatest impact upon shaft deflection; the modulus of elasticity is the same or near the same for almost all com­ mon motor shaft materials and has little effect upon deflection.

Recommendations:

1. Check bearing life in hours for radial shaft load.

2. Verify bearing speed capability.

3. Use balanced, ductile sheaves and matched belts.

4. Check standard shaft deflection, and change diameter if neces­ sary.

5. Open shaft bearing-housing clearance.

6. Be sure belt drive is tensioned properly (glass belts have re­ duced deflection rates for tensioning).

DUTY CYCLE

Because of special product requirements, motors are subjected to duty cycles- intermittent operation with frequent starting, stopping, and often reversing. The associated heating created by high loads for short periods of time often causes the need for higher insulation classes and/or blower cooling.

An approximate determination of required motor horsepower can be made from a cyclic load curve using the rms (root mean square) method,

which is an arithmetic integration of the square of the load curve as follows:

hp is an average hp between T0 (time0) and T10, etc. After arriving at the final rms hp value, it is necessary to take the peak horsepower value encountered during the cycle and convert it to lb ·ft. Then this value can be plotted on the motor speed torque curve to be sure the motor can pro­ duce this torque without stalling.

Recommendations:

1. Calculate rms horsepower.

2. Select a motor with one higher insulation class to cover peak loads.

There are numerous cases where reversing is a part of the duty cycle. Reversing capacity is rated in idle reversals that can be performed with no load or connected inertia. Of course, a motor will always see some con­ nected inertia. Since the reversing capacity of a motor is inversely pro­ portional to the total connected inertia, the following applies:

where

Rx = reversal acceleration with connected inertia

Ri = reversal acceleration idle

WK2 r = rotor inertia (lb • ft2)

WK2 t =total connected inertia (load and rotor)

Reversal capacity is limited by the amount of heat generated by the reversal itself in a particular frame size. Once the data in the preceding formula are determined, reversals can be calculated. All that remains is to assign values of heat units to the cycle. A heat unit is a segment of the cycle to which a heating increment is assigned.

motor acceleration to full speed = 1heat unit

de brake stop = 1heat unit plug stop = 3 heat units

plug reversal = 4 heat units

running losses, inertia, inaccuracies, and

manufacturing tolerances = heat units

By selecting the applicable cycle portions listed, total heat units can be easily calculated. Dividing the connected inertia reversal rate Rx by ·the heat units per cycle yields the loaded cycles per minute.

Recommendations:

1. Low inertia rotors in high reversal rate designs

2. High stator copper content for lower motor temperatures

3. Pinned or keyed rotors for mechanical resistance to reversing

4. Aluminum vent fan keyed to shaft for positive cooling and mechanical resistance to reversing

INERTIA

High-inertia drive motors are those capable of accelerating very large loads from rest to full speed. Typically, these loads are fans or centri­ fuges.

The primary consideration is that of heat dissipation. During start­ ing, heat is generated inside the motor rotor and stator. The degree and effectiveness of heat dissipation is a direct function of the magnitude of inertia that can be accelerated.

During starting, most heat is generated in the motor rotor and stator due to high inrush current and high rotor slip. Since this heat is directly related to the inertia accelerated, the motor must then be able to absorb the heat until heat transfer takes place, allowing normal ventila­tion to take over. Temperature is then directly proportional to pounds of active material (copper and steel). If acceleration time is increased by means of reduced voltage starting, then the heat transfer allows the motor to adequately dissipate the heat generated. (Less inrush current reduces the amount of heat that must be dissipated.) Acceleration times will be lengthened considerably- often from 1to 2 min up to 8 to 10 min.

The important criterion to consider here is that the available torque at reduced voltage must exceed the friction and windage torque of the drive by an ample amount to provide adequate acceleration. Torque becomes the important design factor rather than horsepower as well as start-stop requirements. Star-delta starting as compared to full-voltage starting would yield the following different motor capabilities:

Reduced voltage starting will typically reduce stator temperature to as low as 50-60o/o of the line start value. This is often desirable for pro­ cess requirements in air-conditioned facilities as well as for substantially extending motor life.

Thermal protection is highly desirable, and two sets of protectors are recommended. The first set of protectors is for starting. They are for the maximum temperature condition to eliminate nuisance tripping for two consecutive starts of the high inertia with the motor at operating temperature. The second set of protectors is for the running mode, and they are set for normal rated running temperature. The motor insulation is rated for the maximum rated temperature condition so as to prolong life.

Recommendations:

1. Define inertia and friction and windage.

2. Use reduced voltage starting if inertia values are high (higher than across the line start).

3. Verify motor accelerating torque.

4. Define starting cycles.

5. Define duty cycle.

6. Check starting time.

7. Identify belting (most are belted or geared- direct drive is sel­ dom used) so it can be checked.

8. Use thermal protection for starting and running condition.

SHOCK LOAD

Often a motor will be installed in an area subjected to heavy shock loads. They can range from punch presses to drop forges to vibrating screens and conveyors. It is also common in power plants to design motors for loads imposed by earthquake.

Obviously, magnitude and frequency weigh heavily on the ultimate design considerations. Usually, the stator winding will be most vulner­ able to failure by shock load and failure due to grounding or turn-to-turn short circuiting.

The basic motor frame selection should be for high strength- cast iron housing, bearing brackets, and fan guards.

Recommendations:

Design Condition	Solution
1. Occasional mild vibra­ tion	1. Additional varnish bakes to add mass and rigidity to stator end coils
2. Frequent moderate vibration	1. Additional varnish bakes; see previous solution 2. Bonded insulating tape wrapped around end coils or laced end coils 3. Cast iron conduit box 4. Additional varnish bakes on all hard- ware 5. Vent fan keyed to shaft 6. Rotor keyed to shaft- not shrunk
3. Frequent heavy vibra­ tion	Same as items 1-6, above, plus: 5. Ductile iron castings 6. Laced end coils plus overcoat of heavy epoxy resi•n 7. Clamped or packed leads in housing lead channel

It is often necessary to also consider oversize shafting, special bear­ ings, and heavier-grade grease for the most severe cases. Special bearings usually result in the use of a special heavy-duty ABEC-3 fit roller bearing or spherical roller bearing. Greases frequently used are those with high film strength.

RADIATION

Radiation requirements are most generally associated with nuclear power plants. However, some test labs use radioactive elements as well and therefore have need of motors capable of withstanding radiation dosage.

It is of prime importance that insulating components be of inorganic origin- wire varnish, top wedge, slot liners, phase insulation, and leads. Most frequently silicons, glass, and mica are utilized. The other major consideration is bearing grease. Here again, a silicone base is most often used.

One item that cannot be neglected is the insulating or core plate often used on the lamination steel. Again, a silicone product is selected.

Recommendations:

1. Define radiation levels and submit to the motor supplier. Quan­ tify levels, generally in rads of 1 X 10^x.

2. State required design life necessary in years, if applicable.

Work with the motor manufacturer to select a suitable lubricant that is readily available for normal maintenance .

TEMPERATURE

Most users are knowledgeable of effects of higher-range temperatures on motor life- specifically the stator insulation. A rule of thumb is that motor insulation life is halved for every additionall0°C of operating tem­ perature. However, with today’s higher-grade insulation materials, motors can be built with very acceptable life while operating at higher temperature values.

The three most common conditions causing higher motor temperature are the following:

1. High ambient

2. High altitude

3. Service factor (multiplier applied to normal nameplate hp ratings) Factors such as overload and single phasing are not considered here since they are not a design consideration to be covered by the continuous operation mode.

Even higher-temperature operation can be designed for if necessary. Considerations then are given to very high-temperature insulation (200°C total temperature), very special grease, loose motor fitups to per­ mit thermal expansion, and heat-stabilized bearings. The motor is usu­ ally oversized to allow for lower temperature rise and permit adequate life. Frequent maintenance of bearings is mandatory.

CORROSION

Corrosion is the most frequently occurring condition- the severity and thus the recommendation depend on the actual motor location. Most motors in highly corrosive atmospheres whether acids, bases, or salts will have markedly increased life by utilization of a cast iron exterior. Addi­ tions to this may be as simple as exterior epoxy paint or as special as stainless steel housing shrouds.

Frequently, motors will operate where condensation within the motor occurs due to corrosive contaminant vapors condensing or simple moisture from humidity condensing. The best and most economical means of correcting for these common conditions are the following:

1. Drains to relieve excess moisture

2. Epoxy to protect motor windings from caustic vapor attack

3. Space heaters to keep the motor interior temperature above the dew point

Exterior paints are available for virtually any special application. The very exotic types such as vinyls and coal tar epoxy, to name two, are a small percentage for even the most demanding situation. Most applica­ tions can be met with the standard paint used by most motor manufac­ turers with the remaining portion adequately handled by a standard epoxy-type overcoat.

The small percentage of exterior paints mentioned usually requires sandblasting of parts prior to priming and overcoats of paint in several stages at prescribed intervals for proper bonding and adhesion. Mixing of the paint is often critical for proper application in specific amounts. When these types are utilized, it is mandatory that other equipment con­ siderations such as the motor interior (winding, grease, rotor, shafting, seals, etc.) be examined in detail to verify their adequacy, the point being that the exterior appearance is of little value if the motor experiences premature internal failure.

Seal selection is another prime factor to ensure that contaminants are kept outside the motor. This refers not only to the shaft entrance but bearing housing fit and conduit box fitups as well.

For severe conditions, the following is recommended:

Motor Area	Modification
Housing to bearing bracket fits Shaft	‘0’ Ring seals Stainless material spring-loaded seals where moisture present, rotating seals (rubber) where dictated by environment
Conduit box Housing-lead channel en­trance to conduit box	Gaskets Packing material

Ventilating fans are critical where enclosed fan-cooled motors are used. Three basic types are common: polypropylene (plastic), aluminum, and brass. Where they are unacceptable, highly resistant epoxy coatings can be applied to provide very acceptable life to this critical part.

Conductive dusts are another frequently encountered environment. Attention is focused here on the stator winding end turn to protect the winding from being coated or covered with the material that can short the stator circuit. An overcoat of polyurethane dispersed particles to a 7-mil buildup is applied. This provides excellent resistance to abrasion as well.

ACKNOWLEDGMENTS

I am indebted to Mr. P. D. Preuninger, Special Products Manager, Louis Allis Division, Litton Industrial Products, Inc, for supplying much refer­ ence material used in this chapter. If you have a particular application that is not covered herein, it is suggested that you contact Louis Allis Division at 427 East Stewart St., Milwaukee, WI 53201 for additional in­ formation.