Process Plant Machinery: Electric Motors and Controls (part 2)

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SPECIAL INDUSTRY AND APPLICATION DESIGNS

Capable motor manufacturers can offer suitably modified motors for specific industries. Typical are motors designed for corrosive atmospheres such as those found in the paper, chemical, petroleum, and metals industries. These motors would be available from fractional to 500 horsepower.

Motors for the food and dairy industries must be designed for easy cleaning and hose washdowns to meet the rigid sanitary codes of all government agencies, the Baking Industry Sanitation Standards Committee, and Dairy Standards.

Fgr. 4 depicts an important special application design, a brake motor that combines a motor and an integrally mounted disc brake into one unit. These direct action brakes are spring set, electrically released, and designed for stopping and holding a load.

Energy-saving motor designs are available and have reduced full load motor losses through the use of optimum electrical designs and increased active material. They are also designed to operate at low noise levels and are available up to 300 horsepower.

Multispeed motors are motors with special electrical characteristics for a wide variety of two-speed applications requiting constant or variable torque.

Vertical motors ( Fgr. 5) are flanged, footless designs used in direct coupled vertical applications. Vertical motors are available with normal and medium thrust capabilities on many different frames. They are widely applied in pumps and mixers, and sizes can exceed 1000 horsepower.

A submersible motor is shown in Fgr. 6. These motors are designed for continuous pumping duty submerged in liquids containing a maximum solid content of 10% by weight and 90% liquid.

For use in hazardous environments, these motors are UL listed for use in class I, group D, division I hazardous locations in air or submersible in water or sewage.

MAJOR COMPONENTS OF INDUCTION MOTORS

The major components of an AC induction motor are illustrated in Fgr. 7. They consist of the following:

  • - flame
  • - stator or windings
  • - rotor
  • - bearings
  • - end shield, end bell, or end bracket
  • - cooling fan
  • - conduit box


Above: Fig. 7 What to look for in major components of an AC motor. ( Reliance Electric, Cleveland, OH.)

Frame

Cast iron is the most popular frame material. The frame must be made of a rigid material that will absorb noise as well as vibration because it provides the structural support for all the other motor components. The stator or windings are mechanically attached to the frame prior to the rotor installation. Tight machining tolerances are required on the mating surfaces of the frame and end bells or end shields. These tight tolerances ensure accurate bearing locations that allow the rotor to run at the proper air gap. All of this helps to increase motor efficiency.

Stator and Rotor

The stator is made up of copper wire that is formed into coils that are insulated.

Stator coil construction procedures are sometimes complicated and must be entrusted to capable manufacturers.

The rotor is the rotating member of an induction motor that is made up of stacked laminations. Thin, high silicone steel stampings are aligned on a keyed mandrel and welded in the axial direction in three or four places to form a rigid cylinder. This cylinder is removed from the mandrel and pressed on the motor shaft.

Slots in each lamination of the cylinder are filled with molten aluminum to form the squirrel cage. The cast aluminum bars of the squirrel cage act as conductors for the induced magnetic fields. Some motors are made with copper bars instead of cast aluminum.

The shaft portion of the rotor is precision machined on both ends to fit the bearings. Motors in the 5- to 200-horsepower sizes are usually fitted with antifriction (ball) beatings. Roller or journal bearings are used for very large motors.

Bearings and End Shields

Bearings are used to reduce friction and wear while supporting the rotating element (rotor). The bearing acts as the connection point between the rotating (rotor) and stationary (end bell or end shield) elements of a motor. There are various types, such as roller, ball, sleeve (journal), and needle beatings.

The ball bearing is used in virtually all types and sizes of electric motors.

It exhibits low friction loss, is suited for high-speed operation, and is compatible in a wide range of temperatures. There are various types of ball bearings such as open, single-shielded, or sealed. Although not mandatory, some manufacturers offer a special beating design for oil-mist lubrication.

End shields or end bells cover the ends of the motor frame. They protect the internal electrical and mechanical parts from moisture and dirt and provide support for the bearings.

Cooling Fan

The cooling fan is a small but important part of an electric motor that is often overlooked. It provides the cooling air across the TEFC motor and is made of a non-sparking, corrosion-resistant material. Most motors come with a bidirectional fan so as not to be sensitive to the direction of motor rotation. Some motors have low noise or high-efficiency fan designs and will pump air in only one direction of rotation. This type of fan generally has a direction of rotation arrow cast on the fan hub.

Conduit Box

The conduit box is the metal container on the side of the motor that houses the electrical connections. It must provide a waterproof environment where the stator (winding) leads are connected to the incoming power leads. If oil mist is used to lubricate the motor beatings, a gas-tight seal must be provided around the stator leads at the penetration of the conduit box.

A MOTOR AS PART OF A SYSTEM

Selecting a Motor

Motor selection is a complicated process containing numerous trade-offs. Efficiency is only one of several important considerations. The objective of informed motor selection is to arrive at the best possible installation consistent with minimum cost, horsepower, and frame size for the specified life expectancy, load torque, load inertia, and duty cycle of the specified application.

To satisfy the torque, horsepower, and speed requirements of a large variety of motor applications, polyphase AC motors are designed and manufactured in four groups classified design A, B, C, and D by NEMA. Each classification of motors has its own distinctive speed-torque relationship (Fgr. 8) and inherent expectations regarding motor efficiency.

Motors intended for loads that are relatively constant and run for long periods of time are of low slip design (less than 5 percent) and are inherently more efficient than design D motors, which are used in applications where heavy loads are suddenly applied, such as hoists, cranes, and heavy metal presses. Design D motors deliver high starting torque and are designed with high slip (more than 5 percent) so that motor speed can drop when fluctuating loads are encountered. Although design D motor efficiency can be less than other NEMA designs, it’s not possible to replace a design D motor with a more efficient design B motor, because it would not meet the performance demands of the load.


Above: Fig. 8 Speed-torque curves for a 5-HP motor, NEMA design A and D, and full-load efficiencies. (Reliance Electric, Cleveland, OH.)

The motor with the highest operating efficiency does not always provide the lowest energy choice. Fgr. 9 compares the watts loss of a NEMA design D and a design B motor, in a duty cycle that accelerates a load inertia of 27 lb. ft. 2 to full speed and runs at full load for 60 seconds. During acceleration, the lower curve represents the performance of the design D motor, while the upper curve reflects the NEMA design B motor. The shaded area between the curves represents the total energy difference during acceleration. In this example, this area is approximately 6.0 watt-hours, the energy saved accelerating this load with a design D motor instead of a design B. During the run portion of this duty cycle, the energy loss differential favors the NEMA design B, because it has a higher operating efficiency. In this example, the energy saved operating this load with a design B motor instead of a design D motor is approximately 2.8 watt-hours.

The bar chart shown in Fgr. 10 summarizes acceleration and running loss/cycle on both the NEMA design B and design D. Comparison of the total combined acceleration and running portions of this duty cycle indicates a total energy savings of 3.2 watt-hours favoring the use of the design D motor, even though the design B motor has an improved operating efficiency. The key is the improved ability of the design D motor to accelerate a load inertia at minimum energy cost.


Above: Fig. 9 Energy usage on duty cycle application 5-HP, 4-pole, TEFC accelerating 27 lb-fi 2 inertia. ( Reliance Electric, Cleveland, OH.)


Above: Fig. 10 Acceleration and running loss per cycle on NEMA D and NEMA B motors.(Reliance Electric, Cleveland, OH.)

Other Components Affecting Efficiency

Because a motor buyer selects the most efficient motor of a given size and type does not mean that energy savings are being optimized. Every motor is connected to some form of driven equipment: a crane, a machine tool, a pump, etc., and motors are often connected to their loads through gears, belts, or slip couplings.

By examining the total system efficiency, the component that offers the greatest potential improvements can be identified and money allocated to the component offering the greatest payback.

In the case of new equipment installations, a careful application analysis, including load and duty cycle requirements, might reveal that a 7 89 pump, For example, could be utilized in place of a l O-horsepower pump, thereby reducing motor horsepower requirements by one third. By reducing the mass of the moving parts, the energy required to accelerate the parts is also proportionately reduced. Or in the instance of an air compressor application, the selection as to the size and type of compressor relative to load and duty cycle will affect system efficiency and energy usage. Of course, the most efficient equipment should be selected whenever possible.

Reduced system efficiency and increased energy consumption are also possible with existing motor drive systems due to additional friction that can gradually develop within the driven machine. This additional friction could be caused by a build-up of dust on a fan, the wearing of parts causing misalignment of gears or belts, or insufficient lubrication in the driven machine. All of these conditions cause the driven machine to become less efficient, which causes the motor to work harder.

Rather than replace the existing motor with a higher efficiency model, replacing either critical machine components or the machine itself may result in greater system efficiency and energy savings.

Choosing the Best Applications

Energy-efficient motors may be the most cost-effective answer for certain applications. Simple guidelines are listed below:

- Choose applications where motor running time exceeds idle time.

- Review applications involving larger horsepower motors, where energy usage is greatest and the potential for cost savings can be significant.

- Select applications where loads are fairy constant, and where load operation is at or near the full load point of the motor for the majority of the time.

- Consider energy-efficient motors in areas where power costs are high. In some areas, power rates can run as much as $. 12 per kilowatt-hour. In these cases, the use of an energy-efficient motor might be justified in spite of long idle times or reduced load operations.

Using these simple guidelines, followed by an analysis and cost justification based on various techniques, can yield results that will influence motor choice beyond just-in cost consideration.

Other Determinants of Operating Cost

Voltage Unbalance

Although efficiency is a commonly used indicator of energy usage and operating costs, there are several important factors affecting motor operating costs. Rated performance as well as selection and application considerations of polyphase motors requires a balanced power supply at the motor terminals. Unbalanced voltage affects the motor's current, speed, torques, temperature rise, and efficiency. NEMA Standard MG 1-14.34 recommends derating the motor where the voltage unbalance exceeds 1 percent and recommends against motor operation where voltage unbalance exceeds 5 percent. Voltage unbalance is defined as follows:

Voltage Unbalance (%) = 100 x [Maximum Voltage Deviation from Average Voltage / Average Voltage ]

Voltage imbalance is not directly proportional to the increase in motor losses, as a relatively small unbalance in percent will increase motor losses significantly and decrease motor efficiency as Fgr. 11 shows. An effort to reduce losses with the purchase of premium priced, premium efficiency motors that reduce losses by 20 percent can easily be offset by a voltage unbalance of 3.5 percent that increases motor losses by 20 percent.

Energy cost can be minimized in many industrial applications by reducing the additional motor watts loss due to voltage unbalance. Uniform application of single- phase loads can assure proper voltage balance in a plant's electrical distribution system used to supply polyphase motors.


Above: Fig. 11 Motor loss percentage as a function of voltage unbalance. ( Reliance Electric, Cleveland, OH.)


Above: Fig. 12 Power factor and efficiency changes as a function of motor load. ( Reliance Electric, Cleveland, OH.)

Motor Loading

One of the most common sources of motor watts loss is the result of a motor not being properly matched to its load. In general, for standard NEMA frame motors, motor efficiency reaches its maximum at a point below its full-load rating, as indicated in Fgr. 12. This efficiency peaking below full load is a result of the interaction of the fixed and variable motor losses resulting in meeting the design limits of the NEMA standard motor performance values, specifically locked rotor torque and current limits.

Power factor is load variable and increases as the motor is loaded, as Fgr. 12 shows. At increased loads, normally in the region beyond full load, this process reverses as the motor's resistance to reactive ratio begins to decrease and power factor begins to decline.

In some applications where motors run for an extended period of time at no load, energy could be saved by shutting down the motor and restarting it at the next load period.

Maintenance

Proper care of the motor will prolong its life. A basic motor maintenance program requires periodic inspection and, when encountered, the correction of unsatisfactory conditions. Among the items to be checked during inspection are lubrication, ventilation, and presence of dirt or other contaminants; alignment of motor and load; possible changing load conditions; belts, sheaves, and couplings; and tightness of hold-down bolts.

Total Energy Costs

There are three basic components of industrial power cost: cost of real power used, power factor penalties, and demand charges. To understand these three charges and how they are determined, a review of the power vector diagram ( Fgr. 13)

identifies each component of electrical energy and its corresponding energy charge.

Real Power

The real power-kilowatt (kw) is the energy consumed by the load. Real power-kw is measured by a watt-hour meter and is billed at a given rate (S/kw-hr). It’s the real power component that performs the useful work and is affected by motor efficiency.

Power Factor

Power factor is the ratio of real power-kw to total KVA. Total KVA is the vector sum of the real power and reactive KVAR. Although reactive KVAR performs no actual work, an electric utility must maintain an electrical distribution system (i.e., power transformers, transmission lines, etc.) to accommodate this additional electrical energy. To recoup this cost burden, utilities may pass this cost on to industrial customers in the form of a power factor penalty for power factor below a certain value.

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REAL POWER - KW

~ R:VAAA~I'R IVE DEFINITIONS - POWER FACTOR KW - Kilowatts KVA - Kilovolt- Ampere KVAR - Reactive Kilovolt - Ampere

Above: Fig. 13 Electrical power vector diagram. ( Reliance Electric, Cleveland, OH.)

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Power factors in industrial plants are usually low due to the inductive or reactive nature of induction motors, transformers, lighting, and certain other industrial process equipment. Low power factor is costly and requires an electric utility to transmit more total KVA than would be required with an improved power factor.

Low power factor also reduces the amount of real power that a plant's electrical distribution system can handle, and increased line currents will increase losses in a plant's distribution system.

A method to improve power factor, which is typically expensive, is to use a unity or leading power factor synchronous motor or generator in the power system.

A less expensive method is to connect properly sized capacitors to the motor supply line. In most cases, the use of capacitors with induction motors provides lower first cost and reduced maintenance expense. Fgr. 14 graphically shows how the total KVA vector approaches the size of the real power vector as reactive KVAR is reduced by corrective capacitors. Because of power factor correction, less power need be generated and distributed to deliver the same amount of useful energy to the motor.

Just as the efficiency of an induction motor may be reduced as its load decreases, the same is true for the power factor, only at a faster rate of decline.

A typical 10-horsepower, 1800 RPM, three-phase, design B motor with a full- load power factor of about 80 percent decreases to about 65 percent at half-load.

Therefore, it’s important not to overmotor. Select the fight size motor for the right job. Fgr. 15 shows that the correction of power factor by the addition of capacitors not only improves the overall power factor but also minimizes the fall-off in power factor with reduced load.

Demand Charges

The third energy component affecting cost is demand charge, which is based on the peak or maximum power consumed or demanded by an industrial customer during a specific time interval. Because peak power demands may require an electric utility to increase generating equipment capacity, a penalty is assessed when demand exceeds a certain level. This energy demand is measured by a demand meter, and a multiplier is applied to the real power-kw consumed.

Industrial plants with varying load requirements may be able to affect demand charges by (1) load cycling, which entails staggering the starting and use of all electrical equipment and discontinuing use during peak power intervals, and (2) using either electrical or mechanical "soft start" hardware, which limits power inrush and permits a gradual increase in power demand.

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Power tactoq Fleal Power--KW Total--KVA REAL POWER - KW


Above: Fig. 14 Effect of corrective capacitance on total KVA vector. (Reliance Electric, Cleveland, OH.)

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Above: Fig. 1-15 Effect of capacitors on fall-off in power factor with reduced load. (Reliance Electric, Cleveland, OH.)

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ADJUSTABLE FREQUENCY MOTOR CONSIDERATIONS

Speed control by way of adjusting power frequency is becoming more and more important for economical throughput or pressure capacity variation of modem process machinery. Several key parameters that must be considered when applying induction motors to adjustable frequency controllers include the load torque requirements, current requirements of the motor and the controller current rating, the effect of the controller wave-shape on the motor temperature rise, and the required speed range for the application.

In order to properly size a controller for a given application, it’s necessary to define the starting torque requirements, the peak torque requirements, and the full-load torque requirements. These basic application factors require reexamination because the speed-torque characteristics of an induction motor/controller combi- nation are different from the speed-torque characteristics of an induction motor operated on sine-wave power.

The motor current requirements should be defined for various load points at various speeds in order to ensure that the controller can provide the current required to drive the load. The current requirements are related to the torque requirements, but there are also additional considerations due to the harmonics of adjustable frequency control power that must be taken into account.

Temperature rise and speed range must be considered when applying induction motors to adjustable frequency controllers because this nonsinusoidal power results in additional motor losses, which increase temperature rise and reduce motor insulation life.

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Above: Fig. 16 Speed-torque characteristics of induction motors started at full voltage. (Reliance Electric, Cleveland, OH.)

100 HP, XE Motor STARTING PULL-UP TORQUE

TORQUE ACCELERATING TORQUE

BREAKDOWN TORQUE FULL-LOAD TORQUE

.LOAD TORQUE REQUIREMENT

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Above: Fig. 17 Motor current of induction motor started at full voltage. (Reliance Electric, Cleveland, OH.)

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Above: Fig. 18 Motor current and torque as full operating speed is approached. (Reliance Electric, Cleveland, OH.)

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Before discussing the speed-torque characteristics of a motor/controller combi- nation, it’s useful to review the speed-torque characteristics of an induction motor started at full voltage and operated on utility power ( Fgr. 16). Here we see the speed-torque curve for a 100-horsepower, 1800 RPM, high-efficiency motor. When this motor is started across the line, the motor develops approximately 150 percent of full-load torque for starting and then accelerates along the speed-torque curve through the pull-up torque point, through the breakdown torque point, and, finally, operates at the full-load torque point, which is determined by the intersection of the load line and the motor speed-torque curve.

In this case, we have shown an application such as a conveyor where the load-torque requirement is constant from 0 RPM to approximately 1800 RPM. The difference between the motor speed-torque curve and the load line is the accelerating torque and is indicated by the cross-hatched area.

If the load-torque requirement ever exceeded the maximum torque capability of the induction motor, the motor would not have enough torque to accelerate the load and would stall. For instance, if the load line required more torque than the motor could produce at the pull-up torque point, i.e., 170 percent load torque versus 140 percent pull-up torque, the motor would not increase in speed past the pull- up torque speed and would not able to accelerate the load. This would cause the motor to overheat. It is, therefore, important to ensure that the motor has adequate accelerating torque to reach full speed.

Normally, the motor accelerates the load and operates at the point of inter- section of the load line and the motor speed-torque curve. The motor then always operates between the breakdown torque point and the synchronous speed point that corresponds to the 1800 RPM location on the horizontal axis. If additional load torque is required, the motor slows down and develops more torque by moving up toward the breakdown torque point. Conversely, if less torque is required, the motor would speed up slightly toward the 1800 RPM point. Again, if the breakdown torque requirements were exceeded, the motor would stall.

Fgr. 17 depicts the same motor speed-torque curve, but now the motor current has been shown for full voltage starting.

Typically, when a NEMA design B induction motor is started across the line, an inrush current of 600 percent to 700 percent occurs corresponding to the starting torque point. As the load is accelerated to the full-load torque point, the current decreases to 100 percent full-load current at 100 percent full-load torque. High currents, however, are drawn during the acceleration time.

The amount of time that the motor takes to accelerate the load will depend on the average available accelerating torque, which is the difference between the motor speed-torque curve and the load speed-torque curve, and the load inertia.

Fgr. 18 illustrates a blown-up view of the region between the breakdown torque point and the synchronous speed point, which is where the motor would operate. This is of particular interest because the current for various torque requirements can easily be seen. This would directly affect the size of the controller required to produce a given torque because controllers are current-rated.

At 100 percent full-load torque, 100 percent full-load nameplate current is required. At 150 percent torque, 150 percent full-load nameplate current is required.

Beyond the 150 percent full-load torque point, however, the torque-per-amp ratio is no longer proportional. For this case, 251 percent breakdown torque would require 330 percent current.

Adjustable frequency controllers are typically rated for a maximum of 100 percent continuous or 150 percent for one minute of the controller full-load current. This would generally provide a maximum of 100 percent or 150 percent of motor full-load torque. This would not, however, provide the same amount of torque as the motor could potentially develop if it were operated from utility power, which could normally provide as much current as the motor required.

It would generally be uneconomical to oversize a controller to obtain the same amount of current (torque), since the controller size would actually triple for this example in order to provide 251 percent torque.

Two basic concepts that can explain adjustable speed operation of induction motors can be summarized as follows:

Speed oc Frequency Poles Volts Torque cx Magnetic Flux oc Hertz

The speed of an induction motor is directly proportional to the applied frequency divided by the number of poles. The number of poles is a function of how the motor is wound. For example, for 60 Hertz power, a two-pole motor would operate at 3600 RPM, a four-pole motor at 1800 RPM, and a six-pole motor at 1200 RPM. The torque developed by the motor is directly proportional to the magnetic flux or magnetic field strength, which is proportional to the applied voltage divided by the applied frequency or Hertz. Thus, in order to change speed, all that must be done is to change the frequency applied to the motor. If the voltage is varied along with the frequency, the available torque would remain constant. It’s necessary to vary the voltage with the frequency in order to avoid saturation of the motor, which would result in excessive currents at lower frequencies, and to avoid under-excitation of the motor, which would result in excessive currents, both of which would cause excessive motor heating.

In order to vary the speed of an induction motor, an adjustable frequency controller would have an output characteristic as shown in Fgr. 19. The voltage is varied directly with the frequency. For instance, a 460-volt controller would normally be adjusted to provide 460 volts output at 60 Hertz and 230 volts at 30 Hertz.

A controller would typically start an induction motor by starting at low voltage and low frequency and increasing the voltage and frequency to the desired operating point. This would contrast with the conventional way of starting induction motors of applying full voltage, 460 volts at 60 Hertz, immediately to the motor. By starting the motor with low voltage and low frequency, the inrush current associated with across-the-line starting is completely eliminated. This results in a soft start for the motor. In addition, the motor operates between the breakdown torque point and synchronous speed point as soon as it’s started, as compared with starting across the line, in which case the motor accelerates to a point between the synchronous speed and breakdown torque point.


Above: Fig. 19 Controller output voltage versus frequency relationship for adjustable speed reduction motors. (Reliance Electric, Cleveland, OH.)

Summary

The maximum torque for an induction motor is limited by the adjustable frequency controller current rating. In order to determine the maximum torque that would be available from an induction motor, it would be necessary to define the motor torque at the controller's maximum current rating.

- The starting torque equals the maximum torque for a motor/controller combination.

- The starting torque current is substantially less for an adjustable frequency controller/motor combination than the locked rotor current for an induction motor started across the line. This results in a soft start for the controller/motor combination.

- The motor load inertia capability for a controller/motor is much higher, since the controller can limit the motor current to 100 percent or less. This would result, however, in longer acceleration times than starting the motor across the line.

Harmonics cause additional motor temperature rise over the temperature rise that occurs for sine-wave power operation. As a rule of thumb, for every 10 degr. rise in temperature, the motor insulation life is cut in half. This explains why it’s important to consider the additional temperature rises associated with adjustable frequency control power and to follow the suggested rating curves provided by capable motor manufacturers.

- NEMA design C and D motors are not recommended for use on adjustable frequency control power because these motors have high watts loss due to higher rotor watts loss over design B motors and resulting high temperature rises when operated on adjustable frequency control power.

- Key application points must be defined in order to properly apply an induction motor to a solid-state adjustable frequency controller torque, speed range, motor description, and environment. In order to ensure that adequate torque is available to drive the load and adequate current is available to produce the required torque, the starting torque, the peak running torque, and the continuous torque requirements must be defined. The continuous torque is usually defined, but the peak and starting torques are more difficult to define. For the case of retrofit applications, the speed-torque curve of the existing motor might be used as a reference to define the starting and peak-load torque. Sizing the controller for these points, however, would frequently result in a larger controller than necessary.

- The speed range affects the motor thermal rating. The controllers will typically provide a 10 to 1 speed range below 60 Hertz.

- The motor description will permit selection of a controller size for the motor horse-power, voltage, and current rating. The motor insulation class and design type will permit the motor to be rated properly to ensure that its thermal limitations are not exceeded.

- It’s necessary to consider the environment to choose the proper motor enclosure.

Explosion-proof motors usually have a UL label certifying that they are suitable for the defined classified area. The UL label, however, is suitable only for 60 Hertz sine-wave power. When a explosion-proof motor is operated on adjustable frequency control power, the 60 Hertz sine-wave UL label is voided. In addition, induction motors are normally rated for 40 degr. (104 degr. ambient temperature. Use in a higher ambient temperature may require additional cooling or over-framing.

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