Process Plant Machinery: Electric Motors and Controls (part 1)

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INTRODUCTION

Electric motors are by far the most prevalent "machines" in use in process plants around the world. The engineer, technician, and operator will benefit from an overview-type knowledge of electric motors; accordingly, we have elected first to introduce the reader to this machine category. Basic motor types, their major component parts, selection criteria, and other topics will be reviewed. To assist the reader, we have included a "motor glossary" at the end of this Section.

Squirrel-cage induction motors are the most widely used type in the size range up to 200 horsepower. An induction motor is an alternating current device in which the primary winding on one member (usually the stator) is connected to the power source and the secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries the induced current. There is no physical electrical connection to the secondary winding; its current is induced. Induction motors are simple, rugged, and reliable because they have no rotating windings, slip tings, or commutators. They have good efficiency and high starting torque, but a lagging power factor; the induction motor operates below the synchronous speed that is set by the power cycles and the number of poles in the stator. For example, a two-pole, 60-Hertz (cycle-per-second) motor has a synchronous speed of 3600 RPM, and a four-pole motor operates at 1800 RPM. Normal operating speeds for induction motors are 3550 RPM and 1750 RPM. This deviation from synchronous speed is called slip, and it varies with load. Full load (maximum) slip varies from 1 percent in large motors to as high as 5 percent in small units. Most motors have an average slip of 3 percent.

Synchronous motors are used in cases where a fixed speed is required or to correct a lagging power factor in the power distribution system. A synchronous motor operates at constant speed up to full load, with a rotor speed equal to the speed of the rotating magnetic field of the stator, i.e., there is no slip. There are two major types of magnets on the synchronous motor: reluctance and permanent.

Because of a relatively low inrush current, the synchronous motor has low starting torque and will trip out on overload when started under a load.

Electric motor configurations can be divided into two major categories: horizontal and vertical. The output shafts of horizontal motors are constructed parallel (horizontal) to the ground or mounting base, while the output shaft of a vertical motor will be perpendicular (or at a right angle) to the conventional mounting base. There is a growing interest in the use of vertical motors as pump drivers in the petroleum, petrochemical, and chemical industries, because the entire assembly requires less space than a horizontal pump.

Both horizontal and vertical motors are available in a variety of designs and enclosures. They are classified as standard duty, large heavy-duty, special industry designs, and special application designs. Each of these classifications is available in different enclosures to meet a multitude of environmental requirements, t

The two basic types of motor enclosures are open and totally enclosed. The open motor circulates external air inside the enclosure for cooling, whereas a totally enclosed motor prevents outside air from entering the enclosure. Both types of motors are available in either fractional or integral horsepowers.

Open motors are further classified as drip-proof, splash-proof, guarded, and weather protected, the distinction between them being the degree of protection provided against falling or airborne water gaining access to live and rotating parts.

Drip-proof motors, as defined by the National Electrical Manufacturers Association (NEMA), are never used on outside installations. Integral-horsepower open drip- proof motors now marketed (sometimes called "protected") usually meet NEMA requirements for weather-protected type I enclosures, except they won’t prevent the passage of a 3/4" diameter rod. These enclosures are used on a wide variety of process machinery in outside locations. NEMA-defined weather-protected type II enclosures require oversized housings for special air passages to remove airborne particles. This type of enclosure is not available in the smaller motor sizes.

Totally enclosed motors used on such process equipment as cooling towers are classified as non-ventilated (TENV), fan-cooled (TEFC), air-over (TEAO), and explosion-proof. Whether a motor is TENV or TEFC is dependent on the need for an internally mounted fan to keep the operating temperature of the motor within the rating of its insulation. Air-over motors are TEFC motors without the fan and must have an outside cooling source. Totally enclosed motors are recommended and used for locations where fumes, dust, sand, snow, and high humidity conditions are prevalent, and they can provide a high quality installation either in or out of the air stream provided the typical problems of mounting, sealing, and servicing are properly addressed. In all cases, totally enclosed motors should be equipped with drain holes, and explosion-proof motors should be equipped with an approved drain fitting.

Explosion-proof motors are manufactured and sold for operation in hazardous atmospheres, as defined by the National Electrical Code. The motor enclosure must withstand an explosion of the specified gas, vapor, or dust within it, and prevent the internal explosion from igniting any gas, vapor, or dust surrounding it. The motors are Underwriters Laboratory (UL) approved and marked to show the class, group, and operating temperature (based on a 40 degrees ambient temperature) for which they are approved. In applying these motors, no external surface of the operating motor can have a temperature greater than 80 percent of the ignition temperature of the gas, vapor, or dust involved.

The National Electrical Code defines hazardous locations by class, group, and division. Class I locations contain flammable gases or vapors; class II locations contain combustible dust; and class III locations contain ignitable fibers or flyings.

Group defines the specific gas, vapor, dust, fiber, or flying. Division defines whether the explosive atmosphere exists continuously (division 1) or only in case of an accident (division 2). Motors for division 1 applications must be explosion-proof. Standard open or totally enclosed motors that don’t have brushes, switching mechanisms, or other arc-producing devices can be used in class I, division 2 applications. In some cases, they can also be used in class II, division 2 and class III, division 2 applications.

MOTOR DESIGN

Three-Phase Motors

Three-phase squirrel-cage induction motors have become the standard on the overwhelming majority of process plant machines. They don’t have the switches, brushes, or capacitors of other designs and therefore require somewhat less maintenance. Where three-phase power is not available, single phase capacitor-start motors may be used, usually not exceeding 7.5 horsepower. Concerned process machinery manufacturers will supply motors that are a few steps beyond "off-the- shelf" quality. These motors are usually purchased from specifications developed after comprehensive, rigorous testing under simulated operating conditions.

Two-Speed Motors

Two-speed motors are of a variable torque design, in which the torque varies directly with the speed, with 1800/900 RPM being the most common speeds. Single-winding design motors enjoy greatest utilization, since they are smaller in size and less expensive than those of a two-winding design.

Occasionally, an installation deserves consideration of the use of two-speed motors. Cooling tower fans are a case in point. Whether operated seasonally or year-round, there will be periods when a reduced load and/or a reduced ambient temperature will permit satisfactory cold water temperatures with the fans operating at half-speed. The benefits accrued from this mode of operation will usually offset the additional cost of two-speed motors in a relatively short time.

Additionally, since nighttime operation is normally accompanied by a reduced ambient temperature, some operators utilize two-speed motors to preclude a potential noise complaint.

High-Efficiency Motors

Several motor manufacturers provide high-efficiency designs that are suitable for use on numerous types of process plant machinery. These motors are in the same frame sizes as standard motors, but they utilize more efficient materials. While the efficiency will vary with the manufacturer and the size of the motor, the efficiency will always be higher than that manufacturer's standard motor. Naturally, there is a price premium for high-efficiency motors, which must be evaluated against their potential for energy savings.

Motor Insulation

One of the most important factors contributing to long service life in an electric motor is the quality of the insulation. It must withstand thermal aging, contaminated air, moisture, fumes, expansion and contraction stresses, mechanical vibration and shock, as well as electrical stress.

Insulation is categorized by classes, which establish the limit for the maximum operating temperature of the motor. Classes A, B, F, and H are used in the United States, with class A carrying the lowest temperature rating and class H the highest.

Standard integral horsepower motors have class B insulation and are designed for a maximum altitude of 3300 feet and a maximum ambient temperature of 40 degrees Class F insulation is used for higher altitudes, as well as higher ambient temperatures, and it’s gaining increased use as a means of improving the service factor of a motor of given frame size.

Motor Service Factor

The service factor of a motor is an indication of its maximum allowable continuous power output, as compared with its nameplate rating. A 1.0 service factor motor should not be operated beyond its rated horsepower at design ambient conditions, whereas a 1.15 service factor motor will accept a load 15 percent in excess of its nameplate rating. Usually, motor manufacturers will apply the same electrical design to both motors but will use class B insulation on 1.0 service factor motors and class F insulation on 1.15 service factor motors. Class B insulation is rated at a total temperature of 130~ and class F is rated at 155 ~ More important, a 1.15 service factor motor operates at a temperature from 15 degr. to 25 degr.lower (compared with the temperature rating of its insulation) than does a 1.0 service factor motor operating at the same load. This, of course, results in longer insulation life and, therefore, longer service life for the motor. For this reason, many equipment manufacturers will recommend the use of 1.15 service factor motors for loads at or near nominal horsepower ratings.

Since increased air density increases the load on air movers, an added attraction for using 1.15 service factor motors is that there is less chance of properly sized over-loads tripping out this equipment category during periods of reduced heat load and low ambient temperatures.

Motor Heaters

Although the insulation used in quality electric motors is considered to be non-hygroscopic, it does slowly absorb water and, to the degree that it does, its insulation value is reduced. Also, condensed moisture on insulation surfaces can result in current leakage between pin holes in the insulation varnish. Because of this, it’s advisable on installations exposed to high humidity to keep the inside of the motor dry.

This can be done by keeping the temperature inside the motor 5 degr. to 10 degr. higher than the temperature outside the motor. Motors in continuous service will be heated by the losses in the motor, but idle motors require the addition of heat to maintain this desired temperature difference.

One recommended method of adding heat is by the use of electric space heaters, sized and installed by the motor manufacturer. Another method is single-phase heating, which is simply the application of reduced voltage (approximately 5 to 7.5 percent of normal) to two leads of the motor winding. Both of these methods require controls to energize the heating system when the motor is idle. If low voltage dynamic braking is used to prevent an inoperative motor from rotating, it will add sufficient heat to the motor windings to prevent condensation. A typical application would be in cooling tower fans.

Motor Torques

High starting torque motors are neither required nor recommended for most process machines. Normal torque motors perform satisfactorily for pumps, fans, blowers, etc., and cause far less stress on the driven components. Normal torque motors should be specified for the bulk of single-speed applications, and variable torque in the case of two-speed.

There are five points along a motor speed-torque curve that are important to the operation of many machines: (1) locked-rotor torque, (2) pull-up torque (minimum torque during acceleration), (3) breakdown torque (maximum torque during acceleration), (4)full-load torque, and (5)maximum plugging torque (torque applied in reversing an operating motor). Compared with full-load torque, the average percentage values of the other torques are as follows: locked-rotor torque = 200%; pull-up torque = 100%; breakdown torque = 300%; and plugging torque = 250%.

MOTOR CONTROLS

Control devices and wiring, the responsibility for which usually falls to the purchaser, can also be subjected to demanding service situations. Controls serve to start and stop the motor and to protect it from overload or power supply failure, thereby helping assure continuous reliable equipment operation. They are not routinely supplied as a part of a machinery procurement contract, but because of their importance to the system, the need for adequate consideration in the selection and wiring of these components cannot be overstressed.

The various protective devices, controls, and enclosures required by most electrical codes are described in the following paragraphs. In all cases, motors and control boxes must be grounded.

1. Fusible Safety Switch or Circuit Breaker: This device provides the means to disconnect the controller and motor from the power circuit. It also serves to protect the motor-branch-circuit conductors, the motor control apparatus, and the motors against overcurrent due to short circuits or grounds. It must open all ungrounded conductors and be visible (not more than 50 feet distant) from the controller or be designed to lock in the open position. The design must indicate whether the switch is open or closed, and there must be one fuse or circuit breaker in each ungrounded conductor. A disconnect switch must be horsepower rated or must carry 115 percent of full-load current and be capable of interrupting stalled-rotor current. A circuit breaker must also carry 115 percent of full-load current and be capable of interrupting stalled-rotor current.

2. Nonfused Disconnect Switch: This switch is generally only required if the fusible safety switch or circuit breaker either cannot be locked in the open position or cannot be located in sight of the motor.

3. Manual and Magnetic Starters: These controls start and stop the motor. They also protect the motor, motor control apparatus, and the branch-circuit conductors against excessive heating caused by low or unbalanced voltage, overload, stalled rotor, and too frequent cycling. Starter requirements are determined by the basic horsepower and voltage of the motor. Overloads in a starter are sized to trip at not more than 125 percent of full-load current for motors having a 1.15 or higher service factor, or 115 percent of full-load current in the case of 1.0 service factor motors. Single phase starters must have an overload in one ungrounded line. A three-phase starter must have overloads in all lines. If a magnetic controller is used, it may be actuated by devices sensing certain process fluid parameters. Temperature sensors sensing cooling water temperature would be a typical example.

4. Control Enclosures: NEMA has established standard types of enclosures for control equipment. The types most commonly used in conjunction with process plant machinery are as follows:

a. NEMA Type 1 -General Purpose: Intended primarily to prevent accidental contact with control apparatus. It’s suitable for general purpose applications indoors, under normal atmospheric conditions. Although it serves as a protection against dust, it’s not dust-proof.

b. NEMA Type 3- Dust-tight, Rain-tight, and Sleet-resistant: Intended for outdoor use and for protection against wind-blown dust and water. This sheet metal enclosure is usually adequate for use outdoors on a cooling tower. It has a watertight conduit entrance, mounting means external to the box, and provision for locking. Although it’s sleet-resistant, it’s not sleet-proof.

c. NEMA Type 3R: This is similar to type 3, except it also meets UL requirements for being rainproof. When properly installed, rain cannot enter at a level higher than the lowest live part.

d. NEMA Type 4- Watertight and Dust-tight: Enclosure is designed to exclude water. It must pass a hose test for water and a 24-hour salt spray test for corrosion. This enclosure may be used outdoors on a cooling tower. It’s usually a gasketed enclosure of cast iron or stainless steel.

e. NEMA Type 4X: Similar to type 4, except it must pass a 200-hour salt spray test for corrosion. It’s usually a gasketed enclosure of fiber-reinforced polyester.

f. NEMA Type 6- Submersible, Watertight, Dust-tight and Sleet-resistant: Intended for use where occasional submersion may be encountered. Must protect equipment against a static head of water of 6 feet for 30 minutes.

g. NEMA Type 12- Dust-tight and Drip-tight: Enclosure intended for indoor use. It provides protection against fibers, flyings, lint, dust, dirt, and light splashing.

h. NEMA Type 7 - Hazardous Locations- Class I Air-Break: This enclosure is intended for use indoors in locations defined by the National Electrical Code for class I, division 1, groups A, B, C, or D hazardous locations.

i. NEMA Type 9 - Hazardous Locations - Class II Air-Break: Intended for use indoors in areas defined as class II, division 1, groups E, F, or G hazardous locations.

WIRING SYSTEM DESIGN

The design of the wiring system for the numerous process machines, fans, compressors, pumps, and controls is the responsibility of the owner's engineer. Although the average installation presents no particular problem, there are some systems that require special consideration if satisfactory operation is to result. Conductors to motors must be sized both for 125 percent of the motor full-load current and for voltage drop. If the voltage drop is excessive at full load, the resultant increased current can cause over-load protection to trip. (Although motors should be operated at nameplate voltage, they can be operated at plus or minus 10 percent of nameplate voltage.) In a normal system with standard components, even the larger machines will often attain operating speed in less than 15 seconds. During this starting cycle, although the motor current is approximately 600 percent of full-load current, the time delays in the overload protective devices prevent them from breaking the circuit.

Because of the high starting current, the voltage at the motor terminals is reduced by line losses. Within certain limits, the output torque of a motor varies as the square of the voltage. Thus, under starting conditions, the current increases, the voltage decreases, and the torque decreases, with the result that the starting time is increased. Long conductors that increase voltage drop, low initial voltage, and high-inertia fans can all contribute to increased starting time, which may cause the protective devices to actuate. In extreme cases, the starting voltage may be insufficient to allow acceleration of the fan to full speed regardless of time.

The wiring system design must consider pertinent data on the available voltage (its actual value, as well as its stability), length of lines from the power supply to the motor, and the motor horsepower requirements. If this study indicates any question as to the startup time of the motor, the inertia of the load as well as that of the motor should be determined. This is the commonly known "flywheel" effect (WK 2 factor). Once the WK 2 of the load (referred to the motor shaft) is obtained, the acceleration time can be determined using the motor speed-torque and speed-current curves, compared with the speed-torque curves for the fan. If the calculated time and current is greater than allowed by the standard overload protection of the motor, the condition may be corrected by increasing voltage, by increasing conductor size, or by providing special overload relays. Given no solution to the base problem, special motors or low-inertia fans may be necessary.

CYCLING OF MOTORS

The high inrush current that occurs at motor startup causes heat to build up in the windings and insulation. For this reason, the number of start-stop or speed-change cycles should be limited in order to allow time for excessive heat to be dissipated.

As a general rule, 30 seconds of acceleration time per hour should not be exceeded.

A fan-motor system that requires 15 seconds to achieve full speed, therefore, would be limited to two full starts per hour. Smaller or lighter fans, of lesser inertia, permit greater frequency of cycling.


Above: Fig. 1 Energy-efficient electric motor typical of general purpose AC motors. (Reliance Electric, Cleveland, OH.)


Above: Fig. 2 Totally enclosed fan-cooled motor. The externally mounted fan pushes cooling air over the fin-equipped frame. (Reliance Electric, Cleveland, OH.)

STANDARD INDUCTION MOTORS

Standard induction motors are typically used for industrial applications such as machine tools, material handling equipment, processing lines, pumps, fans, blowers, and countless others.

The protected motor shown in Fgr. 1 (sometimes called the drip-proof or open) is typical of general purpose AC motors. This enclosure is suited for most industrial environments when temperatures are 40~ maximum with ambient air relatively clean and dry.

The totally enclosed fan-cooled (TEFC) motor (Fgr. 2) is the enclosed motor most often selected for indoor or outdoor industrial environments containing dust, dirt, water, etc., in modest amounts that are best kept out of the interior of motors. The external fan is used to cool the motor, since there is not a free exchange of air.

Explosion-proof motors have a dual purpose: to withstand an explosion from within and to prevent the explosion of gases in the atmosphere. Both requirements place special emphasis on motor design. Explosion-proof motors should meet the rigid requirements of UL for most National Electrical Code class, group, and temperature code restrictions.

Motor sizes and dimensions have been standardized by the NEMA. NEMA frames 48, 56, and 140 T encompass single-phase capacitor start and polyphase motors. These motors are designed for continuous duty operations in a 40 degr. maximum ambient environment- obviously not typical plants. Similarly, NEMA frames 180 T through 449 T include standard AC polyphase induction motors and also certain single-phase motors designed for a maximum environmental temperature of 40 degr. They are, however, available in explosion-proof executions for use in class I, group D, and class II, groups F and G hazardous locations.

LARGE HEAVY-DUTY ALTERNATING CURRENT MOTORS

Large heavy-duty AC motors cover the range from 250 to 5000 horsepower and are available in a variety of frames or enclosures. Totally enclosed motors, typically of cast iron frame design, range to roughly 500 horsepower. Weather-protected designs and tube or water-cooled enclosures can be supplied with these motors. They range through 1500 horsepower with cast iron, and 5000 horsepower with fabricated steel construction ( Fgr. 3).


Above: Fig. 3 Weather-protected motor with cooling facilities enclosure and fabricated frame design. (Reliance Electric, Cleveland, OH.)


Above: Fig. 4 Brake motor combining a motor and integrally mounted disc brake. ( Reliance Electric, Cleveland, OH.)


Above: Fig. 5 Cross section of NEMA frame, vertical AC motor. 1 - top-mounted thrust bearing; 2- steel laminations; 3 - rotor fins; 4 - bearing cap; 5 - lower thrust bearing; 6- grease reservoir; 7- grease relief ports; 8 - drip cover; 9 - conduit box; 10 - lifting plates; 11 - cover, with grounding provision.(Reliance Electric, Cleveland, OH.)


Above: Fig. 6 Submersible motor for use in hazardous areas. (Reliance Electric, Cleveland, OH.)

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.

cont to part 2 >>

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