SPEED ADJUSTMENT IN DRIVES FOR THE CHEMICAL INDUSTRY Both capital and operating costs vary considerably according to application ot many years ago, drives with speed adjustments Even today, the majority of electric motor drives are squirrel-cage motor, but the adjustable type is gaining importance. For example, total sales of this equipment to the chemical industry by the Westinghouse Electric Gorp. have almost tripled during the past five years. A major influence behind this growth is expansion of the plastics industry. This article describes briefly the various speed adjustment methods used today, and gives basic characteristics of these drives rather than promote particular drives for specific uses. The scope is limited to general performance characteristics rather than to detailed modifications and regulating systems that can be applied.
N were rare in the chemical industry.
Multirpad InductionM o t o n
Induction motors are available with two or more operating speeds. Reconnecting the windings changes the number of poles. These may be either single- or multiple-windmg motors. Single-winding motors are normally built only with a 2 to 1 speed change-e.g., 3600 to 1800 or 1800 to 900 r.p.m. These are available in three types: constant horsepower; constant torquee.g., 10 and 5 hp.; or variable torque as for fan loadse.g., 10 and 2.5 hp. Two-winding motors are used where speed ratios other than 2 to 1 needed. Typical speed ratios are 1800 to 1200 and 1200 to 900 r.p.m. These also are available for constant horsepower, constant torque, or variable torque. Four speeds can be obtained by winding the motor with two, two-speed, single-winding sets of coils. A typical fandrive motor of this type is rated 10,4.4, 2.5, 1.1 hp. and 1800,1200,900,600 r.p.m. Wound-Rotor Motors
Wound-rotor induction motors sometimes are used in the chemical industry for drives such as blowers, compressors, and pumps. Usually the secondary resistance consists of several fixed steps. Liquid rheostats can be 24
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
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used for continuously variable control but these are expensive. No-load speed is nearly synchronous, regardless of secondary resistance (Figure 1). Woundrotor motors, as adjustable speed drives, are relatively simple and inexpensive. However, the speed adjustment is rough unless a large number of secondary resistance points are used, and the system is not adaptable for precise automatic regulation. Because of the secondary resistance losses, efficiency is proportional to speed. There are also a number of reactor control schemes used with both wound-rotor and high-slip, squirrel-cage motors. Both primary and secondary reactors are used with or without secondary resistors. Primary reactors connected to reverse phase rotation can provide direction (reversing) and torque or speed control. These drives are used most commonly for a.c. crane and hoist drives. They are also used for other material handling jobs such as transfer car drives and occasionally for pump drives. Primary-reactor, reversing control is particularly suitable for applications such as hoists and cranes where very frequent reversing is necessary. These are used mostly for intermittent loads because high losses make
Figure 7.
Wound-rolor ind!Jclion motor rpccd-torque curw
continuous operation difficult thermally. For pump drives requiring a relatively wide speed range, primary reactor control is interesting because the thermal capability of a self-ventilated motor here approximately matches the characteristic pump load curve, especially for a limited speed range. D i n d Curnnt Motors
Fixed-Voltage Power Supply. Where a constantvoltage source of direct current is available, field-controlled, d.c. motors may be used for adjustable-speed drives. The speed range is usually limited to 4 to 1. These drives are suited to constant horsepower loads because the current capability (and kilowatt capability with constant voltage) is almost constant regardless of speed. Regeneration for braking is possible down to base speed. If rectifiers supply power, regeneration is possible only if there are other motors to absorb the braking power. Efficiency is good over the useful speed range of these drives and automatic regulation is readily applied. Armature resistance is sometimes used to provide an intermittent creep or jog speed for motors with a constant voltage supply. Adjustable-Voltage Power Supply. The old standard, adjustable-voltage d.c., is still one of the most versatile drives. This system today uses either an a s . to d.c. motor-generator set with an adjustable-voltage generator or an adjustable-voltage, reactor-controlled rectifier as the armature power source. Inherently these drives have constant torque capability over their speed range unless field-weakening is also employed. Motor-generator set drives can provide regenerative braking down to almost zero speed. Rectifiers cannot absorb regenerative power so dynamic braking is used. Since these drives are controlled by either armature or field voltage or both they are readily adaptable to use with many types of regulators. Performance modifications such as low creep speed, controlled acceleration, regeneration, and reversing are readily obtained. One a.c. and two d.c. machines are required for these motor-generator set drives. The equivalent rectifier drive also has three components, a method of varying the a x . voltage, a rectifier, and the d.c. motor. Efficiency of these drives is fair over their entire speed range. Power factor of the motor-generator set drives depends on the a.c. motor type and load. Rectifier drives adjust voltage by a saturable reactor ahead of the silicon diode rectifier; thaefae.the power-factor is proportional to voltage and hence speed (assuming fixed d.c. motor field). Adjustable voltage drives are useful where several motors must operate as a coordinated drive system. Wound-Rotor Dimt-Curnnt Motor Combination
The major drawback to the extended use of woundrotor motors for adjustable-speed drives has been the rotorcircuit losses. With efficiency proportional to speed, wound-rotor motors are not economical for widespeed range operation. D.c. motors are being used now
to convert these rotor-circuit losses into useful mechanical work. Silicon diode rectifiers convert the wound-rotor secondary current to d.c. for motor use. Both motors are mechanically connected with the d.c. motor powered by the wound-rotor secondary through the rectifier. This Rectiflow drive has been referred to as a modified Kramer drive which used a rotary converter in place of the rectifier. Inherently this is a constant horsepower drive. That is, the inherent capability of the drive over its speed range gives constant horsepower. However, it can be and is used for constant torque and even variable torque-is., fanlike loads. Neglecting losses, the division of power between the two motors is a function of slip: d.c. hp. = total hp. X slip; and a.c. hp. = total hp. X (1 - slip). These equations indicate that the d.c. motor size and rating are a function of slip or drive speed range. The useful speed range is about 3 to 1. The d.c. motor field controls speed as shown in Figure 2. At full field the drive operates at its lowest speed. As the field is weakened the drive accelerates, reducing the wound-rotor motor secondary voltage and increasing the counter e.m.f. until these two once again balance. Top speed is less than synchronous speed because at this point there can be no torque from either machine. These drives have high inherent starting torques; thus, above 15 hp. most use secondary starting resistors between the a.c. and d.c. motors. Fewer steps are needed than with conventional wound-rotor control because of the d.c. motor counter e.m.f. Some special applications require accurately controlled acceleration or a low speed--e.g. threading-below base speed. For these, a small adjustable-voltage transformer is inserted between the a x . motor and rectifier. This control is not used for normal, under-load operation because these applications are usually at light load or are intermittent so the transformer can he small. The transformer adjusts starting torque from zero to the maximum obtainable; at zero-voltage no torque is produced. These Rectiflow drives have been used in a variety of applications. Typical examples are for kilns, adjustablefrequency generator drives, pumps, fans, plastic extruders, and kneaders. Extruder drives are the most common single application. Figun 2. Voltogc-spccdcurucsfor wound-rotor d.c. mlor cornbinotion mith 2 to 1 speed range
WND.-IOTOI S K VOLTA6E
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SPEED VOL 55
NO. 3
MARCH 1963
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Mechanical variable speed drives are economical up to about 25 hp. Mechanical Ad/uslableSped Drives
Mechanical adjustable-speed drives are among the most common and economical small adjustable-speed drives. In this drive an a.c. squirrel-cage motor drives the load through a belt and two adjustable-pitch-diameter sheaves or disks. Each pulley consists of two coneshaped disks. The pitch diameter is changed by changing the axial separation between disks. The belt rides on the sheave (disks) where its width matches the disk separation. One sheave is adjusted by a handwheel or a small motor where remote control is needed. The other sheave is spring-loaded so it adjusts itself to the proper pitch diameter and maintains belt tension. Mechanical, adjustable-speed drives of this type are economical from fractional to approximately 25 hp. Available speed ranges are 2:l to 8:l. Output speeds are varied both above and below motor speed. Maximum output speed is limited to about 4000 r.p.m. Gear units can be mounted integrally to provide almost any desired output speed. The efficiency of these drives is fair (approximately 80%) because, unlike wound-rotor motors and hydraulic or magnetic couplings, no slip losses occur. The belts and mechanical structure are rather complex; therefore, maintenance is higher than for straight electrical drives. They are, however, ideal for many drives such as manually operated machine tools and conveyors which do not require accurate regulation or continual adjustment. The method of speed adjustment is suitable for manual operation and is readily adapted to remote electric operation. A simple motor drive mounted on the “hand wheel” mechanism accomplishes this function. An electric operator also provides smooth acceleration and deceleration between minimum and maximum speeds. This speed-change method is not adaptable to many automatic regulation jobs. Response is slow and continual regulating action would wear out the splines, gears, and other speed-change parts. Specific applications for these drives include machine tools and use8 such as screw and belt conveyors, proportioning drives for blending and mixing, textile machines, small extruders, pumps, and compressors. EkchPrnognetic Coupling Drives
Electromagnetic coupling drives consist of a squirrelcage motor driving the load through a magnetic “slip” coupling. This coupling has a drum driven by the motor, a rotor, and a stationary d.c. field which controls the magnetic coupling between the drum and rotor. Speed can be adjusted from zero speed up to almost full motor speed. Some slip between the drum and rotor is needed to produce torque resulting from eddy currents. These drives have many desirable characteristics: full stepless speed wntml from zero speedup; easy adaptability to manual, remote, or automatically regulated operation; wntrol of acceleration; and torque 26
INDUSTRIAL AND ENGINEERING CHEMISTRY
limiting. The major limitation is poor efficiency at reduced speeds. Motor torque is equal to load torque so the losses are directly proportional to slip; efficiency is proportional to load speed. This principle is also used for braking by making one half stationary. When used as a coupling to the load, braking effort is obtained only above motor synchronous speed with an overhauling load. These drives are inherently constant torque for any given excitation, and consequently have very poor speed-regulating characteristics. Therefore, most of these drives are sold with a speed regulator as part of the usual drive package. Air-cooled, magnetic-coupling drives may be used up to 250 hp. at 1750 r.p.m. if the torque at low speed is not excessive. At other speeds corresponding ratings are available. Larger units, up to 2500 hp. at 1750 r.p.m., and all drives requiring high torque at low speed must be liquid-cooled. Liquid-cooled drives are also preferred for dusty, dirty, corrosive, or hazardous locations. Liquid-cooled drives up to 125 hp., and 1750 r.p.m., and 100 hp. air-cooled drives are available as integral motor-coupling units. Separate motor and coupling are also available for all sizes. These drives are compact, reliable, and have few moving parts (motor rotor, drum, and coupling rotor). Electrical parts are stationary or static, including the control if desired. Typical applications for these drives include fans, pumps, conveyors, winders, and rubber and plastics machinery such as extruders. Inherent functional performance characteristics include the ability to start high inertia loads, low starting current inrush, and isolation to prevent torsional vibration or damage from impact or cyclic loads. AdjusIuble-Fmquency Drives
Adjustable-frequency drives use a.c. motors, usually either squirrel-cage or synchronous reluctance motors, powered by an adjustable frequency a.c. source. The a.c. source is commonly a conventional generator driven by one of the previously mentioned, adjustable-speed drives. Static, adjustable-frequency power supplies using controllable, silicon-rectifier elements are beginning to appear. These are quite expensive because they include both a rectifier for a.c. to d.c. conversion and an inverter for d.c. to variable-frequency a x . In a few years this type of power supply may be popular, but today it cannot compete extensively. Adjustable frequency is most often used where a number of drives must operate in exact synchronism or very close to it. A single, adjustable-frequency power source supplies the individual drive motors. As the speed of (Conlinucd on p u p 28)
Carl R. Olson is Processing Province Engineer, Industrial Systems, Westinghouse Electric Gorp., Pithburgh, Pa. AUTHOR
DEVICES FOR VARYING SPEED OF ELECTRICAL MOTOR DRIVES R#gegsnsrotlur
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Multispeed induction Primary reactor Hi-Slip
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3:1
Infinite; usually intermittent Infinite
Magnetic slip coupling Mechanical adjustable speed
8:l
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Fixed steps Stepless
2-3% Synchron. speed is theoretical no-load sneed
Stepless
Synchron. speed is theoretical no-load meed
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Stepless
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Wound-rotor motor Secondary resistors Primarv reactor and secondary resistors Wound-rotor and integral d.e. motor
D.c. motor Fieldsontrol, voltaee
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1:5:lb 16:l
fixed
Adjustable voltage M-G set Rectifier with reactor control Adjustable frequency
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Usually in steps Stenless Stepless
Yes; between steps
No
I
2-6%
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No chanp-e rate
Synchron. speed is theoretlcal no-load soeed
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58%
No
Yes; down to d.c. motor bane speed Yes; down to base specd
510%
Stepless Infinite
Brakinp
BWP
Stepless
6:l
Stepless
%lo% Theoretical no.load speed is mot. max. volt. speed
Yes; No
0 for synchron. motors; -3% for squirrel cage
Yes
to zero speed
a Znnhnal m-load to fulldoad regdutbn ( f y p i d ) . All molar moy be braked dynnmidly. A.c. prouidar braking mtion aboua gmhromus spmd. A.c. motorr moy also be brokad by applying d.c. to t k strltor. b U w l range, but h i g h isporrible.
FIXED D.C. VOLTAGE
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one changes, they all change. The motors may be one rating or they may be various sizes. If speed adjustment between sections of a drive is necessary, some or all of the drive motors may be supplied with individual mechanical adjustable-speed belt drives. These drives range from simple, relatively inexpensive drives with an unregulated, mechanical, adjustable-speed, generator drive to highly complex, accurately regulated systems with adjustablevoltage, d.c. motor drives for the ax. generator. The typical useful speed range for adjustable frequency is about 6 to 1. With a constant volts-per-cyde source (approximate), adjustable-frequency drives have inherently constant torque capability. Working at constant volts per cycle gives practically constant flux density and current. Except for the reduced cooling effect at low speeds, the torque capability is essentially constant over the speed range. Using semiconductor inverten as the power source opens up the posibiibility of using frequencies well above 60 cycles--e.g., 400 cycles a d above. This meam that these drives can be considered in the future for compact high-speed pump and compressor drives. Other applications will also undoubtedly develop where these high potential speeds will be desirable. Today the most common use for these drives is for the production of manmade or synthetic fibers.
Speed Range. For many of the drives shown in table on page 27, wider speed ranges are possible but those shown are the maximum that usually can be justified for that particular drive. Inherent No-Load to Poll-Load Speed Regulation. There are two broad classifications of droop or speed regulation for the drives shown on page 27. Some, such as the adjustable-voltage, d.c. motor drive have very little droop from no load to full load. Others, such as the
wound-rotor motor and the magnetic coupling drive return to top speed at no load. This characteristic is important iflow speed must be maintained at light loads. Electrical Brakhg. All motors may be electrically braked. Some drives provide regenerative braking inherent with the system. Multispeed, squirrel-cage motors provide this down to the lowest operating speed but not below. Dynamic braking with d.c. and synchronous motors is applied by energizing the field and absorbing the power in resistors. Induction motors may also be braked by applying d.c. to the stator. Efficiency. For adjustable-speed drives efficiency is difficult to estimate. A great deal depends on the type of load-i.e., constant or variable torque, or constant horsepowersize of the drive, and speed range. Figure 3 shows three typical efficiency curves. Field-controlled d.c. motor efficiency shown is higher than for motor-generator set, adjustable-voltage drives because power source losses are not included; often a rectifier is used for constant voltage with higher efficiency than an adjustablevoltage motor-generator set. Curve C'shows how efficiency is proportional to speed for slip drives such as wound-rotor moton and magnetic couplings. Peak efficiency varies with the kind and size of drive. These high, low-speed losses increase power costs and the cost of cooling equipment. Torque Capability. Three characteristic speedtorque curves are shown for constant horsepower, constant torque, and variable torque. Most of the drives discussed have inherent constant torque characteristics. The wound-rotor d.c. motor combination and fieldcontrol d.c. motors have constant horsepower characteristics. Frequently, ventilation must be forced to provide sufficient cooling to obtain the drives' inherent torque capability. Torque capability is important (Figure 4) because the load torque requirements at various speeds affect the over-all size of any given drive.
Figure 3. Efiency us. s p e d curucs. A , 6ypical for constonf nolioge d.c. motor and woundmlor, d.c. motor combination; B, motor-gmrafm set adjutable ooltogr &ne. C, typical for wu~&rotar and o h r slip drives
Figure 4. Inherent torque capability. Conrtanl hp. opplics to &ld control d.c. motor and m u d - r o t m , d.c. motor combination. Constant torque applies to adju6ablc wltagc d.c. wound-roior-mechanicn[, magnrtic, and adjustable frequency. Vm'able torque opplics to some mullixpmd squinel-cagr and self-uentilalcd wound rotor
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