Extrusion drives: time for a change?

Extrusion Drives: Time

There have been significant advances in both a-c motors and electronics that are making a-c drive packages more attractive for extrusion applications. One basic advantage is the simplicity of a-c motors. The common three-phase a-c induction motor normally operates at a fixed speed controlled by the base operating frequency, which is 60 Hz in the U.S. Electronic drives, called inverters, convert the 60-Hz power into variable-frequency a-c, and the motor speed then varies directly with the frequency of power supplied to the motor by the inverter.

One of the more evident advantages of a-c motor technology is potential reduction in equipment downtime and maintenance costs, according to Robert Green, president of C-TAC, Inc., an a-c motor supplier in Cleveland. A-C motors have no brushes or commutators that can be a source of failure in conventional bush-type d-c units. In plastics processing environments, d-c motor failure often stems from one or more of four conditions, says Green:

*The required large volumes of cooling air may contaminate open-frame d-c motors with dirt, moisture, or corrosive chemicals.

*Filters to clean cooling air may reduce air flow if dirt is allowed to accumulate, causing overheating and winding failure.

*Ductwork to deliver clean air to motors may, if poorly designed, reduce air flow and cause motors to run hot and shorten insulation life.

*Finally, insufficient routine maintenance allows brushes to wear out, resulting in commutator failure.

Green claims that a-c motors have certain advantages in cooling arrangements over their brush-type d-c counterparts. In d-c motors, most heat is generated by the armature (the rotating part) inside the motor; motors are therefore usually cooled by blowing air through the motor. This presents an opportunity for contaminants such as PVC dust to be blown through the motor housing, causing brush and winding deterioration. In self-ventilated (a-c or d-c) motors, the fan attached to the shaft provides the right amount of cooling air for top speed. But if the motor is part of an adjustable-speed drive and run at half speed, air flow will be reduced, causing the motor to overheat. Totally enclosing a brush-type d-c motor to protect it from dust is a possible - but expensive - alternative, Green argues, because of the inefficiency of removing heat from the armature.

But, he points out, a-c induction motors with totally enclosed frames are readily available and affordable. A-C motors can be provided in totally enclosed, non-ventilated (TENV) or totally enclosed, blower-cooled (TEBC) versions.

In TENV a-c motors, cooling is by radiation of heat through the frame. This solves the slow-speed overheating problem, because cooling is not dependent on motor speed; but they generally require a larger frame size, which limits their cost-effectiveness to smaller horsepower ranges.

TEBC motors, on the other hand, reportedly are well suited for extrusion applications. In place of a shaft-mounted blower that increases or decreases speed and air flow with motor speed changes, as in self-ventilated motors, TEBC a-c motors have a separately driven blower. This blower operates at constant speed to provide an even, continuous flow of cooling air. Other features may include an internal thermostat and provisions for installation of a tachometer.

Green concedes that the recent extension of brushless d-c motors into higher horsepower ranges also eliminates brush and commutator deterioration in conventional d-c motors, but adds that brushless d-c units still have open frames that may draw in dust. "Winding are subject to the same deterioration problems as brush-type d-c motors, and brushless d-c motors are far less widely available, more expensive, and more costly to repair than a-c induction motors."

In fact, minimum downtime and low repair costs are said to be major advantages of a-c inverter motors. Repair of windings tend to be less labor intensive and less costly, claims Green, and their replacement can take just few days vs. a few weeks on d-c brush-type motors.

A-C DRIVE PERFORMANCE

In addition, he says, a-c drives can equal or exceed performance of comparable brush-type d-c drives. The least sophisticated d-c drives regulate speed in the 2% to 5% range (armature voltage regulation). A tachometer of moderate expense provides brush-type d-c motor speed regulation to 1% of top speed. This can be improved to 0.1% with an expensive tachometer and 0.05% with an even more expensive digital tachometer.

A-C drives offer tight control without the expense of a tachometer. The simplest a-c drives that include a slip-compensation circuit provide 1% speed regulation over a 10:1 speed ratio, typically from 6- to 60-Hz operation. This range can be stepped up to 20:1 using 120-Hz operation, but the minimum speed is still limited to 6 Hz. An additional advantage is that a-c drives are not subject to "cogging" at very slow speeds as are brush-type d-c drives.

For higher performance, an open-loop, flux-vector a-c inverter - an upgrade of the standard pulse-width-modulated (PWM) a-c inverter - provides an operating range of 100:1, with speed regulation of 0.5% over a 10:1 speed range without a tachometer. Speed regulation of 0.05% can be obtained from a PWM inverter with closed-loop speed regulation and a digital tachometer on the motor. Green claims that flux-vector inverters have potential in high-performance extruder and downstream applications. A closed-loop, flux-vector inverter plus TEBC motor, and digital tach can provide complete dynamic response control, speed ranges of 100:1, and 0.05% speed regulation.

One manufacturer that makes a-c drives standard on extruders for up to 75 hp is Crown Machine Div., West Chicago, Ill. According to Crown's extruder product manager Michael Chumura, a-c drives have a higher maximum standard safe speed - 200% of base motor speed, vs. 125% for d-c brush-type drives - which adds flexibility in running a variety of products. Akron Extruders, Canal Fulton, Ohio, reports that it has also supplied a-c drives on primary extruders in the 50- to 75-hp range with good results.

A-C inverter drives may be more economical to run than brush-type d-c drives because of their reportedly favorable power factor, particularly at lower speeds. A-C inverters are said to maintain high power factors of about 0.97 at all speeds. In contrast, says Green, the power factor of conventional d-c drives varies with speed of the motor, from 0.87 at top speed to 0.4 at half speed.

A-C LIMITATIONS & SUCCESSES

Yet a-c drives also have inherent disadvantages in constant-torque applications such as extrusion, say detractors. One reported disadvantage here is low starting torque, which produces slippage as the speed approaches breakover. D-C drives have high starting torques at 0 rpm. Some drive suppliers counter that this drawback can be overcome with electronics. Dan Casacci, of Emerson Industrial Controls, Grand Island, N.Y., claims that many a-c drives today are equipped with an "auto-boost" feature that automatically steps up voltage to compensate for increased draw on current. "With new power devices, a-c drives are getting close to the performance of d-c, with lower cost motors."

Others caution against potential thermal problems with a-c drives, particularly if a-c motors have not been tested for variable-frequency duty. Brian McMahon, product manager at Reliance Electric Co., Cleveland, says, "You need to be careful in forming a drive package to be sure that the motor was designed for variable-speed duty."

One application that has proven to be successful for a-c drives is on air rings and internal bubble cooling (IBC) systems of some blown film lines. The advantage here, according to Matthew Bangert, v.p. of Reifenhauser Film Systems, Peabody, Mass., is that the a-c inverter drives control air flow by varying the speed of the impeller rather than by creating a pressure drop through an obstruction. As a result, these drives reportedly can reduce temperature rise by 10-15 degrees F. Filmaster, Fairfield, N.J., also offers a-c inverter drives standard for this application on its lines.

Another area in which a-c drives may have potential is in extrusion control. Several extruder manufacturers predict more use of digital drives because of their higher accuracy and ability to be tied into line-wide control systems. Of course, digital technology applies to both a-c and d-c drives, but some extruder manufacturers indicate that a-c drives may have an edge. Reifenhauser's Bangert, for example, has seen some shortcomings of brush-type d-c technology in tying all drives in a line together, and adds that digital a-c drives may have potential for this. Louis Faillace, marketing director of HES/Davis-Standard Div. of Crompton & Knowles Corp., Pawcatuck, Conn., makes the point that some a-c drive suppliers, such as Milwaukee-based Allen-Bradley Co., offer drives that integrate well with programmable controllers, which makes systems easier to engineer.

A-C CONTROLLER COSTS

Traditionally, a-c electronics have been more complex than those of d-c drives, making a-c drive systems more expensive. One cost contributor here is that a-c drives require a double power conversion. Some suppliers are saying that two recent trends are closing this price gap. First, microprocessors are eliminating many components in a-c controllers and converting them to software. Second, transistors, the power components in a-c drives, now have higher current-carrying capability than was previously possible.

C-TAC's Green cites an in-house cost survey, conducted last year, that compared average distributor pricing of a-c and d-c systems, including controllers, blowers, and motors, from 11 different drive manufacturers. He claims that a-c systems are now generally less expensive below 40 hp, and that prices are about equal from 40-75 hp. He believes that the cost advantage will begin to shift the market to a-c drives.

BRUSHLESS D-C ALTERNATIVE

The other alternative drive for extruders, brushless d-c, is claimed by Powertec, Charlotte, N.C. - the only major supplier of these drives in higher horsepower ranges - to combine the best characteristics of a-c drives and brush-type d-c drives. The company, which recently extended its product offering from 1/2 hp to 300 hp, says brushless d-c units, like a-c motors, require little maintenance. In addition, brushless d-c drives are said to offer 0% speed error through digital phase-lock circuitry, and a speed range of 100:1.

In a brushless d-c motor, windings and fields are reversed from the usual d-c arrangement: the armature winding is positioned toward the outside and the fields of the motor are put on the shaft. This configuration allows the fields to be turned instead of the windings. One benefit here reportedly is more efficient cooling, because the heat-producing elements are toward the outside of the motor. Efficiency is said to be high enough to allow smaller frame sizes.

Brushless d-c motors eliminate the magnetic field of their brush-type d-c counterpart by placing permanent magnets on the shaft. Putting magnets on the shaft and rotating the fields instead of the armature gives a couple of advantages, according to Powertec v.p. of sales Ed Lee: it reduces the inertia of the rotor and also eliminates brushes. To force current to the right winding (which is done automatically on a brush d-c motor), the brushless d-c motor uses an encoder to read the position of the shaft.

An advantage over a-c motor is claimed in the waveform used by the brushless d-c motor. The brushless d-c stator runs on a trapezoid waveform, while the a-c stator is designed to run on a sinusoidal wave current. In a trapezoidal waveform, when power is switched on, current builds to a level and runs steady at that level; when current is switched off, it decays back down to zero. The practical advantage of this, according to Lee, is that the commutation can be accomplished with simple electronics.

BRUSHLESS D-C

PERFORMANCE

One more advantage claimed for brushless d-c drives is high starting torque, "A brushless d-c drive at a given horsepower will outperform a brush-type d-c of the same horsepower in terms of starting torque," says Lee. "Typically, you get 170% of the rated torque at breakaway."

Also, brushless d-c motors are not subject to thermal problems at very low speeds, which Lee singles out as a drawback of a-c drives. Because brushless d-c drives don't produce slip, they reportedly can run slowly at high torques without problems of overheating. Also, the permanent magnets used in brushless motors eliminate field excitation, which produces heat in a brush-type d-c motor.

High accuracy is said to be inherent feature of brushless d-c drives. Brushless d-c motors incorporate encoders for commutation; thus they are by their nature easily digitally speed regulated, according to Lee. As a result, shaft accuracy of brushless d-c motors has 0% error from large load changes. More important for an extruder, long-term drift of brushless d-c drives is specified at 0.5%, and is typically 0.2%, he adds. This kind of accuracy can be matched by a brush-type d-c drive with a good tachometer, he points out, but is a standard feature on brushless d-c units. Furthermore, shock loads or significant load changes are important in many downstream operations, which may benefit from the favorable transient response of the brushless d-c drive.

Lee claims that the high power factor of brushless d-c drives - 0.95 to 0.98 -is better than both brush-type d-c and most a-c inverter drives. Unlike brush d-c drives, there is no power-factor penalty for operating below base speed. Input current to the brushless motor control is said to be directly proportional to the output power of the motor shaft, which is a product of speed and torque.

Lee disputes competitors' claims that a-c drives have high power factor.

Power factor, he explains, is the ratio of real watts to apparent power (kva), and good power factor occurs when current and voltage peak at the same time and have the same wave shape. In a-c drives with no input inductor - and many do not have the same wave shape. In a-c drives with no input inductor - and many do not have one, he says - the current and voltage do peak at the same time, but do not have the same wave shape. The current waveform is not sinusoidal, and the resulting real power factor is poor despite the current waveform being in phase with the voltage waveform.

Lee says that improvements in magnet technology have made brushless d-c drives more widely available. Improvements reportedly have increased the reliability of permanent magnets and brought their cost down, which in turn has lowered the manufacturing cost of brushless motors. He claims that, unit for unit, brushless d-c motors are now less expensive to make than their brush-type counterpart at 5 hp and up.

Although he says there is no practical limit to brushless d-c motor size, Lee concedes that limitations of power-switching devices presently restrict affordable controllers to 300 hp. Nonetheless, he predicts that breakthroughs in this area will enlarge brushless d-c drive offerings to 500 hp early next year.

A number of extruder manufacturers have reported good success with brushless d-c drives in a variety of applications. Killion Extruders, Inc., Cedar Grove, N.J., offers brushless d-c drives from 5 hp and up as an option at no extra charge to customers. HPM Corp., Mt. Gilead, Ohio, introduced a 40-hp brushless d-c drives as standard on a new 2 1/2-in. extruder and said it would make the brushless drives standard on all units below 125 hp.

Killion's product manager of downstream equipment and turnkey systems, Robert Bessemer, reports that he has seen an increase in requests from customers for greater speed accuracy over wider speed ranges. As a typical example, he points to smaller extruders that tend to have an optimized screw for specialized applications, such as medical tubing. Although one may question the value of a precisely accurate drive on extruders that of themselves are not positive pumping devices, he suggests that the added accuracy of the drives, combined with proper screw design and good temperature control, can indeed make a significant difference in performance.

Edward Smith, sheet-line product manager for Egan Machinery Div. of John Brown, Inc., Somerville, N.J., feels that brushless d-c drives have potential in downstream equipment, such as vertical cooling stacks of cast film lines. "We are finding that customers want to produce in a very wide speed range, from 1-2 mill product up to 40-50 mils. The advantage of brushless d-c drives is that they can precisely control throughout that speed range" without the use of additional gear boxes and transmissions. Another potential application is roll stacks for sheet with high surface finish, he adds, because the brushless d-c drives eliminates gears that may transmit ripple effects from "chatter" onto the product surface.

Although Smith says brushless d-c drives are a bit more expensive than their brush-type d-c counterparts, he adds that the difference is offset somewhat by the reduction in mechanical equipment such as gearboxes.

NRM-Steelastic, Columbiana, Ohio, initially experienced signalling problems with tach response when it first installed brushless d-c drives on a sheet line in its lab, according to process engineer Timothy Womer. He is quick to add that those problems have been corrected and the drives now are performing well. The company has now made a 40-hp brushless d-c drive standard on its 2 1/2-in. profile line.

Womer feels that brushless d-c drives work well, but adds that the motors should be sized for adequate torque requirements. While wide speed range on brushless d-c drives offers the possibility of replacing the gearbox, the motor must be made larger to make up for the torque loss in gearing. In custom sheet applications a machine might still need a right-angle gear reducer engineered into its power base if running in a very low speed range, he adds.

TIME FOR A CHANGE?

Despite claims from makers of a-c and brushless d-c drives, not all extruder manufacturers are ready to write off brush-type d-c systems. One disadvantage of both brushless d-c and a-c inverter drives is that it is difficult to measure power that the drive is consuming, according to Faillace of HES/Davis-Standard. Armature current of brush-type d-c drives is easier to monitor. This may be a minor point in normal production, but is significant if the extruder is used for testing new resins or screw designs.

John Rupert, sales manager of Maplan Services Div. of American Maplan Corp., McPherson, Kans., says that a-c drives may have limited potential extrusion because of possible thermal problems in lower speed ranges. He points to a recent trend among both a-c and d-c motor suppliers to reduce frame size - i.e., upgrade motors to higher horsepowers in smaller frames. "A-C still has the problem that if you drop speed, the energy still must go somewhere - and it goes into extra heat. D-C doesn't have that problem, because if you put volts to it, that is what it reacts to. So inherently d-c is more efficient."

Frank Nissel, president of Welex Inc., Blue Bell, Pa., expects brush-type d-c drives to continue to dominate extrusion because of their wide availability and cost-competitiveness. He disputes the claim that prices are coming down enough to justify the advantages of a-c controllers. And while he concedes that brushless d-c drives perform well, he says that their one-of-a-kind design is, in fact, a liability. "It's a wonderful product," he says, "but it is so unique that it is non-interchangeable. If the motor burns out, you have to go back to one supplier."

The single source of large brushless d-c drives is apparently a concern of other extruder manufacturers as well. Remarks Robert Hitchens of Alpine American Corp., Natick, Mass., "Engineers are looking for drives that are readily serviceable and parts that are readily available." In his view, U.S.-made brush-type d-c drives fit that bill. He predicts that digital technology will become more common on brush-type d-c drive packages to provide better control of blown film lines.

Finally, John Malinowski, d-c product manager of Baldor, Fort Smith, Ark., which supplies both brush-type d-c and a-c inverter drives, points to other alternatives to compensate for the former's shortcomings. For PVC extrusion applications, special brushes are available, at little extra cost, that have a slight abrasive action to scrub the commutator and prevent corrosion. Furthermore, he add, cooling air vented from outdoors is an effective way to prevent contamination of the motor by in-plant dust.

Brush-type d-c drives have an advantage in extrusion applications, he says, because they provide constant torque throughout the speed range required by the extruder. A-C inverters, he says, have a more limited torque range at lower speeds, and may require oversized motors and drives. "There's a crossover point at which inverters become less cost-effective than d-c drives."

He also minimizes some drawbacks in brush-type d-c motor repair. He concedes that such motors with laminated frames may be labor-intensive to rewind, but he adds that some d-c motor suppliers have bolt-in fields that can be replaced easily. Also, he says, a higher degree of technology may be required to repair a demagnetized rotor or feedback sensors on brushless d-c motors.

Wide availability is another advantage. Motors and controllers are interchangeable on brush-type d-c systems, which can make replacements of motors and drives easier.

PHOTO : Brushless d-c drives, now available up to 300 hp, are said to provide 0% speed error with digital phase locking, and have a speed range of 100:1. (Photo: Powertec)

PHOTO : A-C inverter motors contain fewer moving parts, reportedly eliminating brush wear and maintenance problems of standard d-c motors. Reduced a-c controller prices make these drives more competitive.

PHOTO : Powertec's brushless d-c motor includes ceramic magnets permanently bonded to the shaft, simple stator windings, digital position and speed sensor, and fins for heat dissipation. It also has a provision for blower cooling.

PHOTO : Above: Although standard d-c drives continue to dominate extrusion, a-c drives that integrate well into programmable controllers could make inroads. (Photo Davis-Standard). Left: Brushless d-c drives provide independent control to each polishing roll on this multi-configurable sheet line from HPM to prevent overheating.