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Industrial motors: more torque, more integration

06 May 2008

Makers of industrial motors are quick to adapt their products to the needs of manufacturing. In this article we look at direct-drive, permanent-magnet (PM) 'torque motors,' so called 'integrated' motors where the drive is combined with the motor, servo motors large and small, and an extra rugged duty AC motor designed for the harshest environments in process automation.

Figure 1
Figure 1

The direct-drive, permanent-magnet (PM) torque motor is characterised by a large diameter-to-length ratio (giving it the 'flat' look) and large number of magnetic poles to optimise torque production. These relatively low speed motors, usually operating under 1,000 rpm, combine several design features to deliver their intended function. Direct drive—meaning no power transmission elements between motor and driven load—brings advantages of high dynamic motion with essentially no backlash and excellent static / dynamic load stiffness that allows precise motion control. Large numbers of magnet pole pairs in the rotor aid high torque generation.

Baumüller has been a major player on the direct drive technology market since 1991. The increasing use of electric drives in modern production facilities has been driven forward by customer requirements for increased productivity and flexibility, improved options for process control, and increased machine availability and longer service lives.

The elimination of the gearbox has given rise to a drive which is free from backlash and has better smooth running characteristics, thanks to improved control quality. High-torque motors are also characterised by a high overload capability and reduced noise emissions.

With IP54 protection, these motors can be used successfully in harsh industrial environments. Typically, direct drives are used as alternatives to geared and hydraulic motors in the plastics and printing industries.

INTEGRATED MOTORS

In smaller frame sizes, the concept of the integrated motor—where the motor, encoder, and drive are combined in the same package—has become widely accepted. Both inverter motors and servomotors are now finding this packaging concept useful.

A popular inverter motor to use this idea is SEW-Eurodrive's MOVIMOT®, which is a combination of a gearmotor and a frequency inverter for power ranges up to 4 kW. Surprisingly, the combination takes only a small amount of additional installation space than standard gearmotors—a testimony to the good engineering design work that also makes it plug-removable in case it needs servicing. The MOVIMOT can be supplied in all standard versions and mounting positions, with or without a brake. A SafetyDrive package is available as an option.

The revised MOVIMOT D (photo) was shown for the first time at SPS/IPC/Drives in 2007. It is based on the company’s new DR motor line, which replace the DT and DV lines. The DR series integrates all current requirements regarding efficiency (Standard, High, and Premium). SEW-Eurodrive produces over 1.2 million such asynchronous motors every year.

AUTOMATIC FORMAT ADJUSTMENT

Integrated servomotor packages, in small sizes, are also working their way onto the factory floor, but they are coming in by a different route. Many packaging machines, sintering presses, beverage filling systems, tyre production or printing presses are fully automatic and servo-driven in their operation, yet they require manual intervention when changes in production are required, such as when a different size carton will be processed. These manual changes are made by turning small handwheels to adjust the size of rollers, limit stops, or carton erectors. Set up times are correspondingly long for these manual procedures, particularly for more complex machines with many axes requiring adjustment. By replacing the handwheels with small servodrives, batch changes themselves can be automated.

Figure 2
Figure 2

Sick-Stegmann is one motor manufacturer who is pursuing this 'automatic format adjustment' niche with its Hiperdrive (Highly Integrated Performance Drive) portfolio. This application is a good example of how the integrated package becomes important, especially when retrofitting machines. This integrated positioning drive consists of a motor, gears, absolute encoder, and control electronics. The drives themselves don’t need to be large and powerful; typically only 25W to 45W are needed to turn the wheels, but they need to be precise and have good consistency. The compact, integrated package makes it easier for engineers to position them precisely where they are needed.

Larger models have 'self-retention,' that is, they hold their position when the machine is turned off. Some versions have 'jog' buttons that allow manual positioning. The absolute detection of the angle of rotation provides a clear absolute positional value for every axle position, so no reference run is necessary on machine start-up. The drives are networked with a Profibus-DP clip-on module.

Hiperdrives allow the efficient operation of machines because they prevent complicated manual positioning and adjustment work. It represents the optimum alternative to hand wheels, particularly in those machines with several adjustment axes. But it is no replacement for the classical servo drives on process axes on machinery; rather it is a reasonably priced alternative for automating the adjustment of auxiliary axes.

SERVOS BIG AND SMALL

Let's start with the small, and then go to the big. Baldor's new BSM R-series of ultra-compact brushless AC servomotors are small enough to hold in the palm your hand, but offer machine builders a high performance and low-maintenance alternative to stepper motors and DC servos. Packaged in small 40 and 60 mm square bodies, they are capable of delivering peak torques of 3.8 Nm.

The new motors are targeted at automation equipment builders seeking to improve the productivity of machinery in applications such as packaging and labelling, materials handling, semiconductor manufacturing, and winding and printing. Special construction features mean that their action is virtually ‘cog free,’ that is, they provide very smooth motion at slow speeds.

The smallest one-stack motor measures 40 mm square by 59 mm long—about the size of a chicken egg. Power and feedback connections to the motors are made by conveniently long leads.

Users can optionally specify motors with a resolver or a 2,000 parts-per-revolution encoder fitted for positional feedback. A further useful option is the addition of a 24 VDC brake.

HIGH POWER 'SERVO'

They're not really servomotors, but Rockwell says they perform like servos and can be used as servos in high power material handling or web converting applications. They're really induction motors that have motor windings optimised to mimic the speed and torque curves of conventional permanent magnet servo motors. This allows them to deliver a peak torque twice the continuous torque when running at base speeds.

Figure 3
Figure 3

The motors are equipped with single- or multi-turn, high resolution absolute feedback encoders that eliminate the need for time-consuming homing routines, improves velocity regulation, and expands the system's inertia matching capacity. This capability is new to induction motor technology, making HPK-Series motors the first of their kind to offer positioning, acceleration and productivity capabilities comparable to traditional permanent magnet servo motors.

'All together,' says product manager David Hansen, 'this capability is new to induction motor technology, making HPK-(High Power Kinetix) Series motors the first of their kind to offer positioning, acceleration and productivity capabilities that are comparable to traditional permanent magnet servo motors. We've never offered a servo motor solution with power ranges over 22 kW, but the HPK motor extends the normal power range of a traditional servo motor solution up to 112 kW. This allows machine builders to increase acceleration and improve the positioning, precision and overall system throughput, in some cases as much as 25 per cent.'

FOR THE HARSHEST ENVIRONMENTS

Written for severe duty motor applications, the American standard IEEE 841-2001 applies to single-speed polyphase induction motors up to 370 kW. In response to this standard, General Electric has created the X$D 'Extra Severe Duty' Ultra® 841 motor The motor's low temperature rise and low vibration is designed for harsh environments involving highly corrosive materials or where processes place high structural demands on motors, such as in the oil, gas, energy and chemical industries. It has the ability to meet maximum exposed internal and external surface temperatures of up to 200 degrees Celsius under usual service conditions at a 1.0 service factor. Each X$D motor comes with its own test report, demonstrating performance compliance with the IEEE specification.

The 841 motor features a cast–iron construction, including its frame, conduit box, end–shields and fan cover. Internal motor surfaces are coated with epoxyester paint and rotor surfaces are coated with glyptol to resist interior rust. The oversized roller bearings have been designed for 130,000 hours minimum for direct drive loads, and 26,280 for belt–driven loads.

One of a motor's most critical components is its bearing system. This motor's GE-proprietary Six Star Bearing System™, and its low overall vibration of 1 mm per sec. can extend the life of the motor and the connected equipment. The motor’s sealed re–greasable system allows for easy maintenance while protecting and lubricating bearings.

CAPTIONS

Figure 1: SEW-Eurodrive’s integrated MOVIMOT D

Figure 2:Baumüller’s high torque motors

Figure 3: Baldor’s BSM-R series of ultra-compact servomotors


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