Machinedesign 2124 Fig01 Motors Compared Torque Steppers 0 0

Comparing motor options for motion control applications

Jan. 16, 2013
Basic motion designs compete with those produced in emerging markets, but automation reigns where value trumps cost. Key to extracting superior production from such sophisticated machinery is the proper motor and control.

Proper motor, motion control, and automation technology benefits from standard units with easy interconnectivity, scalable design for right-priced solutions, and software with smart wizards and graphic programming tools for setup, programming, and troubleshooting. However, it’s up to the design engineer to initially determine whether stepper, vector, servo, or linear motors are most suitable for a given application.

Stepping motors versus small servomotors
Stepping motors — simple and inexpensive devices used to position loads — typically offer up to 2 hp, though some designs deliver up to 5 hp. Basic designs include rotor teeth (sometimes embedded with magnets) with windings on the stator assembly. When the windings are energized, the rotor teeth are attracted, resulting in a move or indexing in fixed angular increments, such as 2.8°, 1.8°, or 0.8°, for example.
Generally operated in an open-loop configuration, steppers can miss steps and incorrectly position loads.

When used near maximum torque as when moving heavy loads, or for fast acceleration to improve productivity, they can also stall. To address these issues, either a larger motor is used or feedback plus closed-loop control is added. A typical closed-loop stepper application is a belt-conveyor system on which the load can change suddenly.

Stepping motors ease operation by operating open loop, simplify interfaces by responding to digital inputs, and lower cost by typically omitting feedback or using an inexpensive device. Useful for light loads and short moves, steppers are common in industrial applications such as X-Y and rotary positioning devices in numerically controlled systems, process controls, printing trades, and packaging.
Stepping motors compete with small servomotors: A typical stepper’s continuous and peak torques are equal; torque declines from stall value to as much as 50% at 1,000 rpm; no tuning is necessary with an open-loop stepper. In contrast, a typical servomotor’s peak torques vary by up to three to four times continuous; torque is relatively flat until the voltage limit of the drive is reached; tuning is necessary.
Choosing between steppers and servos first depends on the application torque-speed requirement, preferably delivered at maximum efficiency. Consider that a 100-W stepper’s power peaks at about 800 rpm; a 100-W servo’s power peaks at 3,000 rpm. The stepper is designed to provide maximum power at low rpm, and the servo delivers maximum power at higher speeds.

The second consideration is cost. Steppers are less expensive, as servomotors for closed-loop operation require tachometer, encoder, or resolver feedback, rare earth magnets (in the case of brushless motor designs), higher slot fill, precision balancing, and a closed-loop drive.

Vector motors versus inverter-driven motors
Ac induction motors deliver fractional to mega-horsepower. The basic ac “squirrel cage” motor consists of wire inserted into a steel stator and a rotor. When ac power is applied to the stator, current flows and induces a field onto the rotor thus resulting in rotation. Traditional construction incorporates aluminum, but alternative copper rotors reduce motor losses, operate cooler, and shrink motor size by about 20%. These rotors are heavier, costlier, and tougher to cast than aluminum types.

Ac motors operate at a single speed dictated by 50 or 60 Hz input. Variable speed requires that input ac power frequency be changed via an inverter drive (open loop) or vector drive (in a closed-loop system). Note that inverter and vector motors must be designed to handle the high current and voltage spikes generated by the drive.
Inverters are used in centrifugal fans, conveyors, pumps, mixers, and for moving other variable-torque loads. Vectors are used in conveyors, pumps, blowers, web processing, winders, printing, metal processing, and material handling equipment.

When comparing inverters against vectors, designers should consider the application’s controllability and speed range requirements. Inverters regulate speed to approximately ±3% of base speed, with low-end controllable speed starting at about 10% of base speed. (There’s no speed regulation below about 100 to 200 rpm.) They deliver constant torque to base speed, and constant horsepower to 1.5 times base speed.
In contrast, vectors provide full torque from zero rpm to base speed with speed regulation about ±0.1% of set speed, and constant horsepower up to 2.5 times base speed. Vectors — used with closed-loop drives — are also more expensive because they require an encoder or resolver for feedback.

Servomotors versus vector motors in positioning
Dc motors have long been used to deliver adjustable speed. Demands for improved consistency and throughput have spurred development of permanent-magnet dc servomotors, which deliver up to 5 hp — though their brushes for mechanical commutation wear and limit life.
As a result, designers next developed the brushless motor: A typical brushless servo delivers up to 35 hp with rare-earth permanent magnets on the rotor and electrical windings on the stator. A controller determines the motor’s position and provides appropriate electronic commutation with the help of encoder or resolver feedback signals. When a motor winding is energized, the rotor is attracted, resulting in movement; switching power from winding to winding creates continuous rotation.

Brushless servomotors are operated in a closed-loop configuration. As with all such designs, the associated feedback devices increase cost, wiring, and complexity. Even so, servos are essential for accurate positioning to improve consistency and quality in industrial packaging, printing, metal cutting and forming, material handling, inspection, and food and beverage applications.

The alternative — vector motors — is less expensive. However, a servomotor of equivalent horsepower is 20% shorter, 50% lighter weight, and capable of 50% more torque. That said, a vector motor can provide equal accuracy with the appropriate feedback. In addition, vector motors have significantly more inertia — more on this in a moment.
In short, servomotors and vector motors are each suitable for select positioning applications. Vector motors are better for moving very heavy inertial loads, while servos are more suitable for positioning lighter inertial loads rapidly.

When deciding between a servomotor and a vector motor for a positioning application, the determining factor is torque required over the duty cycle — and the time it takes to get into position. This is where inertia comes into play. Inertia is the tendency of an object at rest to remain at rest, and resist moving.
Larger inertias are harder to move and require more torque to accomplish any movement. For motor sizing, the general rule is to determine the load’s inertia and use a motor with inertia 10 to 15 times lighter — for a load-to-motor inertia ratio of 10:1 or 15:1. (Some manufacturers suggest even higher ratios.) This ratio represents a range over which a standard off-the-shelf drive can adequately compensate. To reduce the amount of inertia that the motor experiences, a belt, gearing, or a ballscrew can be integrated into the design.

Rotary motors (plus rotary-to-linear devices) versus linear motors
Linear motors leverage the same basic magnetic theory as rotary varieties, but in an open and flattened form. The rotating shaft that produces work in the form of torque and rotary movement is replaced with a forcer that produces work in the form of linear force and movement. As with rotary varieties, myriad linear motors exist: Linear steppers, linear ac induction, permanent magnet, and brushless. Linear motors also utilize drives, motion positioners, and feedback devices such as linear encoders.

Linear motor benefits include faster speeds, maximum possible acceleration, and much higher accuracy than their rotary counterparts. Consider that replacing a rotary open-loop stepper with a rotary closed-loop servo can improve accuracy by a factor of 80 times; substituting a linear motor can improve accuracy by a factor of 500 times.
Likewise, a typical servomotor and ballscrew with a pitch of 5 rev/in. can move a load at 20 to 40 in./sec; in contrast, a linear motor can provide speeds to 400 in./sec. The same servomotor may accelerate at up to 2 g, while the linear motor accelerates at 10 g.
Finally, the typical servomotor-ballscrew actuator provides accuracy ranging from 0.001 to 0.0001 in., while the linear motor provides 0.0007 to 0.000008-in. accuracy. Note that these figures don’t account for coupling and ballscrew backlash factors. One of the only disadvantages of a linear motor is its initial cost.
Linear motors are used in short-move pick and place and inspection equipment (to 60 in./sec), longer moves and flying shear applications (to 200 in./sec), and roller coasters, people movers, and vehicle launching systems (2,000 in./sec). They are also used in semiconductor and electronics markets, laser cutting and water etching machines, material handling, component insertion, and bottle labeling and inspection equipment.

When investigating rotary and linear motors, application considerations include speed and accuracy. Comparing the relative price (whether rotary or linear), steppers are the least expensive, followed by induction, permanent magnet, and finally brushless motors.
When comparing the costs of linear and rotary motors, keep in mind that the latter requires motor mounting and possibly a gearbox, ballscrew, or belt drive, plus bearings, a slide, and cabling. The linear system requires a bearing system and cabling.

Linear motors can also be easily configured into multi-axis stages — typically most expensive, as they encompass either a single or multiple-axis mechanical system to position the payload, plus linear motor, bearings, encoder, limit switches, cable carrier, and bellows.

John Mazurkiewicz was a product manager at Baldor Electric/ABB for 19 years. Today he serves on several technical advisory boards, including that for Motion of the Penton Design Engineering & Sourcing Group.

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