Boosting the performance of flat-armature motors.
THE AXIAL AIR-GAP, flat-armature servo technology (FAST) motor was introduced to the electronic motion control industry about 25 years ago. The evolution of electronic motion control blossomed as designers saw that direct-drive paper-tape capstans gave the best performance. Demonstrations also showed that direct-drive technology provided the fastest way to read tape and that the FAST motors were reliable. Existing clutch-brake mechanisms and tape-pinch rollers were displaced by the new, higher performance direct-capstan-drive designs.
From paper-tape capstan drives, direct-drive electronic motion control expanded into the computer peripheral market, controlling disk-drive heads, card readers and feeders, print heads, and paper feeds in printers as well as the capstans in magnetic tape drives. Instant video replay was also made possible by the high-performance, direct-capstan-drive technology offered by FAST motors. Electronic motion control has since expanded into industrial, medical, and instrumentation applications, and the industry's growth has led to many$more motor and drive technologies.
The FAST motor can now deliver more power to a load than before, and it has many diverse applications. As this motor technology continues to be refined, new, high-performance materials are becoming available to extend the operating range.
The FAST motor is a dc motor with a permanent magnet and a moving coil. The moving coil distinguishes this motor from those that have iron in the rotor. The rotor in the FAST motor is constructed of copper or aluminum coils that carry the current, and nonmetallic support and insulation materials.
Moving-coil motors have two significant advantages compared with other types of motors. First, they have a low rotor inertia and therefore use much less energy than iron-rotor motors for rapid start-stop applications. Second, because they have no iron in the rotor, moving-coil motors have no magnetic attraction between the stator and the rotor. When present, this attraction puts the rotor in preferred positions and results in detent or togging torques. Cogging torques produce velocity ripple during constant speed operation in velocity servos and cause positioning errors in position servos.
Because the first motors were made with printed circuit rotors, they were called printed-armature motors. Today, virtually all rotors are made of multiple layers of stamped copper or aluminum sheets that are Tungsten Inert Gas (TIG) welded at the OD and ID to make a continuous winding. The only armatures still made by
chemical etching are prototype quantities. The flat-armature winding requires no separate commutator. The flat surface of the windings serves as a commutator and provides many commutator bars per revolution, usually more than 100. This feature keeps the bar-to-bar voltage low for efficient commutation.
Another advantage of not having iron in the rotor is extremely low winding inductance, usually in the micro-henry region. Low inductance makes the FAST motor commutation virtually arc-free, which increases the life of the brushes and the commutator and electrically quiets EMI commutation. Low inductance also produces a low electrical time constant, allowing electrical power to be quickly converted to mechanical power, an important factor in start-stop incremental motion applications.
The magnetic circuits for moving-coil motors are usually very fundamental. In the axial air-gap motor, alternating north-south permanent magnet poles are equally spaced around a diameter so that the magnetic flux is directed across the air gap where the rotor windings are located. Magnets can be located on both sides of the air gap or on only one side. Soft iron plates direct the flux radially between the poles and create a closed magnetic circuit.
The least powerful--or the most powerful--plastic-bonded magnets, can be used by axial air-gap motors. Because continuous torque is proportional to the permanent-magnet air-gap flux, a permanent-magnet circuit is designed to achieve maximum air-gap flux at minimum cost. Using higher energy and more costly magnets, however, is often a good trade-off in those applications where space, weight, and lost power can be saved.
Because the moving-coil design does not have the magnetic saturation typically found in iron-rotor motors, the torque will be strictly proportional to the current in the windings. This torque linearity is useful in control circuits, particularly when applying pulse currents at the start of a move to create a high-acceleration torque for rapid starts.
The ironless rotor design also reduces the magnetic linkage between the stator permanent-magnet field and the rotor field. High magnetic linkage can demagnetize the permanent magnets with high armature current during either high-current accelerations or decelerations. Motors with iron rotors must often limit the amount of current applied to prevent demagnetization, which is especially a problem in conventional motors operating at reduced temperatures.
One apparent disadvantage of not having iron in the rotor is the short thermal time constant. Over-currents or pulse-currents must be carefully controlled, or the rotor will overheat and become damaged. Over-current and thermal measurement detectors, however, can compensate for the low thermal time constant.
Conversely, a low thermal time constant will cause the rotors to lose heat quickly. Therefore, these motors can be effectively air cooled, allowing for substantial increases in power ratings.
In general, a large diameter generates large torque (power). The slim, fiat packaging profile of FAST motors has significant advantages in many applications, particularly in direct-drive applications in which the motor output shaft may be connected directly to the load.
New materials have been developed and introduced that enable the FAST motor to operate at higher temperatures and in severe environments. Raising the safe operating temperature of motor components and the motor itself significantly increases performance.
The new materials, it is hoped, will be used to create motor designs and processes for new, high-volume manufacturing methods. Motors used for high-volume applications are often designed specifically for the task.
The main performance limits with FAST motors (and most others) are the temperature ratings on insulation. Because of the laminated construction of the rotor in the FAST motor, the insulation material must also serve as a structural material.
HIGH-TEMPERATURE ADHESIVES AND INSULATIONS
Typically, rotors are laminated using a fiberglass matting/cloth impregnated with a B-staged epoxy. The glass cloth provides a mechanical spacing between the winding layers, and the epoxy provides adhesion. When thermally cured, this material provides an insulating space for the copper or aluminum windings while securing them into a fixed and rigid position.
Several critical design criteria must be met for this material. For example, the epoxy must flow sufficiently during thermal curing to wet the surfaces of the windings for proper adhesion. However, it should not flow so much that it fills up the space between the windings. The adhesion must be strong because all of the motor torque is transmitted though the cured epoxy and adhesion interface. The coefficient of expansion must be controlled so that it closely matches the copper and aluminum coefficients; otherwise the armature disc may warp or buckle as it is thermally cycled in the motor.
Engineers arc constantly trying to increase the thermal ratings of these laminating materials. Epoxy and epoxy-like materials now in use in armatures have temperature ratings up to 200 [degrees] C. Previously, flat-armature motors had a 150 [degrees] C rating at most.
High thermal ratings are important in high-thermal-ambient conditions. For example, in automotive engine compartment applications where the thermal ambient may be 80 [degrees] C, a 150 [degrees] C rating allows only a 70 [degrees] C rise of the armature temperature. If the rated temperature is raised to 200 [degrees] C, a 120 [degrees] C temperature rise is acceptable; this enables a 30 percent increase in continuous torque.
Higher-temperature materials and ratings also permit higher-power motors, which are useful in automotive under-the-hood applications. For instance, higher-power electric fan motors are needed for cooling the air conditioner condenser. The condenser has grown four times in volume since freon has been replaced with a new, nonpolluting refrigerant.
Expanding the thermal-performance envelope increases speed and torque. Some limited duty applications are now running at up to 12,000 rpm, more than double the previously rated speeds.
The relatively new, high-energy rare-earth permanent magnets used in conventional iron-rotor motors bring new design options and capabilities to FAST motors. Smaller-sized motors can be used with these higher-energy magnets in moderate temperature environments. These magnets have high-thermal-demagnetization coefficients compared with the traditional Alnico magnets, so they are not used in high-thermal-ambient conditions. They also cost more, so they aren't used in most high-volume, cost-sensitive designs.
Other aspects of high-energy magnets are worth considering. Switching from a ferrite magnet to a neodymium-iron-boron magnet increases the flux by a factor of three. Since torque is linearly proportional to the amount of flux, the continuous torque will increase by three as well, or if the torque requirement remains the same the current will be reduced by a factor of three. Armature power will decrease by a factor of nine ([I.sup.2]R, where I=current and R=armature power). For many battery-powered applications, these efficiency gains result in longer life and fewer battery changes, which more than offset the higher cost of the magnets.
For high-volume applications, it is desirable to reduce assembly operations and integrate manufacturing processes. One way of doing this is to convert the sheet-metal endbell into a plastic-molded endbell. The plastic-molded endbell eliminates several positioning and bonding operations and makes the brush holders integral to the assembly, rather than keeping them as separate parts. Although the thermal rating of a plastic end-bell is lower than that of a sheet metal endbell, in many high-volume OEM applications thermal limits are not as critical as costs.
Another way of integrating the assembly is by attaching a molded in-place plastic hub to the fiat armature and the shaft. This assembly integration eliminates bonding operations and a shaft press while creating very good tolerances for disc perpendicularity and shaft concentricity.
The goal of higher-performance products will result in a search for new materials. On the horizon are even higher temperature insulations and new copper alloys with higher annealing temperatures. As these materials arc proven to have high endurance ratings, they will be introduced into a new generation of products that perform even better than current ones.
M. Eugene Bradshaw is chief engineer of automotive/motion control products at IMC Magnetics Corp. in Hauppauge, N. Y.