One cool, light servo

One cool, light servo

Reducing the active magnetic field of a permanent magnet makes servomotors lighter and more efficient.

Reducing the active magnetic field of a permanent magnet makes servomotors lighter and more efficient.

Designed to be an integral part of a machine tool, Parvex HW series spindle servomotors (stator, rotor, and resolver) are driven by a digital amplifier (DPD or Digivex) by means of user-friendly software.

Bearing little resemblance to an ordinary electric motor, servomotors are part of an electronic system that translates an electronic command into a precise and powerful movement. This is done with an encoder, servoamplifier, precision gear-box, and other mechanical components. Permanent-magnet brushless motor technology is recognized as the best available for servomotors because it offers rapid acceleration, high torque-to-weight ratios, and small size. The drawback of this technology is in the difficulty encountered when a very high-speed operation is required.

In order to reduce power losses, the power output of the servomotor needs to be adapted to the mechanical power of the motor. This requires the magnetic field in the rotor to be weakened, which, in this type of motor, is made of punched laminations housing permanent magnets.

While reducing the active magnetic field of a permanent magnet is not an easy thing to do, Parvex, Elmsford, N.Y., has solved this problem by developing a technique called gradual flux control (GFC). Parvex's LV series servo-motors were the first to incorporate the GFC technology. Not only did these motors prove to be more efficient than competing motors, they were also lighter by a factor of two. This in itself had an immediate benefit where weight and size were important considerations.

The second range, the HW series, pushed the GFC concept further and brought about a new market segment of very high-speed electrospindles for Parvex. These spindles, designed primarily for the machine-tool market, provide special loss-free rotors, stator cooling, and built-in encoders. Some of the advantages of the HW electrospindle include good machining quality due to the motor's high-speed capability, an improved multi-functioning capability, and simplified mechanics due to the motor's compactness and multifunction design. With the HW series of motors, the shaft, frame, and high-speed ceramic bearings are assembled by the machine-tool builder and have become an integral part of the machine.

Special tests were set up by an independent German laboratory to compare the HW electrospindle motor against motors from major competitors. A major portion of the test included monitoring the temperature increase in the motor's shaft at full power. When the shaft gets too hot, it expands, reducing the motor's precision capability and the life of the bearings. In the full-speed test, the temperature increase for the HW electrospindle was less than 10 8 C compared with over 100 8 C for other motor types.

Parvex also redesigned the digital amplifier that controls the motor. The amplifier supports the GFC control and uses RISC processor-based signal handling for positioning, current/torque control, and speed control. Via a personal computer, it also provides communications to set parameters. In addition, it controls the status of the logic as well as operational safety.

The HW series of servomotors have ratings up to 100 kW, providing the opportunity to improve machine-tool performance, increase the scope for movement generation and control, and reduce machine downtime.

TAGS: Features
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