Motors & Drives Information

Electric motors are the prime movers for most machine tool functions. They are made in a variety of types to serve three general machine tool needs: spindle power, slide drives, and auxiliary power. Most of them use 3-phase ac power supplied at 220 or 460 V.

THE BASICS

All electric motors use the principle that like magnetic poles repel and unlike poles attract. Current through a coil or permanent magnets creates magnetic fields. Motors deliver toque by shifting the magnetic fields within the motor so the rotor is constantly drawn around.

Initially, all motors used direct current (dc). This current creates magnetic fields in the stator and rotor, then mechanically energizing and de-energizing stator coils cause a moving field that draws the rotor around. With alternating current (ac) motors, the current itself switches in polarity from positive to negative. Sending ac to the stator coils creates fields that draw the rotor around.

For most machine tool applications, versions of the ac asynchronous motor are used for spindle drives, while slide drives are generally synchronous.

A LITTLE HISTORY

The design problem through the years with machine tools and motors has been how to get high torque at a variety of speeds. Initially, mechanical transmissions consisting of ears, belts, and gear/belt combinations gave speed changing capability. Up to 36 speed ranges were common at one time. But all this extra hardware is costly and needs maintenance. In the last decade, as speed requirements have risen the inaccuracy caused by vibration, which was not a problem at lower speeds, made the complex mechanical transmission unacceptable for some applications. Machine tool builders still use mechanical transmission for many applications but, because of more versatile motor speed control, three-speed transmissions are more common.

For today's operation, consider a spindle speed of 3600 rpm as low, with high speed generally 10,000 rpm and greater. Some spindles, such as small-diameter grinding quills, operate at 200,000 rpm. At the same time, motor design and control technology have progressed dramatically. In early motors, it was difficult to alter the power variables so motors could respond to a variety of operating conditions. Now, thanks to computer technology, it's possible to quickly modify motor speed and torque.

# posted by Medical Information : 5:03 AM