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July 1, 2004
High-speed spindles with synchronous motors meet many machining needs.

High-speed spindles with synchronous motors meet many machining needs.

By Jürgen Beck, Precise GmbH
by James J. Benes

Mr. Beck is sales manager, Precise GmbH, Leichlingen, Germany.

Precise SD 60124 spindle with synchronous motor milling aluminum at 20,000 rpm.
A high-speed spindle with asynchronous motor has a working speed range of 500 to 20,000 rpm and a tool-clamping diameter of 80 mm.

Many shops dealing with a variety of jobs need spindles that handle an entire machining operation on a single-spindle machine. For instance, applications may range from driving small-diameter, high-speed tools for micromachining and superfinishing to handling large-diameter tools and taps. An increasingly important spindle feature is the ability to stop at predesignated positions for thread cutting without an adapter collet or for reading barcodes using scanners that identify tools by reading marked toolholders.

High-speed spindles driven by synchronous motors possess these desirable features. They are compact but equipped with the largest-possible tool interface. In addition, they are thermally stable and don't transmit heat to the machine or expand in any direction. These devices also have excellent concentricity, are vibration free, and have comprehensive sensor features. To boot, they're generally durable, reliable, virtually maintenance free, and low cost.

In a synchronous motor, the rotor (armature) locks into step with the rotating magnetic field in the surrounding stator. Synchronous motors are inherently constant-speed motors and operate in absolute synchronism with line frequency. This type is often used where exact speed of the motor must be maintained. Speed is determined by the number of pairs of poles and line frequency.

In contrast, asynchronous, or induction, motors are simple and rugged and consist of a wound nonrotating stator surrounding a rotor assembly. Synchronous speed is the upper limit of motor speed. If the rotor of an asynchronous motor turns as fast as the rotating magnetic field in the stator, no torque is developed. In asynchronous motors, the rotor rotates at a slower speed than the magnetic field, but sufficient to produce enough torque to overcome windage and frictional losses and drive the spindle load. The difference between rotor and magnetic-field speed, called slip, is expressed as a percentage of synchronous speed.

In a synchronous motor, the armature is constructed with permanent magnets, as opposed to steel laminations in an asynchronous motor. Thus, the stator only has to produce the momentgenerating component of drive energy. The permanent magnets provide the field-generating component.

Synchronous motors have greater power density than asynchronous ones and deliver higher output and torque for the same size spindle. The armature of a synchronous motor generates little heat, so the spindle minimally expands resulting in high-axial precision. Bearings and other components in the vicinity of the spindle are also subjected to a low level of heat contributing to increased accuracy. High-speed spindles with openloop asynchronous motors cannot provide defined-stop positions. Only large asynchronous motors produce sufficient torque at slow speeds. However, a large unit cannot generate the high speeds required for small-diameter tools.

Frequency converters for driving synchronous motors are available from a growing number of sources. Speed limits for synchronized drives are no longer limited by frequencyconverter technology but, rather, to the requirement for accurately holding armature-mounted magnets in place at high-armature speeds.

Spindles with synchronous motors typically operate in a speed range from 500 to 20,000 rpm. Newer designs, suitable for high-precision machining, will reach speeds of 30,000 rpm.

Spindles with air-bearings, capable of operating at 180,000 rpm, are for superprecision machining such as drilling printed-circuit boards and milling small components.

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