Brushless servomotorturns electric spindle

Elmsford, NY--Electric spindles offer new possibilities for the machine tool market. Known for constant torque at low speeds, and constant power in the high-speed range, they also feature few components and a compact design. Unfortunately, most electric spindles incorporate asynchronous motors.

Traditional induction or asynchronous motors suffer the inherent drawback of significant rotor loss. As rotor temperature rises, the shaft expands axially, adversely affecting machining accuracy. This, in turn, demands frequent tool corrections on the digital control. In addition, elevated rotor temperatures shorten bearing service life.

Use of a permanent magnet synchronous motor, claims Parvex Inc.--part of BTR's Power Drives Division--can eliminate such concerns. The company's line of HW brushless servomotors, when combined with a new-generation servoamplifier, allow very high torques at low speed, as well as a 1:20 speed range. As there are no currents in the rotor and no iron losses in the rotor laminations, rotor temperature remains steady.

Motor construction. A stack of punched plates with embedded magnets make up the rotor; a wound stator with cooling envelope encompasses the rotor. One of the advantages of this technique, says Application Engineer Randy Urbigkit, is the possibility of obtaining average magnetic flux densities in the air gap which are higher than the magnet's working flux density. This benefit can be exploited either to reduce the motor cost by using ferrite magnets, or reducing motor size with rare earth magnets.

Tolerance rings, fitted to grooves machined on the spindle shaft, locate rotor and shaft; shrink fitting joins them together. Rotors are also supplied with two flanges for dynamic balancing, while a resolver measures real-time rotor position relative to the stator.

Motor control. Resolver signals are processed by the digital servoamplifier to drive stator phase currents and to control motor speed. "Defluxing," or reducing stator flux once above base speed of the motor, lets the user get more speed out of the motor without supplying more voltage from the amplifier. The process is similar to reducing the excitation field on a brush-type synchronous motor, Urbigkit explains.

"Without this flux reduction capability on a synchronous motor," he states, "the servoamplifier must exhibit a current capacity that corresponds to the low speed torque, and a voltage capacity that corresponds to maximum speed at nominal flux." This, claims Urbigkit, requires a servoamplifier 3 to 15 times greater in rating, which is economically unfeasible.

When applied to machine tools, and special machines, Pafvex HW brushless servomotors provide end users the following advantages over induction motors:

- Better surface finishing due to low motor vibration;

- Improved accuracy because of the shaft's low thermal expansion;

- Higher acceleration capability for reduced tool changing time;

- Lower maintenance;

- Increased bearing life due to low temperature rise.

For additional information, contact: James Reynolds, Parvex Inc., 8 West-chester Plaza, Elmsford, NY 10523, Tel: (914) 592-6017, Fax: (914) 592- 5707.

Other Applications

- Lathe machining

- Milling and drilling

- Grinding