August 17, 1998 Design News
From the regional editors
Brushless servomotor turns electric
Flux reduction via microprocessor
control permits economic operation
David J. Bak, International Editor
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
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
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;
For additional information, contact: James
Reynolds, Parvex Inc., 8 West-chester Plaza, Elmsford,
NY 10523, Tel: (914) 592-6017, Fax: (914) 592- 5707.
Milling and drilling