New solid rotor improves resolver performance

Sept. 1, 2000
With a solid rotor that does not use windings, this new resolver provides affordable feedback for industrial automation applications

One of the advantages of typical frameless resolvers is that they give a constant signal that represents the absolute position of the motor shaft within a revolution, even after power failure. One of their disadvantages is that they are expensive, and thus usually used in aerospace and military applications. A new resolver design, however, claims to remove this disadvantage, making resolvers more available for industrial applications.

This design uses a solid rotor without windings, which makes the resolver less complex and therefore less costly to manufacture. It is mechanically and electrically compatible with traditional resolvers, and can be used in original and retrofit applications. According to the manufacturer, Admotec Inc., in Norwich, Vt., the design of the Rotasyn resolver, Figure 1, also offers higher speed operation and better reliability than traditional resolvers.

Built like a small electrical motor, a traditional resolver consists of a wound rotor and stator, Figure 2. The windings on the rotor generate an ac magnetic field with a sinusoidal distribution. This field induces voltages in the stator windings whose amplitudes are dependent on the rotational angle of the rotor. For sine and cosine signals, the two secondaries are wound 90-deg apart in the stator.

A rotating magnetic coupling transfers energy from the stator to the rotor. The primary of this rotary transformer is built into the stator. The secondary is mounted on the rotor and connected directly to the resolver primary, Figure 3. (Slip rings are not recommended because they are subject to wear, generate signal noise, and can compromise the mechanical ruggedness of the resolver). The wound rotor limits the speed of a resolver to 10,000 rpm or less. At faster speeds, the windings tend to fly out of the rotor due to centrifugal force.

The new resolver, called Rotasyn, has primary and secondary windings in the stator, therefore it does not need a rotating magnetic coupling, Figure 4. This design is inherently brushless.

The rotor contains a diagonal section (not perpendicular to the shaft) of high permeability material that varies the magnetic field across the stator as the rotor turns. Transferred energy remains magnetic from the primary coil through the air gap to the sinusoidally shaped poles of the solid rotor. The total flux through the gap is constant—the rotor determines the angular position within the stator bore where the coupling occurs, and thus the relative amplitudes of the output signals.

The primary coil is wound circumferentially between the two stators. The secondary windings are wound in the stator slots in space quadrature, similar to a traditional resolver. Thus, the induced voltage amplitudes correspond to the sine and cosine of the rotor angle as in a traditional resolver.

Benefits of this design:

• Since there are no coils in the solid rotor, it is rated to 30,000 rpm. Higher speeds are a matter of mechanical balance. The top speed (100,000 rpm) is limited by the excitation frequency, and the burst strength of the rotor.
• The single-stage magnetic design means low source impedance, which means the transducer is less susceptible to noise pickup and tolerant of long cable runs. This new resolver can be excited at frequencies to 40 kHz.
• The poles on the rotor have no slots and therefore produce a smooth output with no slot ripple effect.
• A solid rotor requires less manufacturing, therefore reducing costs.

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