Limited Angle Torque Motors/Tachometers

Limited Angle Brushless Motor Description

Limited angle brushless motors are single-phase machines which produce torque over a limited angle without the need for commutation. With a constant current, the torque will follow one of the curves shown in figure 1 – 4 depending on the number of poles, torque constant, angle of regard, and winding distribution. These curves are for reference only and may vary slightly from model to model. Armature reaction effects (Reduction of Kt as a function of the magnitude of current and angle) are not taken into consideration and will distort these theoretical torque vs angle curves.

The limited angle brushless motor is designed without the use of laminations employing teeth. The lamination stack is constructed using smooth cylindrical laminations. Thus the rotor “sees” the same magnetic reluctance which is independent of rotor angle, hence no cogging torque. The windings can be either wound toroidally or can employ a surface winding technique. Most of the motors described are of the toroid variety. In some designs, where higher efficiency is needed, surface winding can be substituted.

The winding configuration is integrated with the magnetics of the rotor to give the back EMF wave forms shown in figure 1 - 4. The windings are toroidally wound and generally potted to form a smooth cylindrical surface. For the surface wound limited angle torquers, the windings are mechanically captured by a non-conductive, non-magnetic structure and then epoxy impregnated. The motors described are non-redundant windings but can be made with redundant windings for a small size and weight adjustment.

The rotors use either Alnico magnets, high energy product Samarium Cobalt or Neodymium-Boron-Iron magnets depending on the requirements. To minimize weight, the stator lamination thickness is designed to fall below the saturation level for the materials. The motors are engineered to develop the maximum torque for the lowest power and the lowest weight for a given form factor. The motors use materials which meet the outgassing requirements of < 1%TML and < 0.1% CVCM required for critical space applications.

Other laminations and materials are available for specific applications, which require lower viscous drag torque, higher torque per unit weight and/or higher temperature operation.

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