A.C. Drug Cup Servomotors are used as low inertia drive motors in automatic feedback control systems. The A.C. Induction Servomotors have distinct advantages over D.C. Servo Motors. The commutator and brush assembly of a D.C. Servomotor has limited maintenance free life. It is more prominent when motors are not easily accessible, or equipment is installed in hostile environments. Squirrel cage or solid rotor design of A.C. Servomotors, can give equally good acceleration, as that of drag cup servo motors. However, drag cup design has an advantage of bare minimum inertia of the rotor, resulting in minimum kinetic energy, stored in the mechanical system. 

The Servomotor is a Drag Cup rotor type 2 phase induction motor. The inertia of rotor is minimized, by mechanically delinking the magnetic circuit from current carrying conductors, on which the force is produced. The rotor conductors are in shape of drag cup, coupled to shaft. The rotor resistance value is so chosen, as to render peak torque at slip of unity or higher, which yields, a torque speed curve, as a straight line. This yields, finite and uniform damping co-efficient, and mathematically well defined transfer function. The design flux density is kept low, to avoid nonlinearity due to saturation. The thermal design permits continuous stalling of motor, at rated current and torque with forced cooling as specified. High precision is required in fabrication as the thin walled copper cup is required to rotate in two annular air gaps, which should be minimized to limit the magnetizing current.