Electro Mechanical Systems is currently stocking an iron-less rotor dc motor from Faulhaber that has a mechanical time constant of just 6ms. The 22mm diameter unit reaches a speed of 4,500rpm in this time, equivalent to a coil surface acceleration of 2,500km/h/s. High efficiency is assured by the use of a rare earth magnet and, depending upon which of six windings is selected, the motor will offer up to 11W, representing a substantial power to weight ratio for a 32mm long unit.