Improving the Speed Control of Magnetic Stirrers The magnetic stirrers commonly found in chemical and biological laboratories generally show unsatisfactory contml and regulation of their operating speed. They tend either to stop altogether or to accelerate to high speed, the latter leaving the stirring bar motionless or wone, jumping wildly about. A sheet of highly conductive metal placed on top of the stirring unit favorably modifies this characteristic, We find that a sheet of aluminum Hs-% in. thick considerably improves control of the stirrer without otherwise affecting its operation. Magnetic stirrers are usually powered by small shaded-pole induction motors whose speed versus torque characteristics favor operation only a t synchronous speed (- 3600 rpm) or a t stall. Speed control is attempted by starving the motor for power until it can no longer accelerate the load presented to it. A sheet of conductive metal placed neaf the driven magnet acts as an eddy-current brake, presenting the motor with a viscous load which increases in proportion to the motor's speed. In contrast to the viscous load presented by a liquid acting on a stirring bar, the motor cannot escape the eddy-current load by acceleration and, hence, the tendency toward averspeeding is eliminated. The additional load also permits use of a higher setting of the speed control with the result that the range and degree of control is enhanced, especially a t low speed. Rutgers Medical School Piseataway, Yew Jersey 08854
Eric F. Eikenberry
Volume 52, Number 6. June 1975 / 385
