A Dynamic Brake for Semimicro Centrifuges 0. L. I. BROWN and C . E . SUNDERLIN United States Narxrl Academy, Annnpolis, Moryland
used for semimicro qualitative CENTRIFUGES analysis usually have no brake, mechanical or electrical. The stopping time for good centrifuges is about two minutes with a semimicro head, three to five minutes with a heavy micro head. Despite admonitions to the contrary, students persist in using their hands as brakes in order to cut down the time of waiting. This is disadvantageous for several reasons. The contents of the centrifuge tubes are jarred and slightly remixed because of the unevenness of hand braking. Also, the unequal pressures exerted on the main shaft may eventually throw i t out of line. The following dynamic, or electrical brake was devised for use with centrifuges operated by a 50 to 60 cycle, 110-volt A. c. induction motor. The wiring diagram is shown in the figure. The double pole double throw toggle switch A, the push button switch B, and the resistor C, are mounted on a small wooden platform. The resistor C i s a 100-watt lamp so that with a 110volt D. c. source approximately 1 ampere will flow. A flexible cord, with plug, leads from one side of A to the 110-volt A. c. source. Another flexible cord, with plug, leads from the other side of A , through B and C, to the 110-volt D. c. source. The center connections of A lead to a socket to which the motor is connected. In operation, the switch on the centrifuge is left closed, and the switch A is used for starting. To stop the centrifuge, the switch A is thrown across to the D. c. side and the push button B is pressed down.
The direct current going through the stator winding of the induction motor brakes the motor to a smooth stop in three to four seconds with the semimicro head, four to seven seconds with the heavier micro head. A push button is used a t B since the toggle switch A has no open position. This ensures that the D. c. will be cut off the moment finger pressure is released.
INDUCTION MOTOR
Any D. c. voltage source (e. g., several dry cells) could be used, provided the resistor C is such as to permit approximately 1 ampere to flow in the circuit. If faster or slower braking is desired, the resistor C may be adjusted to cause greater or less current to flow as long as the windings are not overheated. The authors wish to thank Professor D. G. Howard of the Department of Electrical Engineering, U. S. Naval Academy, for his suggestions.
