TU56/TC11 restoration - VARIAC question

On Mar 18 2005, 10:07, Dwight K. Elvey wrote: "start/run" cap, shifting "run" Simplified explanation: In "capacitor-start, capacitor-run" motors, which run from single-phase AC, there are two sets of coils. The coils need to be fed with AC out of phase with each other to cause the motor to rotate. The AC supply is fed directly to one set of coils, and through the capacitors to produce the phase shift for the second set of coils. To start the motor, you need lots of current and a big phase shift, so the "start" capacitor in parallel with the "run" capacitor is a high value. A centrifugal switch in the motor disconnects that once it gets to some fraction of its nominal speed, and then only the "run" capacitor is left in series with the second set of coils. Capacitors passing AC have an impedance, analagous to resistance but frequency dependant, so they dissipate energy proportional to their impedance and the square of the current. The "start" capacitor is only in circuit for a short time, so it doesn't need to be as robust as the "run" capacitor, which will be dissipating energy all the time the motor is running. Of course if the load on the motor is low, so is the current, so the capacitors may not need to handle too much energy once it's running normally. -- Pete Peter Turnbull Network Manager University of York