Electrolytic Capacitor Function in the Capacitor-Start Motor
Figure 3.31 shows the motor circuitry in the start position with the start switch contacts closed. When the motor is energized, it will accelerate to approximately 75 to 80 percent of synchronous speed in about 1 second. At
this time, the switch contacts open, shutting off the start-winding circuit. The motor now operates with only the run or main winding energized.
Electrolytic capacitors (of the correct size) help create a rotating magnetic field that starts the motor. They control when current flows in the start-winding circuit (in relation to current flow in the run winding). The sine wave in the following illustrations shows why the capacitor has to be the right size.
In Fig. 3.32, there are no capacitors in series with the start winding. The sine wave shows the position of the line voltage, the start-winding amperes, and the run-winding amperes.
Run windings and start windings both have inductance. The run winding usually has more turns and is located deeper in the iron (slot) than the start winding. Both factors (more turns and deeper in the iron) give the run winding more inductive reactance than the start winding. More inductive reactance in the run circuit causes the amperes to flow slightly later than the amperes of the start circuit. This small difference in time between the start and run currents means the motor will have very little starting torque.
Figure 3.33 is an example of too much capacitance. In this illustration, the two currents are flowing at nearly the same time. The result is little or no starting torque.
Figure 3.34 shows an ideal amount of separation. The separation is 90 electrical degrees or 1/240 second on 60-Hz power. Ninety degrees separation in the current matches the 90 physical degrees of separation between the start and run windings. This timing is accomplished using capacitor(s) of the right size in series with the start winding.
