Question

why does the starting capacitor has dual values 100/130 or values in a range whereas the running capacitor has only one value?In starting capacitor which value should be consider in designing higher value or lower value?

Answer 1

Single phase electric motors have 2 kinds of capacitors, a start capacitor, and a run capacitor. Our main concern in the operation of single phase motors is that the operation should be quick such that it shouldn't damage our motor. For quick operation, we use a combination of start and run capacitors.

The start capacitor gives the initial jump start to the motor when it is turned on. It is basically used to energize the start windings quickly so as to speed up the motor. It is only used for a momentary basis and it shuts down as soon as motor attains 3/4 of its full speed. The run capacitor starts acting when the work of start capacitor is done, that is when the motor stabilizes and reached 3/4th of its full speed the run capacitor takes over and creates a rotating magnetic field to keep to the motor in the running state.

The first and foremost difference between the above-mentioned capacitors is the start capacitor has a fairly large value as well as larger size as compared to run capacitor. The reason is that the start capacitors need to provide a large value of starting torque which requires a large amount of charge supply.

Now let us come to the asked question,

If we look at the market products of these capacitors we can notice the difference in the rating of both capacitors. The reason why the start capacitor has a range of capacitance is that we only require a large value of phase-shift of AC current to jump-start the motor and make it reach 3/4 of its full speed. A precise value of capacitance is not necessary because the run capacitor will take over eventually. So, why waste money in manufacturing a very large value capacitor with a precise capacitance value? While on the other hand, the run capacitor should have a precise value of capacitance because the whole motor operation will take place with help of run capacitor.