How to size motor start cap?

I guess it is my time to be educated. Those start capacitors are common but a special case for electrolytics. Good for a short time, but will let loose if AC is applied for very long at one time.

Maybe. There are different levels of quality in such capacitors. Since the heat generated is mostly from the ESR (equivalent series resistance) of the capacitor, a low ESR would heat up less and theoretically last longer. However, that's not what causes caps to die: He starts out putting a run capacitor across 220VAC and leaves it connected. It stays cold and nothing happens. Somewhat later in the video, he does the same to a large value start capacitor. It belches hot steaming electrolyte and then blows the end off. If the centrifugal start switch sticks or is delayed in opening, for example by line undervoltage, the switch stays closed for too long and cap dies. Part of this is ESR and ripple current heating, but the real culprit is junk centrifugal start switches with partly welded contacts. If the switches were good, methinks that even the lowest cost motor start capacitor would last forever.

Some time back there was an old guy whose sole job for 20 years was to make a product in a 1000+ litre reaction vessel with viewing ports, using several steps. Just before he retired, he wrote out all the info needed for the new guy to make the product. But try as he might, with the new guy the process always failed at a certain stage near the end. The company called the old guy and asked him to come in and make a batch with the new guy looking on. He started the process, peering through the viewing port, and the product came out perfectly at the end. The new guy asked him how he knew when to add ingredients at the step where it had always failed before. His answer? "It looked right".

On Trial-by-Combat:

Peter Wieck Melrose Park, PA

Sometimes you just have to go by looks or feel than by the book.

Where I worked there was a device to pull a vacuum on the process. It was very simple. Five seperate stages of a long pipe with what looked like a trumpet horn in the end that steam was put through. Seemed that only me and another could trouble shoot them very well. Others just replaced parts tuil they stumbled on the right one. On more than one ocassion it went like this. The day shift worked on it most of the day and would leave about 11 at night. Then me and the other man would get it going in less than an hour. We had just worked out some tests that were very simple and easy to make, but were not in the book.

Sometimes, this is much like making bread. Flour varies from day to day. Yeast will vary, even the dry stuff. Humidity, temperature, salt content. It comes down to the kneading - and getting that right, more importantly, when to STOP kneading.

Peter Wieck Melrose Park, PA

At a former employer, I would sometimes bring my fencing kit (foils, mask, gloves, conductive jacket, helmet, buzzer, etc) to work. The kit would usually live in my truck, and only be dragged inside for a quick show and tell, or to repair the badly designed contact buzzer. Someone walked into my office, saw the foil, and decided that dueling must be my favored method of settling disputes. Within days, everyone knew that I was armed to the teeth and that I was prepared to do battle with anyone that criticized my decisions. I had no idea that the story had become so distorted.

At the next design review meeting, I was asked to leave all weaponry at the door and that company policy forbids bloodshed during working hours. I still didn't understand what was happening, until someone mentioned the fencing foils. When I proclaimed that my kit was in my truck, everyone seemed to visibly relax.

About a year later, I gave a quick fencing demonstration with a friend in the parking lot during lunch. It went well and we all went back inside when lunch was over. However, someone apparently called the police. At some point, fencing morphed into dueling, which the police interpreted as using dueling pistols. When the SWAT team eventually arrived, there was nothing to see, so they surrounded the building and started to evacuate the neighboring businesses. I'm going to skip over the next hour and just say the police left looking rather disappointed. Of course, management was not thrilled, but eventually concluded that it wasn't really my fault that things escalated out of control.

I thought that maybe the really large capacitance was for higher torque, now I know it that it is. Thanks. Eric

So, if you parallel a mess

not true. if there is current flowing through the cap, ESR of the cap will dissipate power. The purpose of a motor cap is not to correct the PF.

There is no such thing as a single phase motor. Small motors have shaded poles or other tricks to create a phase shift. Medium size motors motors use a cap. Large motors are typically fed with 3 phase.

I think the optimum cap value, is where the current through the aux winding is 90 deg out of phase with the main winding. m

I agree. I screwed up and forgot about ESR.

Also true. However, the original problem is with a 3HP motor, with no run capacitor. I guess for high torque starting, such as with an air compressor, this would be an exception.

Agreed. This explains it better than I could: