Single phase switching capacitor start?
Single phase switching capacitor start?
(OP)
I am trying to design a scaled down version of an industrial machine for home use. I seem to be getting rusty on single phase and not sure how best to do what I want. Here is my problem I will be using a single phase motor 1/4 to 1/3 HP which will need a capacitor to start. The motor will be run intermittently approximately on for 5 sec. then off for 15 sec. The motor I am using will have no problem with the load however the start capacitor will explode under those conditions in a short time. What I would like to do is have a rack of say 4 caps A, B, C, & D. and some way to alternate the load on each start. For example on the first start the motor would use cap. A then on the next cycle start on cap. B and so on giving time for the first to cool before being used again. Any suggestion?





RE: Single phase switching capacitor start?
http://www.automationdirect.com/adc/Shopping/Catal...
This will also save you the cost of constantly replacing the motor's contactor. Alternatively, have you considered a clutch arrangement" They are designed for exactly this type of service.
SceneryDriver
RE: Single phase switching capacitor start?
But I support Jeff's comments and SceneryDriver's suggestion makes a lot of sense. (VFD)
Bill
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"Why not the best?"
Jimmy Carter
RE: Single phase switching capacitor start?
RE: Single phase switching capacitor start?
What's the application? Why so many starts back to back? Given the information provided in the OP the electric clutch idea seems the best way to go, but the follow-ups seem to call that into question...
RE: Single phase switching capacitor start?
Let's see; Four caps, four switching devices, some type of controller to sequence the caps, vs buying the proper capacitor to design a unique DIY device, and the associated bragging rights. When you look at the whole picture, it may be better to incur the added expense, complexity and multiple failure modes of multiple capacitors.
Bear in mind that small motors subject to rapid multiple starts do burn out, but the start capacitor rarely fails. Just go with the standard starting capacitor. The motor will probably protect the capacitor by motor failure before the cap fails.
Bill
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"Why not the best?"
Jimmy Carter
RE: Single phase switching capacitor start?
But Bills point about duty cycle goes beyond just the start caps, motors are designed with inherent duty cycles in mind as well and you are going to stress any motor you use. Over sizing the motor makes up for that somewhat. So if you are going to over size it just for the duty cycle anyway, look for a single phase motor type that does not need caps.
"Will work for (the memory of) salami"
RE: Single phase switching capacitor start?
Let's see, 20 sec duty cycle, that's 3 starts per minute, that's 180 starts per hour. Most of the industrial stuff I deal with is ready to let out the smoke after 4 starts per hour - maybe even three. But I don't deal with 3ph 1/3 hp motors - nor single phase 1/3hp motors. Maybe they are way tougher than industrial grade 3ph normal and can stand 50X more. Have you called the mfg to see if they have a spec on starts per hour? The thermal loading of starting is high. Barring unforsen data from the mfg, my inclination is the windings will be toast long before the first hour is up.
Then there is the start winding switch. I'm guessng it is getting a year's worth of wear every hour. I don't think the light loading nor short accelleration change the arc the contacts see every time they open. Could be the switch needs replacing every 5hrs - 10hrs.
Then again, maybe I am being too pesimistic. If a garage door opener motor is designed for 4starts per day, and ten years, that gives 14,000 starts over the life of the motor --- about 75 hours in this application. Maybe that is enough.
Still, the cap run or split-phase impedance motors suggestions would get rid of that issue. The question here is do you have any data on expected starts over the life of the motor?
I can truely say I have never had to deal with an application that needed 180 starts per hour. Good luck. Let us know how it comes out - I'll be interested
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Single phase switching capacitor start?
RE: Single phase switching capacitor start?
I'm stuck in a motel in Revelstoke BC. All roads out of town closed due to severe avalanche danger. The road may open at
2:00 3:005:00 to 7:00 PM. But the other side of the pass the road east of Golden BC may open tomorrow afternoon. There may be a detour available that will just add an hour.I'll see what tomorrow brings!
Bill
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"Why not the best?"
Jimmy Carter
RE: Single phase switching capacitor start?
RE: Single phase switching capacitor start?
I went the easy route and just doubled the cap size with a Dayton 480mfd.
The prototype has seen around 150 hours run and doing well at between 2 to 4 hours at a time.
So to all the engineers out there don't just go by numbers and ratings for a "Disposable" test.
Millions of dollars are not required especially if it is coming out of your pocket.
"just keep it simple stupid" method works just fine for me with very little effort most times.
If it works then invest in upgrades if it don't you didn't waste much.
Lesson to learn.
RE: Single phase switching capacitor start?
Pleased you got it working.
RE: Single phase switching capacitor start?
Smaller motors can handle far more abuse due to less rotating mass. Single phase compressor duty motors are designed for very high starts per hour under load.
My personal back yard budget does not allow for any wasteful spending. My designing a fully automated injection molding machine from the ground up scaled to work on single phase for a home hobbyist was no small task. Now that I know this prototype worked with only around $400 into it I will be redesigning to use a phase converter with a VFD so it can set run continues without any pause. Few other changes will be made to make more durable. I am now comfortable in throwing a couple of thousand at it. I am still looking at a tight budget as I will be making plans so others can build one for themselves.
RE: Single phase switching capacitor start?
"Will work for (the memory of) salami"
RE: Single phase switching capacitor start?
RE: Single phase switching capacitor start?