Single phase switching capacitor start? 1

[MacGyver15147]

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?

 

[SceneryDriver]

I have another suggestion: get rid of the single phase motor all together. Use a 3-phase motor with a VFD. The VFD will limit the start current to motor FLA (or less if you set it to) and will control your start and stop ramps. A VFD for for a 1/4 - 1/3 HP motor should cost about $120.

http://www.automationdirect.com/adc...l)/GS1_Drive_Units_(120_-z-_230_VAC)/GS1-20P5

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

 

[waross]

Buy a run capacitor instead of a start capacitor. Run capacitors are designed for continuous use. Starting capacitors are designed for momentary use.
But I support Jeff's comments and SceneryDriver's suggestion makes a lot of sense. (VFD)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MacGyver15147]

Let me add some to this question. The motor will never see startup loads much past that of a 50:1 reducer with next to nothing connected. I plan on using a 56c motor that would be normally used in an overhead door for a loading bay. I do have several 3 phase motors and VFD's that would make things easier for me. My goal is to design this to upload plans so DIY hobbyist could build it from scratch on the cheep with easily available parts. I do expect some frequent wear and tear on the relay that would start the motor but with replacements being available at $2 ea. and only taking a few seconds to swap out I think it worth popping one in every few months. As far as the caps. I'm still unsure if it would be better to try to split the startup loads between multiple cheep $5 caps. or try to use one way overrated so it never notices a surge but if that one would go it would get expensive. As far as the clutch idea I have been considering that but can not come up with a very simple design that a self taught home machinist could build without being discouraged. Understand I have seen enough YouTube vids of DIY machinists and trying very hard to make things as easy to build as possible.

 

[crshears]

As always, more information would be helpful...

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...

 

[waross]

The start up load that all motors see is accelerating the rotor up to speed. Too frequent starting cycles will toast an unloaded motor.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

If the load is so light, can it be done with a split phase or even a shaded pole motor? No caps in either of those. Less starting torque of course, but you seem to feel there is little starting torque necessary.

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"

 

[iceworm]

I don't have any suggestions about single phase motors. All of my experience is 3ph industrial. But I do have a few questions.

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

 

[ScottyUK]

You really need a motor designed for intermittent duty. The light load starting doesn't help much because each start puts a lot of heat into the rotor and the motor doesn't spin for long enough for the heat to dissipate via the internal fan and the casing. It is hard on a robust three-phase SCIM but a switched capacitor type will almost certainly have problems.

 

[waross]

Harmless flakes working together can unleash an avalanche of destruction

That hits home today.
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 3:00 5:00 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
--------------------
"Why not the best?"
Jimmy Carter

 

[MacGyver15147]

Reliability and longevity is irrelevant here. This is just meant as proof of concept on 75 bucks of investment to already junk parts out of the trash. If it worked and worked well for an hour I would invest more money in the idea.

 

[MacGyver15147]

So an update,

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.

 

[ScottyUK]

Everyone doesn't work in a backyard tinkerer environment. I expect my motors to be good for at least 40,000 hours continuous duty, and they should really make 80,000 although that's currently a bit of a reach. If I get things wrong and something goes ka-boom as a result then the plant inventory has the potential to wipe out the entire district. Your experimental methods couldn't be used in my industry, but I almost certainly have a bigger budget to draw upon. Maybe don't be too quick to condemn those who work in similar industries, and perhaps we should remember that some people have tiny budgets and can cut some of the corners to keep cost down?

Pleased you got it working.

 

[MacGyver15147]

I have also work with many 3 phase motors up to 250hp in hammer mills that would start to leak smoke after 2 failed starts in 15 minuets during ramp up time pulling around 1500 amps. Unskilled operators would make one go kaboom around once a month.

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.

 

[jraef]

... I will be redesigning to use a phase converter with a VFD ...

Just so you know, a VFD already IS a phase converter and if it is under 3HP (2.2kW) at 230V, most do not even need de-rating.

"Will work for (the memory of) salami"

 

[MacGyver15147]

Yes I will be using a VFD that has a single phase 220v input and has a 3 phase 230v out. If that is what you meant to say?

 

[LionelHutz]

You may want to consider publishing the option of a permanent split capacitor motor with your plans. This motor only has a run capacitor and no starting capacitor or centrifugal switch and typically has low inrush currents. Th downside is lower starting torque.