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jeremy13
05-13-2009, 11:11 PM
This was given to me after the compressor unit got 3 phase power to it. It was on a 15hp propane vapor compressor. This thing is huge 24 X 30 cabinet and the start caps are the size of beer cans all 10 of them. It did come from oil country. But how do the name brand guys get away with a converter the size of a lunch box? And can you over power a smaller 71/2 hp motor with it.

http://i40.tinypic.com/2u62quf.jpg

J Tiers
05-13-2009, 11:51 PM
If it was for 15 HP, it probably needs some capacitance removed to work well with a 7.5 HP motor.

I can offer no further opinion about that one, functionally, but it appears to represent quite a few hundreds of $$ of "stuff"..... and visually good build quality, although I don't see wire markers anywhere.... I do see what looks like a maker's data plate.

At least you are surely getting your money's worth.....

As for size, it's possible they could have made it smaller, but good practice with industrial stuff is to have it reasonably open and accessible inside..... That one is more so than it needs to be, but maybe they use that size box a lot.

It says to me that the makers went over-spec and used heavier-duty parts and general construction in order to assure the long service life.

they also included inside stuff which would need to be put outside, added to, a commercial item, like motor starter, overload protection, etc.

"Name Brand" folks have lots less total stuff inside, and are making things to meet a cost, for cost-conscious consumers and small shops. Larger shops have 3 phase already....... They cut a lot of corners, and accept a failure rate, limited lifetime, etc.

They almost never include the motor starter, overloads, etc.

Carld
05-13-2009, 11:51 PM
Hmm, I miss read the thread. That's a nice looking panel you have and I suppose it can be modified to do what you want.

Mcgyver
05-14-2009, 06:46 AM
looks like you have everything there to put a motor on and have a rotary phase converter. proportion of start caps vs run caps seems reversed, but of course not knowing their values, saying that doesn't mean much.

jeremy13
05-14-2009, 11:58 AM
I could have the amount of caps reversed. Just asumed that since you wear makeing up a third leg you would need alot of caps to get things moving.I found a free 25hp 3ph motor for a roto phase converter. But I don't think this static will start it. Then agen the compressor did start under load and the 25hp will be freeweling. If that makes a differance.

Rich Carlstedt
05-14-2009, 11:27 PM
Jeremy
What you have is a great three phase convertor !
First,
at the top of the two rows of start caps ( 2x 5 ) are the cross wires connecting the banks. If you cut those two wires, you reduce the start capacity in half which will get you close to the 7 !/2 HP you want.
You should disconnect half the run caps too !
Second.
the circut has a step down transformer to reduce 240 to 120 for relay and timer, and it is fused ( hooray )
Third.
You have a timing relay for control. this is the best of all worlds !
It controls the motor contactor (relay) so it will not burn its sensitive contacts.
The motor contactor in turn makes the contact for the start relays which make big sparks..but the contactor is built to handle that.
Now all you do is adjust time to give you the best start characteristics.
That means, if the motor starts and growls, either turn down the time from say 1.5 seconds to 1.1 seconds, OR disconnect a Cap !
If the motor is stalling, add caps, or increase time.
I would not go more than 3 seconds, or you risk blowing a start cap.
The cabinet you have is the perfect setup for experimentation.

The size may be because they had the cabinet on hand..also in industrial applications, there is never "too much room "

Rich

Okuma2540
05-18-2009, 10:24 AM
Why Would you need to reduce the size to 7.5hp? When I was looking into getting one for my shop all the dealers told me to add up my hp from each motor that i might be running at one time and get a converter that size or larger. They said i couldn't get too big but i could go too small? But i am no electrician and was just wondering if it is true.

Rich Carlstedt
05-18-2009, 02:40 PM
Jeremy mentioned running a 7 1/2 HP motor on the unit. Thats why I mentioned it.

What the dealers told you is correct. You want the convertor = or larger than the total load. Even being equal cuts it too close ( I like 150%)

Running a phase convertor is not without cost or considerations.
If you plan to run multiple motors, you should consider this:
Rather than putting a run capacitor on the phase convertor , mount it
on each motor ( enclosed of course) and balanced to that motor.
This prevents high currents and "growling" of the slave motor which can damage it.
If you are too small ( RPC), the accumulated currents will overheat the pilot motor (convertor) and smoke it..(some fellows like to balance voltages, but the amperage loads can/will cause failure in a unbalanced mode.)

Running a high horsepower convertor with only light loads means a heavy bill for your electricity. I wired up a friends commercial shop, and he runs a 10 HP convertor all the time, for his 1-2 HP grinders. Since he has 5 grinders he used a 10 HP setup.
but, he only runs one grinder most all the time and as a result his electric bils are more than double what they would be if he had a VFD or a smaller convertor.( exclude lighting /heat) but he does have the advantage of being able to run all his equipment. He said the convience makes it worthwhile.
To each his own

rich