RPC Sizing

1coollift · Apr 6, 2011

Did a lot of searching and still confused. I apologize if this has been covered.

I have acquired a Stenhoj auto lift. It is all electric and powered by a 3 phase 220v AMB motor.

I assume in this application the 3hp motor is considered "hard start". (?)

I also assume a RPC would be the best way to go here. (?)

If I use a RPC how big of one do I need? ...I have gathered conflicting info...

(For now I plan on this being a dedicated RPC that would not be used for anything else.)

A local shop can build me a 3hp rpc and says that I do not want anything bigger because it will produce too much currect/power???

The issue of having to big of a rpc is contained in some of the forums so there may be some validity to it...

The rpc retailers (Temco, Phoenix) say I need a 10 hp unit. They are sales people referring to company charts that I too can access...
..............................................
I guess I could swap out the 3 phase for a single phase motor but this is an integral unit with the up/down switch panel nicely built in. I have no idea how difficult the wiring issues would be.

Got this lift for free...but I guess nothing is free :rolleyes5:

ben80 · Apr 6, 2011

yes

the shop is right, a 3 hp rpc would do the job. Thier rpc would do the job that is.

ben80

ben80 · Apr 6, 2011

yes

Hi,

my battery on my laptop ran out, had to shut down.

I wanted to add, that any manufactured made RPC of 3 HP would work. You don't have to buy from just one source.

You might search about where would be a good place to buy and a good price on this site. I really don't know the answer to that.

I would guess that it will cost about $450 plus shipping and the shipping could add up.

Some of the confusion comes from the fact that we can make our own RPC's , but that's a whole other thing.

good luck.

ben80

1coollift · Apr 6, 2011

why confused---

this is from Garage Journal Forum---

3 different posters---

"A lift must start at full load (consider: car is up a certain height, and you want it up more: you hit raise and it (should) go up.) But that requires more starting power than a smaller converter would be able to provide... "

"I use the X2 the Tool hp, in sizing rpc for hard start machinery. I say a 7.5hp rpc should be fine. Not an expert though. Build a larger rpc and get a 3 phase compressor."
__________________

"a converter that is a 3hp one will power a 1-3hp motor from that
a converter that is a 10 hp will power up to a 10hp motor but is better used on more like a range of 5-10hp motors
short answer is dont get the 10 unless you plan on adding other 3ph equipment"

go figure..... :willy_nilly:

t.jones · Apr 6, 2011

Coollift welcome to the site. Yes a hoist is a hard start ( so is a compressor ) I built my own convertor with advice from this site, it's easier than you might think . I also have a hoist that I got for free. ...Thanx Trevor

racer55 · Apr 7, 2011

It's not an exact comparison to what I have but here goes:
I built my own self starting balanced 7.5hp RPC form designs found here as well as other sources and run 2 pieces of equipment from it so far.
My 2hp bridgport style mill starts and runs with no effort from the RPC and is considered easy start.

My 4+hp 2 speed motor gearhead lathe(its labled in Kw and I forget exactly what at the moment) starts fine in the lower speeds but nearly stalls the RPC in the high speeds and is considered hard to an extremely hard start load,I need to either run the mill as as second idler to aid in high speed starts, build a second RPC or build a bigger RPC for that situation.
I plan on building a second 5hp RPC wired in parrallel for the hard start lathe and allow for further expansion.My capacity would then be 12.5hp combined for starting and 30+hp running load and should be sufficiant. After the loads are started up to 3 times the nameplate hp can be run from a good working RPC as I understand it.

This is from the American Rotary site:
"For starting, figure at least one size up from load hp for a light load, 2 times larger for a hard load, and 3 times for an extremely hard load. Converter must be additionally oversized for multiple speed motors, older motors (pre-T-frame), foreign motors, and custom motors."

If your application is hard start 2 times bigger RPC is needed,extremely hard start like my gearhead lathe-3 times larger RPC is required.

Torque1st · Apr 7, 2011

IMO- Go with the 10HP units that the companies recommend. At least if it fails to run the lift you have someone to make it work. It will also be great for future tools and equipment.

Forrest Addy · Apr 7, 2011

Looks like a VFD should be considered at least just to compare costs and merits with an RPC. 3 HP VFDs are often single phase rated.

Here's one you might consider: Shopping Cart - Dealers Industrial Equipment, LLC $184 and its quiet and efficient.

1coollift · Apr 7, 2011

this is the lift in lower right of link...Stenhoj Maestro

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MOTOR PIC-

ben80 · Apr 7, 2011

good

Did Not think of the hard start problem. I have only been working with 1 and 2 HP running off a 3 HP RPC.
Thanks for joining and stoped him from making a mistake !!!!!!

My question would be, is the vfd be a better match for any hard start?

ben80

t.jones · Apr 7, 2011

Coollift does the hoist have 2 motors? ...Thanx Trevor

1coollift · Apr 7, 2011

one motor drives pulley on threaded shaft...@ bottom of threaded shaft a chain gear...chain goes across floor to bottom gear of other shaft.....

t.jones · Apr 7, 2011

Coollift I asked because the picture of the one in the brouchure looks like it has 2 motors. I would just want to be sure this is a good working current design before spending too much. Years ago I saw lots of problems on the Acme thread style hoists. Was this hoist operating before you got it ? ...Thanx Trevor

1coollift · Apr 7, 2011

-yes it was working

--looks to be very well made from what I have seen

1coollift · Apr 7, 2011

Forrest

can u give me a quick run down on the pros cons of a VFD vs RPC?

1coollift · Apr 7, 2011

sooo

I'm leaning towards purchasing a 7.5hp RPC with digital controls from American Rotary...

I just don't think I will have the time to run around collecting all the spare/surplus parts to build this thing.

It is a little pricey ($675) for my "free" lift but I was very impressed with the "local" nature/excellent/advice I got when I contacted American Rotary.

Service & support is everything to me.

racer55 · Apr 8, 2011

I would not actually call that lift an extremely hard start like a hydraulic lift would be,more like a med-hard start.

A friend of mine has a hoist like that and it is powered by a single phase reversible motor a round 2 or 3 hp-not sure.
It starts pretty well,there is a quite a bit of mechanical advantege with them.He says the best part is when the power goes out he can still use it with a large monkey wrench and some patience.

One note: my buddies hoist had the problems with the lift nuts wearing to slivers in very short order until he had them custom made from what he called "high wearability bronze"no problems since and its been about 20 years,the originals didn't make it 2 years.

1coollift · Apr 8, 2011

racer55-

What/where are the lift nuts?
.........................................
Now thinking the ~ $700ish for new RPC a little steep. Can anyone help me with locating a 5 or 7.5 hp 3 ph surplus motor in Georgia?

racer55 · Apr 9, 2011

As far as I know the lift nuts are on the screws and under the carriages,I belive they fit in a slot in the carriages,if you lower too far they can come undone and get the sides lifting at different heights,be carefull when lowering.
My friend stops a fraction before the arms hit bottom.

Since the lift operates like a screw jack it has nuts on the screws in each leg.

RPC Sizing

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