Thread: Another RPC Question

When slecting a 3 phase motor for a RPC does motor speed make any differance ? 1725 or 3450 ?Is one better than the other ?

Either motor speed will make an acceptable converter. I have used both and there are pros and cons for each. The most important pro is availabilty and price.
3450 rpm does a little better job of generating however a 3450 TEFC motor will make a lot more noise than a 1750 TEFC. The best choice is an open drip proof design (ODP) if you are putting the motor inside. If you put it outside then use a TEFC motor.
A 1750 motor is quieter, slightly larger, slightly heavier and will make a very good RPC when balanced properly. I built one 1750 that required no start circuit once it was balanced per Fitch Williams technique.
I generally use 3450 rpm motors for 5 hp and smaller RPCs and I use 1750 rpm for larger ones.
This has to do with used motor availability and the noise generated by a 3450 rpm motor--not any electrical theory.
A lot of folks try to mount a box full of contactors and capacitors directly onto the idler motor. This is not necessary and takes some extra effort. Just get an electrical enclosure about 12" x 18", or larger, and mount it on the wall near your breaker panel. Mount the idler motor anywhere you want within 50 ft or so. I like to put them outside the shop maybe in another room or under a shed (you can build a doghouse for it). You can mount multiple start / stop stations at convienent locations in the shop. Mount one start / stop /run light station near your door and light switch. This way you can see if the RPC is powered up when you turn out your lights. If you need more help, just contact me. I will not have computer access for the next week though. When I take a vacation I leave the newspaper, phone, television and computer behind.
When I return I will finish building a 5 hp I just started and I have a request for a 10 hp.
I also have a request to fix a 20 hp someone half a@@@d built.
Stay away from the so called RPC panels as the ones I have repaired were all about 1/2 the capacity stated in the ads. You can build a much superior RPC. The factory built RPCs, such as Kay Industries ,are very good.
Good Luck

In my limited experiance with RPC's, I have found that the lower speed motor require less starting capacitance. I have a formula to calculate the needed starting capacitance if any one is interested. I have developed this formula from empirical data and do not know if it will work in all instances.

Kent

If you are talking about a 10 hp or smaller RPC, then starting with capacitors is no problem. It does not even take a very large start capacitor. I haven't built any above 10 hp.
One rule of thumb to consider is the circuit size you need. Your RPC will pull 5 amps per hp from the single phase supply. I use a 40 amp circuit for a 5 hp and a 60 amp for a 10 hp. I can run 7.5 hp total on my 5 hp but only 10 hp on the 60 amp 10 hp idler.

What about an 1170 rpm 10hp motor for an RPC?

It will work and it will run quieter than the 1750. I have not used one myself but know of them being used.

I have used many in 1170 rpm.if they are cast iron in large frame sizes,they take less start capacitance to get them started,than as would a 3450 in most applications

Scott

why does the rpc take 5 amps per hp, i have never heard that theroy before, is this correct, so a 5 hp rpc is gonna idle at 25 amps, for some reason them numbers just dont seem correct....isnt it 2.8 amps or something per hp on 220 volt?

On a full frame 5 H.P. rpc,the appoximate idle current is 3 amps,a 10 h.p 7amps and a 15 hp 8 amps.

Scott

full frame is tefc corrct? and thats 3 amps per leg right?

tefc stands for totaly encloesed fan cooled,and it is only a referance to the motors encloesure.
And has nothing to do with the motors performance,with the execption of how is is cooled.

For example,what would be considered a full frame in the T-frame motors for 5 h.p. would be a 184 t frame,this motor would go appox 80# and have a 1 1/8" shaft.A oversized frame in the same h.p. would be a 215T frame.if was in a cast iron instead of rolled steel frame would go appox 130# and have a 1 3/8" shaft.

on the other side of things in 5 h.p./ is the 56 frame,35# 5/8" shaft many times with no service factor as well.

and if you see sp or spl on the nameplate. if its in a 56 frame size.save it for another project,not a rpc.

bottom line,larger frame,more copper more steel,better motor,better rpc.

The rot is a full frame motor fully loaded,one expects it to draw around 2.8 amps per h.p.
on compressors,presses and many other applications,quite often it will draw that plus go into the service factor which would be the 2.8 X 1.15 and is very commen.

as i stated,On a full frame 5 H.P. rpc,the appoximate idle current is 3 amps

hope this helps

Scott

The 5 amp per hp is the size single phase circuit you need to provide sufficient amps to operate your conveter at full load. This has nothing to do with idle current. In short: if you do not have the power available you sure won't get it out. Most RPCs are underdesigned and are on circuits too small except those supplied by RPC MAN as his are correct.
If you look at the calculations for single phase horsepower versus three phase horsepower you will notice a factor of 1.732 (square root of 3). When you operate a 3 phase motor (with a RPC), from a single phase supply, you must have 1.732 times the three phase nameplate amps available in your single phase circuit so that you have the same amount of power. Power out can not be more than Power In.
In general: a 5 hp RPC needs a minimum circuit of 30 amps but I prefer to build them for 40 amps
a 7.5 hp needs a minimum of 40 amps but I build them on 50 amp ciruits
A 10 hp needs a minimum of 50 amps but I build them on 60 amp circuits.
I design RPCs to run motors as large as the idler and sometimes have to do so.

toolnut,i follow your science,I responded to idle current.
Quote:
"Most RPCs are underdesigned and are on circuits too small except those supplied by RPC MAN as his are correct"

I do this every day,either repair or rebuild units from all over the country,comercial or home built.I do this hands on everyday..

And i do the 50 questions,all nameplate data,full loaded amp draws on machines fully loaded,then wire size,distance of runs,what will be ran at the same time.its just the begining of what i ask.sometimes i insist another machine is left to run in idle.

Many of the systems are running a 15 h.p. load from a stout 5 h.p. r.p.c. ideler. with no problems.and some times even more...there are so many things that vary from application to application.

my point is i work from the back of the plant to the front,and size the service size last.

Scott