Rotary phase converter idler oversize

D

DukeOfSavoy

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Looking for an idler motor to supply the 7.5hp 3phase motor on my monarch 10ee, and I think most recommendations say to double the load hp for the idler. My question is this: would it be problematic to use a motor larger than the doubled hp? There is a 25hp motor locally for cheap and I was wondering if i would run into problems with that large of a motor? Would the 25hp draw the full load amps if the other motor draws much less at full load?
 
You might not want to idle a 25hp motor on single phase, it will be woefully inefficient.

A better plan may be utilizing a VFD from single phase input to 3 phase output

The VFD will directly control the motor
 
25hp wouldn't be too big but as Mike said the bigger motor will have more efficiency loss. A 15hp might be a better fit, maybe even a 10hp could work if you can start the lathe motor without any load on it, ie a clutch.
The efficiency is also related to how well you balance the capacitors, I'd think you could probably get it under 10% loss.
 
For a lathe, a 10hp idler is sufficient for a 7.5 hp motor. The lathe motor is not heavily loaded upon start up.
I used a 7.5 hp RPC to run my entire shop for 6 years.
I often started unloaded 10hp motors for testing purposes on the 7.5 hp RPC.
I don’t recommend the practice to others as your results may differ ( I didn’t care if i blew my RPC up, i could easily make another)

I was able to safely run multiple 3phase motors from the RPC simultaneously
 
I disagree. The inrush on my 8 hp BAM Brazil lathe motor is about 18 hp. Apparently, Brazilian motors are known for higher than normal inductance, so I'm running my second RPC on a 20 hp system. I'd say for 7.5 hp on an asian motor that a 15 hp RPC build is warranted. You don't have to take my word for it, use an online calculator from an RPC mfgr and see what comes out. Load under startup is only part of that equation, inrush is the real issue at play.
 
I guess we disagree
I didn’t experience any failures or loss of equipment so I don’t see the need to double overkill
 
I have a North America phase converter and just took a look at their website as they have sizing calculators, etc. They list their Pro Line 10hp model as being appropriate for 7.5hp loads that are easy starting. Since we're talking about a 10EE I'm going to assume it has a quality motor that's working properly. Short version...I wouldn't hesitate for a minute to go with a quality 10hp phase converter for the 7.5hp motor on a 10EE.
 
A ten horse would be fine. If you build a 25, I assume you would build it so it is capable of using the full 25. So there are larger wires, more capacitors, larger contactor and generally just more expense. I have a 2hp and a 25hp. The 25 is just for the welder, I have started a 3hp unloaded motor with the 2hp converter.
 
I have made over 60 RPC's and Static Convertors , but have never run or worked on a 10 EE.
Using a 10 HP Idler for a 7 1/2 Hp motor should do fine but let me explain
One of the considerations that most folks ignore is whether the lathe has a clutch or not (important)
If your lathe has a clutch , then it will always start without a load ( assuming the operator knows good practice) and
not have any issues with a 10 HP for the idler . If the Lathe does not have a clutch then starting loads increase
and reflect the work at hand, like trying to start in high speed with a 20 pound chunk of steel in the chuck.

Now, I did have a problem one time with a WEG motor, made in Brazil ,
that was a tough cookie and was in a Nardini lathe and was 2 speed 7 1/2 HP motor.
It also had a auto-brake and no clutch. I found the difference in load amps between low and high speeds as incredible.
I think a 25 HP idler is way overkill , and stay away from WEG motors
Don't forget, that you can add some running Caps on the slave motor in addition to the RPC to get balance

Rich
 
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