# Need Help On Converter Choice



## chrishoag (Jan 12, 2017)

Ok I bought a leblond 15c lathe a few weeks ago. It has a 7.5 hp motor on it. My question is what is the best way to get 3 phase to this machine? I have 100 amp service in the garage. Static converter? RPC? Change the motor out for a smaller one? I have a used FUJI VFD that I can also use. I have found a 25 hp RPC for a good price but I'm afraid that it will be too much to start on my 100 amp service. What kind of startup amperage will this draw? I may purchase a compressor, or a bandsaw somewhere down the road so that has me leaning toward a RPC. 

Suggestions?


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## markba633csi (Jan 12, 2017)

Chris: The startup amperage might be a little much for 7.5hp on 220 volt/100 amps service- I'm not totally sure though. Myself I would try the Fuji if it has the capacity. 
Mark S.


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## randyjaco (Jan 12, 2017)

Stay away from the static converter and go with the RPC.  For multiple devices the RPC is the only way to go. 

Randy


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## tq60 (Jan 12, 2017)

We have a L&S 16 X 54 with 7.5 hp motor and spc.

Came with it and person converting failed to install larger heaters in motor protector, being gear drive it trips protector while running unloaded on higher speeds due to oil friction. 

A 7.5 hp motor on a VFD would require a substantial one.

If you already have a vfd rated for 7.5 hp with single phase input that would be best option as you already have it.

A RPC can operate multiple machines but a large one is large $$ unless you make one.

If time or budget is short the spc can get you by for temp.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk


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## JimDawson (Jan 12, 2017)

If the Fuji VFD is at least 10HP, it should work.  I recently installed a 20HP RPC on a 100 amp service, it dims the lights on startup.  Not sure what a 25 HP would do.  If you spun it up with a pony motor before switching it on then the start current would be much less and then should work fine.


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## Keith Foor (Jan 12, 2017)

A 25 running a cap start is really gonna tax a 100 amp service.  Couple options. 
First option is a pony motor start.  First out what the motor is rated in RPM then setup a single phase motor with the same RPM and a one to one drive to spin the motor up.  Once spun up just turn it on.  Remove the start circuit in the RPC and you will be golden.  I run a 15 HP RPC on a 50 amp circuit and start it with a 3/8 drill motor in the same fashion and it has never given me issues.  I do this because the motor is fully sealed and is class 1 explosion proof rated.  Meaning the 15 HP motor is physically the size of a 75 HP open frame motor.  

Second option.  
If the 25 HP RPC is reasonable enough in price buy it and then switch out the idler motor with a 15.  You will need to remove some of the run capacitors to get the thrid leg voltages down and correct but that's an option

Third option is build one from scratch.
This is MY personal preferred method but I build the things all the time to fund machine purchases.  
 There is really no black magic in one.  If you have knowledge or 3 phase wiring and basic industrial controls then an RPC is cake. 

Now a couple things you should be hunting for if you are building anything over 10 HP and planning to have more than one machine on it. 
The biggest thing is a 3 phase panel.  A sub panel without a main breaker is fine.  The breaker feeding the RPC will be fine for safe control of the output.  You can then wire proper outlets to the panel with independent breakers for your equipment. 

Good luck and remember to ask questions on here as you progress if you need help.   Several others and myself are always willing to answer RPC and wiring questions.


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## JohnG (Jan 15, 2017)

Very good, practical advice from Keith.  For a while my convertor was a pair of 7.5hp motors run off a 50 amp circuit.  A standard starter circuit got one going, and then a power relay on a time delay brought the second motor in a few seconds later.  This avoided the very high starter current required for a single big motor.  The combined pair would power the start up of a 5hp air compressor while other machines were running under full load without the voltage falling  to where a motor starter on a running machine would drop out.  The motor pair also seemed to have better balanced voltages.  I first noticed this effect when I had 2 other guys working with me.  All the tools seemed to run smoother when several were going at once.

There have been several well written posts on Hobby Machinist about using VFDs with older motors.  The output is not a smooth waveform, and the rapid voltage spikes in it can damage the windings.  I could not improve on the clear explanations I've read.  They are worth finding.


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## Ulma Doctor (Jan 15, 2017)

both motor starters and true RPC's can easily be made from spare components, rather inexpensively i may add.
the VFD is an excellent option as well, that comes with the added bonus of frequency control.
this bonus doesn't come free.
older (USA) motors were not designed to operate at varying frequencies. they were designed to run on 60Hz, supplied in North America.
when the windings are given frequencies above or below 60 Hz, the motor windings take the brunt of the assault through higher winding temperatures.
the fans are generally insufficient to cool the overexcited windings below 30 Hz or above 90 Hz.
i have run scores of old motors on new VFD's in the specified envelope with very minimal casualties
i work with motor controls, & controllers in harsh environments, water has caused more failures than frequency deviation in my experience, 
your experiences may differ


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