# Amp pull at startup



## Phils69 (May 11, 2014)

Hello again, I used your guys help a couple of years ago to put together a RPC based on a 7.5 idler and it runs great. I am buying a house next month and need to upsize my RPC to accommodate some larger machines which will following me there. The largest machine motors I have right now is a power hacksaw, mill and a lathe with 5 hp each. I went with a friend to buy a shaper last week and saw a converter there which I bought. It's a shop made balanced converter with a 30hp idler. The shop I am moving to has 200 amp service. Will this be enough for the startup pull of the 30hp motor?


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## John Hasler (May 11, 2014)

Phils69 said:


> Hello again, I used your guys help a couple of years ago to put together a RPC based on a 7.5 idler and it runs great. I am buying a house next month and need to upsize my RPC to accommodate some larger machines which will following me there. The largest machine motors I have right now is a power hacksaw, mill and a lathe with 5 hp each. I went with a friend to buy a shaper last week and saw a converter there which I bought. It's a shop made balanced converter with a 30hp idler. The shop I am moving to has 200 amp service. Will this be enough for the startup pull of the 30hp motor?



Barely.  Be sure the water heater and oven aren't on when you start it.  Are you sure you need that much?  What's the biggest motor you intend to run?


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## Phils69 (May 11, 2014)

I probaly wont need one that big but knowing me I might one day. It's a detached garage and it has its own 200 amp panel and the house has its own as well. Not sure how that works exactly but shouldn't the garage panel be able to supply 200 amps on its on?


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## John Hasler (May 11, 2014)

Phils69 said:


> I probaly wont need one that big but knowing me I might one day. It's a detached garage and it has its own 200 amp panel and the house has its own as well. Not sure how that works exactly but shouldn't the garage panel be able to supply 200 amps on its own?



Yes.  I suggest that you reserve the existing panel for single phase loads.  Add a 100 amp breaker fed directly from the service to supply the RPC and then install a three-phase panel supplied by it for your three-phase loads.   Put a 100 amp main in the exisitng panel.


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## Phils69 (May 11, 2014)

Sounds like a plan. I have a big 3 phase panel I'll be using for all the 3 phase machines. I'll try to remember and post a pic once it's installed. Now the question would be should I use the 30hp RPC for everything or only the larger motors and the 7.5hp RPC for everything else? I could run the 7.5 to supply the big 3 phase panel to meet most of my needs and the 30 to a small panel just for the larger motors? Would there be enough difference in operating cost to justify doing it that way?


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## TOOLMASTER (May 11, 2014)

Figure 2x running amperage to start


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## JimDawson (May 11, 2014)

Phils69 said:


> Sounds like a plan. I have a big 3 phase panel I'll be using for all the 3 phase machines. I'll try to remember and post a pic once it's installed. Now the question would be should I use the 30hp RPC for everything or only the larger motors and the 7.5hp RPC for everything else? I could run the 7.5 to supply the big 3 phase panel to meet most of my needs and the 30 to a small panel just for the larger motors? *Would there be enough difference in operating cost to justify doing it that way?*




There might be.  The no-load current for a 30HP motor @ 240V is 27 Amps, that's about 6.5 KW at idle, but that is running on 3 Phase.  I have no idea how to calculate the no-load current in a RCP configration.  Nor am I sure what the power meter would actually see as "real" power, due to the regeneration effects.

Once the system is operational, it would be interesting to take some power measurements (with an actual power meter) to see what is really happening.


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## John Hasler (May 11, 2014)

TOOLMASTER said:


> Figure 2x running amperage to start



Right, but the startup transient won't trip a 100 amp breaker.


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## xalky (May 11, 2014)

Phils69 said:


> Sounds like a plan. I have a big 3 phase panel I'll be using for all the 3 phase machines. I'll try to remember and post a pic once it's installed. Now the question would be should I use the 30hp RPC for everything or only the larger motors and the 7.5hp RPC for everything else? I could run the 7.5 to supply the big 3 phase panel to meet most of my needs and the 30 to a small panel just for the larger motors? Would there be enough difference in operating cost to justify doing it that way?



I just upgraded my 3hp RPC to a 15hp RPC. I can run the smaller 1 1/2 and 2 hp machines on it without a problem. The 15hp RPC does put out some heat compared to the little 3hp that I had. I agree with putting the 3 ph output to a seperate 3 phase panel.

At idle with no load on the RPC, the only difference will be the extra friction encountered in the larger RPC, so that would be the difference for extra draw at idle.

As far as amp draw goes, Lets say that the amp draw from a machine is 30 amps, It'll be roughly the same whether its single phase or 3 phase. The only difference is that in 1 phase circuit the amperage gets divided between 2 legs (15 amps per leg),and on a 3 phase circuit the amp draw would be divided up by 3 legs (10 amps per leg), assuming the voltage is the same. which means that you can run a smaller wire theoretically to the machine on the 3 phase side. The 3 phase jolt on startup of the large motor does draw quite a bit more than running load, for this reason I ran some 8 guage wire (mainly because I had it laying around) to my 10hp lathe. The previous owner only had 12 guage running to the machine and on startup, especially when put into high gear, the machine struggled to ramp up. At my house with the 8 Guage wire it ramps right up with no struggle.

If it was my place, I'd just run the 30hp RPC for simplicity sake, though it might cost you a little bit more in electricity. I have no idea how much, but there's the overcoming inertia on startup on the larger RPC plus the additional friction.

Marcel


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## Ray C (May 12, 2014)

Hmmmm...  30 HP is a lot.

1 HP is 746 Watts and I usually use 750 to round-up the numbers.  Also, Watts (power) is equal to Voltage x Amps.

30 x 750 = 22,500 Watts

22,500 = 240 x Amps; therefore, Amps = 93.75.  ... That's a lot of current and for reference is about 2.5 times as much as a central air conditioning unit in a typical 3 bedroom house.  1 Watt is equal to 3.4 BTU so, that's the equivalent of a 76,500 BTU heating or cooling device.

Those Amps are when the motor is pulling the full 30 HP -which hopefully doesn't happen all that often.

Anyhow, under full load the power from the meter going into the RPC will be at least this much plus any inefficiencies from the PRC itself.  The best quality motors are about 95% efficient and typical values range anywhere from 75 to 85%.  I would roughly estimate a 10-15% loss in the RPC so, the actual power coming out of the meter is right around 100 Amps (give/take a little).

The startup current on a motor can be quite high -especially if there's a load.  An unloaded motor has a very brief draw of about 1.5 x the rated current and something like a compressor motor (which usually has a high load) can be upward of 3 to 4 times the rated current...

In your shoes, yes, I'd get a dedicated panel box for this and depending on how much you load the machine, you may need to look into "slow-blow" type breakers or possibly a VFD to use as a "slow-start" device if at all possible.

Ray


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## ironmonger (May 12, 2014)

xalky said:


> <snip>
> As far as amp draw goes, Lets say that the amp draw from a machine is 30 amps, It'll be roughly the same whether its single phase or 3 phase. The only difference is that in 1 phase circuit the amperage gets divided between 2 legs (15 amps per leg),and on a 3 phase circuit the amp draw would be divided up by 3 legs (10 amps per leg), <snip>
> Marcel



Current is a series circuit is equal. The 220V circuit must supply 30 amps, the current in each 'leg' (see * below) is 30 amps. The neutral is irrelevant in this example as the voltage is measured between L1 and L2 of the mains circuit, but the equivalent would be two 110v circuits in series, in each of which 30amps would flow and would be 220V at 30 amps when series connected.

paul

*(the 'legs' represent L1 and L2, which are the secodary winding leads of  the distribuition trnasfomer that sits on the ground or or your pole)


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## JimDawson (May 12, 2014)

Ray C said:


> Hmmmm...  30 HP is a lot.
> 
> 
> In your shoes, yes, I'd get a dedicated panel box for this and depending on how much you load the machine, you may need to look into "slow-blow" type breakers or *possibly a VFD to use as a "slow-start" device if at all possible.*
> ...




I remember way back an old guy(about my age now I suppose) ran his 3 phase machines from a single phase supply.  He did have a RPC that consisted of a large 3 phase motor.  The difference was that he used a small single phase motor to spin up the 3 phase motor before applying power to it.  Had it on a rocker mount with a belt drive and a foot pedal, start the motor, apply foot pressure.  Seemed to work pretty well, and eliminated the start up load.  I guess this is a pretty good way of doing a soft-start.


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## Phils69 (May 12, 2014)

Thanks for all the feedback guys. I know it's more than I will probaly ever need but It was one of those deals that I felt like I couldn't pass up. I was thinking of the same soft start idea using a small motor to spin it up to operating speed then hitting the breaker on the RPC. I'll probaly try it if I have any issues. My friend has a 20 hp idler on his and is running it off of 100 amp service and it starts fine so maybe the 30 won't be too bad on the 200 amp panel. Btw I went to a one man shop in VA last year that was closing and he had some big machines, full size Carlton radial and a King VTL, that he was running off a RPC. It was a dubious looking setup but he was running a 60hp idler, it was an old motor also so it was huge! I would love to have heard that one fire up. I am scheduled to close on the house end of June so it will probaly be end of July or August before I have the 30hp RPC running. I'll do a temporary setup with my 7.5 so I can put power to my small mill and lathe. I feel naked without them operable LOL. On another note I am thinking of mounting the RPC outside of the building, along with a air compressor,  under a attached shelter behind it. Other than possibly having to upsize the gage wire for the extended run would there be any other issues to worry about? Would the moisture be too bad for it? I'd sure like to have as little noise as possible but I could put it up in the rafters if I have to have it inside.


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## John Hasler (May 12, 2014)

JimDawson said:


> I remember way back an old guy(about my age now I suppose) ran his 3 phase machines from a single phase supply.  He did have a RPC that consisted of a large 3 phase motor.  The difference was that he used a small single phase motor to spin up the 3 phase motor before applying power to it.  Had it on a rocker mount with a belt drive and a foot pedal, start the motor, apply foot pressure.  Seemed to work pretty well, and eliminated the start up load.  I guess this is a pretty good way of doing a soft-start.



That's a reasonable way to start an RPC.  If the motors are matched for speed there's no need to disconnect the single phase one.  Use a time delay relay or centrifugal switch to apply power to the three-phase machine.


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## John Hasler (May 12, 2014)

Phils69 said:


> Thanks for all the feedback guys. I know it's more than I will probaly ever need but It was one of those deals that I felt like I couldn't pass up. I was thinking of the same soft start idea using a small motor to spin it up to operating speed then hitting the breaker on the RPC. I'll probaly try it if I have any issues. My friend has a 20 hp idler on his and is running it off of 100 amp service and it starts fine so maybe the 30 won't be too bad on the 200 amp panel. Btw I went to a one man shop in VA last year that was closing and he had some big machines, full size Carlton radial and a King VTL, that he was running off a RPC. It was a dubious looking setup but he was running a 60hp idler, it was an old motor also so it was huge! I would love to have heard that one fire up. I am scheduled to close on the house end of June so it will probaly be end of July or August before I have the 30hp RPC running. I'll do a temporary setup with my 7.5 so I can put power to my small mill and lathe. I feel naked without them operable LOL. On another note I am thinking of mounting the RPC outside of the building, along with a air compressor,  under a attached shelter behind it. Other than possibly having to upsize the gage wire for the extended run would there be any other issues to worry about? Would the moisture be too bad for it? I'd sure like to have as little noise as possible but I could put it up in the rafters if I have to have it inside.



The RPC should be fine in the shelter as long as it's protected from the weather.  It shouldn't be very loud, though.


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