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Large rotary phase converter Q.


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  #1  
Old 01-30-2013, 08:05:23 PM
930dreamer 930dreamer is offline
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Default Large rotary phase converter Q.

I've built a few rotary phase converters (RPC) and currently have a 10hp running the shop. I want to install a 30hp RPC and power a three phase panel (have it already) as this will cover all future requirements.

My plan is to add an 100 amp 240v circuit off the shop panel to start the 30hp idler. The current 100 amp disconnect I have is only rated for 15hp on single phase/ 30hp on three phase. 200 & 400 amp disconnects are rated for 15hp max also. Wire will copper 1/0 gauge that I already have.

I can start the 30 hp idler with a pony motor to reduce the inrush current to nominal amps. I know this setup is vague with NEC codes and want everything correct. but I'm having some trouble finding a solution. Thank you.
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Old 01-30-2013, 09:51:22 PM
Mike Dennis Mike Dennis is offline
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Default Re: Large rotary phase converter Q.

According to the chart I looked at, 100 amp circuit willl be inadequate for 30 HP.
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Old 01-30-2013, 10:20:42 PM
SeymourIron SeymourIron is offline
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Default Re: Large rotary phase converter Q.

I would look for a combination motor starter. This will come with the disconnect and overload protection for the RPC built in, this will also handle the start and stop issue which is why your switch is only rated for 15HP as the contacts in the starter are rated to handle the arcing of dropout under load. The switch portion of the combo starter is strictly for safety and lockout for servicing. The fuses in the switch portion will protect the wiring and circuit, while the overload heaters will protect the motor.

Bill Jarvis
WJ Electric LLC
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Old 01-30-2013, 10:27:16 PM
930dreamer 930dreamer is offline
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Default Re: Large rotary phase converter Q.

The idler (30hp) will pull about 30 amps as an idler.
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Old 01-30-2013, 10:41:18 PM
Mike Dennis Mike Dennis is offline
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Default Re: Large rotary phase converter Q.

Yes, but all the three phase motors it operates will use at least their name plate amps times 1.73 in addition to the current for the idler. All together it is likely too much for your 100 amp source.

I agree on the motor starter idea. Any RPC should use start / stop type control to prevent an automatic restart after power failure, not just turn on a breaker to start it. A starte also leads to much better breaker life as the breaker does not close under start-up conditions. NEC starters are vastly better than IEC stuff!

On mine, I go a little further and have an output contactor that is operated by a timer from the derived phase. Only about 15 seconds but idiot proofs the RPC and 3 phase machines from trying to start on single phase. Works great for me.
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Old 01-30-2013, 10:46:56 PM
930dreamer 930dreamer is offline
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Default Re: Large rotary phase converter Q.

Quote:
Originally Posted by SeymourIron View Post
I would look for a combination motor starter. This will come with the disconnect and overload protection for the RPC built in, this will also handle the start and stop issue which is why your switch is only rated for 15HP as the contacts in the starter are rated to handle the arcing of dropout under load. The switch portion of the combo starter is strictly for safety and lockout for servicing. The fuses in the switch portion will protect the wiring and circuit, while the overload heaters will protect the motor.

Bill Jarvis
WJ Electric LLC
I'm trying to grasp all of this, I have a control panel(salvaged) with a nema 2&3 magnetic starter, transformer etc.
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  #7  
Old 01-30-2013, 11:03:41 PM
SeymourIron SeymourIron is offline
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Default Re: Large rotary phase converter Q.

I love the timer idea with the contactor.

Also, for anyone running three phase air compressors on phase convertors, avoid using the "cheated" phase in the pressure switch as the compressor won't start. Use the two positive phases as I have had this trouble call before and people were about ready to throw their phase convertors in the trash
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Old 01-30-2013, 11:06:40 PM
Mike Dennis Mike Dennis is offline
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Default Re: Large rotary phase converter Q.

The #3 starter would be marginal, IMHO.
A single phase 15HP motor requires a #3.
A 3 phase 30 HP requires a #3.

Now, with a pony motor for starting, if the RPC is brought to full RPM before applying power to it then it is more like part winding start. There will still be a short duration high current until the RPC synchronizes but not nearly as long as a normal start. A loose belt to the pony is a good idea. Once the RPC is running, it is generally best to disconnect power to the pony.

I am not a real pony motor expert as everything I have built is 15 HP or less - and uses start capacitors and across the line start.



Are you REALLY going to have 30 HP of three phase motors on line at once? More correctly, are you going to start a 30 HP three phase motor? The RPC needs only be as large as the largest single phase motor you are going to start. The sum HP of all three phase motors on line at once can exceed the idler size by quite a bit. Fact is, an idler can be SMALLER than the largest motor to be started IF it starts unloaded.

As an example, I ran my 3 HP clutch type lathe on a 2 HP RPC for a couple years. Only difference I noticed when I went to a 5 HP RPC was the lathe motor would start a little better. Even on the 2, my lathe and Bridgeport could run together just fine. If the Bridgeport was already running, the lathe started about the same a it now does on the 5 HP RPC.
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Old 01-30-2013, 11:14:15 PM
930dreamer 930dreamer is offline
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Default Re: Large rotary phase converter Q.

The answer is no, I've put two in parallel (current 10hp and a 7.5hp). I plan to add my industrial Kellogg American 7.5 hp air compressor in the future for a blast cabinet. I know I'll need a bigger circuit if I use a larger rpc.
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Old 01-30-2013, 11:23:23 PM
Mike Dennis Mike Dennis is offline
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Default Re: Large rotary phase converter Q.

Seymore Iron has a GREAT point!! (and I love the handle!)

NEVER use the derived or artificial phase for ANY single phase load. Yep, I violate the rule with my timer but the current is tiny and seems to cause no trouble. I chose that method so I could verify the derived phase was probably present.

Same as the wild leg on a delta system - the one that is 208 to ground - it should NEVER be used for a single phase load!

An air compressor is a demanding load because even "unloaded" there is a lot of inertia to overcome.

BTW, never apply an RPC to a hermetic refrigeration or AC compressor - they demand 4% or less phase imbalance and will go belly up with more in most cases.

---------- Post added at 10:23 PM ---------- Previous post was at 10:14 PM ----------

If you are using a 10HP and a 7.5 HP three phase motor on a 10 HP RPC all should work fine. To add another 7.5 HP might push it but a 15 HP RPC would probably be plenty.

Not the only manufacturer but here is a wealth of information: http://www.arco-electric.com/Sizing.aspx
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Old 01-30-2013, 11:29:51 PM
930dreamer 930dreamer is offline
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Default Re: Large rotary phase converter Q.

I added a 7.5 hp idler to my current 10 hp rpc. I have a pallet of three phase motors, 30 hp being the largest. I have a 15hp from my Ingersoll Rand compressor. I can't stop messing around with these.
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Old 01-30-2013, 11:58:03 PM
Mike Dennis Mike Dennis is offline
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Default Re: Large rotary phase converter Q.

That 15 HP with a lot of oil capacitors may work just fine for all three of your loads.

Rule of thumb: If motors START Ok and none runs HOT you are probably OK. If breakers or overloads trip, promptly look into the cause as it may be serious. Put an amprobe on derived leg at the idler and see if it exceeds name plate rating of the idler.

RPCs can mess with selection of motor starter heaters - expect it!

I prefer old, slow motors for RPC as they have more iron and copper, both of which are your friends. Admittedly, some motors serve in RPC idler service better than others.

KISS is certainly VERY important. I also like things automated so I can't screw up. Even more important is safety so no one else screws up. Not only to protect you and yours from shock and burns but also protect your machinery from damage.

Here is another link you may find useful: http://www.cerusind.com/catalog/docu...ngFormulas.pdf
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Old 01-31-2013, 09:32:21 AM
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Culy Culy is offline
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Default Re: Large rotary phase converter Q.

The RPC (30hp) at the farm to run the grain leg worked good for quite some years, but as it was added on to it had to be upgraded. Now it has a 50hp and a large panel full of capacitors and a large load tranformer sitting just outside the leg control room. Kinda scary when you open it up. The electricion that wired it did a nice job of running the connections. I noticed it doesn't have an idler motor for starting, it sure pegs the amp meter on the control panel when starting. Would adding a idler help with this?
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Old 01-31-2013, 10:26:21 AM
Mike Dennis Mike Dennis is offline
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Default Re: Large rotary phase converter Q.

Leave it be!

The rotary converter IS the idler. A "pony motor" is used to start a primitive RPC that does not have start capacitors and relay. They CAN reduce starting current but make start up more complex. In a situation where the available source is inadequate like 930 Dreamer has, the lower inrush CAN be important.

Culy, you probably have a commercial converter like a Roto Phase or other brand. They are really a large 3 phase motor with a capacitor bank - internal for smaller machine, external for larger, like yours. The cost of that short period high starting current is, for all intents and puposes, nil. Most commercially made converters do not have a shaft extending outside the machine anyway, as a safety meansure. Home - made ones often do but it should be shielded in some way.

Billing is power over time (Kilowatts per hour) and the time is too short to matter. Demand, sometimes called "ratchet" is normally not applied to farms, at least around here. It is integrated over a shorter time, usually 15 or 30 min. Shows on bill as KW and IS expensive. However, again, 6 seconds, even of only 15 min period doesn't amount to much. It is actually smaller than the running load in KW so will have no effect on billing.

In other words, start load is too short to cost you any money, contrary to a lot of old wives tales!!
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Old 01-31-2013, 10:47:35 AM
930dreamer 930dreamer is offline
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Default Re: Large rotary phase converter Q.

Can I safely use the 100 amp disconnect for the 30 hp idler?
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Old 03-19-2013, 10:31:37 PM
930dreamer 930dreamer is offline
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Default Re: Large rotary phase converter Q.

Here's the 100 amp disconnect wired up and the 30 hp idler under power. Click the motor pic for the video.




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Old 03-20-2013, 06:34:24 AM
Rick McKay Rick McKay is offline
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Default Re: Large rotary phase converter Q.

I have to do an interesting exercise in cold weather to start my iron worker (5 hp). My 5 hp idler starts with a relay that drops out some of the cap's after starting. I then start the 7.5 hp turret lathe (starts easy because it has a clutch), then I start the big 10 hp lathe (also has a clutch). With the three motors humming, I am now able to start the ironworker at 10 degrees. It is quite a load to start a hydraulic pump with 10 degree oil. In the summer it will start right up with just the 5hp idler.

Maybe I should move south.
3' snow drift outside my door right now from last night's snow. Worst storm of the year, and today's the first day of spring!

Rick
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