wiring run caps in RPC

Hi Investigator,
(all dependent on load and service voltage!!!)
Answer:1- you will have 2 different capacitor values when balancing a RPC.
phases A and B will be supplied voltage (for this purpose) their running voltage is 240vac between poles
phases A and C will have one supplied leg and one generated leg, their running voltage may be as low as 200vac between poles
phases B and C will have a generated leg and a supplied leg, their running voltage may be around 226vac between poles
if you were to treat phases A-C, exactly like Phases B-C, you would throw the motor back out of balance by application of too much capacitance on the B-C phase that may only require 30 or 40 mF of balancing capacitance.
phases A-C in this scenario would require the most capacitance maybe as much as 80 or 100 mF (all dependent on load and service voltage and rotation!!!)
.

Well, got everything wired up and running. I didnt put any run caps in, only using the idler motor. I turned it on and took some readings with my meter, hopefully folks can tell me if and what I need to do to make improvements.

With just the idler on, and no other motors i have:

ground/com -L1 123Vac
Grnd/com-L2 124Vac
Grnd/com- L3 170Vac
L1-L2 249V
L1-L3 220V
L2- L3 208V


When I turned on the lathe (Logan 12") and took readings at slowest and fastest speed, the readings between l1-l3 and l2-l3 were the same as without the lathe on.
 
you did everything correctly!!! :aok:
those are normal readings for a small unbalanced RPC idler in motion @240vac
your lathe could run like this and it wouldn't cause catastrophic problems.

use L1 and L2 as the power legs to transformers or the control circuit, the generated leg is not suitable for these circuits
 
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You could add some run capacitance across L2-L3 to try to balance the voltage a little better.
 
You might take a look at this thread
http://www.hobby-machinist.com/threads/another-rotary-phase-converter.63204/#post-520986

The capacitors are roughly calculated by this formula:
CL1-L3 = 4 to 5 times the motor horsepower = 75 (20+20+10+10+10+5+5+5+5)
CL2-L3 = 10 to 15 times the motor horsepower = 180 (60+60+20+20+10+10+5+5+5+5)
PF COR = (10+5+5+5+5)

This is not to say that I am correct in the way I'm doing this. We'll see later this week when I fire it up :cautious:

This is the way my 10hp rotary phase converter is wired and it has been running great for the past 20+ years. All 3 legs are within a few volts of each other. If L1 - L3 = L2 -L3 I don't think the legs would be balanced very well. I recall trying about 3 capacitance values each for L1 - L3 and L2 - L3 before the voltages were in a range I felt comfortable with. I spent longer finding a value for the start capacitor that started the idler in a reasonable period of time without a massive start current.
 
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I need to make sure understand, since I am about to wire things together.
I have a 7.5hp idler motor for my soon-to-be-built RPC. My questions concern the run cappacitors

Question 1: When sizing the run caps to help balance the 3rd leg, my calcualtions show I need between 90 and 120 mF. Does that mean I need that value per leg or total. For instance, I have 2, 50mF run caps and plan to put one between L1-L3 and the other between L2-L3. Is this correct that it will give me the 100mF I need, Or do I need 100mF on each leg?

Question 2: My run caps are duel run 50/5mF. Where to plug wires? From L1 to Com on cap, out HERM on Cap. to L3?

I'm going to come in from left field here and totally mess this up. For one thing, I never was comfortable with the notion of two-legging a 3-phase motor for purposes of deriving three phases. It is inefficient, taxes the devil out of three-phase motor, incurs incredible complication wrt power factor capacitors, and in general - does not make sense. I say it does not make sense because the typical RPC uses two motors anyway - a drive motor which everyone refers to as an Idler, and the larger 3-phase motor itself. If you have two motors to begin with - why not run a motor-generator arrangement where the single phase motor drives the 3 phase motor through a mechanical coupling and the rating of the system is limited by the drive motor rating. Using this arrangement, you do not need power-factor correcting capacitors, but if you want to be dandy, you can go ahead and use them anyway to keep your currents low. I just did a design for a client with a 5 hp 3-phase Rix compressor using this MG arrangement and two 5 hp motors (a 1-phase and 3-phase) and it works without need for pf compensation. Granted that it is compressor duty over continuous, but you folks are not running NEMA design C motors in your lathes.
 
3ph, or any induction motor, does not have any permanent magnets. Don't you need to excite the armature coils some how so they produce a magnetic field that will induce a current in the primary windings? Then wouldn't you also need some mechanism to regulate the output voltage? Seems like there would be more involved than just spinning a 3ph motor with an engine or another motor?
 
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I think we are going in several different directions here. Jacque: The "typical" RPC is comprised of only one motor called an "idler". The synthetic 3 phase output produced from this idler is used to power the lathe or milling machine motor in question. The idler usually has a horsepower rating equal to or larger than the load it powers. Some RPC installations have an idler so large and with so much mass that a second smaller "pony" motor is employed to help get the idler up to speed and reduce the huge startup current surge. This pony motor is then disconnected after a few seconds and freewheels with the idler.
As far as I understand it, a motor/generator system like you mentioned is less efficient than the RPC scheme, hence they are not used much anymore. The RPC is rugged, relatively inexpensive and decently efficient.
Mike: I believe a 3-phase motor does not make a decent generator due to the lack of excitation. The alternator in your car requires a small amount of excitation from the car battery or there would be no output. 3-phase ac alternators have either permanent magnets (as you mentioned) or a separately excited field winding.
Some generators operate from "residual magnetism" in order to self-excite, I believe these are DC units although there may be AC ones too.
Mark
 
3ph, or any induction motor, does not have any permanent magnets. Don't you need to excite the armature coils some how so they produce a magnetic field that will induce a current in the primary windings? Then wouldn't you also need some mechanism to regulate the output voltage? Seems like there would be more involved than just spinning a 3ph motor with an engine or another motor?

You are on target and excitation is necessary. In your standard RPC, excitation is derived from the external single phase applied that also serves to rotate the motor. Typically, capacitors are used in parallel with the armature to offset the lagging vars just as in pf correction. High-efficiency motors will usually self-excite without need to momentarily overspeed the motor. However, regulation can be problem with the biggest problem being overvoltage if the system is not tuned for a specific load or if the load is changed out. In retrospect, the MG arrangement would indeed be more involved and more complicated than your conventional RPC.
 
I think we are going in several different directions here. Jacque: The "typical" RPC is comprised of only one motor called an "idler". The synthetic 3 phase output produced from this idler is used to power the lathe or milling machine motor in question. The idler usually has a horsepower rating equal to or larger than the load it powers. Some RPC installations have an idler so large and with so much mass that a second smaller "pony" motor is employed to help get the idler up to speed and reduce the huge startup current surge. This pony motor is then disconnected after a few seconds and freewheels with the idler.
As far as I understand it, a motor/generator system like you mentioned is less efficient than the RPC scheme, hence they are not used much anymore. The RPC is rugged, relatively inexpensive and decently efficient.
Mike: I believe a 3-phase motor does not make a decent generator due to the lack of excitation. The alternator in your car requires a small amount of excitation from the car battery or there would be no output. 3-phase ac alternators have either permanent magnets (as you mentioned) or a separately excited field winding.
Some generators operate from "residual magnetism" in order to self-excite, I believe these are DC units although there may be AC ones too.
Mark

You are correct, the standard RPC is far more reliable and less complicated than what I had proposed. Aside from the extra efforts needed to induce excitation, the voltage regulation in response to varying loads would introduce far more complexity than needed.
 
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