Capacitance required to balance RPC
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  1. #1
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    Default Capacitance required to balance RPC

    Hello, I'm fairly new to the site and have been reading up on making a RPC.
    There are many plans out there for 5hp and up idlers. I'm using a 3hp 3 phase to power a 2hp Bridgeport Series 1 CNC. I'm going with cheap and easy as I'm not looking to make parts on this machine. I just need it to run so I can sell this one. I have another identical machine that will get a full rebuild. I don't plan on needing the RPC for that one.


    This is all for the joy of learning something new, so please don't be going off on me about wasting my time with junk! It's my time! I work full time managing a gage lab and enjoy making things work in my spare time. Cars, Jeeps, Bikes (street & dirt) 4 wheelers, Personal Watercraft, Arduino projects. Now CNC! Starting with an RPC.

    With that out of the way I'll get around to my project.
    I'm leaning toward the Fitch design.
    I have a Jacobs 3hp 3 phase motor wired for the low voltage setting.
    First I thought I would need a base reading so; one leg to 3-9, and the other to 1-7, with 2-8 to be the generated leg. Rope start the motor and throw on the power.

    generated leg to Black = 209V, to Red = 198V, to Neutral = 166


    Measuring the other two legs with it running read the stock incoming voltages of B/R = 233 and 113/119 to neutral.


    Since this is such a small idler I planned to use a SPP6 Hard Start kit as the potential relay and start cap. SPP6 Hard Start Capacitor for Supco Relay 1/2HP-1HP | eBay
    The run capacitors have me stumped. I've read that anything over 250V will work (preferably oil filled) but the amount/value of microfarads required to balance each leg would be helpful. All the balancing posts I've read are a little vague on how much cap is needed to raise a certain voltage. There's no store round here to just go buy caps, and I don't want to buy a pile of various sizes and "hunt & peck" to find what works.

    Does anyone have the values or the formula to balance these readings?

    The power running to my barn is 2 hot legs and a ground as neutral, with a separate ground rod staked at the barn. I was told this is common, but so is DEATH!
    I read of the control transformer trick to get a real neutral that's not also the ground and I made one up. Is this a valid method? Or should I look into running a Neutral to the barn?


    Thanks in advance for any helpful information. I take constructive criticism as helpful, especially concerning high voltage!

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    Bump it up!
    Anyone put together a 3Hp RPC? What run caps did it take?

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    it will depend on the idler..... There is not really a "universal generic" default value.

    You may not even need the balance capacitors, depending on the nature of the load. An oversized idler has less need, as the voltage drop is typically less with a big idler.

    In some cases, it may actually be better to use a boost transformer instead of balance caps. That more directly addresses what you really want..

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    Quote Originally Posted by JST View Post
    it will depend on the idler..... There is not really a "universal generic" default value.

    You may not even need the balance capacitors, depending on the nature of the load. An oversized idler has less need, as the voltage drop is typically less with a big idler.

    In some cases, it may actually be better to use a boost transformer instead of balance caps. That more directly addresses what you really want..
    At first RPC build, I played with balance caps until I collected quite a number of them. I had jumper wires galore!.

    Slowly, I started removing the caps, the latent danger, even with bleed resisters, concerned me. I fond nothing amiss with the absence of "balance caps". Start caps remained even if only re-purposed run caps. The motors I have would start themselves with a nudge from a foot.

    Now, with the advent of VFDs, I only keep one RPC, it's a three HP GE sleeve bearing motor that sits on the floor behind the mill. The mill has a two speed spindle motor, a Horizontal shaft motor, a table feed motor and a coolant pump. I thought that combination would be too much for my simple brain to fit a VFD.
    OH! Plus the same RPC powers the Wade lathe with PLUGGING for spindle reversal. Ouch!

    There are NO run caps in this set up. Starting of the idler is by a "Phase-o-matic" kit built "phase converter" which is nothing but a start cap and a starting relay (voltage senstive, AKA Steveco)

    The set up works PERFECTLY, and has for more than 20 years. Well, I can't say perfectly. Sometimes, when the mill spindle motor is set on low speed, and nothing else is energized. I need to hold the spindle OFF button for the count of two before the relay drops.

    I get around this by energizing the table feed motor, or the coolant pump, or both.

    An expedient to capacitors for balanced power from a RPC is added motors!

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    Thanks for responding.
    Quote Originally Posted by JST View Post

    In some cases, it may actually be better to use a boost transformer instead of balance caps.
    Now you got me thinking I may not need to balance the input to this old CNC. I can't find a print for this model, but the power goes directly into a three winding transformer that knocks it down to 80VDC and 24VDC per leg. Then straight to three more transformers with no values labeled on them.
    These old Bridgeport Textron CNCs have some serious power management in them, lots of transformers and big blue VDC caps.

    Would it damage anything to run the machine with a low voltage condition as described above?

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    The low leg will be "lazy"..... not supplying full current. So the others will have to pick up the slack. At low power, that will not be an issue, but could be if the machine is asked to produce full power for long. It is a sort of "single phasing" problem.

    The internal transformer is not a direct fix for that, As generally used, it just repeats the voltages that come in.

    IF you can re-set the transformer to different voltages per winding, in small increments of a few volts, THEN it may be possible to "balance" the load with the internal transformer. You want to be careful though, you do not want to forget what is what and get into trouble later if moving the machine to a different power source, etc.

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    Ok, one of the E.E. techs at work told me low voltage would start drawing more current, and I don't want to start popping stuff.
    I checked with the previous owner of these machines, and he had a balanced/tuned RPC.

    I don't know enough to start changing the winding of a transformer. I have a MOT to play with but I haven't cut into it yet. I need it to heat a swing-arm bolt on a 4-wheeler that's seized.

    So I'll try to get some caps to start with.
    I'm just tossed as to get 15Uf or 50Uf to start with, and how many.

    Any suggestions?

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    Low voltage does pull more current if talking about a motor that has a certain load on it. Power has to come into the motor in order to be turned into shaft power. So if the voltage is lower, the current has to be higher. If the motor was already fully loaded to max at normal voltage, then if the voltage drops, the current has to go up. If the motor was already at full load current, it will be in current overload. And, 10% extra current means over 20% more heating. That can fry motors.

    CNNC often has a DC bus and servos with drives running off the DC. Rectifiers act differently than motors. the highest voltage phase tends to charge the caps to a higher voltage than the others, and then the others, since they are lower voltage, only charge the bus during the time their voltage is higher than the DC bus, which is less time than the highest voltage phase.. So the highest voltage phase does more work and draws more current, working those diodes harder than the others, etc.

    If the previous guy ran OK with an RPC and balance caps, I wonder how hard he ran the machines. If you don't run them hard, there may be no issue.

    If you are not going to run at full power, then I'd not fool with the transformer etc. You can go ahead and try to balance the RPC and just run the thing.

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    I found a local HVAC supply house that carries caps in this range.

    After some trial and error I ended up with 2 - 50Uf on Black L1 to the generated leg, and a 25Uf on Red L2 to the generated leg with equal voltage of 247. (And some spare caps, not bad for less than $30)

    The current has me a little puzzled. Black = 9.35, Red = 5.05, Generated = 11.35! The motor is rated 9.6 at 240V.
    I tried adding different Uf between the L1 - L2 as the Cpf with almost no change so I left that out.


    The Bridgeport was under power for about an hour with the spindle running and Y - Z axis moving just fine. X axis moved a little bit then the stepper just humms while the readout displays movement. The gib is loosened and ways are clean, so I'm thinking it may be power related.

    Thoughts?

    Oh, I'll post v/a readings of while it's running later.

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    Quote Originally Posted by BDGiven View Post
    I found a local HVAC supply house that carries caps in this range.

    After some trial and error I ended up with 2 - 50Uf on Black L1 to the generated leg, and a 25Uf on Red L2 to the generated leg with equal voltage of 247. (And some spare caps, not bad for less than $30)

    The current has me a little puzzled. Black = 9.35, Red = 5.05, Generated = 11.35! The motor is rated 9.6 at 240V.
    I tried adding different Uf between the L1 - L2 as the Cpf with almost no change so I left that out.


    The Bridgeport was under power for about an hour with the spindle running and Y - Z axis moving just fine. X axis moved a little bit then the stepper just humms while the readout displays movement. The gib is loosened and ways are clean, so I'm thinking it may be power related.

    Thoughts?

    Oh, I'll post v/a readings of while it's running later.
    I don't know what context the man told you the lower voltage increases current to the motor, this is only true when all three phases are low voltage and when the load is higher than the optimal voltage for that load. so for instance, an induction motor at no load, the line amps will decrease until the voltage drops to on the order of 20-30% of nameplate volts. at 75% load the optimal voltage is about 70-90% of nameplate voltage. at full load, anything less than nameplate volts will increase current.

    In the case of an induction motor rpc, the generated leg is going to produce less current into the driven motor because its voltage is lower and lagging behind where it should be. but at a given load, the total amps into the motor will usually be higher because the third phase is weak. adding capacitors works up to a point but to find the optimal value probably requires a watt meter as well. simply adding capacitors until the third leg amps into the load are equal to the other two phases may not be the optimal solution.

    The amps you are measuring of 9.5, 5, 11.35 are nearly 2-3 times as high as they should be for a 2-3 hp motor at no load. where did you measure these amps at? do they include any current flowing through the capacitors?

    if you remove the capacitors and use an rpc to generate the third leg, you will have no amps on the third leg once the driven motor is up to speed if it has no load and is similar in size to the rpc.

    50uF is 4.65 amps of reactive current at 247 volts btw.


    but regarding the servo drives, if the incoming 3 phase voltage is simply rectified to dc, then it will work on single phase, same as any vfd will but there are thermal limitations which will inspire the capacitors or rectifiers to fail (unlikely). And dc ripple limitations which may impact performance depending on how well the dc ripple is removed from the pwm loops. (not unlike the 60 and 120hz hum from an old guitar amp with failing capacitors) and this 120hz ripple will only show up when trying to max out the torque at high velocity in a program.

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    Here's my record of values starting with the Lincoln AC motor TEFC, 182T Frame, 1750 RPM, 3 Hp, 3 Ph, 8 INS, Max Amp 40, 200/400 Volt Amp 9.6/4.8
    Wired to the low voltage setting.
    Motor wire to L1 Black (B), L2 Red R, Generated G, Ground/Neutral N.
    3-9=B, 1-7=R, 2-8=G

    1st pull start (I only have a cheap volt meter)
    G-B= 209, G-R=198, G-N=166, Incoming B-R/R-B=233, B-N=119, R-N=113

    Add 50Uf to B-G
    G-B=234, G-R=217, G-N=191, B-R/R-B=237, B-N=121, R-N=115

    Add 25Uf to R-G. (Bought a Clamp Meter)
    G-B=233, G-R=228, G-N=197, B-R/R-B=237, B-N=121, R-N=115
    Current Measured at the wires coming out of the 3ph idler motor (RPC).
    B current=7.2, R current=6.4, G current= 5.3

    Add 25Uf to the 50Uf on B-G, and change the 25Uf to 50 Uf on the R-G.
    Connections between capacitors is made with jumpers to the existing cap.
    B===Cap1===Cap2=open
    G===Cap1===Cap2=open
    Not;
    B===Cap1===Cap2===G
    Readings: G-B=237, G-R=251, G-N=214, B-r/R-b=235, B-N=120, R-N=114, B current=6.8, R current=7.05, G current=10.66

    Change B-G to 2 50Uf caps, and R-G to 1 25Uf cap.
    G-B=247, G-R=247, G-N=219, B-R/R-B=233, B-N=120, R-N=112, B current=9.35, R current=5.05, G current=11.35

    Notice the generated leg to neutral value rapid rise. What's up with that?

    I plan to swap the leg wires to the mill to see if a different axis is affected, because I noticed voltage dropped to about 213 and 203 measured B-G and R-G with the mill running.
    Should I add more to the lowest leg to try to make it more balanced while running a load?

    As you can tell; I really don't know what I'm doing!
    That's why I'm here!

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    i presume you have 120/240v single phase, but your readings are consistently measuring 6 volts difference from one side to the other? while this may not be a problem it could be in the future if you have a bad neutral.

    Secondly, this 200/400 volt motor that you are using as an idler is that a 50hz motor? if so that's fine. if its a 60hz motor then it is running saturated. 200v motors do exist for 60hz they are designed for 120/208 three phase, not 240v three phase.


    anyhow, the generated leg to neutral should read 208 for your application assuming 120/240v single phase input, and yes adding capacitors causes that voltage to climb rapidly, that's the whole point.

    your initial voltage of G-B= 209, G-R=198, G-N=166, Incoming B-R/R-B=233, B-N=119, R-N=113 seems really low for a 3 hp motor.


    if you're certain this is a 1750 rpm motor then using it as an rpc at 60hz 240 explains the high amps and the low volts with no capacitors.

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    Quote Originally Posted by johansen View Post
    i presume you have 120/240v single phase, but your readings are consistently measuring 6 volts difference from one side to the other? while this may not be a problem it could be in the future if you have a bad neutral.

    Secondly, this 200/400 volt motor that you are using as an idler is that a 50hz motor? if so that's fine. if its a 60hz motor then it is running saturated. 200v motors do exist for 60hz they are designed for 120/208 three phase, not 240v three phase.


    anyhow, the generated leg to neutral should read 208 for your application assuming 120/240v single phase input, and yes adding capacitors causes that voltage to climb rapidly, that's the whole point.

    your initial voltage of G-B= 209, G-R=198, G-N=166, Incoming B-R/R-B=233, B-N=119, R-N=113 seems really low for a 3 hp motor.


    if you're certain this is a 1750 rpm motor then using it as an rpc at 60hz 240 explains the high amps and the low volts with no capacitors.
    It is a 60 Hz motor. What Do you suggest? Can this Work?

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    Further above, you mentioned a 240V motor, which I thought was the idler.... maybe that was the load motor?

    Using a 200V motor in a 240V situation is not going to work well. I agree that the currents are going to be distorted by the overvoltage condition, and you just need to use a 240V motor for the idler.

    An unloaded motor usually draws about 30 to 50% of full load current, depending on design issues, etc. As an idler, it may draw a little extra, since just one phase is being powered.

    ANY normal motor used as an idler motor is going to produce low volts on the generated leg. The "output voltage" is really just the "back EMF" of the motor. That HAS TO BE lower than the actual line volts, or the motor would not draw any power. So the generated voltage starts out low, and sags farther under load.

    The whole idea of the "balance" capacitors, is to cancel out some of the motor inductance that causes voltage drop, and possibly to get close to resonance with the motor inductance, which will raise the voltage. But the resonance condition is affected by load, so the boost effect is damped down by the load motor. A sufficient capacitor to raise the voltage under load, may produce a very high voltage at low load, or a lighter load, which is not desirable.

    One can also boost the generated leg by a special idler motor that is wound to produce more generated leg voltage, OR by a boost transformer that raises the output voltage to equal, or slightly exceed the incoming line volts. Those approaches do not have the disadvantages of the capacitors, but are rarely used.

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