440v 3 Phase Rotary Converter Help - Page 2
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  1. #21
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    Capacitors in system currently are:
    2 - 30 MFD 370 Vac (silver metal)
    1 - 270-324 MFD 220/250 Vac (black plastic start cap.)

    I traced the wiring in the control box and came up with the schematic that I attached. The relay/switch in the dashed box is rated for 260Vac. It is not the greatest drawing..
    Attached Thumbnails Attached Thumbnails 3-phase1.jpg  

  2. #22
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    It is a good thing you didn't get it to start.

  3. #23
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    Attached is a revised schematic diagram incorporating the following changes:

    1) Exchange motor lead 2 for lead 3,

    2) show the PFC capacitors as a representative example of this and the two other run-type capacitor composite units,

    3) show resistors across each capacitor, for both voltage equalizing and for bleeding, and

    4) show the internal "protection" resistors.
    Attached Thumbnails Attached Thumbnails revised_460_converter.jpg  
    Last edited by peterh5322; 10-02-2008 at 02:43 AM.

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    Lounsbmw
    Based on the information that you supplied and using peter's diagram. You should have two run caps rated at 17.5mfd @ 370 in series between A and B, two more run caps rated at 12.5mfd @ 370 in series between C and B, two more run caps rated at 3mfd @ 370 in series for Cpf circuit. Your start caps circuit should have two start caps rated at 145mfd @ 250vac wired in series, make sure that all the capacitors have discharge resistors.

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    Lounsbmw
    If your on a tight budget, you can use the two 30mfd run capacitors in series between A and B and dont have any run capacitors between C and B and A and C ( Cpf circuit ).

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    A good source of capacitors at reasonable prices is Burden Sales Co (Surplus Center) in Lincoln, NE.

    Here ...

    http://www.surpluscenter.com/sort.as...tname=electric

    ... is a listing of all 370 VAC motor run capacitors presently available.

    (Scroll down to find the larger value capacitors. Usually, you can ignore the dual capacitors as these have a small value and a large value capacitor in one case, for HVAC applications. If the two values are identical, then these would certainly be suitable for the above application).

    Alas, Burden Sales seldom has as good a selection of 250 VAC motor start capacitors.

    (Currently, they have none).

  7. #27
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    I was pricing the capacitors on Ebay as well. That website seems to have pretty good prices too. Are the caps. from that website protected? The don't seem to list it in their description.

    I edited the schematic from Peter to show physically what the circuit should look like and added the capacitor values macplus stated. Could you guys review it to make sure I got it correct? Thanks again.

    I will be ordering some caps. soon, as well as the 90-66. That way I can get going on building this. I think I am going to have to get a bigger control box on the RPC or go to a separate control box for all these capacitors...lol
    Attached Thumbnails Attached Thumbnails 460_converter.jpg  

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    Yes...dont forget to add resistors to all the capacitors.

  9. #29
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    Ever since Pyranol®, GE's fireproof insulant, was outlawed because of its PCBs, capacitor makers have had to use a substitute which is PCB-free, but is not fireproof. One name for the PCB-free, but not fireproof insulant is Dielektol.

    One solution to this safety issue is "protected" capacitors, where the capacitor has within it a protective device, possibly a "fusible" resistor, which will open if the capacitor overheats, thereby preventing a fire.

    Every modern run capacitor which I have examined has stated "PROTECTED" on it. Also, sometimes "CONTAINS NO PCBS".

    The GE 97F series of run caps is pretty much the "gold standard", but there are other makers, too.


    Yes, of course, there should be a bleeder across every cap.

    This resistance is also there to equalize the voltage across the capacitors as these are in series connection to form a single, composite capacitor.

    The failure mode of the integral protective device is such that the capacitors fail "open".

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    Another idea is use 600vac run capacitors, this way you would not need to series the run caps and the the resistor across them. I did check my stock on the run caps and have better prices than surpluscenter and graingers potential relay ( about 25% less and 40% less )

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    macplus,
    I take it you are part of an electrical supply store? Where are you located in Western NY? I am in that area of NY also. Do you also carry the resistors I would need (15K ohm 5 W)? I would prefer to drive to a store and get everything from one location. That usually saves on shipping cost from the internet, as long as it is not too far away. Thanks.

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    "Another idea is use 600vac run capacitors, this way you would not need to series the run caps and the the resistor across them"

    A dangerous idea for a run cap, but OK for a start cap.

    Run caps must be rated (singly or in series connection) at least 1.56 times the RMS line voltage.

    1.56 * 240 = 374.4 (370 is fine).

    1.56 * 480 = 748.8 (two 370s in series connection are fine).

    Start caps must be rated (singly or in series connection) at the RMS line voltage.

    250 is fine for 240.

    500 (two 250s in series connection) are fine for 480.


    Note: whenever making a composite capacitor by placing two in series, the capacitors must be of the same rated value (not necessarily the same actual value, as caps are made with a tolerance). Otherwise, the "series capacitor law" has to be calculated in each case of unequal capacitor values:

    Ctotal = 1 / ( ( 1 / C1 ) + ( 1 / C2 ) )

    It is generally a bad idea to place two dissimilar caps in series connection as the voltage across the individual cap will divide unequally.

    Resistors are already recommended even for the series connection case of equal value caps. The resistors become even more important for the unequal value case.


    A side remark: one reason those cheap static converters have such a bad reputation is these generally have only 125 volt caps in them, for 240 volt SPCs.

    Some makers DO offer what they call a "heavy duty static", but all that usually means is they substitute a 250 volt cap for the 125 volt cap, with NO other changes.

    The "magic smoke" Genie is just waiting for the opportunity to be let out. Installing an under-rated cap is one sure way to let that smoke out!

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    Peter
    I am almost sure that the rms is already figured in when the manufacturer design a AC capacitor....so a 240vac rated capacitor can be used on a 240volt ac circuit or a 600vac rated capacitor can be used on a 480volt ac circuit. Now a DC capacitor would need to have the rms figured in.

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    "I am almost sure that the rms is already figured in when the manufacturer design a AC capacitor....so a 240vac rated capacitor can be used on a 240volt ac circuit or a 600vac rated capacitor can be used on a 480volt ac circuit"

    I am not sure what 600 volt caps were intended for, possibly with the exception of 380.

    The 370 volt caps were designed specifically for 240 volt service.

    The requirement is the capacitor must be designed for the the peak voltage, not the RMS voltage, plus a ten percent margin.

    Converting from RMS to peak is accomplished by multiplying by ( 2 ) ^ 0.5 = 1.414.

    Applying the safety factor is accomplished by multiplying by 1.1.

    1.414 * 240 * 1.1 = 1.56 * 240 = 373.35, for which 370 is fine.

    1.56 * 380 = 591.14, for which 600 is also fine.

    Using a 600 volt cap on 480 would provide just one-half of the RMS-to-peak requirement, and no safety factor.

    If a cap is in the circuit only intermittently, such as during motor starting, then you can dispense with the RMS-to-peak allowance, and the safety factor as well.

    I see no problem using a 600 volt cap in 480 RPC starting service, but the run caps, being continually in the circuit, need the RMS-to-peak allowance and the safety factor.

    As a 480 RPC needs just one-quarter the starting capacitance, it may make a lot of sense to use a 600 volt run cap in that application.

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    Quote Originally Posted by peterh5322 View Post
    "I am almost sure that the rms is already figured in when the manufacturer design a AC capacitor....so a 240vac rated capacitor can be used on a 240volt ac circuit or a 600vac rated capacitor can be used on a 480volt ac circuit"

    I am not sure what 600 volt caps were intended for, possibly with the exception of 380.

    The 370 volt caps were designed specifically for 240 volt service.

    The requirement is the capacitor must be designed for the the peak voltage, not the RMS voltage, plus a ten percent margin.

    Converting from RMS to peak is accomplished by multiplying by ( 2 ) ^ 0.5 = 1.414.

    Applying the safety factor is accomplished by multiplying by 1.1.

    1.414 * 240 * 1.1 = 1.56 * 240 = 373.35, for which 370 is fine.

    1.56 * 380 = 591.14, for which 600 is also fine.

    Using a 600 volt cap on 480 would provide just one-half of the RMS-to-peak requirement, and no safety factor.

    If a cap is in the circuit only intermittently, such as during motor starting, then you can dispense with the RMS-to-peak allowance, and the safety factor as well.

    I see no problem using a 600 volt cap in 480 RPC starting service, but the run caps, being continually in the circuit, need the RMS-to-peak allowance and the safety factor.

    As a 480 RPC needs just one-quarter the starting capacitance, it may make a lot of sense to use a 600 volt run cap in that application.
    Wow....still confused on this matter, here is a link on capacitors http://books.google.com/books?id=kdd...DKY3Ke_9LW7oS8

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    I am throughly intrigued by the design you guys put together. I have been toying with the idea of building a 460 volt RPC and you just saved me several hours and a lot of head scratching before I figured out how to use the Steveco.

    Keep the brilliant ideas coming!

    Bruce Norton
    Kingsport, Tn

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    "Wow....still confused on this matter"

    1) meters read the average voltage, not the peak voltage (certain expensive meters read "true RMS", but most meters read "average" as a close approximation of RMS),

    2) a capacitor's insulation system is specified for its peak applied voltage, any higher and the capacitor may fail,

    3) to convert from the observed measurement ("average" or RMS), one must apply the a conversion factor, which is 1.414 for a pure sine wave,

    4) the capacitor must be rated to withstand at least this peak voltage, but it is customary to apply an additional factor of 10 percent, for safety, therefore

    1.1 * 1.414 * Vrms = Vmin

    For Vrms of 240, the Vmin is just over 373 volts.

    Therefore, run capacitors employed on 240 volts must be rated at least 370 VAC.

    The reason start caps may be rated only 250 is the voltage is applied for a brief duration, plus the electrolytic type caps are designed to be "self-healing", within reason.

    Electrolytic type caps are also rated for a maximum number of cycles over its lifetime, and a maximum number of cycles in any 15 minute period.

    Therefore, for applications where a machine is frequently started and stopped, or is frequently reversed, an oil-filled cap is used instead of an electrolytic, for starting.

  18. #38
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    "I have been toying with the idea of building a 460 volt RPC and you just saved me several hours and a lot of head scratching before I figured out how to use the Steveco"

    The most obvious choice is placing a 2:1 step-down transformer between B-C and the P.R.'s coil (but that requires modifying the P.R., too).

    However, an induction motor is really a rotating transformer, so we can use the transforming ability of the motor itself to effect the 2:1 step-down.

    If you draw straight lines paralleling the motor's windings, then the windings will have the same electrical symbol as a transformer winding.

    So, we're taking one-half of the B-to-star winding and one-half of the C-to-star winding, and getting the equivalent of a 240 volt output from 480 volts input.

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    "The most obvious choice is placing a 2:1 step-down transformer between B-C and the P.R.'s coil (but that requires modifying the P.R., too)"

    The reason an autotransformer is NOT used in this application is this: should the common end of the primary fail "open", or should it be mis-connected, also as "open", then the full 480 will be imposed on the P.R. and it won't work as expected, and as required.

    By using the inherent transforming action of the induction motor itself, we are able to eliminate a part, save cost, and improve performance and reliability.


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