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Thread: What's difference between 'run' and 'start' capacitor

  1. #1
    vdmmedia is offline Plastic
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    In looking at the 'integrated' phase convertor design, there are both 'run' and 'start' capacitors listed. I realize that the 'start' capacitor is to produce a kick to get the idler motor started and the 'run' capacitor produces the phase shift necessary to manufacture the third phase.

    But, are they actually the same (physical) design or are they two different constructions. Are they different from other types of capacitors which list their voltage rating a VDC instead of VAC.

    In other words, "what's da deal here"?

    If one were to build a large converter (say 30hp), is there a rule of thumb (or maybe an equation) for the value of the capacitors on a converter of this size running on 220V or on 440V.

    Since a 30hp converter would have a rather large inrush (about 5 times the running current, I'm told) this would most certainly suck my residential neighborhood voltage down (and make me justifiabably unpopular). Would it be practical to clutch connect a 5hp single-phase motor to bring the idler up to near speed before powering the idler. It would then seem that the 'start' capacitor circuit could be done away with in this case simplifying the converter while at the same time reducing the inrush problem.

    Also, are far as motors go, is the rotational speed of the motor significant? There are motors available in about 1170 rpm, 1800 rpm and 3600 rpm flavors. Does the rotating mass of the armature provide a 'flywheel' effect?

    Questions, questions, questions...

    Thanks in advance!

    John
    vdmmedia

  2. #2
    peterh5322's Avatar
    peterh5322 is offline Diamond
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    Completely different construction, and design limits.

    Start caps are actually electrolytics which are designed for operation on ac. These have a limited number of starts, and are only allowed to be in the circuit for a few seconds.

    Run caps are actually oil-filled caps. These have a virtually unlimited lifetime, and are allowed to be in the circuit continuously.

    Start caps have a very high capacitance to volume ratio.

    Run caps have a low capacitance to volume ratio.


  3. #3
    bnelson is offline Senior Member
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    John, some great questions. Peter gave you the differences between start and run caps. Might add that the way rated voltages are specified is also different between the two. This has nothing to do with differences in the physics, just the way they are traditionally listed in the catalogs and stamped on the cap body. If you plan to use a RUN cap on a 240 volt circuit, you'll need to buy one rated at 370 volts. Run caps are rated in peak inverse volts (PIV), which is 1.414 times the root mean square (RMS or 'line') volts. 1.414 * 240 = 339 PIV.

    START caps, on the other hand, are simply rated in line RMS volts, so you'd need a 240 volt start cap on a 240 volt line.

    My rule of thumb for figuring cap sizes is to take the nameplate hp and multiply it by the service factor SF on the motor to get the "actual horsepower" of the motor. Then I figure 75 mfd per actual hp for the start caps, and 15 mfd per actual hp for the run caps on ONE side; for a 'balanced' design, which gives much better overall regulation and power factor, you'd need about that same value on the other side as well; the actual 'split' is usually in a 60:40 ratio and is determined by taking measurements and moving caps by trial and error.

    Peter has a nice method for converting the required capacitor values between 220 and 440 volts:

    http://www.practicalmachinist.com/ub...ML/000086.html

    Your idea about using a start motor is very feasible, see the recent thread:

    http://www.practicalmachinist.com/ub...ML/000119.html

    Might add a veteran converter builder told me last year he had built a 30 hp rotary using the standard start cap circuit, and if he ever built another that big or bigger he'd use a start motor. We don't have demand pricing here in Indy either, so that was not his reason.

    Your last question deserves some discussion on this forum (Peterh, Forrest, others???). You can use any rpm induction motor to build a phase converter. If you look at outputs on a scope, the 3600 rpm motors give closer to the desired 120 degree phase separation than the lower rpm motors. On the other hand, for bigger converters I like the 1800 rpm motors better because they're so much quieter (I almost always use TEFC fan cooled motors for the idler, and at 3600 rpm that fan sounds a bit like a jet airplane on the tarmac).

    We need a good discussion here about the pros and cons of high speed vs lower speed idlers. With the low prices of big motors today, this should be the user's choice based on converter fundamentals.

    [This message has been edited by bnelson (edited 04-08-2003).]

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    There are usually two options for the idler motor: 1) 3600 rpm, and 2) 1800 rpm.

    In some sizes, particularly the larger ones, there may be three options: 1) 3600 rpm, 2) 1800 rpm, and 2) 1200 rpm.

    Often, a 3600 rpm motor and an 1800 rpm motor cost about the same.

    A 1200 rpm motor is usually considerably more expensive than the other sizes mentioned.

    There is some controversy as to 3600 rpm vs. 1800 rpm.

    I don't believe it has been proved that one is inherently superior to the other, as to conversion performance.

    Certainly, it takes a 3600 rpm motor longer to achieve near synchronous speed, and that can influence the premises electrical system, as a motor, any motor, draws a significantly higher current during subsynchronous operation (i.e., during starting), and that can contribute to nuisance trips.

    Perhaps it is for this reason that many experimenters favor the lower rpm approach.

  5. #5
    Rob Russell is offline Member
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    John,

    One comment about the start vs. run caps - you can use run caps as start caps but not vice versa. I was lucky enough to buy 2 dozen new 100 uF/330vac run caps on ebay for cheap. Because of that, I'm actually using a bank of run caps as my start cap. The only problem that causes is I need a larger enclosure to house all of the caps, but that wasn't really a big deal.

    In terms of the idler motor RPM, noise can be overcome. I'm building a sound hood for my 3600 RPM/15HP idler. The idler will be enclosed and cooled via a muffin fan that pulls air in through some baffles. That way the whine of the idler won't be as irritating.

    I would be interested in what comes out of the discussion on 1800 vs. 3600 rpm. It's a moot point for me because I have the motor and it was a scrounge job based on price, but it would be nice to know for building future (smaller) convertors when I want to run lower HP load motors.

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    joshuaz2 is offline Plastic
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    i would like to re-ask the question of caps rated in vdc vice vac. can these be used? what is the difference?

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    peterh5322's Avatar
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    "i would like to re-ask the question of caps rated in vdc vice vac. can these be used? what is the difference?"

    Capacitors intended for use in ac circuits are always rated "VAC" (volts, ac), never "VDC" (volts, dc) nor its equivalent "WVDC" (working voltage, dc).

    A capacitor which is used for motor starting can be, and usually is, an electrolytic capacitor. Not a dc-type electrolytic, but an ac-type electrolytic.

    Such a capacitor must be rated for at least the line voltage.

    A capacitor which is used for motor running must be an oil-filled capacitor. Not a dc-type oil-filled, but an ac-type oil-filled.

    Such a capacitor must be rated for at least 1.56 times the line voltage.

    Note: the line voltage is always specified in RMS (root-mean-square) volts, not in P-P (peak-to-peak) volts. For conversion from RMS to P-P volts, in circuits with zero or very low harmonics, one multiplies RMS by 1.414 to get P-P.

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    Webb Wyman is offline Stainless
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    Hi There,

    On the subject of idler motors, what about internal construction? I mean which is prefered: Delta wound or Wye wound motors for an idler?

    Regards,
    Webb

  9. #9
    howard d is offline Cast Iron
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    One difference not mentioned is that run caps are usually in metal cans and start caps usually in black Bakelite containers. As Peter said, start caps have much higher capacitance per unit volume than run caps. Start caps should not have yhe voltage applied for more than a second or two.

    Having built and sold about a 100 rotary phase converters (40 hp is the largest I've built), I prefer the 1200 or 1800 rpm motors because they require less time to reach speed compared to the 3600 rpm motors and make less fan noise. Functionally there is no difference in performance in running the load motor.

    I also prefer the delta wound (when I can get them) because they draw less current when idling (i.e., no load motor). Here again, there is functionally no difference between the two.

    As to ODP (open drip proof) vs TEFC (totally enclosed fan cooled), again there is no difference in performance. The preference here depends upon environment. In a wood work shop with a lot of dust, the TEFC is best. I run a 15 hp ODP in my machine shop and have it located where flying chips can not be sucked in by the cooling fan. The ODP are prone to suck in normal floor dust.

    Other "rules-of-thumb" not mentioned concerns the load motor. Motors staring under a load (e.g., air compressors) require a converter motor about 3X the size of the compressor motor. This is also true for CNC machines running off a converter. And finally, I recommend to all my customers that the lagest load motor be no more than about 80% of the converter motor (up to 1.5X the converter motor if all load motors are small ones).

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  10. #10
    bnelson is offline Senior Member
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    Agree with above. With the 3600 rpm idler motors, you can decrease the time it takes to get up to speed simply by increasing the value of the start cap. Cheap solution. However, if you make it too big, you may trip a breaker or blow a fuse during starting. By far the biggest advantage of a lower rpm motor to me is NOISE.

    Use HACR rated breakers to supply the idler; these are less prone to nuisance tripping during starting. Even better are so called 'C' or 'D curve breakers, but these cost $$$$$.

  11. #11
    howard d is offline Cast Iron
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    Another important thing to keep in mind is that a rotary phase converter has a "high leg." This means that the third leg (also called the ghost leg) has a higher voltage to neutral or ground than the other two. Two of the legs are the 240 vac 1 ph supplied by the utility, with each leg at 120 v to neutral or ground. The third leg will be 208 v to neutral (if you are perfectly balanced, i.e., 240 v between each of the 3 legs). The only time this is important is when using 120 v appliances - for lights, DROs, power feeds, computers, etc., or if using a 240/120 v stepdown transformer to get the 120 v. Just ensure you are not using the third leg, because it is high with respect to neutral (important for appliances)and the voltage will drop as load motors are turned on (important if using a stepdown transformer).

    In my search for a solid state dc drive, most manufacturers I contacted stated that their three phase dc drive units should not be powered be phase converters because of the high leg. Baldor, Warner, and Boston units require "balanced voltage with respect to ground", which a converter does not have. That's why I decided to go with a single phase dc drive - you do not get the full 230 vdc to the armature, but it still powers the motor nicely.

  12. #12
    howard d is offline Cast Iron
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    Sorry, forgot that is was not on the Monarch 10EE topic. That was what I was addressing in my last paragraph about dc motor speed controllers, when converting from a vacuum tube controller to a solid state controller.

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    "In my search for a solid state dc drive, most manufacturers I contacted stated that their three phase dc drive units should not be powered be phase converters because of the high leg. Baldor, Warner, and Boston units require 'balanced voltage with respect to ground', which a converter does not have. That's why I decided to go with a single phase dc drive - you do not get the full 230 vdc to the armature, but it still powers the motor nicely."

    ... and ...

    "Sorry, forgot that is was not on the Monarch 10EE topic. That was what I was addressing in my last paragraph about dc motor speed controllers, when converting from a vacuum tube controller to a solid state controller."

    Love those 10EE topics.

    You can get 230/115 from a KB controller by boosting the input from 240 volts to 288 volts, using autotransformers.

    See the 10EE thread on drive replacements for many details.

    Emerson makes a 10 HP drive which accepts 277 volts single-phase (as from one phase of a 277/480 volt Wye source) and produces 230/115 volts. A bit of an overkill for a 10EE which is quite happy with 3 or 5 HP.

    Anyway, I have a Ward-Leonard 10EE coming perhaps next week, and I'm going to try either a dual KB drive or an Emerson drive on it, and if successful, then a second Emerson will be used on my present WiaD 10EE.

    I have a 240:480 7.5 KVA transformer in house for the WiaD, but it isn't wired up as yet, although the 60 amp branch circuit for it is.

    No time to conduct tests as I've been overseas on family business, and some of my posts have been from "Internet cafes" in various places in Europe.

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