start and run capacitors

Discussion in 'General Electronics Chat' started by FrenchConnection67, Sep 10, 2013.

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  1. FrenchConnection67

    Thread Starter New Member

    Sep 9, 2013
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    Hi everyone,

    I am actually writing to you because I would need to shed some light on start and run capacitors.

    I am working in automatic doors and most of the single phase motors with work on have capacitors (just one per motor).

    Up to recently, I have always thought that those capacitors were start capacitors because I was taught this by the most experienced engineers in work and it also very common to say that capacitors help the motor to start so for me it was just making sense.

    However, I have recently realised there were loads of confusion between start and run capacitors and not a lot of people can actually make the difference and identify them properly. I went to a electronic component retailer last week and the guy at the counter didn't know the difference and could only read what was on his catalogue.

    But what really confused me was that I went on a job last week where there was a gate on which the left motor would hum and not move, after checking visually the caps. I quickly noticed that its cap was melted at the bottom and some grey matter has come out and gone hard.

    Reading the motor technical manual, they are talking about a starting capacitor however following the spec of the cap and the motor, I believe it is actually a run capacitors and not a start capacitor.


    So now, I am interested to hear your opinions and would appreciate if you could (un)confirm my findings/believes.


    Motor spec:
    FAAC 400 (hydraulique ram)

    Max Traction / Thrust Force: (daN) 620
    Effective Rod Stroke: (inches) 10 1/4
    Linear Rod Speed: (inches/sec) 0.4
    Operator Weight: (lbs) 19
    Use Frequency: (cycles/hour) 70
    pump Flow-Rate: (liters/min) 1
    Hydraulic Locking: (1) = Closing (2) = Opening and Closing (1)
    Max Leaf Length: (feet) 16
    Power Supply 115 VAC ± 10% or 230 VAC +6% -10% 50/60 Hz.
    Absorbed Power (W) 220
    Absorbed Current (A) 1 (230V) or 2 (115V)
    Electric Motor (rpm) 1400 - 4 poles
    Thermal Protection on Winding (°F) 248°
    Thrust Capacitor 25uF / 115 V or 8uF / 230 V
    Ambient Operating Temperature Range (°F) - 4° to 131° Protection Class IP55

    As you can see there are talking about a thrust cap but in another paragraphe of the manual they mention it as a starting cap.....

    But if I follow the identification process, it comes up as a run cap.

    Very confusing indeed!

    The cap spec are:
    8mfd -+5%
    AC 400V B 25/85/21
    AC 450V C 25/85/21
    50/60 Hz



    thanks
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Are you interested in the basic principle or sizing capacitors etc?
    The general difference between Start an run capacitor is usually the technology used.
    A start capacitor generally needs to be larger (capacity) than the run capacitor, to make a capacitor of this size, constructed in the manner of the run capacitor which could be oil filled paper would require considerable physical size.
    In order to construct a large value in a smaller package, often electrolytic are used and consists of two capacitors back to back in the same can in order to conduct AC, the down side is, they must only be energised for a very brief period, due to the heating effect due to a low ESR (Equivalent Series Resistance).
    So these are rated AC Motor start capacitors.
    The run cpacitor is usually lower in capacity value so can be constructed of Oil filled paper and have a High ESR and can be connected continuously.
    The run cap can be used in a start situation but not vice-versa.
    These are rated AC motor run.
    If a motor only has one capacitor, this is generally just a start capacitor, no run.
    As to the theory, single phase ( split phase ) motors require some kind of phase shifted winding to start revolving, otherwise the field oscilates at 180° and result in a stationary rotor.
    Max.
     
    Last edited: Sep 10, 2013
  3. FrenchConnection67

    Thread Starter New Member

    Sep 9, 2013
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    Thanks for your reply max. According to your information, it just confirms my primarily belief that most of cap on single phase motor are start caps.

    however, what started to confuse me was that when I followed the identification process looking at this webpage, my cap comes up as a run cap.

    http://www.electricneutron.com/electric-motor/single-phase-capacitor-sizing/

    what do you think max?

    thanks
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There are motors where the start cap is kept in circuit so it is a combined start/run cap.
    So as the link bears out, when two caps are used, the smaller one is the run cap.
    There are a few different configuration combinations of single phase motors.
    But one thing you can be sure of is that if a motor only has one cap, it is either start only, or combined start run.
    Max.
     
  5. FrenchConnection67

    Thread Starter New Member

    Sep 9, 2013
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    Right, I ve had the cap specs + its picture examined by a cap specialiot and it confirmed it was a run cap.
     
  6. PackratKing

    Well-Known Member

    Jul 13, 2008
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    It is a run cap, on a permanent split-phase capacitor motor [google] which makes it reversible based on which leads and terminals the cap is wired in on...
     
  7. #12

    Expert

    Nov 30, 2010
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    Confirming. You have it right.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I am absolutely unqualified to answer ;0
     
  9. Smoke_Maker

    Active Member

    Sep 24, 2007
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    Is it true that almost all motors with start caps have contacts inside the motor that open after the motor get to run speed and are wired through the contacts??
     
  10. PackratKing

    Well-Known Member

    Jul 13, 2008
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    That's how they work... The start cap is wired in series with the start windings, and the centrifuge kicks the start switch open at approx 80% rated speed
     
  11. shortbus

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  12. MaxHeadRoom

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    Jul 18, 2013
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    I got tired of changing the centrifugal switch on my air compressor, I did a Triac mod on it, so the same switch just did the low level turn on of the triac.
    Still going strong.
    Max.
     
  13. #12

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    +1 on shortbus
    There are also "current" relays. No electronics allowed inside the compressor case. That would whack the reliability too much.
    Recent changes in the industry use a PTC thermistor on a piece of ceramic as a start winding energizer for refrigerator compressors in the 1/4HP and less range. They fail A LOT!

    I haven't replaced a start relay on a refrigerator compressor in 20 years, but the parts store has 6 cases of ceramic PTC starters on the shelf.
     
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  14. PackratKing

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    Jul 13, 2008
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    I absolutely love the "hard start " setup in the link you provided...
    We had call to do a truckload of these mods at the HVAC shop I worked with !!

    It leaves me wonder why capstart motors even still have internal switch and centrifuge setups...
     
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  15. MaxHeadRoom

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    One of the reasons is the rpm actuation point is more accurately defined.
    Max.
     
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  16. #12

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    Besides that, the actuation point is entirely under the control of the motor manufacturer, and they like that.

    Secondarily, the motor needs zero external components that would kill it if they fail.

    And third, it costs less to install a motor with no external parts. You don't even need a metal box to house them, let alone the skills to wire it correctly.
     
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  17. PackratKing

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    Thank you both, Max and #12 for lending substance to what I suspected...

    12, you pointed up... " let alone the skills to wire it correctly. "
    this is one of the fine points we got an occaisional roar out of, is the DIY or incompetent contractor snafus we often got called on to rectify...:D
     
  18. #12

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    Tracing wiring errors is a lot more time consuming than finding bad parts.
    I used to work for a guy that couldn't wire a thermostat to save his life, but he tried. We just jerked all his wires loose and did it over because it was quicker than trying to trace out his mistakes.
     
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