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Question about failing starter capacitors on electric motors

Discussion in 'Hobby Electronics' started by Rod Out Back, Jan 30, 2007.

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  1. Rod Out Back

    Rod Out Back Guest


    Do starter capacitors ever fail gradually, or do they either work or not work?

    I have a rooftop evaporative aircon, which is playing up. The motor starts, but
    accompanied by loud humming and the fresh smell of burning electrical
    Starter capacitor in the control box (it is a variable speed controller) is
    nominated as 25uF (+- 5%), and reads with a multimeter as 22.74uF. No bulges or
    deformities on the starter cap.

    Should this reading be exactly as printed on the cap?

    I have checked all the wiring into the control box, and out to the various
    accessories (fan motor, water pump motor & downstairs control box -with the
    user-selectable switches in it for the aircon), and there dont seem to be any
    problems there.

    Water pump starts happily, with no humming, and no burning electrical smell.

    My suspicion is I have a fan motor about to expire. The electrical smell doesnt
    manifest itself in the rooftop control box, and only appears when the fan is

    Info on the death of starter capacitors appreciated.



    Rod.....Out Back

    For a selection of my pics from the past 12 months,
    visit the web site below:
  2. Lord Garth

    Lord Garth Guest

    Exactly as printed + or - 5%
  3. Mr.T

    Mr.T Guest

    Yes, and when coupled with the uncertainty of the multimeter figure, it may,
    or may not be, in spec :)

  4. Bob Parker

    Bob Parker Guest

    I've been told that some air-con technicians/mechanics use ESR
    meters to identify motor start caps which have gone bad....

  5. Ross Herbert

    Ross Herbert Guest

    An ideal use for your ESR meter Bob. Polypropylene (MKP) starter caps
    measure very low when they are in good condition.
  6. In another life I used to work in a motor rewind and repair shop, personally
    I would regard the reading you got as reasonable (meter accuracy etc.) but
    the burning smell is a worry since it comes from overheated windings. Once
    that happens a few times there is every chance your motor is fritzed !
    The situation really needs checking by experienced persons which is not easy
    for you out there, also it may well be the cost of a new motor and
    controller would be similar to that of a callout.
    Perhaps you could send the parts into town for testing and take it from
    Incidentally technically that is a "Run" capacitor and of different
    construction to a "start" cap. Run caps usually last longer than start caps.
    Hope you find a solution Rod .
  7. Phil Allison

    Phil Allison Guest

    "Ross Herbert"

    ** It weird how poly film caps sometimes go high ESR.

    It aint like there is any liquid electrolyte in there to dry out.

    Got a 22nF Wima class X cap here now that measures about 5 kohms in series
    with 2 nF.

    Not visibly damaged.

    Pulled it out of a Marshall amp, was driving output valve grids.

    ......... Phil
  8. Bob Parker

    Bob Parker Guest

    As Phil said, it's weird that this kind of cap would fail with high ESR.
    BTW and just for everyone's info, DSE are flogging all their kits at
    ridiculously low prices at the moment. The ESR meter is just under $25
    when at one time it was up near $80.

  9. Bob Parker

    Bob Parker Guest

    I dunno what's going on with the Kiwi DSE website. I can't find the
    ESR meter by catalog number. All I managed to find by searching for
    'ESR' was the chart for the first version's front panel.
    I couldn't find the K7205 LOPT tester either. Looks like DSE's not
    stocking a lot of their kits over there now.
    Does anyone know anything about this?

  10. Rod Out Back

    Rod Out Back Guest

    Question about failing starter
    capacitors on electric motors",

    "Rheilly Phoull" <> ordered the talented chickens to
    cluck & peck the following:

    Thanks to all who offered suggestions. I spoke to the electrician (original
    installer of the aircon), and his assessment is it sounds like the motor is
    dying. The fan motor cops a hiding in it's location (pretty corrosive bore water
    used in the aircon), and I replaced the same motor about 4 years ago.

    I ran into town this arvo and got a new motor. The whole fan/motor assembly
    comes out of the aircon in about 5 minutes, and the connections are clearly
    marked on the control board(also took plenty pics before removing connections as
    well), and on the side of the motor.

    MY next 'challenge' was removing the plastic fan off the old motor shaft. I
    hadnt remembered the compressible collet that sits between the fan & shaft, and
    of course it fell apart as we remove the cover plate. No chance of making one
    up, so I expect we will be in touch with electrician again tomorrow....sigh.

    Thanks again for all the ideas. I might pick up another capacitor as well when
    picking up the collet. May as well change it out, and then I know they are both
    right to go.


    Rod.....Out Back
  11. I'll probably never use it, but I am going to go and buy one if they
    are still at that price!
  12. Bob Parker

    Bob Parker Guest

    There might be other really cheap kits you'd have more use for? The
    ESR meter is awfully specialised.
    If you decide to get one, be aware that DSE's selling the Mk1 (1996)
    version of the meter for the same price, and the Mk1 and Mk2 cartons
    tend to get mixed up on the shelf.
  13. Thanks Bob. I will go and have a look today. Its one of those tools
    that occasionally you say 'I wish I had one of those'. Whilst i very
    rarely have anything to do with hardware it does still arise from time
    to time.

    Are DSE clearing out all there kit stock?
  14. ron

    ron Guest

    I think DSE NZ don't stock this kit. I purchased one a year or
    so ago and had to use mail order from DSE Australia. It's proved
    to be a useful addition to my workshop.

    cheers, ron
  15. budgie

    budgie Guest

    Sounds like they need more shelf space for the farting ashtray lines ;-(
  16. There were two of each on the shelf in the Box Hill (Melb) store,
    tagged at $49.95 - but the counter check showed they were $25.
    Thanks for the heads-up, I almost bought one six months ago at >$70.
    Now I have a MK2 to build.

    What're the main differences in capability between the MK1 and MK2?

    What else can it be used for apart from testing electro's?

    Clifford Heath.
  17. Phil Allison

    Phil Allison Guest

    "Clifford Heath"
    ** Not a great lot

    The Mk2 has better protection for the meter .

    ** Resistors from 0.01 ohms to 100 ohms - but must be non-inductive,
    lengths of wire or coils are out.

    The ESR of cells and batteries - indicates condition, plus approximate
    state of charge or remaining capacity.

    Just perfect or button & lithium cells etc.

    Do a Google search - plenty of stuff on Bob's site and elsewhere.

    ....... Phil
  18. Geoff C

    Geoff C Guest
    Hi Bob,

    I came across this review website and wondered if you think the
    comments and tests on your design are fair?
  19. Phil Allison

    Phil Allison Guest

    "Geoff C"

    ** How about this quote then:

    " But a good 1 ufd, 200 volt aluminum electrolytic can have an ESR of 100
    ohms or more. That is beyond the measurement range of all of the meters
    listed above! "

    I just checked a number of old but unused electros bought from WES in a bulk

    Values ranged from 0.47uF to 1.0 uF with voltages from 250 to 400 volts -
    physical size all about 12mm by 8mm dia.

    None tested more than 30 ohms on Bob's Mk1 meter.

    Any that test over 100 ohms are FAULTY !!!

    ......... Phil
  20. Geoff C

    Geoff C Guest

    Good to know, thanks.
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