Connect with us

Starter Capacitors

Discussion in 'Hobby Electronics' started by Rod Out back, Sep 3, 2004.

Scroll to continue with content
  1. Rod Out back

    Rod Out back Guest

    All,

    House pressure pump died yesterday arvo. After ascertaining the unit was
    still able to turn easily, I discovered the starter capacitor had blown.

    After much frenzied searching through electric motors for something to get
    us through the night, I discovered a starter capacitor that got us through
    the night. Luckily, I found that there is someone in town (Longreach) who
    has another starter cap I can replace the original with. However, I was
    advised that the starter cap I used was way too small, and could damage the
    motor. Unfortunately, I didnt have any alternatives at the time, and so far
    the motor continues to run...A quick dash to town this morning to get the
    replacement cap is in order.

    Now, in an interest of understanding the gear we have here, and appreciating
    I dont always have the luxury of calling in an experienced electrician at a
    moments notice, I wondered if people could enlighten me how you decide a
    replacement Starter Cap size. I would assume the starter cap size is
    relevant to amount of work that the motor does from start. Hence, an
    air-compressor motor is likely to have a dedicated starter winding, or a
    large starter capacitor to assist in producing a fair amount of torque to
    start things moving. A pressure-pump doesnt have a huge load, but obviously
    more than the bench grinder I raided the cap out of.

    I understand the starter cap produces an out-of-phase current, and this
    allows the motor to start to spin up(in lieu of a dedicated starter winding
    in the motor). Is this correct?

    What is the cause of possible damage to the motor if I replace the cap with
    something of the same voltage, but smaller capacity (like about half the
    value)?

    What happens if the starter cap replacement is too large for the
    motor(correct voltage but double the value)?

    I ask these questions not because I want to go into the motor repair
    business, but because this equipment tends to fail when it is most needed,
    and it tends to when there is no chance of getting an expert in. Best
    example is when it's been raining for 5 days, and the road to town(100 kms)
    is non-existant. It generally falls to me to resolve the issue one way or
    the other.

    Any thoughts appreciated.

    Cheers,

    Rod.......Out Back
     
  2. Ross Herbert

    Ross Herbert Guest

    On Thu, 02 Sep 2004 23:02:01 GMT, "Rod Out back"

    |All,
    |
    |House pressure pump died yesterday arvo. After ascertaining the unit was
    |still able to turn easily, I discovered the starter capacitor had blown.
    |
    |After much frenzied searching through electric motors for something to get
    |us through the night, I discovered a starter capacitor that got us through
    |the night. Luckily, I found that there is someone in town (Longreach) who
    |has another starter cap I can replace the original with. However, I was
    |advised that the starter cap I used was way too small, and could damage the
    |motor. Unfortunately, I didnt have any alternatives at the time, and so far
    |the motor continues to run...A quick dash to town this morning to get the
    |replacement cap is in order.
    |
    |Now, in an interest of understanding the gear we have here, and appreciating
    |I dont always have the luxury of calling in an experienced electrician at a
    |moments notice, I wondered if people could enlighten me how you decide a
    |replacement Starter Cap size. I would assume the starter cap size is
    |relevant to amount of work that the motor does from start. Hence, an
    |air-compressor motor is likely to have a dedicated starter winding, or a
    |large starter capacitor to assist in producing a fair amount of torque to
    |start things moving. A pressure-pump doesnt have a huge load, but obviously
    |more than the bench grinder I raided the cap out of.
    |
    |I understand the starter cap produces an out-of-phase current, and this
    |allows the motor to start to spin up(in lieu of a dedicated starter winding
    |in the motor). Is this correct?
    |
    |What is the cause of possible damage to the motor if I replace the cap with
    |something of the same voltage, but smaller capacity (like about half the
    |value)?
    |
    |What happens if the starter cap replacement is too large for the
    |motor(correct voltage but double the value)?
    |
    |I ask these questions not because I want to go into the motor repair
    |business, but because this equipment tends to fail when it is most needed,
    |and it tends to when there is no chance of getting an expert in. Best
    |example is when it's been raining for 5 days, and the road to town(100 kms)
    |is non-existant. It generally falls to me to resolve the issue one way or
    |the other.
    |
    |Any thoughts appreciated.
    |
    |Cheers,
    |
    |Rod.......Out Back
    |

    You didn't give any particulars of your pump, ie. make, model, motor
    hp or watts. Is your capacitor Start Capacitor or a Start/Run
    Capacitor? Usually, start caps are electrolytics of fairly high value
    and they are switched out of circuit several seconds after the motor
    is running by a centrifugal switch. Start/run caps are usually film
    type (mylar/polypropylene) caps of much lower values of up to approx
    50uF.

    If it is a domestic pressure pump it will probably be around 1.0 -
    1.5HP and will most likely be a permanent split capacitor motor
    http://www.engin.umich.edu/labs/csdl/ME350/motors/ac/induction/1-phase/index.html

    This type of motor is commonly used on Davey Dynajet pumps
    http://www.davey.com.au/files/D864-1-5K-0301-GPW.pdf used for domestic
    water supplies.

    The series capacitor will probably be not more than a 30uF/440V unit
    if your pump is of similar size (up to 1.1kW).

    Since the capacitor is permanently in series the reactance of the
    capacitor will largely determine the starting torque available. If
    too small a capacitor is used the motor may stall when trying to start
    into a load requiring high torque and this may damage the motor.
     
  3. Rod Out back

    Rod Out back Guest

    Ross,

    Thanks for replying. From your details I expect it is a start\run
    capacitor, and the value on the capacitor is 16uF. Cannot remember the power
    rating for the motor, but I think it is around 1hp (~750w). The local fridge
    mechanic in town had some spare 20uF units that seem to have done the job
    (now I have a spare one as well). All working now, and we have water again!

    The capacitor I installed last night to get me out of trouble was only a 5uF
    one, but it was all I could find (and after ransacking 5 motors to find this
    one!). I notice that the motor hummed a lot louder when running this 5uF
    cap, and was very quiet when I replaced it with the 20uF unit at lunch time.
    I would assume it was having a lot of trouble producing the torque to turn
    the motor with the small cap??

    Thanks again for your reply.

    Cheers,

    Rod.......Out Back
     
  4. Kevin Ettery

    Kevin Ettery Guest

    Most single phase induction motors have a dedicated starter winding, which
    is the winding a starter capacitor is wired into. The theory's a bit
    complex, but essentially the starter winding (including capacitor)
    interaction with the main winding produces a torque that allows the motor to
    begin spinning - without it the motor is likely to just sit there and hum
    (and eventually overheat) unless you happen to switch the power on at the
    right instant. It is possible to start the motor by moving the shaft/pulley
    by hand (WATCH OUT FOR THE FINGERS THOUGH!).

    Dedicated starter winding, unless they've been especially designed to
    remain in circuit, usually are not rated for continuous use and so if the
    start capacitor is not large enough to start the motor turning (or the
    cetrifugal switch doesn't open) you'll cook your starter winding fairly
    quickly.

    Ciao



    Kevin
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-