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Capacitor for induction Motor

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  1. #1
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    Default Capacitor for induction Motor

    G'day,

    I've picked up an old blower which has what looks to be a small induction motor. based on the 4 leads and the remnant of the wiring diagram (not exactly clear) it looks like a pretty standard setup with a start winding (probably with centrifugal switch) that should have a capacitor on it.

    Unfortunately it just has the four leads, and none of the associated switches and starting capacitor it would have had. My problem is I don't know what size capacitor I should be using (or if it matters alot).

    No nameplate on the motor, but it is about the size of my fist ... couldn't imagine it being much bigger than 1/6hp. Got it locally, so I am assuming its 240V.

    I don't have any caps lying around and would rather avoid buying a huge selection. Anyone have a rule of thumb or similar motor?

    Ps, the blower fan is integral to the motor, so would prefer to get it up and running rather than replacing.

    Any hints appreciated!

    Chris

  2. #2
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    I will probably get shot down in flames but I will have a go.

    The Capacitor is to prevent too much current being drawn on startup and once its spinning it is taken out of circuit by the switch.

    The smaller the capacitor value the lower the starting current so I would go for a low value, you could probably scrounge one from just about anything left out on council cleanup day.

  3. #3
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    i think you're a bit off on this one. my understanding of the capacitor in single phase motor was to offset a winding enough to create a phase difference to allow the motor to start.

    you seem to be thinking more of power factor correction than motor starting.

    From the top of my head, and without seeing the motor, two wires would go to a winding with heavier gauge wire, the other two would go to windings with lighter gauge wire. Take one wire from each winding, connect these. They're your neutral. I think they need to be of the same polarity in terms of the winding. Good luck with that.

    The other two wires, they each go the the capacitor. I think the heavier gauge wire is the start winding as it has less inductance than the other so it is easier to offset. The lighter gauge winding would then be the run winding, so this is connected to the active and the other side of the cap.

    i'll have a gander at my school books, but i think it'd be something like that, or a basis to start upon. Bear in mind, it may be a cap start-cap run, not just cap start.

  4. #4
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    Default Capacitor Start Motor

    Unfortunately it just has the four leads, and none of the associated switches and starting capacitor it would have had.
    Are you sure it had a switch ( centrifugal )? It may be a Capacitor Start-And-Run Motor which has no centrifugal switch.These type of motors used a low value capacitor usually in the range of 3 to 16 microfarads (paper or oil filled). Because of the low value capacitor these motors have a low starting torque and are suited to applications like fans.

    cap-start-run-motor.jpg

    If it is a Capacitor Start-And-Run Motor I would probably try a 3F to start with and see if you get enough starting torque out of it.

    Cheers
    Juan


    "If the enemy is in range, so are you."

  5. #5
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    Thanks for the response guys, that's helpful. The wires are all the same gauge, so that's not much help but the remnant wiring diagram at least tells me which wire is neutral. I wondered if it was a capacitor start and run, as the centrifugal switch is not obvious, but it has a hub that looks suspiciously like it may contain one. The whole motor rotates, and the blower sucks air over it for cooling. The shaft/hub is stationary in this case, and is where the wires feed in.

    The low capacitance thing makes sense, so I'll start with a low value. Once it's going I suspect things will become clearer! I might have a sticky-beak at the capacitors my other workshop motors use.

    Thanks again.

  6. #6
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    if you spin the shaft, you should be able to hear the centrifugal switch open and close. If not, put an ohmmeter across it and try again. if that doesn't work, i'd be thinking pretty confidently that there isn't one

  7. #7
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    Quote Originally Posted by thatirwinfella View Post
    if you spin the shaft, you should be able to hear the centrifugal switch open and close. If not, put an ohmmeter across it and try again. if that doesn't work, i'd be thinking pretty confidently that there isn't one
    You'd have to spin it quick to lift a switch if it had one. I think your ego might be writing cheques your body can't cash

    I dare say its just a permanent cap motor. With an ohmmeter you should be able to identify two sets of windings. The set with the lowest resistance is the run set. connect one wire from each set together as the common and put the cap in series with the start windings.

    The value of the capacitor will determine the starting torque, being connected to a centrifugal blower you won't need much.

    Edit: just saw the pitcure Juan posted. could have saved me this post

  8. #8
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    Default Capacitor may not be needed

    Quote Originally Posted by acton View Post
    G'day,

    I've picked up an old blower which has what looks to be a small induction motor. based on the 4 leads and the remnant of the wiring diagram (not exactly clear) it looks like a pretty standard setup with a start winding (probably with centrifugal switch) that should have a capacitor on it.

    Unfortunately it just has the four leads, and none of the associated switches and starting capacitor it would have had. My problem is I don't know what size capacitor I should be using (or if it matters alot).

    No nameplate on the motor, but it is about the size of my fist ... couldn't imagine it being much bigger than 1/6hp. Got it locally, so I am assuming its 240V.

    I don't have any caps lying around and would rather avoid buying a huge selection. Anyone have a rule of thumb or similar motor?

    Ps, the blower fan is integral to the motor, so would prefer to get it up and running rather than replacing.

    Any hints appreciated!

    Chris
    Hello Sir,

    It may not need a capacitor fore starting. Examine it carefully.
    Attached diagram below.

    Thanks and Regards


    Split-phase
    The split phase motor, also called an induction-start/induction-run motor, is probably the simplest single-phase motor made for industrial use, though somewhat limited. It has two windings : a start and a main winding, Figure 1. The start winding is made with smaller gage wire and fewer turns relative to the main winding to create more resistance, thus putting the start winding's field at a different angle than that of the main winding, and causing the motor to rotate. The main winding, of heavier wire, keeps the motor running the rest of the time.
    A split-phase motor uses a switching mechanism that disconnects the start winding from the main winding when the motor comes up to about 75% of rated speed. In most cases, it is a centrifugal switch on the motor shaft.
    The split-phase motor's simple design makes it typically less expensive than other single-phase motor types made for industrial use. However, it also limits performance. Starting torques are low, typically 100% to 175% of rated load. Also, the motor develops high starting currents, approximately 700 to 1,000% of rated. Consequently, prolonged starting times cause the start winding to overheat and fail; so don't use this motor if you need high starting torques.
    Other split-phase motor characteristics: Maximum running torques range from 250 to 350% of normal. Plus, thermal protection is difficult because the high locked-rotor current relative to running current makes it tricky to find a protector with trip time fast enough to prevent start-winding burnout. And, these motors usually are designed for single voltage, limiting application flexibility.

  9. #9
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    Quote Originally Posted by Learner View Post
    Hello Sir,

    It may not need a capacitor fore starting. Examine it carefully.
    Attached diagram below.

    Thanks and Regards


    Split-phase
    The split phase motor, also called an induction-start/induction-run motor, is probably the simplest single-phase motor made for industrial use, though somewhat limited. It has two windings : a start and a main winding, Figure 1. The start winding is made with smaller gage wire and fewer turns relative to the main winding to create more resistance, thus putting the start winding's field at a different angle than that of the main winding, and causing the motor to rotate. The main winding, of heavier wire, keeps the motor running the rest of the time.
    A split-phase motor uses a switching mechanism that disconnects the start winding from the main winding when the motor comes up to about 75% of rated speed. In most cases, it is a centrifugal switch on the motor shaft.
    The split-phase motor's simple design makes it typically less expensive than other single-phase motor types made for industrial use. However, it also limits performance. Starting torques are low, typically 100% to 175% of rated load. Also, the motor develops high starting currents, approximately 700 to 1,000% of rated. Consequently, prolonged starting times cause the start winding to overheat and fail; so don't use this motor if you need high starting torques.
    Other split-phase motor characteristics: Maximum running torques range from 250 to 350% of normal. Plus, thermal protection is difficult because the high locked-rotor current relative to running current makes it tricky to find a protector with trip time fast enough to prevent start-winding burnout. And, these motors usually are designed for single voltage, limiting application flexibility.
    When was the last time you saw a split phase motor on a blower?

    Blower = low start torque. Permanent Cap setup is the simplest and usually most reliable.

  10. #10
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    Quote Originally Posted by Timmo View Post
    When was the last time you saw a split phase motor on a blower?

    Blower = low start torque. Permanent Cap setup is the simplest and usually most reliable.
    Hello timmo Sir,

    I think you are confused. Split phase motors are used for typically for blower/fan applications with low starting torque.

    http://www.iprocessmart.com/leeson/l...se_article.htm

  11. #11
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    Quote Originally Posted by Learner View Post
    Hello timmo Sir,

    I think you are confused. Split phase motors are used for typically for blower/fan applications with low starting torque.

    http://www.iprocessmart.com/leeson/l...se_article.htm
    Learner, don't get your knickers in a knot but keep calling me sir and we'll all get along fine.

    No, not confused. Just not common in small motors around ~120watts as this one is. If it had a cap it wouldn't have a centrifigal switch on a low torque situation now would it?

    Split phase motors are usually reserved for larger blowers and pumps from the 1/2 hp mark upwards.

    I'm sure a pic of the motor in question could clear it up.

  12. #12
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    Quote Originally Posted by Timmo View Post
    Learner, don't get your knickers in a knot but keep calling me sir and we'll all get along fine.

    No, not confused. Just not common in small motors around ~120watts as this one is. If it had a cap it wouldn't have a centrifigal switch on a low torque situation now would it?

    Split phase motors are usually reserved for larger blowers and pumps from the 1/2 hp mark upwards.

    I'm sure a pic of the motor in question could clear it up.
    Well prove your point.

    I disagree with you at present in a friendly way of course.

  13. #13
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    Quote Originally Posted by Learner View Post
    Hello timmo Sir,

    I think you are confused. Split phase motors are used for typically for blower/fan applications with low starting torque.

    http://www.iprocessmart.com/leeson/l...se_article.htm
    Learner,
    so tell me, do you know this because you are an electronics engineer as you claim in one post or because you are an auto electrician as you claim in another post?

    Mick
    "If you need a machine today and don't buy it,

    tomorrow you will have paid for it and not have it."

    - Henry Ford 1938

  14. #14
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    Definitely a capacitor involved in this case, as that much was preserved on the limited remnant wiring diagram. Whether it is permanent or switched in is a little academic anyway by the sounds of it. I haven't made much progress on this, but have identified some bearing noise which suggests I'll be looking at the inside of this one after all! It might end up being more worthwile to rig up an alternate arrangement.

    I'll take a photo of the offending motor once I find the wifes camera. It's definitely built for the purpose. German made apparently. My guess it is out of a rangehood style extractor arrangement, although industrial.

    A quick peek at the easily accessible motors in the workshop revealed 100uF as the most popular value. these were all much bigger and dodgy chinese built motors, so no surprise there.

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    Quote Originally Posted by dougpurd View Post
    I will probably get shot down in flames but I will have a go.

    The Capacitor is to prevent too much current being drawn on startup and once its spinning it is taken out of circuit by the switch.

    The smaller the capacitor value the lower the starting current so I would go for a low value, you could probably scrounge one from just about anything left out on council cleanup day.
    Disregarding what type of motor it is, a smaller size capacitor should be avoided. In this situation & if in doubt, a larger size capacitor should be used. Why? The lower value capacitor may allow the winding to approach resonance & thus create a dangerously high voltage (can be in the thousands of volts).

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