3 HP TEFC Motor Starting Capacitors: Need help picking new capacitors

Discussion in 'Power Electronics' started by SEGACDX, Oct 12, 2018 at 5:56 AM.

  1. SEGACDX

    Thread Starter New Member

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    I have a Air Compressor motor that I need to get new capacitors for right away.

    The motor is a Brook TEFC 3 HP Single Phase 60 HZ 115V/230V, 34 amps at 115V, 17 Amps at 230V, 1725 RPM. It only has Two starting capacitors in parallel and no running capacitors. I added an image of the pair of capacitors from the motor.

    I can't find any good info on it or really any good info on similar type motors to compare and make a good choice.

    I've been looking online for answers but I am not finding much but bits and pieces of info.

    So here is where I am at, I have the motor running on 240V two pole with one ground on a 60 amp breaker it's a 115/230v type motor, the caps in it where Two 590-708 MFD or uF 110 volts 109mm Tall and 52.5mm in diameter.

    I was doing testing on my compressor before I put it back into service after a complete restoration job on the tank, motor and air pump, while testing and setting the pressure switch the caps vented a bit the next day after another test and I noticed the smell of electrolyte, then motor having trouble getting to speed and engaging the centrifuge switch. It was fine before but those caps are very old and not the right voltage so no wonder they went.

    So I've soured some new caps these CB60 800-960 MFD/uF 220-250V Caps 111.12 mm tall and 46.02 mm diameter , I was told 500 MFD/uF per HP under high starting load like an air compressor.

    I need to put two of them in parallel to get about 1500 MFD/uF. I was also told I needed to use a 1.5X volt rated cap compared to the line voltage so 165V for 110V and 330V for 220V.

    I've also found these CD60 450V 500MFD/uF Caps 100 mm Tall And 50 mm diameter, so I am wonder what I should go with.

    If anyone has some input I'd really appreciate it before I make a decision, I am leaning to the CB60 800-960 MFD/uF 220-250V since they are the closest to the caps I have in size and type without being 330V, Everything in 330V I found is ether low MFD/uF, super pricey or only on Alibaba and I can't order form them the shipping alone would be 250+ USD for a few caps.

    These's two are the best I could find that are affordable, fit my size requirements and are available.

    https://www.aliexpress.com/item/590...pm=a2g0s.13010208.99999999.265.e32a3c00fAwa1U

    https://www.aliexpress.com/item/AC-...pm=a2g0s.13010208.99999999.259.114c3c00klsTie
     
    Last edited: Oct 12, 2018 at 6:06 AM
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Definitely the 110vac ones are unsuitable.
    I would have considered 1000μF a tad high.
    But you do need 350v motor-start rated capacitors.
    250v is taking a chance on early failure.
    Item two is the best option.
    Max.
     
  3. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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  4. SEGACDX

    Thread Starter New Member

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    Thanks for the advice I'll get the 450V ones they are not to much more then the 250V.

    The two old caps where both 1180 uF combined so 1000 uF would be close to what was already installed.
    Just out of curiosity what would a normal uF be fore a modern 3 HP motor similar to mine, the one I have is old but in good shape.
    I wish I had a manual or any info on it but brook has no info on it.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A modern 3HP motor is usually 150μF to 200μF/Kw
    Older motors may be higher, the necessity is to get the start switch to operate before 3 seconds maximum
    Preferably around 2 sec's, otherwise damage to the caps can occur.
    You can check with a volt meter across the cap as to time it switches.
    Max..
     
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  6. SEGACDX

    Thread Starter New Member

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  7. SEGACDX

    Thread Starter New Member

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    I'm going to order two of those 450v caps and try running just one at first just to see if it works fine on just one, I'll make sure to have my multi-meter hooked into it while it's starting.
    I know modern capacitor start motors also have a run capacitor to improve efficiency, which my dose not.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes, surprising for a motor of that size!.
    Max.
     
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