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IndustryArena Forum > CNC Electronics > CNC Machine Related Electronics > Capacitor calculation for toroidal transformer and rectifier bridge
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  1. #1
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    Capacitor calculation for toroidal transformer and rectifier bridge

    Hi,

    Sorry if this is the wrong place for this question.

    I have a large toroidal transformer. Input is 220 VAC and output is 17.6 VAC 3kw. I also have a 150 amps rectifier bridge to convert the transformer output to DC. Output of the bridge is 24 VDC.

    Since the output power is too high which and how many capacitors (wired in parallel) do I need to add to the system?

    Thanks in advance.

    Best,
    Suat

  2. #2
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    What do you mean by 'Output Power too High'?
    If you mean how much capacitance to maintain that load, you will need many 10's of thousands, what is the exact nature of the load?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  3. #3
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Output of the transformer is 17 VAC 3000VA and the bridge that is connected to it outputs 24 VDC 3kw which is high I guess. I need to add capacitors between the bridge and the driver.

    I'm not an expert as you noticed but that's all I know yet. What I don't know is how many and what mf (or uf) capacitors do I need for a nice and steady 24v? And It must handle up to that 3kw load.

    I tried to explain as best as I can with my poor English.

  4. #4
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Yes but was is the exact nature (type) of load? Motor etc?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  5. #5
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    130a 24v brushless motor and its speed controller.

  6. #6
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    The bridge does not necessarily output 3kw only what the circuit or device needs, IOW, it may be seldom the full motor current is required, depending on load.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  7. #7
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Yes, motor will not pull a steady 3kw. That's the peak load.

  8. #8
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    A motor of that size would typically be much better used with a 3 phase supply and rectifier, the ripple on rectified 3ph is only ~5% instead of 100% with 1ph.
    Capacitance needed on 1ph probabally be in the 100.000's ufd.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  9. #9
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    245

    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    As Al mentions, the primary concern is the quality of the output signal.

    This is a somewhat standard AC to DC power supply, although the currents are quite large. The bridge rectifier converts AC power into positive pulses. Assuming Turkey has 50 cycle AC power, the rectifier will provide 2 pulses per cycle, or 100 Hz. The capacitor needs to hold this charge between pulses, while the load drains away the stored charge. A small capacitor will have a lot of voltage droop before the next pulse from the rectifier. A larger capacitor will hold more charge, so it will have less droop. Do you know how much droop is allowed?

    3 phase power provides 6 pulses per cycle so the capacitors only have to hold the charge for a short time. Not sure if this is an option in your case.

    Steve

  10. #10
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    The single phase transformer I have is very expensive so I prefer not to buy another one

  11. #11
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Quote Originally Posted by steve323 View Post
    Do you know how much droop is allowed?
    Steve

    The speed controller is a CC Edge 130a brushless speed controller. Normally this thing is designed to work with LiPo batteries so voltage droop is not a problem.

    Simply, can we say the more uf and v on the capacitor is the better?

  12. #12
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Quote Originally Posted by Azalin View Post

    Simply, can we say the more uf and v on the capacitor is the better?
    Yes, but it is not that simple. If your load is too high and the capacitors are not large enough then the capacitors make not much use. You will need to use several very large ones, connected in parallel.

    Also, what's not been mentioned here is a word of warning. You are not very familiar with electrical works, and these very large capacitors are potentially dangerous if you are not doing things right. They can literally explode in your face, endangering your eyes, so be careful. Rule number one: Use only quality products, not cheap eBay capacitors. Rule number two: Be EXTREMELY careful when you solder these, the polarity is very important, connect one wrong and it may blow up. Rule number three: When you switch the power off and if you must handle the capacitors then you must regard those as still charged, so make sure they are really discharged BEFORE you start working on them.

    The voltage droop of a battery is different from a PSU because a battery is ALWAYS DC, just less and less as it discharge. If you load a PSU above what the capacitors can manage to smooth out then you will have a pulsing DC out, not just drooping. Drooping should not be the case, unless you overload the PSU. OK, no load voltage is a bit higher, but there should be no significant drooping.

  13. #13
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    You may be better off and safer to use a series inductor (choke) in combination with a capacitor to smooth out the supply. Increasing the capacitance will increase the off load voltage significantly. Large charged capacitors store lots of energy and should have a discharge resistor across the terminals for safety.

  14. #14
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    But I still don't know what transformer I need to use.

    Sent from my MI 5s Plus using Tapatalk

  15. #15
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    I thought you said you had it already?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  16. #16
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    umm, no? That's why I'm here

  17. #17
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Quote Originally Posted by Azalin View Post
    The single phase transformer I have is very expensive so I prefer not to buy another one
    Why this ? I took it you had it already!
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  18. #18
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Quote Originally Posted by Azalin View Post
    But I still don't know what transformer I need to use.

    Sent from my MI 5s Plus using Tapatalk
    You describe the transformer you are using in your first post or are you planning on getting another one ?.

  19. #19
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Ahh I'm sorry. I meant I need to know what capacitor I need. I don't know why I wrote "transformer".

    I have the transformer but I don't have a cap yet.

    Sent from my MI 5s Plus using Tapatalk

  20. #20
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    Re: Capacitor calculation for toroidal transformer and rectifier bridge

    Considering that they are no expensive, you may be better of just using trial and error. Most powers supplies of that nature would be using 50,000 uf rated at 40v (at least), just try one and see what happens. They cost around £5 GDP or about 6 Euro's, If it works fine, if not add another. Make sure you measure the offline voltage before you connect to the servo, also be aware that large capacitors will have a large inrush of current when you first switch on which may trip your breakers.

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