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Thread: Capacitor calculation for toroidal transformer and rectifier bridge

1. 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?

Best,
Suat

2. 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.

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

Yes but was is the exact nature (type) of load? Motor etc?
Al.

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

130a 24v brushless motor and its speed controller.

6. 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.

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

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

8. 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.

9. 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. 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. Re: Capacitor calculation for toroidal transformer and rectifier bridge

Originally Posted by steve323
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. Re: Capacitor calculation for toroidal transformer and rectifier bridge

Originally Posted by Azalin

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.

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