Hi,
I have a high amperage 12V AC transformer (30 to 50 amps). I would like to use this to power a few DC motors. The problem is that the capacitor after the full wave rectifier has to be insanely large. A 10.000uF cap is giving a lot of ripple and also heats up a lot so it will probably explode after 5 mins. Can DC motors run without a capacitor filter or is there another solution? Or do i need a 1.000.000uF capacitor or smth?

 

Can DC motors run without a capacitor filter ?
For the most part yes. You make no mention of what you are using for rectification or any link to the diodes. You make no mention of a circuit diagram? You make no mention of the current draw of the motor which would be instrumental in choosing a capacitor if a cap is even needed. Your capacitor should not heat up at all assuming a good capacitor with the correct working voltage rating.

A 10.000uF cap is giving a lot of ripple and also heats up a lot so it will probably explode after 5 mins.

Giving a lot of ripple is not very descriptive. You really need to provide a better description of what you have and exactly what you are trying to do.

Ron

 

I added the schematic in the attachment. The circuit is just a simple unregulated power supply. The transformer outputs around 12V RMS AC, but can be adjusted (there are different primary inputs for 220-230-240V). The bridge rectifier is 1 IC called GBPC5008, rated for 50A continuous and 800V peak. That one is mounted to a big heatsink. The approximate current draw would be around 30 amps, but of course the inrush current of the motors is much higher. (it are 10 seperate motors btw, so around 3A each). The capacitor i am using right now is a 10000uF with a maximum voltage rating of 35V, but due the high charging and discharge currents i dont think it will survive. The ripple i didnt actually measure but i know for sure it will be a lot.

 

Your capacitor is bad.

 

Your capacitor is bad.

uhh it has never been used before so probably not

 

Your capacitor is bad.

I agree, your capacitor should not be warming up or getting hot unless it is breaking down. Again, the capacitor should not be necessary. Try running your motor(s) less the capacitor.

Ron

 

uhh it has never been used before so probably not

Uhh, if it is heating up like you say while operating, then probably so. They heat up due to dielectric breakdown, which means that you probably bought a counterfeit cap with a 35V rating that is really a 15V (or less) rating.

 

well i thought capacitors of such size were not intended to charge/discharge more than 10 amps. The motor seems to run fine without a capacitor as i dont see a lot of difference apart from that it runs just a bit slower. Is it really just fine to feed the rectifier output directly into a motor?

 

uhh it has never been used before so probably not

Then you explain why a capacitor rated for 35 VDC coming off a 12 VAC rectifier is getting hot? With a 50 Amp load coming from 50 or 60 Hz mains power a filter cap would start around 1,000 uF. The fact that a capacitor is new in box really means nothing. Have you even looked at filter capacitor requirements and how values are derived?

Yes, it is just fine to run the motor less a capacitor. The only thing a capacitor will do is maybe facilitate and easier motor start. The overall motor speed should not change with or without a capacitor assuming the motors draw their rated current.

Unloaded coming off a 12 VAC (RMS) source less the diode bridge forward voltage drop your capacitor will charge to about Vpk (Peak Voltage) of about 1.414 * 12 = 16.968 VDC.

Ron

 

I get it, i will just run it without the capacitor than, thanks

 

If your voltage without cap is lower now than the motor plate voltage you can add a few over-winds on the secondary usually.
Max.

 

Yeah the transformer has multiple primary inputs so i can adjust it to the right rms value

 

Yeah the transformer has multiple primary inputs so i can adjust it to the right rms value

Reread Max's post, secondary NOT primary.

 

well if it has multiple primary inputs to step up the output voltage i dont need to adjust the secondary windings as max says

 

Normally you cannot do this, but in this case, range of the taps is <10% so you could try lowering a range and monitor for overheating.
The better way is to add an overwind.
Max.

 

well if it has multiple primary inputs to step up the output voltage i dont need to adjust the secondary windings as max says

You're looking at it wrong. Those primary taps are to get the same output/secondary voltage from many input/primary voltages. For your purpose you would pick and use the primary consistent with your local voltage, then remove from the secondary to get the output you need. Plus it is usually easier to remove winding's from the secondary because they are the outermost set of winding's, primaries are usually closer to the core of a transformer.