Capacitor holding HV for a long time

Thread Starter

einstamas

Joined Aug 2, 2020
6
Hey. I am crafting a super simple "trash-made" rectifier with a puffer capacitor, for a 220vdc motor.
I tried cabling the "circuit" to the multimeter instead of the motor(in case i mess up something, it is a smaller problem) and experienced that i have to wait about 15-20 minutes before the capacitor drains back to 0 or close.
What might solve my problem? I am thinking about placing a resistor parallel to the capacitor, might it help? This is my current design:
1596366742818.png

How to choose the wattage and size of the resistor? I'll use a 300w motor from this, it means no problem to my diodes as they are extra durable and hardy, but i am afraid of "overloading" the resistor and putting it to the flames.
 

dendad

Joined Feb 20, 2016
3,625
You can work out the wattage by W=(VxV)/R.
But if you use a 100K 1W resistor it will dissipate around 400mW and that will be ok.
Caps can hold a charge for quite a time. Where I used to work, Radio Australia transmitter site, the high voltage caps, like 30KV, some the size of a 2 gallon oil can or bigger, were stored with their terminals wired together to prevent them from accumulating and holding a dangerous charge.
 

Thread Starter

einstamas

Joined Aug 2, 2020
6
You can work out the wattage by W=(VxV)/R.
But if you use a 100K 1W resistor it will dissipate around 400mW and that will be ok.
Caps can hold a charge for quite a time. Where I used to work, Radio Australia transmitter site, the high voltage caps, like 30KV, some the size of a 2 gallon oil can or bigger, were stored with their terminals wired together to prevent them from accumulating and holding a dangerous charge.
Thanks for the help, i just attached some resistors as i was waiting, you might know the feeling when you just can't wait and want to progress :D
Other thing is that when i use the multimeter in VDC it claims that the output is 300vdc, when i use it in vac mode it claims that the output is around 600vac, both of those are quite impossible as i use a 230vac network. I think it might be because i use a cheap multimeter and my signal might be wavey, not totally dc.

Do you think it is safe to link that power to the 220vdc motor, or should i keep working on it?
 

dendad

Joined Feb 20, 2016
3,625
If you have 230VAC coming in, the DC after the rectifier will be ACV x 1.414 , so 230 x 1.414 = 325VDC.
A rectifier will charge the cap to the peak voltage of the incoming AC wave form. For a sine wave, that equates to the magic 1.414 number. Your multimeter is calibrated to display the "RMS" or Root Mean Square voltage.
Have a read up on some AC theory. It is pretty interesting.
Also, if your motor is running, there is likely a lot of spiky noise that will confuse your meter.
 

Thread Starter

einstamas

Joined Aug 2, 2020
6
If you have 230VAC coming in, the DC after the rectifier will be ACV x 1.414 , so 230 x 1.414 = 325VDC.
A rectifier will charge the cap to the peak voltage of the incoming AC wave form. For a sine wave, that equates to the magic 1.414 number. Your multimeter is calibrated to display the "RMS" or Root Mean Square voltage.
Have a read up on some AC theory. It is pretty interesting.
Also, if your motor is running, there is likely a lot of spiky noise that will confuse your meter.
I am aware to link the motor to that rectifier yet, as it is made for 220-240vdc. I don't want to kill the motor, because i live in a really poor country and you can't get something like that every day. If i understand you correctly, then the output of my rectifier is actually 300v+? What should i do, use a resistor "splitter"(idk the english name, same thing as potmeter) to lower the voltage?
 

dendad

Joined Feb 20, 2016
3,625
For some DC motor applications, it does not need the capacitor.
But what is your motor type? Can you find a data sheet of it?
Do you have any speed control?
Does it need to reverse?
What is your application?

EDIT:
I'm off to bed now, so maybe someone else can help you further...
 
Last edited:

Thread Starter

einstamas

Joined Aug 2, 2020
6
For some DC motor applications, it does not need the capacitor.
But what is your motor type? Can you find a data sheet of it?
Do you have any speed control?
Does it need to reverse?
What is your application?

EDIT:
I'm off to bed now, so maybe someone else can help you further...
it is an mcc 52/64 - 148/LG2 washing machine motor. No datasheet, i only found some russian videos showing it's connectivity etc. I have no speed control, nothing. I want to use it for a blade saw or something like that later, 12000rpm is enough for that. it was spinning a heavy drum before, so i think it must be no problem. But had to dispose the original controlling circuit because it was damaged and filled with gel anyways.
so tldr i have close to 0 information about this motor, i am just trying to put life into it, safely.
 

Thread Starter

einstamas

Joined Aug 2, 2020
6
Problem solved, i just saw a polish guy directly linking it to the network, damn; it works.
That's how life works. You struggle with some problems and a slavic or indian guy pops up with the strangest and easiest solution, and teach you something new. Anyways thanks for the help, I'll look for a speed controller because this is a BEAST :'D
 

dendad

Joined Feb 20, 2016
3,625
I did a quick Google search on my iPad (in bed) and found a number of Youtube videos using that motor. It looks to be an AC motor.
The iPad will not let me copy and paste the links, sorry.
Have a look and see what you can find.


EDIT: Oh, I see you found something :)
Have fun, and be careful to keep safe!
 

Thread Starter

einstamas

Joined Aug 2, 2020
6
I did a quick Google search on my iPad (in bed) and found a number of Youtube videos using that motor. It looks to be an AC motor.
The iPad will not let me copy and paste the links, sorry.
Have a look and see what you can find.


EDIT: Oh, I see you found something :)
Have fun, and be careful to keep safe!
Yeah, strange thing is that the first thing i checked was that its a 220vdc motor, the label said. Looks like it's not :D
 
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