How much voltage overhead is there in large electrolytic capacitors?

Thread Starter

J-Erbes

Joined Jun 16, 2019
27
NOTE: A SUMMARY OF THIS THREAD WILL BE FOUND IN POST #79.

I'm putting together a DC-Injection braking circuit for my brother's big 120V band saw. This thing will coast quietly for a good 30 seconds when turned off. Very dangerous.
Anyway, I've got some 6,600 MFD 150 volt electrolytics to use for the braking circuit. They will get charged up through a 1K resistor and diode while the band saw is running, and then discharged back through a 20 to 50 ohm resistor and the 120 V motor windings when the saw is shut off.
120 volts rectified AC will put 170 DC into the capacitor.
The electrolytics are marked for 150 V.
If the band saw is only used a few minutes at any time for a total of a couple of hours per year, how long can I expect the caps to last?
The caps will be fully discharged most of the time and only charged while the band saw is running.
 
Last edited:

BobTPH

Joined Jun 5, 2013
8,974
None.

A diode + capacitor rectifying 120VAC will produce 170V. 150V is too low, you need a 200V capacitor.

Wouldn’t simply shorting the terminals give you enough braking?
 

Thread Starter

J-Erbes

Joined Jun 16, 2019
27
None.

A diode + capacitor rectifying 120VAC will produce 170V. 150V is too low, you need a 200V capacitor.

Wouldn’t simply shorting the terminals give you enough braking?
Yes, if it was a universal motor, which it is not.
It is an induction motor, like most table saws, band saws and drill presses.
Only those 10" miter saws seem to use universal motors.

I agree that I need a 200v cap, which I don't have.
I do have, however, about forty 6,600mfd 150v caps lying around.

My question still remains: What is the voltage overhead on an electrolytic? 5%? 10%? 15%? 20%?
Inquiring minds want to know...
 

djsfantasi

Joined Apr 11, 2010
9,163
Yes, if it was a universal motor, which it is not.
It is an induction motor, like most table saws, band saws and drill presses.
Only those 10" miter saws seem to use universal motors.

I agree that I need a 200v cap, which I don't have.
I do have, however, about forty 6,600mfd 150v caps lying around.

My question still remains: What is the voltage overhead on an electrolytic? 5%? 10%? 15%? 20%?
Inquiring minds want to know...
Your question has been answered.

 

nsaspook

Joined Aug 27, 2009
13,282
Yes, if it was a universal motor, which it is not.
It is an induction motor, like most table saws, band saws and drill presses.
Only those 10" miter saws seem to use universal motors.

I agree that I need a 200v cap, which I don't have.
I do have, however, about forty 6,600mfd 150v caps lying around.

My question still remains: What is the voltage overhead on an electrolytic? 5%? 10%? 15%? 20%?
Inquiring minds want to know...
If you have forty then make a 300V series/parallel equivalent from 4 of your 150v caps.
 

Thread Starter

J-Erbes

Joined Jun 16, 2019
27
If you have forty then make a 300V series/parallel equivalent from 4 of your 150v caps.
Yes. That, of course, is the obvious solution I had forgotten about as my 77-year-old brain is losing brain cells rapidly.
I did some additional research with the capacitor gurus at Panasonic and found that the 'overhead' or maximum surge voltage of electrolytics rated at <250v is 15% above the listed voltage. For a 150v cap this works out, conveniently, to 172 volts.
Each surge should be less than 60 seconds in duration with a rest period between surges of >five minutes.
Repeated over-voltage at the surge level results in a loss of capacitance over time.
Repeated over-voltage at >50% above the rated voltage typically results in an eventual capacitor short with accompanying fireworks.
For now, I'll try the 150v caps and see how long they last (probably longer than I will).
Thank you, to everyone that replied.
 

nsaspook

Joined Aug 27, 2009
13,282
Yes. That, of course, is the obvious solution I had forgotten about as my 77-year-old brain is losing brain cells rapidly.
I did some additional research with the capacitor gurus at Panasonic and found that the 'overhead' or maximum surge voltage of electrolytics rated at <250v is 15% above the listed voltage. For a 150v cap this works out, conveniently, to 172 volts.
Each surge should be less than 60 seconds in duration with a rest period between surges of >five minutes.
Repeated over-voltage at the surge level results in a loss of capacitance over time.
Repeated over-voltage at >50% above the rated voltage typically results in an eventual capacitor short with accompanying fireworks.
For now, I'll try the 150v caps and see how long they last (probably longer than I will).
Thank you, to everyone that replied.
This is a safety feature. Are you sure the risk is worth it?
 

Hymie

Joined Mar 30, 2018
1,284
If your motor produces 170V, charging a 6,600µF capacitor – the resulting energy stored by the capacitor will be less than 100J; or to put it another way, less than 100W of braking load for 1 second (I very much doubt that this will slow the saw blade rotational speed noticeably).

I would suggest you try resistive braking, switching in the load when the mains is disconnected.
 

MaxHeadRoom

Joined Jul 18, 2013
28,690
I would suggest you try resistive braking, switching in the load when the mains is disconnected.
That does not work that well with Induction motors, they do not self generate at switch off as a DC version does, normally it is done with DC injection.
 

Jon Chandler

Joined Jun 12, 2008
1,051
A 2 volt margin based on some guy's say-so, IF the voltage is 120VAC or less, which it very well may not be.

Maybe being careful around the coasting down blade will be safer than what happens when you're started by the flash and noise of an exploding capacitor?
 

Thread Starter

J-Erbes

Joined Jun 16, 2019
27
A 2 volt margin based on some guy's say-so, IF the voltage is 120VAC or less, which it very well may not be.

Maybe being careful around the coasting down blade will be safer than what happens when you're started by the flash and noise of an exploding capacitor?
It wasn't some random dude's say-so, the 15% surge overhead came off of Panasonic's Data Sheet on large electrolytics.

On the other hand, what if you are someone who happens to like the flash and noise of exploding capacitors? I mean, seriously, who doesn't?
You're not one of those guys that switches off your breakers when swapping out wall switches and receptacles, are you?
I don't, but yet here I am, still living (so far) at the ripe, old age of 77.

To be serious, in 62 years of electronics hobbying I have only had two capacitors sputter and pop on me--with no explosions. The caps were in a really old HP signal generator I bought at a swap meet. I should have brought the voltage up gradually, but I didn't.
Oh well, live and learn, I suppose.

Anyway, since you can buy a hundred 1N4007 diodes for $1.92, shipped, from China, and electrolytics are damn expensive these days, I'm just going to mount 30 or so diodes on a perf board to drop the charging voltage down to below 150V.
That way I can parallel-up a bunch of those 6,600 mfd 150v caps.
Wish me luck (or don't). I'm good either way.
 
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