Capacitors

[Glyn]

I recently got help off members regarding a capacitor for my pillar drill. It's sorted now but I can't help thinking, what exactly do capacitors do? I've never really thought about them until now but..... There's one on my drill, my compressor and when I take the side off my mig there's 5 of them in there.
Yet, open my other mig and there's none! Why would one welder need them and the other not? And do they do the same job in the welder as they do no my drill? Now, who's going to enlighten me?

 

[rtbcomp]

A capacitor is a short-term means of storing electrical charge.

In the welder the dc produced by the rectifier varies between zero and the output voltage. The capcitor(s) will charge as the voltage rises and discharge when it decreases, thus evening the voltage out and giving a smoother arc.

The better quality MIGs have them, the cheaper ones don't!

On AC they charge, discharge and reverse charge thus effectively allowing AC to pass through, but they don't dissipate power so they remain cool unlike a resistor would. The "resistance" or impedance of a capacitor depends on its value and the frequency of the AC.

They also provide a phase shift on AC, ie the voltage rise accross a cap will follow the current into it (a coil or inductance does the opposite, the voltage leads the current)

My AC motor theorey is non-existant, so someone else will answer that bit for you!

 

[shenion]

As rtbcomp says, caps store charge or are used to adjust phase.

In motors, if you have a single winding and apply AC power, it will just vibrate. There is nothing to make the motor go in any direction. On induction motors, if you manually spin it, it will run.

The easy solution is to use 3-phase and 3 pairs of poles. The core will follow the magnetic field as it runs around the poles.

So, what to do with only single-phase power? You use caps to generate a second phase. That way, the core will have a magnetic field that instead of moving back and forth between two poles, the field moves around the poles in sequence and the core can just follow it.

Motors may have 1 or 2 caps. It may have a "run" capacitor. This will provide a delayed phase all the time. A "Start" capacitor will create a delayed phase only during spinup. Some motors can have both. These caps are usually 2 to 160uf.

For MIG, TIG and Arc welders, the caps provide 3 functions:

1. Smoothing the output voltage. They also increase the current in short-arc transfer when the wire touches the weld pool. These are usually very large DC capacitors and in banks. A total over 100,000uf is not uncommon. These large caps are expensive so they are only included in higher-end welders. A large bank alone can cost more than a cheap welder.

2. Reducing noise. Rectifiers, and welding is non-linear. This creates large surges and harmonics on the power lines. Caps on the transformer and rectifiers will surpress this from being emitted on the lines. Cheap MIGs leave these out and can cause damage to electronic equipment on the power lines. They also protect the rectifiers from damage due to spikes. These are usually about 0.5uf or so.

3. Power factor correction. Welders tend to be inductive; that is, the current lags the voltage. That causes the current X voltage to be larger than the power. This is unfreindly to power companies and it increases the losses in the power lines with no billable gain. Adding caps to the power will add a leading current that will offset the inductive current. The net is the current is closer in phase with the voltage. Note: this comes at the expense of a higher idle current.

 

[Glyn]

Well Rtbcomp, Shenion, thanks for taking the time to reply but there was a loud wooshing noise as all that went flying over my head! I guess I'm not 'ready to know' (too dumb) what they do. BUT.... I take it when I use the welder without the capacitors, (Miller 150 amp) at medium to high, the arc starts with quite a violent pop? I sort of understood the part about the motor vibrating shenion, I can get a picture in my head of what they do to make it go round but...They also provide a phase shift on AC, ie the voltage rise accross a cap will follow the current into it (a coil or inductance does the opposite, the voltage leads the current) and (Smoothing the output voltage) and(Reducing noise.) and ( Power factor correction.) All too much for me! But hey, thanks for replying.

 

[shenion]

Capacitors store charge. With a constant voltage, the draw no current. With a increase/decrease in voltage, the draw/supply current. On AC, caps responed to the slope of the sine wave, that happens to be 90-degrees ahead of the voltage (to go positive, it first has to slope up). So the current leads the voltage.

Inductors store current in a magnetic field. The voltage across the inductor sets the slope of the current. Apply DC to an inductor and it will start at 0 current and rise up to infinity (in theory.) So the current lags the voltage.

Both can be used to create a different delayed/advanced current on AC.

As for smoothing, when you rectify AC to DC you get pulsed DC. Capacitors store charge during the pulses and provide it when the voltage drops down.

Power factor is hard to visualize. At any instant, the power is voltage x current. With AC there is power factor. Suppose the current is not in line with the voltage. You end up with the power going through 4 states in each cycle:

Voltage +, Current + = power transferred to load.
Voltage +, Current - = Power fed back to source
Voltage -, Current - = power transferred to load
Voltage -, Current - = Power fed back to source.

A pure inductive or capacitive load will spend equal time in all 4 states. The net result is power is going back and forth between the source and load with no net power being delivered.

A pure resistive load (heaters, incandescent lights) are always in states 1 and 3.

you want power to be in the 1st and 3rd states (power delivered to load.) Power factor is the fraction of time power is in these states. I.e a power factor of 0.8 will have 20% of the power bouncing back and forth and 80% getting delivered to the load.

A low power factor means ther is excessive current flowing that does no work. This higher current increases losses in wires, switches etc.

If you look at the big Miller TIG welders, there will be normal an "P" versions. Both provide and equal net input and output power but the "P" versions (with Power Correction) will draw less current as less power bounces back and forth between the supply and load.

 

[Swifty]

Capacitor

So as I understand it if I put a capacitor on the output of the rectifier it will make my welder work better.
If this is the case what size and voltage will I need?

Swifty

 

[shenion]

They need to be after the rectifier and before the reactor.

I used two 30,000uf 50V caps on my Miller. I figure they are effective to about 50 amps. They do increase the minimum current significantly. Not a problem on mine as I can set the voltage down to about 11v.

Rectified AC has a 71% effective voltage of the peak. Adding caps will increase the average close to the peak. So, if your minimum is 18V/30A, adding big caps will increase the voltage to 25V at >40A. May not be desired if you do body work.

A smaller value will add stability while not significantly increasing the voltage at lower currents. A 5000uf/50V or so will do. It will help keep the arc going between cycles.

These need to be industrial grade caps, preferably rated at 105C temp. 50V is minimum, higher voltage is better.

you should also add a bleed resistor, a 180 ohm, 5watt will do. The bleed resistor will remove the charge on the caps when you release the trigger. Mine is 250 ohm, it takes about 15 seconds before the voltage goes back to 0.

Adding a large bank of caps will increase the inrush current significantly. May cause issues with fast circuit breakers.

 

[R Kraft]

The start capacitor lowers the AC series resistance of the start coils windings, to allow greater current flow.
The torque creating magnetic field that starts the motor is proportional to the amps flow times the windings.

On a DC welder they allow greater instantaneous current when starting an arc and increase the open circuit voltage, making it easier to start an arc.
They also allow the reactor/choke to function better,sort of like a center fed antenna system.

The capacitors need to be the right voltage for the application .
Too low a voltage rating and they may explode.
Too high a voltage rating and they may start to deform to a lower voltage rating at a higher capacitance.
If you get oldish electrolytic capacitors they should be reformed with the working or peak voltage at a low current to prevent heating and damage.
Robert

 

[Glyn]

Well chaps, you cured me! I no longer care or have the slightest desire to know what those silver tubes with wires going in and out do!
BUT..... I feel even more confident that if I've got any problems someone on here can help!
now, anybody want to know anything about plastering??? Nah, didn't think so.
Thanks anyway for TRYING to educate me.

 

[rtbcomp]

Well chaps, you cured me! I no longer care or have the slightest desire to know what those silver tubes with wires going in and out do!
.......

Works every time!

 

[Glyn]

My son said he's found a load of thermistors, do I want any he asked???? Err....NO!

 

[bill_gj]

So, what's a good way of making the inductor/reactor?

I was intending to wind some welding cable (say 20 turns) around an inch-ish bit of plastic waterpipe stuffed with soft iron wires (florist wire). Not very scientific, I know.

 

[shenion]

Here is one i made:

About 36 turns of #4 wire wrapped around some 1.5" cast iron pipe. Then a frame of mild steel square tubing. It worked well. There are 2 windings as I only had 2 lengths of #4 wire.

I will probably build another using the same 1.5" pipe and windings with 1.5" wide aluminum flashing with insulating paper soaked in epoxy..

 

[Glyn]

Pah! Thats nothin. I put a new plug on the toaster the other day!

 

[Tom_M]

Had some bad experiences with electrolytic capacitors while in the TV business. Gas build up in them, when faulty, can turn them into missiles

Tom

 

[madkayaker]

had good fun at school reverse connection electrolytics and ramping up the voltage seeing how far you can fire the top

 

[bill_gj]

Thanks Shenion - I think I was on the right lines, but my idea is a bit poofy compared to your pic.
I must wrap some steroids aroid my design.

 

[Weldingartisan]

Well chaps, you cured me! I no longer care or have the slightest desire to know what those silver tubes with wires going in and out do!
BUT..... I feel even more confident that if I've got any problems someone on here can help!
now, anybody want to know anything about plastering??? Nah, didn't think so.
Thanks anyway for TRYING to educate me.

Each to their own, I'm OK with metal, wood & plastic - but wet trades - absolute nightmare! W/A