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Topic: What's the best way to determine the quality and stability of your AC power?

Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 01-13-2005 09:04 PM

I am not talking about measuring AC ripple in DC current. I mean the AC power lines which feed the equipment. Apart from simply looking at the waveform on the oscilloscope, what other methods would you recommend to establish how stable your AC power is? And what are the criteria for deciding if and when it is necessary to take measures such as installing filtering devices?

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 01-13-2005 09:34 PM

Power companies can place a power monitor on your line. They can plot "events" that occur on all three phases plus the neutral (significant neutral currents indicate an unbalanced panel).

That is the best way to look at it...at any one point in time you are unlikey to find significant problems that the equipment wouldn't have shown you anyway.

As far a spikes and surges...they are un predictable but will happen so it best to protect against them...series mode is preferred but MOVs will due if you realize they do degrade as they get hit.

Some protect individual equipment...while others, like Mark G. will go for protecting the full panel.

Steve

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Thomas King
Expert Film Handler

Posts: 119
From: Sheffield, Yorkshire, England
Registered: Oct 2004

posted 01-14-2005 07:40 AM

We have all the sensitive stuff (dolby box, amps, lighting/masking control unit, etc) run through two power conditioners. I'd imagine there's not much point filtering the supply to the projector and rectifier.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 01-15-2005 12:08 AM

Bzzzzt. Wrong, the rectifier is a big item to have power conditioned. The biggest cause of diode failure is noisey power in a rectifier.

Steve

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Thomas King
Expert Film Handler

Posts: 119
From: Sheffield, Yorkshire, England
Registered: Oct 2004

posted 01-15-2005 07:09 AM

I imagined wrong then. Is that a fault in the rectifier, in that it provides noisy powers, or the fact that the rectifier is on an unfiltered supply?

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 01-15-2005 07:39 AM

Rectifiers older style inductor types can and do introduce some phase warping but the diodes are still seeing noise that passes through the transformer
Switcher types are like computor switching powersupplys and like clean power

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 01-15-2005 09:13 AM

Its amazing that no one has built a rectifier with torodial trannys. Toroids are very good at filtering out power line noise and are the heart of many power limne conditioners and are also in the neighborhood of 40% higher efficiency than iron core type. Toroids are requirements in all hospital applications due to their good filtering, regulation, and efficiency characteristics. They and also allot lighter in weight... the down side is higher inruch current draw but there simple methods to control that.

Mark

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 01-15-2005 10:43 AM

So what is typical power line noise? Is it distortion of the 60Hz/120V waveform or addtional components superimposed on it? Up to what point is it acceptable, and when does it start to create problems?

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Thomas King
Expert Film Handler

Posts: 119
From: Sheffield, Yorkshire, England
Registered: Oct 2004

posted 01-15-2005 11:09 AM

This is going to sound stupid, but what exactly is 'diode failure'? I know what diodes are/do, but what is their function in this situation, and what happens if you do get a failure?

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 01-15-2005 11:17 AM

Basically, diodes help making DC out of AC because they allow electrical current to flow only in one direction. That is why you normally have 6 diodes of which each one is "assigned" to let one half-phase of the three AC lines pass and block the other half-phase. If a diode is open, it doesn't allow current in either direction anymore, if it shorted, current passes uncontrolled in both directions.

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Frank Angel
Film God

Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999

posted 01-15-2005 11:32 AM

quote: Steve Guttag
MOVs will due if you realize they do degrade as they get hit.

Absolutely: this is a serious problem and shouldn't be taken lightly. We have power in the big theatre that ranges from 123vac to 129vac....way higher than is comfortable for many pieces of equipment. And although surge protectors will do nothing to reduce high voltage, we have them on most of our signal processing and computer equipment, as do most people, for spikes and momentary anomolies. But here is a major problem with that, at least in our environment of unusually high, steady-state voltage. As Steve says, as the MOVs get hit and as they age, their threshold for clamping down spikes gets lower and lower. When the voltage is as high as ours, we found out (the hard way) that at some point these surge protectors will simply clamp the high voltage and stay there, overheat....and then fry everything in the unit. A sudden high spike when they are in this "clamped" condition will actually cause them to flame out. I had TWO fires in as many months.

One day in our box office, there was a line spike -- big enough to see lights flicker -- and then three of the surge protectors that computers were plugged into shot fire out of the socket holes while making a nasty hissing sound. This was bad enough, but a few weeks later, one of the big UPS units that weighs about 50lbs which services our main telephone switch, same thing....some "event" on the power line make the aging MOV in it actually go on fire. I was right there to witness it so I unplugged the thing, but flames and smoke continued to pour out of it. I used the handy fire extinguisher that was in the room; while the yellow powder that the extinguish was pumping out was aimed directly at the flaming USP, the fire was quenched, but as soon as I stopped the extinguisher, the damn thing burst into flames again. So here I am, choking on the smoke which quickly filled the room and the offices as I dragged the thing by its AC cord to get it out of the building. Who knows what PCBs were spewing out of that thing along with the smoke. If I weren't in there, that thing would have cause a major fire.

BE CAREFUL surge protectors, especially those in UPSs because, whereas the little power strips will only burn up the few caps and whatnot that are inside them, the MOV that's built into UPS will cause the batteries to burn and once that starts, continue eventhough it is disconnect from any power source. That's not to say that those power strips can't be very dangerous depending where they are located when they decide to fry. If they are placed where is typical, under desks with all kinds of flammable materials pile around them and even on top of them....that little flame that shoots out of the sockets can start a fire if luck isn't on your side.

I looked into this more closely and it seems that a new UL rating has been established that requires there to now be thermal protection in surge protector circuits. Problem is, you don't know if what you are buying has the thermal protection because the old UL spec which DOESN'T have it can still be sold grandfathered. And it will still say UL on it.

Oh, and parenthetically, that fire extinguisher that the college supplied which I used in the electronic equipment closet....that yellow power that it sprayed all over everything....it is corrosive to metal. Any piece of equipment that is anywhere near that powder will be coated with it inside and out because it migrates over everything. The CAT5 patch bays, all the equipment terminals, every electrical connection in the room had to be thoroughly cleaned with solvent. A call to the extinguisher manufacturer confirmed that eventually that coating on any electrical contact surface will corrode it. Nice, huh? So I advise STRONGLY to check what kind of fire extinguishers you are being supplied with. Make sure they are CO2 and not the dry chemical type if they are going to be used anywhere in the vicinity of electrical equipment. If they are found in your projection booth, get rid of them ASAP.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 01-15-2005 11:49 AM

quote: Michael Schaffer
So what is typical power line noise?

Put your scope on a line and have a look see. You will find different types of noise in every different location you check. There are literally hundreds of types of noise that can be on an AC line.

quote: Steve Guttag
MOVs will due if you realize they do degrade as they get hit.

This is the main reason why they require their own dedicated circuit breaker when you install them as a whole panel protection device.

Mark

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Michael Schaffer
"Where is the
Boardwalk Hotel?"

Posts: 4143
From: Boston, MA
Registered: Apr 2002

posted 01-15-2005 04:27 PM

Which of those, if any, noise patterns can seriously affect the performance of your equipment? The actual voltage in most of our locations is usually between 116-119, rarely higher, so it is within the 5% normally deemed acceptable. But I am concerned about slow degradation of the equipment by unclean power patterns, if that actually has an effect.
Do you normally install surge protection in your projects? What's the most common cause for high voltage spikes in the line?

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Tony Bandiera Jr
Film God

Posts: 3067
From: Moreland Idaho
Registered: Apr 2004

posted 01-15-2005 04:41 PM

quote: Frank Angel
Who knows what PCBs were spewing out of that thing along with the smoke.

Actually, no PCB's (Illegal in electrical gear since the '70s) but a lot of other nasty toxic byproducts! The burning of the UPS battery probably put out heavy metal fumes. (Great name for a rock band maybe? [Big Grin]

)

quote: Mark Gulbrandsen
Put your scope on a line and have a look see. You will find different types of noise in every different location you check.

Be careful!! Many 'scopes (older) cannot handle a direct connection to the AC line! And you have to make sure you don't connect the probe's common or ground to a hot line, many scope's probe commons/grounds are connected to scope chassis.

Mark, do you have a good method to check AC line noise with a scope? So far, everyone I've talked to repeats my caution above and can't give a good answer otherwise! [Smile]

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 01-15-2005 05:27 PM

quote: Michael Schaffer
Do you normally install surge protection in your projects? What's the most common cause for high voltage spikes in the line?

Yes, There are two ways to do this. Install a giant surge protector at the panel feeding the booth sub panels or install a smaller one at each booth sub panel. I prefer the latter method because if one largish protector blows there is not a risk of the whole place shutting down. Also another reason is that it can result in a more stabilized voltage in each operating booth.
A good example of how well they work was during the install at Thanksgiving Point Stadium 8. Right next door is the Giant Screen 15/70 3D theatre in the Dinosaur Museum. This is in Utah County and they have very poor power down there for what ever reason. During the middle of one afternoon there was a massive power bump that knocked the 15/70 off the screen and damaged one of the projectors claw mechanisms and about 200 feet of film between both the projctors.... Those Museum guys that made the decisions had decided to skimp and not install any surge protection or UPS supply to keep things running as normal...... the "Museum Guys" should have installed them though and learned the hard way. Right next door with 6 of the cinemas running we were unaware that there was any power bump at all until Stewart Anderson came over to tell us he was off screen and one projector had been damaged. All of the 6 Stadium 8 cinemas kept on running as normal because they were equipped with surge protection devices. The 15/70 theatre has since installed a whole building surge protection device and the projection system now has a sizable UPS supply that can run all the 15/70 stuff long enough to effect a safe shut down of the lamps and projetors... this all after ruining thousands of dollars of 15/70 film.

quote: Tony Bandiera Jr
Mark, do you have a good method to check AC line noise with a scope? So far, everyone I've talked to repeats my caution above and can't give a good answer otherwise!

You're right about that, its imperative that you KNOW which is hot and which is neutral. You can also use a small torodial isolation transformer on the scopes input along with a X10 probe with just about any scope. The toroid won't introduce any noticable distortion. My Tektronix 465B for what ever reason works fine for looking at AC, most likely its input grounds are floating and not connected to chassis ground....would have to take a look at its vert. amplifier schematic but I can bet thats why. Most modern scopes should be able to do this but possibly not all of them. You may still need the isolation tranny to be on the safe side though.

Mark

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