Tuesday, October 27, 2009

October Activities

This month has been a busy month for Power Quality Monitoring.  I have done studies at two different facilities and have two more to do before the month is out.
So far, nothing very unusual to talk about.  Mid-level Harmonics causing some nuisance problems on signal cables coupled with RFI coming from long output leads from Variable Frequency Controllers.  The RFI can be solved with an output filter in most cases as was the case at one of the locations.

At another site the customer has a couple of Siemens 4300 power meters that have not been working.  These were programmed for PT inputs and are actually connect directly to the 480 supply.  They also read in negitive numbers.  This is due to the Current Transformers being installed upside down.  This happens all the time as most don't know that most CT's are directional.

We finished a Motor Control Center with VFC's for a project in the new Downtown "City Creek" Center.  I will post some pictures of this on the website as soon as I can.  Nothing special, just a nice job.

Madhu flew to Indonesia to work on a VFC on an Oil Rig in the ocean.  He fixed it the first day he was on site.  It seems that the customer had reinstalled one of the plugs on the drive incorrectly and cause a failure of the power supply.  While there, he has been doing some training for the people that work the oil rig.  He also will fly home to visit with family as he is not far away from India prior to coming back to Salt Lake City.

Looking forward to what November has to offer.  Some big things in the works but we'll have to see how they all play out.


Friday, October 9, 2009

Transient Voltage Surge Suppressors

There has been a change in the way TVSS's are being tested now by the addition of new specification required by UL 1449 3rd Edition.  Measured limiting voltage testing is now preformed at 6kV/3kA.   Also, this test is now an ANSI standard which has not been the case in the past.

There has also been a change in the terminology from Transient Voltage Surge Suppressors to Surge Protection Devices.

There has been the addition of Nominal Discharge Current to ratings and markings on the devices.

This testing now includes Duty Cycle testing at nominal discharge current.

UL 1449 3rd Edition also gives four designations to surge protective devices (SPD's) depending on where in the electrical system the device is connected.

Type 1 - Permanently connected device installed before of after the service disconnect overcurrent device and intended to be installed with no external overcurrent protective device.  This type of SPD most closely relates to devices tha were called secondary surge arrestors prior to 3rd Edition.

Type 2 - Permanently connected device installed after the service disconnect overcurrent device.  This type of SPD most closely relates to devices that were called transient voltage surge suppressors prior to 3rd Edition.

Type 3 - Point of use SPDs that are installed with a minimun of 30 feet of conductor length from the service panel.  These 30 feet of conductor lenght does not include conductors used to attach the SPD.  Some examples of Type 3 SPD's are cord connected, direct plug-in and receptacle type SPD's.

Type 4 - Component SPD's and component assemblies.

There is quite a bit of information about this that can be found on the web.  If you want to know more about the testing, do a Google search and UL 1449 3rd Edition and several good articles can be found.  EC&M has a great article on this and Eaton does as well.

The point of this is that all manufacturers must now meet this testing with the product they are producing at this time.  They still can sell all the product they have produced prior to this going into effect on 9-29-2009.  Some manufacturers were building in fuses to meet the 2nd Edition criteria which was a cheap way of getting around it because if the fuse blows your protection is lost and most times you don't know about it.

TPS has met the new standards and is still the leading manufacturer for the best and most reliable TVSS/SPD's on the market.

Thursday, October 8, 2009

Transient Voltage Surge Suppression Rant

Transient Voltage Surge Suppression (TVSS)

Just a few words about TVSS’s.

Whether we believe it or not, surges and spikes occur on our electrical systems every minute of every day and we don’t see them. They can occur when we flip on or off a light, the A/C comes on, a refrigeration compressor comes on or off, an elevator is in use or when a host of other things happen. Prior to everything being controlled by solid-state circuits, we didn’t give this much thought. The Television would go out and we would go down to the corner store, test the tube transistors, find the bad one and replace it (for those of you who are old enough to remember that). But now that everything in our homes, offices and factories are controlled via solid-state (PC board mount) circuitry (which most of us have no idea how to fix) we have to replace it with a new one or we call in someone to make a costly repair of whatever is down. We need to do something about this and we have the tools to do so.

We still act as we did years ago like nothing is happening. Now when something goes out we just say, “oh well, guess I need to replace it”. Why have we become a society of “use, dispose and replace”? We should not think that way. Computers, Lights, Televisions and all other electrical items we use on a day-to-day basis are made better today than they were 10 to 15 years ago but we have been indoctrinated to have a throw it away mentality.

Let me give you an example of what is happening:

Just the simple act of turning on or off a bank of florescent light in an office can cause a spike in excess of 1000 volts on a 120-volt system. I didn’t believe this until it was shown to me using a digital oscilloscope during a training session at a factory. Wow, what damage is occurring with other loads connected to this panel when this happens? Slowly the other connected loads are taking abuse and will fail.

Installing TVSS’s can greatly increase the life of all our electrical devices. Just a simple application of a single unit on a panel servicing lights in a warehouse can save hundreds of dollars a year in material and replacement cost of bulbs and ballasts.

Attached are a few examples of what others have done in their facilities and the cost savings they have enjoyed. We need to stop sitting on our hands and do something. All this talk about “Going Green” is just that; TALK, unless we join in and quit being a disposable society.

What will you do? Just needed to rant a bit.

docs/TPS_Flyer Green Sustainable.pdf

docs/Preventing Power Surge Problems is Key to Medical Device Operations.mht

Single Phase to Three Phase Power

Using a Variable Frequency Controller for this application

Most people will tell you that you can take a three phase VFC and feed it with single-phase power and operate a three-phase motor. Well, here are some facts to consider.

Inputting single phase on a VFC that is designed to have a three-phase power source input is at best risky. Some will tell you that you need only to double the horsepower and it will work. Will it? Lets see.

Lets take a normal 5 horsepower three-phase motor at 230 VAC.
Normal current for a typical squirrel cage induction motor is 15.2 amps at 230 volts.

We know that if we multiply the 15.2 by the square root of 3 (1.732) we should be able to come up with the amperes that will be required on the input of the VFC.

15.2 x 1.732 = 26.3264 amps
This is calculation is only valid considering a good 3-phase system where the power factor is above .92

The rating of a TECO N3 VFC at 230 volts, 10 HP is 35 amps but this is the output amps. This looks to be well over what one would need to drive the motor. The input amps of the VFC are not published in the price book so is the input the same as the output? Not usually.

Due to the power factor on the input of the drive being at .96 or greater, which is created in the VFC by the Capacitor Bank, most manufacturers input is rated lower than the output of the VFC. Also, excessive bus ripple is also created on the DC power supply in conjunction with the capacitor bank causing even more deration needed on the input.

The other thing to consider is that there will be considerably more harmonic current on the input of the drive due to the single-phase source. This is additive to the RMS current of the input demand.

If a VFC comes equipped with a DC link reactor, this can help with the ripple on the bus and one could possible assume that you could use the VFC above to do the job. Does this unit come with a DC Link Reactor? The answer is NO.

A purpose built single-phase unit usually also comes with added bus capacitors. This adds storage capacitance for the output to make up for the lost phase on the input.

This VFC will probably run the load for some time before a failure occurs but be certain the in time the front-end of this unit will suffer from the stress of the demand and the excessive ripple and you will be left with a VFC that has blown input rectifiers and possibly more than that.

Most manufacturers offer a VFC purpose built for single-phase input applications. Most are only three horsepower and below. If you need a unit to do more than this, you better do your homework and seek out a unit that is rated for the horsepower that you need and is purpose built for single-phase input.

Scott Gordon

P.S. We have the answer for the above application!

Glenmoor Service Call 8-11-09

Saturday Service Call to Glenmoor Golf Course

Saturday morning at 8:00 AM, I drive to the Golf Course because the main pump VFC for the watering of the course is down. It didn’t display anything because the Keypad was blank. Due to this, we had no fault diagnostic.

I first verified that the power was off. Then I did an ohmic check of the Input Rectifiers, the Transistors and check the Personality boards.

I then did a visual check of the Capacitor Bank. All looks good so far. I ohmed out the coil on the Pre-charge Contactor…. it looks good.

I visually inspected the Control Board and it all looks good. I visually inspected the backside of the Keypad and it looks good as well.

So what has happened? Then it dawns on me, Pre-charge Resistors. Sure enough, one of them does not read through. It had opened in a small spot and these are connected in series so no power was being applied to the bus.

Now, seeing this the first thing you would think it that the Power Board Transistor failed and dropped the Contactor. When this happens the current flow to the capacitors is across the resistors. So is the Contactor bad as well? Not usually unless it is in a very dirty environment and even at that, it usually just needs to be cleaned and it can be put back into service so long as the coil is good.

So I change the Power Board and the Pre-charge Resistors. Funny thing, the Power Board later checks out okay. So what happened?

This customer turns the VFC off when not in use. The constant cycling of the power is probably enough to weaken the resistors as the resistors are acting as the “Shock Absorber” of the system when the power is turned on.

I have only had one other drive do this so it is not common but well worth noting I think.

Scott Gordon