Single Phase to Three Phase Power

I thought I would bring this out of the archives.  I wrote this about 5 years ago and it still applies today.  Since I was asked this question today and it still comes up all the time, maybe it is a good time for a review.....

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.

Have a need?  Call us.

Scott

Forums on the Inter-web

Every notice when you search for something on the Internet, you will most always find a "forum" on the item or subject you have searched for?  Sometimes by accident you click on it and then realize that it is not what you were after?

So I end up on a HVAC forum and the subject is a service person at an office building in Utah (SLC area) has ABB drives on the air handling equipment.  He has been having failures on them and has slowly been changing them out to Toshiba units only to have the failures repeat on them as well. 

To figure out the problem, he gets on a "forum" and writes that the failures are occurring at about 6:00 am each and every time this happens.  His solution was to just jump to another brand when all he really needed was real guidance from someone with experience on drives and related problems.

Here's the deal, there were probably 10 plus responses to his question about what could be happening.  The answers where all over the place but way off target.  One even from a "ABB Technical Engineer", but no one addressed the real problem.

In most areas of the Wasatch Front, our utility uses capacitors to strengthen the lines during the day and takes them off-line at night when the demand for power goes down.  The "switching on" of these capacitors causes spikes in the line and if your system has no protection for these events, guess what happens?  The input of the drives on this building was and is still being pounded with these spikes every morning, and you know what time the utility normally switches these on,  yep.....6:00 am.

His problem could very well have been solved with the application of a simple line reactor on the input of each of these ABB drives.  But instead he received a bunch of advice from more people guessing what to do.  That resulted in spending money on the new drives (not to mention the labor to change them out, shipping, downtime and so on) but having the same problem. 

Have a problem and need help?  Your first source for help should be locating someone who is familiar with your application, equipment, area environment and has experience in what you need.  The Internet can be a great tool but like everything else, it needs to be used properly and wisely.

Scott 

Pressurized Irrigation Systems

Lately, we have provided several Variable Frequency Controllers for constant pressure water system and I have run into the same problem with a few of these.  The problem is that there needs to be a means of holding pressure in the system and we are finding that this is not being installed or recommended prior to installing the drive. 

The purpose of installing the drive is to maintain constant pressure in the system.  This includes when there is no water being used and the drive is at rest or “asleep”.  If we don’t have a means of check or holding pressure, the water just races back down the well when the drive stops thus causing pressure to drop and the drive to start right back up.  At this point it once again builds pressure to the set point and when the systems is satisfied, shut down repeating this process again and again and again.  This not only defeats the entire application but is very harsh on the drive, pump and motor.

A typical system should look similar to what is depicted here:



This is a bit over-simplified but non-the-less shows the correct placement for the check valve in the system.  Without the check valve, there is no “system” as it is an integral part of the plan.

Scott

 

Grounding Shields

I was on a start-up for a customer last week when one of the other people from the district showed up and was talking about a problem at another site.  It would seem that they had two small drives running some dosing pumps and whenever the pumps would run, the signals from their metering would "wig-out".  I just injected a single thought into the conversation and told them to check their grounding.

Earlier this  week, that customer call me and asked if I would come to the site where they were having this problem.  I arrived at the site and they proceeded to show me that they had installed a filter on the drives.  They showed me that the power and signal cables had previously been ran in the same wire gutter but that they had added another small gutter above the existing one and had moved the signal cables into it.  They also showed me that they had shielded wire going to the motors from the drives.  Well, here are my thoughts and observations:

1. The filter was a TCI K-series which is a RFI filter that was installed on the common input feeder to the drive cabinet.  Humm, great but pretty much not needed.  It is however a good idea to install a simple Line Reactor at this point for drive protection and some harmonic mitigation back onto the systems service and other connect load(s).  Not to mention it would have been cheaper and more effective.

2. It was great that an effort had been made to get the signal cables out of the same gutter as the motor leads and other power conductors for other equipment in the building.  The problem was though, they ran the new gutter right on top of the existing one and it ran parallel to it for about 25 feet.  Anyone see the problem here?  Can you say inductance?  This was a valiant effort but really a big waste of time.  If they would have added some separation space, this would have been worth the effort.  Right together, not so much.

3. The shielded cable....ah, another great idea but done incorrectly.  See, the shields where tied together in the VFD cabinet.  Then the other end of the shield was attached to the ground lug in the motor which also is where the ground conductor for the motor was tied and this was at best a bad connection because there was some corrosion on the lug to motor attachment point.

Looking at this, I had the customer clip off the shields in the motor conduit box and re-make the grounds for the motor conductors.  I then grounded the shields in the VFD cabinet and what the heck do you know.....problem solved!  Due to the fact that the shields where tied together in the VFC cabinet and were poorly attached in the motor connection boxes, it was creating a huge loop from the output of the drives to the motors and back to the feeder panel where the ground wires were connected to ground.  Yes, connect to ground there as the grounds came into the VFD cabinet but only to a terminal strip and then continued to the feeder panel.

I can't tell you how many problems I have solved just starting with just a check of the basics of good wiring practices...mainly grounding.  It is worth the time to do this right the first time.  And please, I ALWAYS ground all my shields in the drive cabinet and cut in the field.

  Remember this:

Fly the field and ground the garage!  My instrumentation teacher gave me that years ago and they are words to live by.  Albeit stupid, but easy to remember.

Scott

Rural Water Association Utah

It's that time of year again for the Rural Water Association of Utah's conference in Saint George, Utah. The event will be the week of the 25th and will be held at the Dixie Center as usual and is a by appointment or invitation only conference and show.
Every year, the RWAU invites people from various walks of life that work or teach in the water field or train members of the association in the many aspects of water extraction, collection, storage, distribution, reclamation and treatment of our most precious resource.....water. This gives many members the ability to take advantage of required training that is needed to do their day-to-day jobs in regards to water. Much of this training is required by the state or federal government for state approval for distribution to the public and is required by the federal government for funding that may be needed on a specific project.

The groups of people that attend come from cities, towns, counties, state or even special services districts that account for most all of the water that is collected or extracted for public use. Whether that use is for potable use or for irrigation, the majority is managed and maintained by one of these groups. The only water that is not controlled by them is private water users that hold water shares through the state for personal use, for individual residential use, farming and or ranching, Industrial facility, or it can be for developments with a Home Owner Association with a shared resource for a collective community use. At any rate, in most instances if you are a member of the association you are trained to do various jobs that are needed to be done in this field.

Vendors are invited to set-up in the main hall for two of the five day event, to show off their wares to the attendees at the conference....that's where we come in. We will again this year be there showing some of the new and exciting tools we have in our "bag of goods" for these folks to do their jobs better and more efficient while also helping them save money. The savings can come from being more efficient or by purchasing a product that will do the same job only better and be more reliably in the long run.

One of the new and exciting items we will be showing off this year is our radio control units we have now from Weidmuller. These units are very flexible, cost efficient and are very easy to incorporate into an existing system. The other great thing is the fact that they can be powered by a solar source and has a built-in power regulator for that application. So you just add a solar panel that is rated for the unit, a bit of interconnection to existing controls, a little programming and away you go. These are especially great for remote tank locations where there are no power resources. But that is just one application and there are virtually hundreds of uses for this powerful, flexible yet cost friendly product. If you are there, come check them out at our booth. We will be in 405 as we always are. If you won't be there, you can always call or stop by the office for more information.

For some of you, we'll see you there.

Scott

This years Super Bowl

Whether you are a football fan or not, most of us watch the "biggest game of the year" either for the game or the commercials.  For those of us that were watching the Super Bowl last weekend, we were able to see just what happens when a power disturbance occurs and you are not prepared for it....the lights go out.

Right after we watched Jacoby Jones run a 108 yard kickoff return back for a touchdown the lights went out in the Mercedes-Benz Superdome.  You would think that a company that builds some of the most prestigious cars on the planet would make sure that the stadium they put their name on was built and protected with the latest and greatest equipment money could buy, right?

I have been preaching about power quality and surge protection for years and yet we see that this can even effect what is now being called the "Blackout Bowl", and for good reason.  If the people in charge of this stadium would have installed surge protection equipment, the effects of the surge would have been minimized to a less severe event.  In fact, if the surge equipment would have been strategically installed at the right locations, they probably would not even noticed the effects of the power event and the game would have gone off without a hitch.  Instead, they experienced a 34 minute delay for breakers to be reset, for the power to be restored and for those stupid lights to warm back up so they would be at full brightness for the game to proceed.

Surge Protection equipment works when installed correctly, applied correctly and located in the right place.  That said, I cannot think of a home, business, factory, hospital, school or even sports arena that would not benefit from the installation of SPD's.  Think about it people, companies spent millions to advertise during the Super Bowl to promote their goods, we had 34 minutes of FREE advertising about the issues of power quality, the adverse effects of a spike or surge and what we should be doing about it.

What the kids said in the 90's.....WORD

Scott