Monday, November 25, 2013

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.


Thursday, November 14, 2013

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.