Question about Compensation circuitry in 1.5 KVA isolation transformer - Page 1
Forum section: electronics / projects

author: rbm
author_rank: Regular Contributor
timestamp: December 30, 2017, 12:06:45 am
content: Hi,I picked up a 1.5 KVA isolation transformer today, a unit built by NCR from the looks of it, several decades ago.There are three windings on the core - primary, secondary and a third winding terminated in three oil-filled AC capacitors. The third winding is electrically connected to a tap on the secondary. I'm not certain the purpose of the third winding but I'm guessing that it is either to improve Power Factor of the device or for transient suppression. First photo shows the tertiary winding with the attached caps; the second photo shows the three windings. Primary is on the right; note the jumper between the secondary (centre) and tertiary (left) windings. Any help appreciated.

author: duak
author_rank: Super Contributor
timestamp: December 30, 2017, 12:54:35 am
content: I think it's a voltage regulating transformer. Part of the core is designed to be saturated and basically clips the voltage peaks on every cycle. Google "ferroresonant transformer" and you should see various ways it is done. In your case one winding is connected to the capacitors and forms a resonant tank circuit.Cheers,

author: cvanc
author_rank: Frequent Contributor
timestamp: December 30, 2017, 01:02:46 am
content: Yeah, it's probably a regulating transformer. Also try Googling "Sola CVS" - Sola is (was?) a major manufacturer of constant voltage transformers.

author: rbm
author_rank: Regular Contributor
timestamp: December 30, 2017, 06:59:29 pm
content: @duak, @cvanc: Thank you for clearing up this mystery. I understand the ferroresonant transformer theory now.

author: SeanB
author_rank: Super Contributor
timestamp: December 30, 2017, 08:20:35 pm
content: Be wary if you want to measure the voltage across those capacitors, it can be as high as 1kV under certain load conditions, as this winding is used to saturate the part of the core the secondary is attached to so it regulates.

author: tecman
author_rank: Frequent Contributor
timestamp: December 30, 2017, 08:30:50 pm
content: It is known as a ferro-resonant transformer. It provides voltage requlation (plus/minus a few percent) as well as surge and noise isolation.paul

author: rbm
author_rank: Regular Contributor
timestamp: December 31, 2017, 12:59:13 am
content: Quote from: cvanc on December 30, 2017, 01:02:46 amI opened the case today to do some cleanup and changes to the grounding. There was a Sola PCB inside the case, containing MOVs on the line input. Interesting; this might be a Sola CVS unit rebranded NCR.I made the following changes:- debonded the transformer's secondary Neutral wire from chassis ground- disconnected ground pins on the four outlets from chassis ground to let them float- added a SPDT-centre off switch so that I can have (1) outlet grounds tied to chassis ground, or (2) outlet grounds floating, or (3) outlet grounds tied to transformer's secondary Neutral wire.

author: floobydust
author_rank: Super Contributor
timestamp: December 31, 2017, 01:20:48 am
content: Why monkey around with safety ground? You're going to tie it to a floating (secondary) neutral?As I remember, these transformers run with a saturated core and get quite hot.

author: rbm
author_rank: Regular Contributor
timestamp: December 31, 2017, 01:46:01 am
content: Quote from: floobydust on December 31, 2017, 01:20:48 amThe chassis along with the transformer and capacitors are still tied to safety ground through the input lead. The floating secondary Neutral was tied to ground in the unit when I got it; I removed that connection to let the secondary Neutral float.I need this transformer to perform the job of an isolation transformer that will float a unit under test. I'm following directions given by SDG Electronics in a youtube video.This video details similar modifications he made to a medical grade isolation transformer. He chose banana jacks whereas I have chosen to use a switch. All the modifications I've made so far are completely reversible, if you feel that I've followed an incorrect path, I'd be open tocriticismenlightenment.

author: duak
author_rank: Super Contributor
timestamp: December 31, 2017, 02:06:15 am
content: Were you planning to leave the capacitors wired up in the tank circuit? If not, you might get a lower secondary voltage than expected because the core isn't being saturated. You might be able to put the the secondary windings in series should this happen.BTW, these types of transformers are often used in old fashioned battery chargers because they have another interesting characteristic. Because the secondary doesn't intercept all the magnetic flux from the primary it becomes a current limiter which is quite handy in case of a short circuit. If you notice that the secondary voltage is dropping off at high current draws, this is probably why.Best o' luck & Happy New Year

author: floobydust
author_rank: Super Contributor
timestamp: December 31, 2017, 06:00:11 am
content: Lifting the output ground can give surprises, such as it floating to a high voltage compared to primary safety-ground.It depends on this transformer's stray capacitance, between primary-secondary, and secondary-frame. Some transformers have significant inter-winding capacitance, at 60Hz it's enough to make leakage currents.You can measure first with ohmmeter, then powered up with AC voltmeter or ammeter if you are courageous.

author: oldway
author_rank: Super Contributor
timestamp: December 31, 2017, 07:08:05 am
content: What you are doing is completely wrong and very dangerous.This old CVT transformer is not intended to be used as an isolation transformer and can not be modified because:- there are high voltages much higher than the mains voltage produced by the type of ferroresonant operation, which increases the danger of electrocution- these transformers work at high temperature especially when they work without load: this results in premature aging of insulation materials .... you can not rely on the quality of the insulation to ensure your safety.This transformer has nothing to do with the medical grade transformer of the video.You are in the process of making a serious and dangerous mistake .... DO NOT MODIFY THIS CVT TRANSFORMER, IT IS NOT A SAFETY ISOLATION TRANSFORMER.

author: rbm
author_rank: Regular Contributor
timestamp: December 31, 2017, 01:17:25 pm
content: Thanks. I've reversed all the changes back to original.

author: oldway
author_rank: Super Contributor
timestamp: December 31, 2017, 03:12:58 pm
content: You have not asked the question but you may be tempted to want to power your test bench from this CVT transformer.Is it a good idea ?No, because the output voltage of a CVT transformer is very strongly distorted (THD> 10%) and is more like a square wave than a sine wave.This can influence the operation of the circuits under test and give you erroneous RMS voltage measurements if you are not using a true rms multimeter.To power test and measurement circuits, it is necessary to use a special CVT with THD <3%These CVT transformers have H3 and H5 filters added to reduce the distortion rate.