Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Oct Tue 26, 2021 9:40 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 7 posts ] 
Author Message
 Post subject: LESSON LEARNED CHEAPLY
PostPosted: Sep Mon 06, 2021 1:58 pm 
Member

Joined: May Wed 15, 2019 10:50 am
Posts: 250
Location: Charlotte, NC 28205
Some time back, I was asking if a 3-phase transformer [with all 12 leads accessible] could be used to make a high-voltage, single-phase, plate transformer by paralleling the low side windings & seriesing the high side. It just so happened, that at this week-end's hamfest in Shelby [NC], I found the exact very small x-former to do the experiment. I disconnected all sec. leads & phase paralleled the primaries. Results:
Using a variac, Increasing the voltage to 20 volts, gave a current draw of 9 Amps. Conclusion:
Because of the single iron core common to all three windings, the currents bucked each other.
My question was answered for only $8.00.

Jack

PS - Hams will do anything to save a buck.


Top
 Profile  
 
 Post subject: Re: LESSON LEARNED CHEAPLY
PostPosted: Sep Mon 06, 2021 2:40 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11014
Location: Long Island NY
Not so fast. What you have to consider is the phasing of the windings. For the sake of argument, each winding has a "start" and a "finish." If you are going to connect them in parallel, then the "starts" and the "finishes" have to be connected together. That way the magnetic fields all go the same direction and they add together. If one winding is flipped compared to another, the fields will go in opposite directions and cancel. This is the equivalent of a magnetic short circuit and nearly an electrical one since the overall impedance will be very low.

What you could try is connecting two of the primaries together with a dim bulb tester in series. If the bulb glows brightly the windings are connected opposing, so flip the wires from one of them. The bulb should now glow dimly. Then the third winding can be connected up and tested in similar fashion.

It might also be pointed out that in three=phase transformers where all the windings are on a common core, less iron is used since the three phases are never active in the same direction at the same time. For the same reason, three phase motors are often smaller and lighter than their single phase counterparts. So you may find that the particular transformer you have can only tolerate two primary windings connected single phase; if you try to connect the third it will drive the core into saturation. At that point the transformer will draw more current, hum, and get hot.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: LESSON LEARNED CHEAPLY
PostPosted: Sep Mon 06, 2021 3:06 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5272
Location: Rochester NY USA
You can use two primaries with proper polarity in series or parallel; connecting the third in either polarity will buck one or the other and transformer will basically act as a short.

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: LESSON LEARNED CHEAPLY
PostPosted: Sep Mon 06, 2021 3:50 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11014
Location: Long Island NY
Please elaborate, not sure how that is possible.

I am assuming three identical primary windings on a core with six wires brought out, not delta or wye connected.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: LESSON LEARNED CHEAPLY
PostPosted: Sep Mon 06, 2021 6:42 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5272
Location: Rochester NY USA
Three magnetic paths in the core, any two primary windings (if connected "aiding") will produce a flux in one path and cancel in the third leg. Secondaries on the two energized legs can be used, third will be near zero. Energizing the third leg will buck one of the two, essentially producing a short. And so will a delta connected secondary - the portion on the unenergized leg must be disconnected. Two outer coils should be used for best flux cancellation in the third (center) leg and of course, better heat dissipation.

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: LESSON LEARNED CHEAPLY
PostPosted: Sep Mon 06, 2021 7:29 pm 
Member

Joined: May Wed 15, 2019 10:50 am
Posts: 250
Location: Charlotte, NC 28205
Okay, When connecting coils 1 & 2 out-of-phase [coil #3 totally open], I get 2.25 amps at 120 volts input; NO load. Afterwards, when reconnecting coil #3 in the line-up [only coil #2 is out-of-phase], I get 9 amps at 120 volts; NO load.
BTW; I uploaded a picture of the transformer; Where is it ?

Jack


Top
 Profile  
 
 Post subject: Re: LESSON LEARNED CHEAPLY
PostPosted: Sep Mon 06, 2021 9:01 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11014
Location: Long Island NY
Quote:
Three magnetic paths in the core, any two primary windings (if connected "aiding") will produce a flux in one path and cancel in the third leg. Secondaries on the two energized legs can be used, third will be near zero. Energizing the third leg will buck one of the two, essentially producing a short. And so will a delta connected secondary - the portion on the unenergized leg must be disconnected. Two outer coils should be used for best flux cancellation in the third (center) leg and of course, better heat dissipation.


Right. So the usual method is to connect the two outer coils together in phase and the middle coil out of phase. Only problem is, with many transformers this will drive the core into saturation, in which case you can just leave the middle coil disconnected. And yes, connecting the secondaries in delta will surely cause a problem!

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
Post New Topic Post Reply  [ 7 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB