×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

240 208 transformer connections.

240 208 transformer connections.

240 208 transformer connections.

(OP)
I'm working on a milling machine.  It came out of Lockheed Corp.

It has a 3kW dry type transformer in its control panel.

This was wired as a 480 supplied system.  My customer wanted me to rewire it for 208.

It has the standard two windings on the primary side and the secondary side.  The nameplate shows the expected 480/240 connection drawings.   I was about to switch the 480 to the 240 but was handed the schematic for the machine - a rare treat -.

It was drawn as a 480 connected machine but had a note stating "see drawing 2 for other voltages".

Down in the corner was drawing 2.  It showed 240 as the expected L1-H1,H3 and L2 H2,H4.

BUT!! It also showed 208V as being L1-H1 and L2-H2. H3, H4 not connected or used.

Has anyone seen this?  I don't get why using half the 240 windings gets you a different voltage.  It would only seem to me to get you a greatly reduced transformer rating.  

Keith Cress
kcress - http://www.flaminsystems.com

RE: 240 208 transformer connections.

May be 208V needs proper tap and the taps are on the H1-H2 half. Only way to be able use them is to use only that half of the winding, without the non-tap H3-H4 winding in parallel.

Rafiq Bulsara
http://www.srengineersct.com

RE: 240 208 transformer connections.

I think that it is a typo. But I have been wrong before. Going to google lockheed milling machine. First or second hit will probably be Eng-Tips. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 240 208 transformer connections.

Another guess Keith. If H3 and H4 are not shown on the drawing it may be that 208 Volt use requires the 480:240 Volt transformer to be replaced with a 208 Volt transformer. (Dedicated 208 with only one winding.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 240 208 transformer connections.

Bill:

Subject to Keith confirming it, I thought that this is not a 480:240V transformer. He is talking about primary voltages. Keith did not mention secondary (control) voltage. 480V primary needs H1-H2-H3-H4 in series. For 240V primary -in parallel. 208V would require one half with a tap. Secondary voltage remains the same.

Rafiq Bulsara
http://www.srengineersct.com

RE: 240 208 transformer connections.

Rafiq and Keith;
I have seen equipment with a standard transformer with a 240:480 Volt primary. For 208 Volt operation, a different transformer was supplied with a 208 Volt primary with just H1 and H2. A drawing may show both options even though the 208 Volt option requires the transformer to be replaced rather than being reconnected. If a transformer has a tap suitable for 208 Volt operation, then generally the taps will be H1, 480 Volts, H2, 240 Volts and H3, 208 volts, no reconnection just change the tap.
In your instance Keith, I suspect that the connections for 208 volts require a transformer change.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 240 208 transformer connections.

Well, if you connect your transformer to a 416 system voltage, it should throw out 208V at secundary windings, i think so...

If there was no electricity there would be no internet. Good point, don´t you? :D

RE: 240 208 transformer connections.

Welcome to Eng-Tips mavd02.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 240 208 transformer connections.

thx

If there was no electricity there would be no internet. Good point, don´t you? :D

RE: 240 208 transformer connections.

(OP)
Thanks guys.

I set it up as the drawing showed with no mention of transformer replacement, etc.

I was subsequently called by the customer because of "a whole lot of buzzing coming out of the control cabinet".

I thought maybe the transformer with the screwy wiring and an SCR drive that had been subsequently brought on line were not playing well and the transformer was complaining.

However when I got there and had them "make it buzz", it was obvious a power relay that isolates an axis motor was the "buzzer".

I measured the control voltage. 108V.  I rewired the transformer L1-H1,H3 and L2-H2,H4 to look for an improvement,  and re-measured the voltage.  108V.. (both measurements in the unloaded state)

Since the relay is only getting about 108V instead of 120 I thought that might be the reason.   I isolated the relay and powered the coil with 120V.  No change.

I tapped it with a mallet and each tap reduce the noise by 1/5.  Until there was no buzzing.  Subsequent re-energizing brought back the buzz at full strength.

So I swapped out the relay.

As for the transformer, they must assume you'll use a different one.  Lame! since there was no mention of a different one in the drawings which were just of the primaries.

As the machine will be sold into an older building it may be a 240V facility so they don't want to change everything to 208V.(yet)

Keith Cress
kcress - http://www.flaminsystems.com

RE: 240 208 transformer connections.

A 120:12 Volt auto-transformer on the 108 will get you back up to 118.8 Volts Keith. You only need about 0.3 KVA on the auto-transformer.
OR
Just pull a neutral and use 120 Volts directly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 240 208 transformer connections.

And if you really want the correct voltages for milling head motors, use my old favorite, the open delta auto-transformer connection. Cheap easy and dependable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources