Contact US

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!

*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

Small transformer question

Small transformer question

Small transformer question

I ran into some industrial saws that I think are not properly wired. It has a transformer wired for 480 volt primary and a 240 secondary and that is all well and good. However some of the controls on the inside use 120 volt coils and there is no connection to x2-x3 on the secondary so they have to be flowing current through ground. Center tap is not grounded and one one machine they got 75v to ground on one leg and 220v on the other and was tripping a breaker after 7 years of running ok. A similar machine next to it had 120v to ground. It doesn't make since to me at all how either are working since the secondary isn't grounded. Is the power returning through the core?


RE: Small transformer question

Having trouble with some controls being 220V and others being 120V? Never seen that mess before. Are you sure some are 220V and not just indicator lights?

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

RE: Small transformer question

The 240 V secondary is serving 120 V loads that are connected in series, through the ground connections. The voltage across each "leg" depends on the impedance of each leg to ground. Say you have two 120 ohm loads, one connected from X1 to ground and one from X4 to ground. The voltages will be 120 V across each since the loads are equal. Current flowing from X1 to X4 will be 120/240 = 0.5 A. If you had 120 ohm from X1 to ground and 360 ohm from X4 to ground, total 480 ohm drawing 120/480 = 0.25 A. Voltage from X1 to ground would be 0.25 x 120 = 30 V. Voltage from X4 to ground would be 0.25 x 360 = 90 V. Total 120 V.

There will be some capacitance to ground, making things a little different and causing some current flow and voltage to ground even without loads.

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! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close