×
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

Size 5 NEMA Starters

Size 5 NEMA Starters

Size 5 NEMA Starters

(OP)
We have a size 5 starter operatinga 225 h.p. motor. The starter has c.t.'s (300:5)installed in the back of it to monitor the current going to the motor. The secondary makes up to heaters installed in the front of the starter. The motor normally runs anywhere from 40 amps to 135 amps throughout the day. The current on the secondary can be calculated by cross mutiplication. My question is, is there a mathematical way to figure out what the voltage should be across the heaters. Thanks for any help.

RE: Size 5 NEMA Starters

I don't think you should really care what the voltage is.  

To size the overload heaters, take the motor FLA and divide it by the CT ratio (60:1).  This will give you the motor full load amps in secondary amps.  The overload heaters are rated in amps.  

If you measure the dc resistance of the heater (out of the circuit), you can get a rough idea of what voltage to expect - just I^2*R.  The ac impedance will be a little different than dc resistance, but it will be close.

But maybe I didn't really understand your question.  

RE: Size 5 NEMA Starters

dpc is right, but what would that info be useful for?

"Venditori de oleum-vipera non vigere excordis populi"


RE: Size 5 NEMA Starters

     v = I * Z

     Z = SQRT( R^2 + Xl^2 + Xc^2)

      Xl & Xc ----> 0

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