×
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

Motor T-Lead Sizing

Motor T-Lead Sizing

Motor T-Lead Sizing

(OP)
What is the basis for large motor t-lead sizing?  We have a 7000HP, 4000V, 900FLA motor that had to be rewound.  During the repair, parallel 4/0 conductors were used per phase.  The motor originally had 3/0 conductors.  This seems like it would but a large thermal load on termination lugs for the guy used to sizing such things per NEC.

RE: Motor T-Lead Sizing

Well the replacement conductors are bigger, and the NEC doesn't apply to internal motor leads. They typically have a much higher current and temperature rating than the NEC allows for branch circuit wiring.
You may not have a problem
respectfully

RE: Motor T-Lead Sizing

The cable manufacturer tables should be consulted. Be aware that elevated ambient temperature into the motor frame will affect the ampacity. We have successfully used the following ratings for 7.5 kV, 150°C EPDM Cables on motor terminal leads.
4/O AWG---- 275 Amperes
3/O AWG---- 235 Amperes
2/O AWG---- 203 Amperes
1/O AWG---- 176 Amperes

According to above values, for 900 Amperes you should have 4 cables 3/O in parallel or any other practical combination per phase.

RE: Motor T-Lead Sizing

(OP)
I knew that seemed a little small.  This is a motor from a pretty well known and respected large motor manufacturer.  We started having problems after a rewind.  A random lug in the main terminal box would burn off every few months to few years.  This was thought to be from bad crimps at the repair facility.  I cut the insulation back on one of the t-leads and found heavy VPI contamination all the way to the lug.  We cut off the remaining unfailed lugs and found that the lugs were installed after the VPI process.  

The motor has been sent to a different repair facility and is ongoing complete lead replacement.  I want to specify a larger lead size or more than 2 parallel 4/0 per phase, but didn't know what the manufacturer used as the basis for sizing.  I asked, but wasn't given this information.

The last lug failure resulted in the catastrophic destruction of the terminal box; after the lug burned off, the open ended lead contacted a grounded component in the box.

RE: Motor T-Lead Sizing

Laplacian This seems like an ampacity  wire problem,because the amperes rating used by each manufacturer depends of designs and the material .Normally for electric motors you could find Hypalon, EPDM or Silicone Glass Class F & H.

Ussually in our Shop for 4160 Volts motors  we use EPDM Class F - 7500 V manufactured By BELDEN for this manufacturer  wire  the 3/0 Ampacity is 457 and  4/0 wire:  529 Amps.(Maximum values for single conductor in free air @ 30 C) If the motor was designed by the maufacturer with  parallel 3/0 per phase, I think this could be a short design. If the motor was rewound with two Parallel 4/0 and failled maybe the motor´s temperature is too high and this could be derrating the wires.

the shops could use 13.2 KV 250KCMIL wire.

Regards

Petronila

RE: Motor T-Lead Sizing

(OP)
The motor runs pretty cool even at full load.  The parallel 4/0 installed after rewind were impregnated with the stator VPI resin; insulating the inner strands from the outer strands resulting in much less current carrying contact at the lugs.  

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