Welcome to the new Schneider Electric Community

It's your place to connect with experts and peers, get continuous support, and share knowledge.

Close
Important Announcement: WELCOME to the Schneider Electric Community! We have moved to a new address: community.se.com — bookmark it today and contact SchneiderCommunity.Support@se.com if you have any questions!
Invite a Co-worker
Send a co-worker an invite to the Exchange portal.Just enter their email address and we’ll connect them to register. After joining, they will belong to the same company.
Send Invite Cancel
82757members
350545posts

How do I size a Xfmr for a motor load

NEMA Engineers & Designers Forum

A community for the free exchange of ideas and information related to engineers, specifiers and designers within USA and Canada or those who design with North American products.

Solved
JLStanton
Cadet
Cadet
0 Likes
1
506

How do I size a Xfmr for a motor load

1.  Sizing Xfmr for Motor Loads

2.  Formula –Motor LRC / Xfmr Secondary Current * Xfmr Impedance (As a Whole Number) = Voltage Drop

3.  Assume a 40 HP Motor at 208V, and you have a 480V System.  FLA of Motor is 120A, and assuming LRC is 6 times FLA, you would have approximately 720A. 

4.  If you based Xfmr Size on FLA, you would assume a 45 KVA Xfmr would suffice.  Let’s look at the Voltage Drop.

5.  Secondary Current of a 45 KVA Xfmr at 208V is 125A, and assume impedance is 5%.

6.  720//125 * 5 = 28.8% Voltage Drop

7.  Let’s look at a 75 KVA Xfmr with 208A of Secondary Current.

8.  720/208 * 5 = 17.3% Voltage Drop

9.  You would have to verify whether a 17% Voltage Drop would allow the motor to come up to speed.  If not, you may need to go to 112.5 KVA Xfmr.  Which is much bigger than most people would assume.

Tags (1)

Accepted Solutions
JeffMMiller
Lt. Commander Lt. Commander
Lt. Commander
0 Likes
0
500

Re: How do I size a Xfmr for a motor load

Thanks for the helpful information.

See Answer In Context

Tags (1)
1 Reply 1
JeffMMiller
Lt. Commander Lt. Commander
Lt. Commander
0 Likes
0
501

Re: How do I size a Xfmr for a motor load

Thanks for the helpful information.

Tags (1)