It is currently Thu Jan 27, 2022 11:25 pm

Author Message

 Post subject: SKM Motor Contribution CalculationPosted: Wed May 06, 2020 1:26 pm

Joined: Thu Jun 09, 2016 6:17 pm
Posts: 6
Good Afternoon,

Within SKM, Xd" and X/R values utilized for motor contribution calculations can be defined in a few different ways. These values can be calculated utilizing user defined parameters consisting of LRA/FLA and the starting PF. Additionally, typical values can be utilized from sources including IEEE 399, IEEE 141 and Conrad St. Pierre's Short Circuit Calculation Guide.

For those of you who utilize SKM, what approach do you usually utilize? Exact parameters required are not always obtainable, especially for existing installations.

Thanks.

Top

 Post subject: Re: SKM Motor Contribution CalculationPosted: Thu May 07, 2020 9:23 am
 Sparks Level

Joined: Thu Apr 18, 2019 11:42 am
Posts: 65
I use the guidance in the Red Book.

Mike

Top

 Post subject: Re: SKM Motor Contribution CalculationPosted: Mon May 11, 2020 1:12 pm
 Sparks Level

Joined: Tue Oct 26, 2010 5:00 pm
Posts: 201
Location: Maple Valley, WA.
If it is a existing installation, you can find the impedance of the motor by looking at the NEMA starting code on the motor nameplate. This is a letter ranging from A - V. The per unit impedance will be the reciprocal of the associated lock rotor number. Make sure that you are not looking at the NEMA enclosure code! These are not the same.

For Chillers and some other types of motors, look for the locked rotor or starting amperes. Per Unit Impedance = Full Load Amps / Lock Rotor or Starting Amperes.

_________________
Robert Fuhr, P.E.; P.Eng.
PowerStudies

Top

 Post subject: Re: SKM Motor Contribution CalculationPosted: Tue May 12, 2020 4:35 am

Joined: Tue Dec 31, 2013 12:19 pm
Posts: 37
Robert is correct about using the motor code to determine the subtransient impedance Z. In order to get to the subtransient reactance Xd", you need to also adjust the starting PF. IEEE 399 indicates that is around 0.20 for large motors and references "Industrial Power Systems Handbook", 1955, by Donald Beeman. I got a copy of that reference which by the way is a good reference and is referenced multiple times in the Color Book series. It has a graphical figure for starting PF at around 0.62 for 5 HP and on a (starting PF Linear Y-Axis x HP log scale X-Axis) is near linear to about 300 HP at 0.22, and then curves to 0.16 pf at 1000 HP.

For new motors under design, you can use motor manufacturers LRA or motor code and if the subtransient reactance is not given, use the Beeman figure for starting pf.

Top

 Display posts from previous: All posts1 day7 days2 weeks1 month3 months6 months1 year Sort by AuthorPost timeSubject AscendingDescending
 Page 1 of 1 [ 4 posts ]

 All times are UTC - 7 hours

 You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot post attachments in this forum

 Jump to:  Select a forum ------------------ Forum Library / Articles The Lounge    Question of the Week - What Do You Think?    Arcflashforum.com Feedback and Announcements    Off Topic Discussions    News in Electrical Safety Arc Flash and Electrical Safety    General Discussion    Electrical Safety Practices    Equipment to Reduce Arc Flash Dangers    Personal Protective Equipment (PPE) Arc Flash Studies    General Discussion    Arc Flash Labels    Software for Arc Flash Studies    System Modeling and Calculations    NEW! Electrode Configuration Library – 2018 IEEE 1584 Codes and Standards    CSA Z462 Workplace Electrical Safety    EAWR Electricity at Work Regulations, HSE - Europe    OSHA CFR Title 29    IEEE 1584 - Arc-Flash Hazard Calculations    NFPA 70 - National Electrical Code - NEC (R)    NESC - ANSI C2 - National Electrical Safety Code    NFPA 70E - Electrical Safety in the Workplace    2015 NFPA 70E Share It Here    Arc Flash Photos    Your Stories    What's Wrong Here? by Joe Tedesco
© 2022 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883