×
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

Calculating Starting Currents

Calculating Starting Currents

Calculating Starting Currents

(OP)
Good Afternoon.

Could anyone tell me how to calculate the starting current for a motor with only a KVA rating and PF value?

Obviously depends on starting method (VSD, Star/Delta and DOL)

Any Thoughts

Regards

JM264

RE: Calculating Starting Currents

If you're referring to the normal running kVA and power factor, then you can only guess at the starting current.  

If you know the **locked rotor** kVA and the locked rotor power factor, you can calculate a good approximation of the starting current.  

RE: Calculating Starting Currents

The starting current (and torque) also depends on the rotor design, in particular the shape and electrical resistivity of the rotor bars.  In the US the induction motor starting torque (relative to full load torque) is classified by NEMA, in the rest of the world I don't think there is a specific IEC classification (but I could be wrong).

The best way to find out is to look at manufacturers data, e.g.
http://www.brookcrompton.com/pdf-files/2100e_waluminium_v3e.pdf
- the ratio of starting current to full load current is given for each machine.

RE: Calculating Starting Currents

Hello jm264

The Full Voltage start current will initially be equal to the Locked Rotor Current and then gradually fall as the motor accelerates, only falling significantly one the motor reaches about 80% speed. If you do not have the rated LRC of the motor, then you have a difficult task, except the range of LRCs of motors is generally between 550% and 900%. You will find some, very few, that fall outside this range.
Low slip motors (High efficiency motors) tend to have a high LRC and low efficiency motors tend to have a low LRC. Larger machines are often higher on LRC also.
Provided there is sufficient start torque available on reduce voltage to accelerate the motor to full speed, the current will reduce directly with the voltage reduction. (Torque reduces with voltage reduction squared.)
There is more information on my web site at www.LMPhotonics.com

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Calculating Starting Currents

On U.S. made motors there should be a starting code letter from A to V that indicates how much starting KVA per horsepower of the motor. The table for decoding the code letter is in National Electrical Code article 430. The lowest code letter motor that I have installed is a code B (low starting torque) and the highest was code S for a 1/2 HP submersible well pump.

Motors for submersible well pumps have code letters that are all over the place. For a motor that fits into a 4 inch (100 mm) well the code letter tends to around R or S. This is because in a small diameter submersible motor the oil gap between rotor and stator is abnormally small and the designer cannot use a double squirrel cage or other rotor design that results in reasonable starting current. Motors for 6 inch wells are more like code F and G (normal range) because there is more design freedom.

Mike Cole mc5w at earthlink dot net

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