Motor Inrush Current 1

[TheBlacksmith]

We have several in-house publications citing that an electric motor in-rush current should be 500% of rated current. Is this rule derived from some obscure agency specification I can't find or just someone's rule of thumb for a good design basis.

 

[rbulsara]

Firstly, what you are referring to is the locked rotor current or the maximum current the motor will draw, which is when it is stalled trying to start a load at full voltage start. Or in other words the maximum starting current, which in turn depends on the starting torque requirement of the load.

Inrush is technically just the magnetizing current which could be much higher but very short lived.

The LRA is the inherent characteristic of a general purpose induction motor. It is not "required" by any standards but natural characteristic of the beast. For a 3 phase motor it is generally between 500% to 700% of its rated load current.

There are design aspects that can be tweaked to change the LRA somewhat, generally at expense of something else such as available torque, efficiency, etc. They are standardized (categorized) by NEMA code letters, A through V, which relate to the designated multipliers ranging from 3 to 22. Most common being the F, G and H (5, 6 and 7).

The starting current changes (reduces) with reduced voltage starting but at the expense of available torque.


Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Look in the Cowern papers for a reader friendly description of electric motors.

http://baldor.com/support/literature_load.asp?LitNumber=PR2525

Bill
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"Why not the best?"
Jimmy Carter