×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*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.

# LRA - Inrush Current Definitions4

## LRA - Inrush Current Definitions

(OP)
For an Induction Motor with across the line starting.

Some indicate that the initial instantaneous Inrush Current is much larger than Locked Rotor Amps (LRA ), while others state that they are the same thing.

Which is correct?

### RE: LRA - Inrush Current Definitions

2
The initial motor current is a combination of magnetizing current that lasts only a couple of cycles or so plus locked rotor current that lasts until the motor has accelerated to a speed at which the current has significantly reduced. When people talk about inrush current being the same as locked rotor current, they are really not talking about the instantaneous inrush but the more sustained initial part of the acceleration current.

### RE: LRA - Inrush Current Definitions

2
The instantaneous peak during the first quarter cycle after motor energization can be up to 2.82 times the rms locked rotor current.  The actual peak depends upon the phase of the voltage at time of circuit closing (will vary between closure attempts) and on the circuit decay constant.

This is due to the presence of an exponentially decaying dc component.  Model an a series R/L circuit with suddenly applied voltage of magnitude. The response has a steady state sinusoidal component and an exponentially decaying dc component.  The initial value of the dc component in the worst case phase closure has the value of the peak of the sinusoidal component.  This can just about double the peak.  Doubling the normal peak of sqrt(2) times rms LRC gives 2*sqrt(2) times LRC max instantaneous peakl

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

### RE: LRA - Inrush Current Definitions

Excellent responses.
This is a pet peave of mine. Industry professionals misue the term "inrush current" when they mean "starting current" all the time.

JRaef.com

#### 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.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!