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Programming Application - 3 Phase Motor Startup

Article ID: 14
Last updated: 16 Nov, 2003

This might be a seem like a simple application fix, but more times enough I've seen some beginner programmers take the lazy approach when starting a bank of three phase motors. A motor is a pretty big inductive load, and with inductive loads we get inrush current.

Let's say we have 10 three phase motors at HP each. For the purpose of our example, let's assume our incoming voltage is 460 Volts 3-phase.

Each one of our motors would have a full load current or (FLA) of about 1.0 AMP each, so about 10 AMPs all together when all the motors are running. Inrush is seen at motor startup right when you first turn the motor on or energize the motor starter coil to start the motor. Typically inrush on motors can be as high as 300% or more of the FLA rating. That means that your single HP motor could generate a pull of about 3 AMPs when the motor first starts. Now back to the lazy programmer. If you were to put all your motor starter coils in a parallel branch in your logic to start all at once, can you guess what the inrush current would be on your electrical system for 10 motors? You guessed it right if you said 30 AMPs.

A simple startup delay between each motor is common programming etiquette when starting a bank of motors. Simply solved with a timer. Included in the programming example are start and stop controls to work in conjunction with your motors. This programming example uses a global "Running" bit that does not go true until the last motor is running, typically used in a conveyor line startup. You may also notice in this program example that the last motor is started first, and the first motor is started last. This is because in this example the last motor is furthest down stream in a conveyor system, so you would want to start your downstream conveyors first working your way upstream the conveyor system. This keeps your product on the conveyor flowing nicely downstream.

Download the 3 Phase Motor Startup Logic RS LOGIX example, PDF included. Winzip Required.

Article ID: 14
Last updated: 16 Nov, 2003
Revision: 1
Views: 34496
Comments: 4
This article was:   Helpful | Not helpful
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Programming Application - Photoeye Make and Break Logic...     Principle of Operation - Dynamic Braking

Jim Barnes | 08 Sep, 2005 12:00 AM
I found this article very helpful. The machine file was excellent - well written and commented. The sequenced start routine with the 8 motors gave my students something to study without doing a lot of typing at the beginning of class. I liked the twist where the start sequence could not be stopped until it was completed. It stressed the value of the MCR. I also challenged the students to study the code and figure out how to make the stop work immediately.

Very well written example.

Thank you,
Jim Barnes
Posted: 12 years 9 months ago   | Permalink
Jeff | 26 Apr, 2006 12:00 AM
Is the Stop Pushbutton (XIO I:2.0/15) supposed to be a NO or XIC? Seems like in this example someone is holding the button down or am I analyzing it wrong?
Posted: 12 years 1 month ago   | Permalink
Jonathan Rafaiani | 15 May, 2006 12:00 AM
for jeff:
i was not able to dl the file, but i'm guessing that you are referring to a NC stop button. for safety's sake, stops are NC. that way, if a wire breaks, the machine stops. we wouldnt want an E-stop to be dissables and not know it. that could happen in a NO switch.
Posted: 12 years 1 month ago   | Permalink
Erwin | 19 Jan, 2012 12:00 AM
This article is very helpful to me, since I am only a beginner in handling RS Logix programs. The program's printable version is in PDF format. Can I know what are the procedures in documenting the RS Logix programs into pdf format. I will be using this for learning and documentation purposes. Your detailed instructions are highly appreciated.

Thanks in advance,

Posted: 6 years 4 months ago   | Permalink

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Programming Application - Photoeye Make and Break Logic...     Principle of Operation - Dynamic Braking