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Programming Application - 3 Phase Motor Startup

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.