Basic Motors using Ladder Logic

Basic Motors

This section covers some of the basic Start / Stop Motor logic. The purpose of this section is to give PLC programmers a sense of some of the basics in Motor control in ladder logic. We have included some of the most used controls. More advanced section for motor section is available later on in the training section.

Example 1

Assume we have a motor that could be started and stopped from 3 different places. What this mean is that we have 3 sets of Start/Stop buttons that can start and stop the motor.

Solution:

Rung 0: Here we have put Star1/Start2 /Start3 in parallel and then pass it thru a one shot. We are assuming that the start button could also have a maintained (1) signal as well as momentary (2) signal. So to cover both cases, this will prevent the motor from starting automatically after any of the 3 stop buttons is pressed. It is just a precaution and better as far as security is concerned.

Rung 1 : Here we have the stop buttons in series, since pressing any of the buttons should stop the motor and will disallow any of the start button to pass thru.

Note

• Maintained signal is a signal that stays on when a push button is pressed. Typically a selector has a maintained signal. Some type of push buttons has a maintained signal. When you press on lets say on a start push button, the start bit will stay on until the stop button is press.
• A momentary signals is a signal that will go on for as long as the operator pushes the button. When the operator release the button the signal will go off. That is why most of the time we use the OSR instruction to lock on the signal.