We have a s7-200 plc and we need to control the speeds of three dc motors with it. As we have never done this before we looked all over the internet for ways to do this and we found many approaches including using scr drives, using pwm output, and using vfd. As we are very new to this we did not understand much of the jargon and failed to comprehend what was given and how to proceed further. It would be nice if someone could guide us step by step what to do.

We are versed in ladder programming. Its the hardware part that we are clueless about.

 

As you noticed, there are several different ways to do so. A lot depends on your application, the HP and voltage of the DC motors, whether they are PM or not, and your budget.

--
Bob
http://ilbob.blogspot.com/

 

The Interface between PLC S7-200 and speed controller for DC motor must include Analog speed signal +/- 10V (PLC Analog Output), Enable (PLC Digital Output) and Fault (PLC Digital Input). If you need low speed range, than use scr drives. In case in high speed range, PWM drives will be suggested. And I hope that your DC motors have taxo or encoder for speed feedback.

You may contact me directly: [email protected]

 

And the answer depends on: How big are the motors, and a lot of other factors.

Regards
cww

 

Assuming a small hp or fractional hp motor, possibly permanent magnet from Bodine, Leeson, Bison, etc., you can get inexpensive SCR and PWM drives from KB Electronics, Bodine, Minarik, Dart, etc. Some inexpensive drives even have field supplies if your motors have a wound field rather than PM.

These drives will take a 0-10V or +/-10V signal, or sometimes 0-20 or 4-20ma, alone or by the addition of a signal converter/isolator.

Then you need a matching analog output on the PLC to control the speed.

One factor that may trip you up if you're not aware. I've found the output speed to not always be very linear vs. input signal, though repeatable. You may have to do tests and derive an equation that gives the analog output for a desired motor speed. There is some old curve fitting software for DOS called Fit2, and you can get others for PC or smartphone, to derive the equation relating speed and analog command value.