PLC controlling a dc pump using PWM output

Hi All

I'm very new to the whole PLC world, and I was thinking of an 'easy' project to test a PID control loop. I'll share my thoughts on the project so far:

About me:
I am a graduate chemical engineer and want to up-skill/get a bit more hands on experience, so I understand more about control systems - as I am required to produce process control narratives for others in my current role.

So far I have a MicroLogix 1100 BBB (24vdc model with some high speed inputs and outputs). I also made a little PLC tester box with some switches to act as inputs.

The Project:
My idea is to control the level in a tank via feedback from a Pressure Transmitter (0-10V signal) using a small 24vdc Brushless Submersible Pump to be controlled via PWM output from the PLC. The water flow out of the tank will be controlled using a manual valve so it can be adjusted. (It will all be bench scale stuff).

As far as I can tell the Pressure Transmitter should be pretty easy to wire up, (paying special attention to only ground the sensor at one end). However I'm not sure what is the best way to wire up the 24vdc Pump for variable speed control.

Concerns/Questions:
My feeling is that because the ML1100 has no Analog outputs, the easiest way would be to use a SSR (Solid State Relay) in order to use the PWM of the output to control the speed of the pump.

The pump has a working voltage range of ~5-24vdc and a max load current of 1A.

What considerations should I use when selecting the SSR - I assume that the peak load of the pump would be during startup? What amperes rating should I use - 2A - 10A? (FINDER 34.81.7.024.9024 maybe?)

Also - I hear that I should use a flyback diode? Can anyone explain this abit better?

Can anyone think of a better/different approach to this problem?

Also I found this - http://www.robotshop.com/en/cytron-13a-5-30v-single-dc-motor-controller.html - would this work instead of a SSR? Would the fact that it is designed for a brushed motor create any issues?

I hope this makes some semblance of sense. My fallback idea is to do a simple RTD / Fan control loop.

Warm Regards
Jeremy

http://www.plctalk.net/qanda/showthread.php?t=67529

The above is a previous thread on the PWM instruction.

Hope this helps you out.

Regards,

Brushless DC motor must have some internal electronic control, which means its speed does not necessarily depend on the voltage applied.
You may, however, implement low frequency PWM, by starting-stopping the motor for controlled periods of time, without controlling its rotation speed.
Such control should be sufficient for tank level control task.
Be aware, that DC SSR are different from the AC ones.
For the motor peak current see the motor specs.
For the flyback diode- the Wikipedia rules.

gclshortt said:

http://www.plctalk.net/qanda/showthread.php?t=67529

The above is a previous thread on the PWM instruction.

Hope this helps you out.

Regards,

Click to expand...

Noted, Thanks for this.

Will be helpful when I get to programming.

Sergei Troizky said:

Brushless DC motor must have some internal electronic control, which means its speed does not necessarily depend on the voltage applied.
You may, however, implement low frequency PWM, by starting-stopping the motor for controlled periods of time, without controlling its rotation speed.
Such control should be sufficient for tank level control task.
Be aware, that DC SSR are different from the AC ones.
For the motor peak current see the motor specs.
For the flyback diode- the Wikipedia rules.

Click to expand...

Thanks for the comprehensive response!

Very helpful! Will let you know how it goes.

Are you operating alone from beginning to end?


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