PWM speed control

hi

can anyone help ? i am doing a project and i would like to control a 12v dc motor using PWM and i have no clue as to how to go about it

Use an analogue output on a PLC, into a Pulse Width modulator
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More info 12VDC ???A
PLC's can most certainly do PWM but you need an amperage driver board or H-bridge control. More specs mate. speed, fwd, rev etc.
Some guys on this forum actually work with motion all the time and have specific modules to drive motors and get commands from controllers be it a PLC.
S

the motor will be able to rotate forward and reverse for about 15 seconds at a time , it is rated 4.5 - 15 vdc, so i am looking to control it directly from the PLC outputs if it is posible its of very low amperage so curent drawn should not be a problem

Assuming your PLC outputs are relay. If you have a center tapped (or two) power supply you could do it + for forward - for reverse.
You need to keep in mind that the inrush current for the motor is much higher than the running current so you need to limit it in some way or you will wind up welding the contacts together.
I have never tried using a plc as PWM but it might be possible using a reactor and diode similar to a switching regulator.
Regards
Roy

hi Roy
the problem is this is my first PLC project and i've never had any training before . i only know a bit of Ladder. hence on the S7 200 where do i find the the function to generate a PWM output i know that Q0.0 and Q0.1 is were i connect my wires . as for the function that i can use to generate the PWM i have no idea

reversing a motor this often especially DC and especially 12 V may result in motor burnout. Is 4.5 the current rating??

PWM may be your only option to do this.
Does it have to be 12V - can you use off shelf 90V for which there are all kinds of PWM controllers. If absolutely stuck with 12VDC then Penny and Giles may be able to help you with PWM.

http://www.pgdt.com/

Dan Bentler

A PLC isn't fast enough.

There is much more to PWM than just sending pulses. They must be timed and changed frequently to match the position of the motor armature.

See this
http://focus.ti.com/lit/an/bpra048/bpra048.pdf
There is no way a PLC can do this

I bought one of these for one of our engineers to play with
http://focus.ti.com/docs/toolsw/folders/print/tmds2mtrpfckit.html
If you really want to learn how to control a BLDC motor then this is a bargain.

Now if you simply want to send pulses and integrate the pulses using a RC circuit then that is another matter but the response will be limited by the RC circuit. It will be slow and almost uncontrollable.

Originally posted by Peter Nachtwey:

There is much more to PWM than just sending pulses. They must be timed and changed frequently to match the position of the motor armature.

Click to expand...

Based on the limited description I don't think this is a BLDC motor. I think it is a good, old-fashioned brushed motor. So commutation isn't a problem.

Having said that, if you try and do PWM to a motor armature with a set of relays those relays won't make it long.

Also, unless your PLC supports a PWM function out of high speed outputs you will have a very hard time keeping the velocity ripple to a reasonable level. Most 'standard' PLC outputs won't update a wole lot faster than 5 msec from off to on and about the same going the other way. This makes your minimum PWM pulse length 5 msec. If you want any resolution at all you are looking at a PWM cycle time in the 500 msec area. That's pretty long.

Finally, keep an eye on the plc output current rating. It is pretty easy to exceed this going to a motor, even a small one.

Keith

I think in terms of what happens every pulse.

If the output is updated every 5 millisecond then one must look at how much the velocity will change between updates. If the time constant of the motor and load is 20 milliseconds then the ripple will be exp(-0.005/0.020)=0.78 which means the velocity will drop to 78% of the desired speed in the 5 milliseconds.

Small motors have very short time constants. If the small motor is attached to a fly wheel then much of the ripple can be smoothed out. If the motor and load time constant is 1 second then the speed will change by about 0.5% providing the updates are still every 5 milliseconds.

One should do the math before starting.