PLC control motor 's speed

yeah, I just see that PLC turn on/off motor before, never think of its quality of controlling speed, is it possible ? need something between PLC and motor ?
thanks !!!

You usually need a variable speed motor control. You can use analog controls in the PLC or do step speeds like in hoisting applications.

The PLC does not control the speed directly but tells the drive what speed to run at.

Some PLC's have the ability to control small servo motors directly. They use a PWM output.

with DC motor, they use AFD (adjustable frequency driver), I think PLC can change the opening angle of thyristor then change the value of DC voltage applied for amature and yielding changing speed, but how can PLC recognizes the speed at which motor is currently running at ? Could you give me some ideas !!!
thanks !!!

You could try to do this but I don't know that you want to.

A plc isn't going to be fast enough to directly control the trigger point of an SCR. You won't be able to correctly sync to the AC line. At the very least you will need to use a solid state phase control relay. This will allow you to send an analog signal from the plc to control firing angle.

However, unless you want to run the risk of burning up your motor, you will also need some current limit method. This will require another analog input to your plc so you can limit the desired firing angle based on motor current.

As for speed control, you could use either armature voltage feedback, a DC tachometer or an encoder. No matter what you use you will need yet ANOTHER analog input or counter card and will need to further modify your firing angle to achieve the speed you desire.

You will probably also want a phase angle feed forward structure to get your phase angle reasonably close to what you need based on speed command before any control elements are even required.

This all assumes that your plc scan time will be fast enough to give you the speed control bandwidth you want AND (even more of a stretch) that the analog inputs and outputs on your plc are fast enough to get the information to/from the physical system as quickly as you need to.

Or you can forget all that and, as Leadfoot said, get an adjustable speed drive. This unit would contain the SCRs, the pulse firing logic, the speed control, the current limits and the enable control. All you would do from the plc is send an analog signal proportional to the desired speed to the drive and the drive will do the rest. And these things are pretty cheap.

Keith

I have these little amplifiers ( about 200w ) that convert +/-10 volts to current. ( A transconductance amplifier ). My amplifiers use PWM instead or using analog output because PWM is more efficient. Note, these are simple amplifiers. There is no velocity loop so it isn't stable unless the controller uses lots of derivative gain to add stability.

Thanks, I being with my final plan at university, so I have to design a circuit using with PLC for motor's speed controlling (I must think of that just because I need some programming work in my plan), suggest I have way to get speed's feedback value at digital form, so I have to move these signal into microprocessor of computer and outputs them via RS485 pins to PLC inputs, is my way right ?
Well back to PLC, I see that you say analog signal, for e.g with 22x generation, I see that just only 224XP have 2 analog inputs and 1 analog output, I want to ask about the case that these kind of input(output) are used ?
Thanks !!!