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Motor Speed Control using PLC and VFD

Motor Speed Control using PLC and VFD

Motor Speed Control using PLC and VFD

Hello Everyone!

I've seen many posts that are related to this topic, but i haven't yet found a thread that can answer my question so any help would be so greatly appreciated.

I have a motor which i want to run at at different speeds at different time periods automatically. I can connect a 4-20 mA analog output of the PLC to a 4-20 mA analog input of the VFD and thus control motor speed but how do i incorporate the time factor into this? Should this be included in my PLC program? If so, can anyone please tell me how i should do that?

Sincerely looking forward to any helpful replies!

RE: Motor Speed Control using PLC and VFD

A little more detail would be helpful.. What's the motor driving?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor Speed Control using PLC and VFD

Thank you very much Sir for your quick reply!

Actually this is an effort to automate our Extrusion Lamination machine which runs with three motors. One motor drives the Extrusion unit , the second motor drives the Re-winder unit and the third motor drives the Silicon and Mirror Roller.

Sir what i want to do with PLC is :
1. Synchronize all the three motors at different specified ratios for each motor,
For example : when i start the machine, the second and third motors that i mentioned should accelerate gradually to 100 rpm. This speed needs to continue for three minutes and then ramp up to 1300 rpm. After the period of three minutes , the extrusion motor should also accelerate to 1000 rpm.

Any help on how to create a PLC program and wire the VFD's will be of invaluable help!

RE: Motor Speed Control using PLC and VFD

You've got a lot of work cut out for you!

Yes, you can use a PLC's analog output to do what you're considering. It will mean you'll need at least three analog outputs. I like to use 0-10V rather than 4-20mA but either will work. I don't know of any PLCs that offer that many analog outputs without buying an analog out module of some sort. Typically add-on modules come with four analog outputs. You then set the VFD's parameters to take the speed input from an analog input.

Ultimately the VFD's parameters control everything, so things like acceleration and deceleration speeds are set by, and controlled by, the VFD. Other important things include maximum speed which you can limit regardless of what you send from the PLC.

Alternatives abound you can also control the VFD with digital inputs. All VFDs have the ability to set specific speeds to specific digital inputs. Then when that input is active the VFD goes to that preset speed. Some applications work fine with a couple of preset speeds and then you simply use the PLC digital outputs to control the speed selection. One thing to note about that is that VFDs almost always want their own internal logic power source fed back to them on the digital inputs. This is not a problem with PLCs because you always feed PLC output modules with a voltage supply that they then send out their outputs, (often in groups of 4 outputs). You'd bring the VFDs logic power over to feed the PLC's digital output module.

The last alternative is Modbus. Lots of VFDs will take in Modbus communications. This offers maximum flexibility as typically you can set virtually every VFD parameter over Modbus including the speed.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor Speed Control using PLC and VFD

I prefer the digital communication method that Keith mentioned above. Other protocols are available such as ethernet I/P, profibus, devicenet and the like, to suit whatever flavor of PLC you have. Some drives even support 'gearing' speeds between drives, but I have never done that. Whatever source you use for motor control, make sure to set the configuration for safe state in case the signals are lost, whether that stops everything, puts it in a safe speed or mode, or whatever your application requires.

Brad Waybright

It's all okay as long as it's okay.

RE: Motor Speed Control using PLC and VFD

Some sort of communications method from/to drive is necessary. The most used interface is ethernet. If possible i would go to this interface, it gives you ease of seeing parameters to the plant plc level as it relates to data collection, and interfacing to other machines.

I guess my question is how do you know that this motor is going at this speed? Some way is needed to make sure your motors are at the correct speed. A measurement device is needed for all 3 conveyors.

Existing equipment probably does not have the motor shaft at the end bell where you could easily mount an encoder on the motor. Otherwise you could mount a hollow shaft encoder on an idler shaft on the conveyor. Looks like your application is somewhat at a temperature where you need to guard the feedback device from wear and heat.

Some how your using some form of closed loop application for a motor/feedback device.

There is a range of VFDs that can do closed loop. Just pick something that your familiar with or have had some experience in the past. Or maybe this is a chance to learn something new.

RE: Motor Speed Control using PLC and VFD

@KeithCress :
Sir thank you so much for your great feedback! With the knowledge you shared with us, we were able to achieve something we have been trying for a long time. We were able to synchronize the speeds of two asynchronous motors at any specified ratio by connecting the analog input terminals of the master VFD to the analog terminals of the slave VFD and using a potentiometer to vary the speed.

Sir,however is it possible for the PLC to act as a potentiometer so that instead of manually controlling the speed, we only have to press a button and the speed controlling process automatically happens?

We haven't used Modbus communication before, but we'll certainly utilize this method for our future projects.

thanks again for helping us out :)

RE: Motor Speed Control using PLC and VFD


Thank you Sir for your helpful feedback! We will definitely take into account these measures when we are implementing communication protocols for our AC drives.

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