Analog vs Digital outputs for motor control

[mikeyp]

Hi guys this is my first time posting here. I have an Allen Bradley Micrologix 1000 PLC with all digital outputs. I also have a Baldor VS1ST Microdrive for the motor. The specific PLC model that I have only has digital outputs. Is there any way I can vary the motor speed with this combination using digital instead of using an analog output? Or do I pretty much have to switch my PLC for one that has an analog out. Thanks!

 

[Lancie1]

Is there any way I can vary the motor speed with this combination using digital instead of using an analog output?

Yes, there is a way, but whether it is suitable can only be determined by knowing exactly what speeds you need.

For example, most motor drives have preset speeds that can be controlled with digital inputs (PLC digital outputs) to the motor drive terminal blocks.

I see that your Baldor Microdrive has 2 digital inputs that can be used for preset speeds, and has up to 4 preset speeds that can be defined by the user. Using these 4 preset speeds, you could use 2 of your Mircologix 1000 digital oututs to control 4 different speeds of the motor. Then you could use 1 other output to Start/Stop the drive. Therefore, you can control Start, Stop, Speed 1, Speed 2, Speed 3, and Speed 4.

Using 4 preset speeds, and by setting the internal drive parameters for Acceleration and Deacceleration times, and Min and Max speeds, you could achieve fair control over the drive.

However, to get variable speed control from 0 to 100%, you would need a PLC with some type of analog output.

 

[mikeyp]

Thanks for the quick response! The application is an automatic door controller. So using preset speeds could work it will just be slightly more annoying programming the speeds from the motor driver instead of through the touchscreen connected to the PLC. Thanks again!

 

[Lancie1]

. . .it will just be slightly more annoying programming the speeds from the motor driver instead of through the touchscreen connected to the PLC.

You could still use the touchscreen if you want. Program some buttons on it to select the preset speeds, then send those to the PLC, which then controls the Baldor motor drive.

You could also set up some ramps, or steps, where when a RAMP touchscreen button is on, the PLC automatically steps the Baldor drive from 0 to Speed 1, 2, 3, or 4, or any combination as desired, over individual time periods that could also be set from the touchscreen.

Or you can buy another PLC with an analog output.

The application is an automatic door controller.

Why would you need anything other than max speed, with an Acceleration time set in the drive to control how fast it goes from 0 to Max Speed? In other words, WHEN would the speed need to be faster or slower when opening a door?

For a door controller, what you probably WILL need is Forward and Reverse digital inputs for the Baldor Drive terminals, to switch the door from Opening to Closing, as well as a Start/Stop input. Looking at the Baldor manual, it appears that you can use Terminal 2 for Stop/Run (0 = Stop, 1 = Run) and Terminal 3 for Forward/Reverse (0 = Forward, 1 = Reverse). That still leaves Terminals 4 and 6 than can be programmed as Preset Speed Inputs.

 

[mikeyp]

Well since I am using the micrologix 1000 it doesnt have a RS-485 port or use RJ-45 to connect to the motor driver. Without that I dont think there is a way to program/access the preset speed register?

As for needing anything other than max speed, the guy installing the systems likes to vary everything until he gets each door the way he wants it. So if i make him program the speed from the motor driver instead of the touchscreen he will probably throw a fit.

Thanks again for taking the time to even look up the baldor specs. I will probably be using the forward and reverse and start stop just like you said.

 

[Andy6]

You shouldn't have to change the preset speed parameters once you know whats the slowest speed you can start at.

On the Baldor drive,
- input 1 can be called low, eg. 30hz
- input 2 can be called medium, eg. 45hz
- input 3 can be called high, eg. 60hz
- input 4 can be called turbo, eg. 75hz

And if your installer likes to complain,(about nothing at all really, adjusting freqency is no hard task) tell him the other PLC you would have had to buy was going to come out of his pay.

 

[Tom Jenkins]

Most VFDs also have an input that will increment the speed up or down, with the contact closure acting just like the up and down arrows on the VFD keypad.

 

[Lancie1]

. . .Without that I dont think there is a way to program/access the preset speed register?

I conclude that you do not understand what a motor drive preset speed is, or how it is turned on and off. Once the pre-set speeds are set IN THE DRIVE in PROGRAM mode ahead of the actual running - that is why they are called "preset" - then in RUN mode, the drive only needs some digital inputs (which your Micrologix 1000 can supply) to select WHICH of the 4 preset speeds to run.

The guy installing the systems likes to vary everything until he gets each door the way he wants it. So if i make him program the speed from the motor driver instead of the touchscreen he will probably throw a fit.

He is a real nerd then. When I test a system for min and max speeds, I like to do it at the driving force (the motor drive in this case), not at some remote device that, duirng prove-out, may or may not be giving the correct commands, and may or may not be showing the real speed. Once the maximum speed is set in the Baldor Microdrive paramaters file, and the running speed is set with the drive keypad, then I can switch it to remote terminal-controlled mode and run it at that speed.

Besides, if you are using a MicroLogix 1000, you are not going to be able to run very many motor drives. You do not have enough outputs.

Tom, the best I can tell from searching the manual, this Baldor VS1ST drive does not have any way to remotely simulate the Up and Down speed-change arrows.

And if your installer likes to complain,(about nothing at all really, adjusting freqency is no hard task) tell him the other PLC you would have had to buy was going to come out of his pay.

Exactly what I would tell him, Andy. If he is too lazy or stupid to change the speed from the drive keypad, then he should have to pay for any additional cost.