Speed control on PowerFlex 70

Hello everyone,

I am new to the forum as well as the world of PLCs. I have not had any sort of formal training and am very green so if you guys could dumb it down as much as possible (easier said than done) I would appreciate it very much.

Let me start by describing my issue. I have a PowerFlex 70 controlling the motor speed of a shaker conveyor. The PLC program (RSLogix 5000) has two speed setups. It has a running speed and a stall speed. The stall speed is used to keep the motor running when there is no product so that it doesn't have to ramp all the way up from 0.

What I need to do is set up a potentiometer to control the RUNNING SPEED ONLY. The way our network is set up is that we have CompactLogix controllers in our control cabinet that communicates to a ControlLogix in a master cabinet. They communicate with DeviceNet.

So I am assuming that I would have to send the analog signal from my pot to the controller and then throw in some code to make that control the speed reference for the VFD. Any ideas on how to set this up? Any advice is greatly appreciated!

You are going to have a couple of problems. First, if one of the PLCs is controlling the speed of the drive, then it has "ownership" (as the Powerflex User Manual calls it) of the speed control. If you add a pot, then how are you going to switch ownership of speed control from the PLC to the pot?

On the other hand, if you manually set the Powerflex parameters so that the potentiometer input always has ownership, then the PLC loses its ability to control the speed at all. How will that affect the operation of your process?

Because your Powerflex now is controlling 2 speeds, it probably is set up to have 2 preset user-defined speeds which are controlled with 1 or 2 terminal-block wired inputs from PLC outputs. If you switch the speed control ownership to another device (pot), then you will lose the existing 2-speed capability.

There are ways to switch the ownership of the speed control input back and forth, but sometimes that creates more issues than it solves.

Any ideas on how to set this up?

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Yes, first get a copy of the Powerflex 70 & 700 Reference Manual, and read it from conver-to-cover, then go back and look at all the Parameters that are used to set speeds. Then decide if there is any way to allow your pot to only work when you need it, but still allow the existing controls to work at all other times!

If I remember, there is a way to set up an analog input (from a pot maybe) to act only as a trim reference, so that it does not delete or override the normal speed reference signal, but merely "trims" it to be plus or minus about 10% of the actual existing speed signal. That might work for this case.

Could'nt you use the Pot to a analog input and then MOV that value to the PLC output for VFD Frequency Setpoint Value?

If you have a HMI close by you could forget about the Pot and the AI and do the whole thing with only key strokes.

Of course this is only if it is using analog values and not just using 2 fixed frequency setpoints via digital input to the drive.

BCS

Could'nt you use the Pot to a analog input and then MOV that value to the PLC output for VFD Frequency Setpoint Value?

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Yes, but then what or who sets the 2nd "Stall" speed that JRSnydley mentioned? If the PLC is now controlling 2 fixed speeds, then adding a pot means the pot would have ownership and be the controlling device. If you want BOTH devices to have control, you have to set up an automatic method to switch ownership from one to the other. I think that cannot be done while the drive is running.

The pot can be set up as only a trim device that adds to or subtracts from the existing speeds.

Lots of unknown's.
Without knowing Exactly how it is set up to start with we are all just guessing.

Something is triggering the inverter to go to stall speed now.
If it is done through analog values then it is probably something like:

No product detected then MOV "Stall Speed Value" to AO value.
If this is the case then leave this in place and when product is present MOV Pot value to AO value.

If speed change is being done using digital input at the VFD via 2 fixed frequency set points then it is a moot issue anyway.

OP can you provide more input on your existing set up?

BCS

Not sure on this particular VFD, but most have discrete inputs that can be programmed to go to a preset speed or release the VFD to analog input control. Check with your local A-B distributor - they usually have good support.

Another option is to use a signal conditioner to convert the pot to a 4-20 mA or such, and tie that to an analog input to the PLC. Then the PLC is always sending the command to the VFD, and the PLC logic can determine if that is the preset idle speed or echoing the command from the pot.

Bering C Sparky, I am attempting to use the MOV function. Lancie1 the speed settings are two seperate tags in the program. There is an HMI hooked up but it's not in the same area as the shaker conveyor. We want the operator to be able to observe the speed adjustment. I think since the tags are separate I should be able to use the MOV function just to control the Running Speed? Thanks for the input guys

If you run into any trouble try to post your program (zip it first) and someone will be able to help you out pretty easy.

Also try to tell what lad and rung your running speed output value is on so it will be easy to find.

Lancie and Tom are both spot on Gurus that can point you in the right direction quickly.

Good Luck
BCS

Welcome to the forum!

I think that perhaps Lancie has misunderstood how your system works. This is my understanding of how it works, tell me if I'm right?

The drive gets a speed signal from a certain PLC at all times. The PLC itself only ever sends two speeds; normal speed and slow speed. Let's call them 50Hz and 20Hz, for the sake of argument.
The PLC monitors the process, sends the "50Hz" speed value to the drive during normal running, and the "20Hz" speed value when there is no product on the line.

If this is the case, then yes, you're on the right track. First, you'll need a potentiometer. Then you'll need the PLC to have a spare analog input that you can connect the potentiometer to. If you have both of those things, it's just a matter of putting all the pieces together and writing some code!

First, wire your pot into the analog input. Make sure you read the documentation on your particular card and do the maths, so that you spec the right size pot, and wire it in the right way!

The next thing you'll need to do is scale your analog input from the potentiometer into meaningful units, e.g. 0-100(%) or 0-50(Hz), etc. Store the scaled value in a tag, which for the sake of the example I'll call "Potentiometer_Speed"

Then, where the PLC writes the "fast" value to the motor speed reference, I would assume you've just got a MOV block, moving a fixed value to the speed reference tag. Change the fixed value to "Potentiometer_Speed".

That's all there is to it. Of course, if you're new to all this I'm sure there's plenty of challenges in there but hopefully this will get you started. Post back when you get stuck and we'll try to help you out some more!

Whilst all of the above address your actual question I would be careful about changing the speeds on this type of system. The vibratory conveyor tray is suspended on springs (probably flat plastic leaves) and the system should be tuned to run at a particular speed which will generate the required amplitude for the tray. This speed normally spins some out of balance weights at just below the resonant frequency of the tray assembly. If the speed is changed away from this value then this near resonance will be lost and the conveyor will stall (this is what happens at your normal stall speed). The bigger problem, however, is if the speed is increased slightly to the actual resonant frequency when the system will become unstable and you will probably break the springs. You cannot use the drive speed to control conveying speed on this type of conveyor.

I have a 1769-IF4 analog input module. I am assuming that I hook the pick-up of my potentiometer to the V/I in 0- terminal with my other two leads on the V in 0+ and the I in 0+. Is that right? I have my module set up in RSLogix 5000. (I started a new project just to test it before I monkey around with the existing project) When I go online with the controller and look at my input in RSLogix, I do not have any values in the value field. I also do not measure any voltage on the module terminals. I have the dip switch set for bus voltage instead of external voltage. Does that matter? The module must be getting voltage because the OK light is on and it is communicating with my controller. I am using an A-B 800T-U29 pot if that's worth anything.

One end of the pot goes to +24 on the module (since you have selected bus voltage), the other end goes to V/I in 0-. The wiper of the pot (the pick-up as you have called it) goes to the V in 0+ terminal.
You may need to set the module within your I/O configuration to Voltage measurement NOT current.

jrsnydley said:

I have a 1769-IF4 analog input module. I am assuming that I hook the pick-up of my potentiometer to the V/I in 0- terminal with my other two leads on the V in 0+ and the I in 0+. Is that right? I have my module set up in RSLogix 5000. (I started a new project just to test it before I monkey around with the existing project) When I go online with the controller and look at my input in RSLogix, I do not have any values in the value field. I also do not measure any voltage on the module terminals. I have the dip switch set for bus voltage instead of external voltage. Does that matter? The module must be getting voltage because the OK light is on and it is communicating with my controller. I am using an A-B 800T-U29 pot if that's worth anything.

Click to expand...

I'm not sure the post above is correct. Dan seems to be assuming that when bus power is selected, the +24VDC terminal will provide loop power for the devices. My understanding is that it doesn't - this terminal is only used if you want to provide external power to the card. Quote from the manual: "The 1769-IF4 and -IF8 modules do not provide loop power for analog inputs. Use a power supply that matches the input transmitter specifications".

With that in mind, this card can get it's power through the PLC bus (which is what you've selected), or through an external 24VDC power supply connected to the appropriate terminals. So yes, selecting bus power is fine.

As a side note, the "OK" light on the analog cards just means it's powered up and not faulted. It doesn't tell you anything about whether your analog inputs are healthy/within range/there at all.

Check out page 2-19 to 2-12 of this document

The "V In +" terminal for each channel is the positive connection for a VOLTAGE transmitter (typically 0-10VDC). The "I In +" terminal for each channel is the positive connection for a CURRENT transmitter (typically 4-20mA). The "V/I In -" is the negative/ground connection for BOTH types of transmitter. The card is designed to be universal - each input can take a voltage transmitter OR a current transmitter. You are using a potentiometer, which will give you a variable voltage signal, so as the post above suggests, you will connect one of the "fixed" ends of the pot to a positive voltage, the other "fixed" end to the "V/I In -" for whichever channel you have picked, and the wiper, which will have a voltage that varies depending on the position of the pot, to "V In +" for whichever channel you have picked.

One important thing to consider from that document: "Voltages on Vin+, V/Iin-, and Iin+ of the 1769-IF4 and -IF8 modules must be within ±10V dc of analog common". So you need to be careful with your supply voltage across the potentiometer. The most likely supply you have available to put across your pot is 24VDC, but if you do this and then wind your pot right to one end, you'll have 24VDC on your card. I don't know exactly what it'll do, but "work" will probably not be it your best option is a 10VDC power supply, failing that perhaps you can find a pot that doesn't wind all the way down to zero ohms. Ohms law will tell you what resistance range you need to take a 24VDC supply down to a range of 0-10V.

Then you will need to ensure you have selected voltage input rather than current input on RSLogix 5000. Right click on the card and select properties, and have a poke around in there - there's a few interesting things to look at.

Thanks for the info guys! I think I should be able to figure it out!

Good luck! Post back if you get stuck, or even if you don't - we like to hear how things went and it'll probably help someone else out in the future