# AB PowerFlex 70 from PLC control to Manual Mode



## seekon (Oct 4, 2014)

Hi,

We are having an intermittent problem with one of racks that causes our Siemens PLC to STOP, shutting down all processes. All other processes can be managed, but we would like to have just 1 pump to run outside of the PLC control. So if it fails, this pump will continue to run.


*We have put the PowerFlex 70 Drive in Manual Mode.*
Manual for this drive: PowerFlex 70 

*Changed drive parameters to: *(not sure if this correct method to remove control from PLC, but our electrician set these up thinking this was)
 
Parameter 361, Digital Input = 4. “Stop - CF”
Parameter 362, Digital Input = 5 “Start”
* 
Original Parameters:*
Parameter 361, Digital Input = “Enable”
Parameter 362, Digital Input = “Run”

With these new settings, the relay can be removed, and the drive will still keep the pump running and the HMI is also unable to stop the drive or change its speed, but when the PLC STOPs due to the fault condition, so does the drive / pump.

I am assuming there are other settings we need to change, any help would be greatly appreciated.


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## gpop (May 14, 2018)

Just add a relay from a plc output then loop power through a N/C contact back into the run relay. Program this as always on in the code. you can also use a second set of contacts to switch the drive to a preset speed. (again N/C). With out seeing the wiring diagram its hard to tell what other controls you have wired.

If the drive says not enabled when a plc crash's then a output is connected to the enable somewhere
if the drive says running but the motor is not then you lack a input to a pre-set speed and it sees a analog signal 0/ 3.8 /4 ma which is fault or run at zero speed or a lack of a pre-set speed digital input

Plc fails all outputs go to safe (off). relay closes and power is now sent to the run relay (make sure power is the same potential E.G common on output card is same as power that the relay contact is using). Add a maintained stop button unless you have a e-stop or disconnect aux wired to the drive enable.

Some plc may have the option of setting a output to fail on and/or a analog to remain at state, Fail high/low.


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## paulengr (Oct 8, 2017)

What you want is “three wire mode” but it will not do what you expect.

You had 2 wire mode with two inputs with different names but the effect is that they worked in series. Loss of either one shuts the drive off. There may be differences in whether it coasts to stop or decelerates but effectively they work the same. So both are NO inputs.

Also by the way generally speaking you don’t wire up both to a PLC output. The PLC operates the run input only. The enable on the tube output PF70 can be done three ways. First there is a hardware jumper. If you remove it the enable is hard wired. Even if the VFD software crashes, it disables output gates. This should be connected to the output of an external E-Stop relay so that you have a hard wired stop that is immune to software failures. If you aren’t doing this and you leave the jumper in you can simply disable this input function. Third jumper it out which is the factory default. No reason to disable the drive when you just want a stop.

Now getting to your issue. You are pursuing 3 wire mode but here is why it won’t do what you expect. 3 wire mode simulates a 3 wire start-stop push button station. In this case pulsing the stop input stops the VFD and pulsing the start input starts it. BUT the stop input is also inverted so if the PLC goes offline so does the VFD.

What you really want is hold last state. Since the PLC action is to de-energize outputs on fault, we need to work with that.

The solution is quite simple and cannot be done within the VFD alone. Go back to your original settings. Use a latching relay to control the VFD. This type has separate set and reset coils. Or alternatively use the 3 wire mode in the drive but put an interposing relay with NC contacts on the stop input. In this way you can pulse both the start and stop inputs. In this way the pump holds its last state,

You can also easily add manual start and stop push buttons to the above options or at least a stop button if you need manual control.


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## seekon (Oct 4, 2014)

Thank you for your prompt replies on this:

To help understand the current wiring, see attached image;


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## paulengr (Oct 8, 2017)

A couple things stand out immediately. For one you have a disconnect in the path between the motor and the VFD. If you open it with the VFD running you stand a good chance of destroying the VFD from the resulting surge. Either you need to get rid of it or put an early-break aux contact in the disconnect wired back to the enable input and program the enable as a coast-to-stop (cut off power ASAP).

Second as mentioned earlier...the ENABLE input should be for emergency stop functions, NOT whatever you are doing with the PLC. If the PLC goes crazy, how do you shut things off? In your case you'd have to open disconnects but that's what the ENABLE input is for. If you're not going to use it, disable it in the drive (if you can) or connect it to the voltage source so that it is hard wire jumpered out.

Third, you also have an analog input. Even if you fix the run command issue, what happens if you send a "0" speed reference to the VFD? If you have it programmed so that 0 to 100% means 0-60 Hz, guess what happens if you lose your analog input? Yep, same thing...it stops. This is a far harder problem to solve if you want "hold last state" to work. One option is to program a minimum speed...it drops to say 15 Hz and stays there. "0%" now means operate at that minimum speed. Another option is to program another PLC output that toggles the VFD between two different speed commands. The second speed could be a potentiometer input or an internal drive speed preset speed. You just leave this input on all the time in the PLC.

Still yet another option which makes all of these options easier is to use say an Automation Direct CLICK PLC with 24 VDC inputs, 24 VDC outputs, and analog inputs and outputs. This is available for around $100. It's a stupid solution to have a PLC driving a PLC but since you can't seem to fix the Siemens PLC, this is the best alternative. So in this scheme the Click inputs are wired up to the analog input and the two digital outputs from the Siemens PLC. The Click programming simply sends the Run command input and the analog input to the VFD when the ENABLE input is on. Wire up the VFD enable so it is disabled (don't need it) as suggested earlier. Then if the ENABLE turns off, HOLD LAST STATE...don't change anything on the outputs to the VFD. One small gotcha with this one is a potential race condition with the Siemens PLC outputs. If the stop input and/or the analog input changes states to off/zero before you see the Enable input drop, you will end up shutting the VFD off first THEN holding last state, which is not what you want. So you will need to add a small time delay so that the Click PLC doesn't change the outputs until say 0.1 seconds after it sees a change of state of the run and analog inputs.


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## gpop (May 14, 2018)

paulengr said:


> A couple things stand out immediately. For one you have a disconnect in the path between the motor and the VFD. If you open it with the VFD running you stand a good chance of destroying the VFD from the resulting surge. Either you need to get rid of it or put an early-break aux contact in the disconnect wired back to the enable input and program the enable as a coast-to-stop (cut off power ASAP).
> 
> Second as mentioned earlier...the ENABLE input should be for emergency stop functions, NOT whatever you are doing with the PLC. If the PLC goes crazy, how do you shut things off? In your case you'd have to open disconnects but that's what the ENABLE input is for. If you're not going to use it, disable it in the drive (if you can) or connect it to the voltage source so that it is hard wire jumpered out.
> 
> ...



Sb38_38-2 is a disconnect aux contact on the relay used for the enable so im not seeing a problem with the disconnect.

you can program a digital input (lets say 363 input 3 set to 18) to put speed control in auto/manual (hopefully wont mess with run command or you have to mask dpi so the drive ignores it). Select analog 2 as auto and dpi port 1 as manual (096) then set the him display to required speed. (it will remember where you set it) 
Add a relay in parallel with 39k6 to force manual run, Also have it close input 3.

The other option is using the digital inputs (3 of them) to select where the speed reference is coming from. This can be a pain as you may have used a input already. Using the him you can tell who has speed control so ask the drive who its using for speed control if you have a problem. 

This should work but with out testing its hard to tell if something else needs to be changed.


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## seekon (Oct 4, 2014)

paulengr said:


> A couple things stand out immediately. For one you have a disconnect in the path between the motor and the VFD. If you open it with the VFD running you stand a good chance of destroying the VFD from the resulting surge. Either you need to get rid of it or put an early-break aux contact in the disconnect wired back to the enable input and program the enable as a coast-to-stop (cut off power ASAP).
> 
> Second as mentioned earlier...the ENABLE input should be for emergency stop functions, NOT whatever you are doing with the PLC. If the PLC goes crazy, how do you shut things off? In your case you'd have to open disconnects but that's what the ENABLE input is for. If you're not going to use it, disable it in the drive (if you can) or connect it to the voltage source so that it is hard wire jumpered out.
> 
> ...


Thanks for your detailed explanation, we will review these options accordingly. Much appreciated...


gpop said:


> Sb38_38-2 is a disconnect aux contact on the relay used for the enable so im not seeing a problem with the disconnect.
> 
> you can program a digital input (lets say 363 input 3 set to 18) to put speed control in auto/manual (hopefully wont mess with run command or you have to mask dpi so the drive ignores it). Select analog 2 as auto and dpi port 1 as manual (096) then set the him display to required speed. (it will remember where you set it)
> Add a relay in parallel with 39k6 to force manual run, Also have it close input 3.
> ...


Thanks gpop, much appreciated, I will forward this info to our electricians to review as well...


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