Baldor vfd signal problems

[Thedroid]

I'm installing several baldor drives to run grinding aid pumps for ball mills. I'm using an analog in 4-20mA to control the speed and an analog out 4-20mA for a speed reference to the plc. Using an altek calibrator to provide a signal, I noticed that the output was measuring higher than the input. 4mA in 4.22 out. 20mA in 20.77 out. I was able to match the 20mA by adjusting the scale to 95%.

Everything works fine using the calibrator, but when I let the plc control the drive I have problems. If i give it a 20% run command the drive ramps up to 20% and all is good. If I change the speed while it's running the analog output spikes and causes a fault.

I'm using a shielded cable that contains two twisted pairs that aren't idividually shielded. The other drive i installed has two seperated shielded cables, and it runs fine. I don't understand why it runs good if i give it a signal from the calibrator, but not using the plc. Any ideas?

 

[MikeHalloran]

Make the cable like the one that works.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Is the drive output frequency slightly higher than the demand to compensate for slip? For example you want 1500 rpm, the motor runs at 1470rpm at 25Hz, so the drive compensates for the 30rpm of slip by raising the output frequency slightly so the shaft rotates at 1500rpm?

Are the drives configured identically? Many will allow 'cloning' so if you have this option and a working parameter set then copy it and dowload it to the other drive. That would be the easiest way to eliminate a configuration error. I can't imagine that an analog output can be causing a fault within the drive, more likely the analog out is following the internal parameter and reproducing a spike on whatever signal is driving it. Does this happen with a local mA source and meter connected directly to the drive or only when the cable is part of the circuit? Has a careless ground loop been inadvertently created through a signal cable screen? Is the earthing around the drive less than perfect, especially for high frequency currents?



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Thedroid]

It doesn't happen with a local mA source using a meter a reading the feedback in the control room. It only happens using the plc as the source. The cable only has one shield and it is connected at the supply side only. How is a ground loop created? I'd love to pull another cable, but it's a long pull inside an older cement plant mill building. Probably 200ft half conduit half cable tray. Pulling through the existing conduit can be damn near impossible, and running new conduit would be a nightmare, 20ft ceilings, and 50+ years of existing conduit, channel, and cable trays in the way. Everything is well taken care of, just hard to add to. Tomorrow I'm going to pull a separate signal wire on the ground, if automation doesn't find something wrong in the pid parameters, and if it works then I guess I'll get to work.

 

[ScottyUK]

Does the problem happen wif you inject a signal rmeotely using the mA source and reading remotely using the meter, so the cable is in circuit both ways?

Ground loops can occur when screens are earthed at both ends. If you have a bad HF earth on the drive then currents at output frequency can turn up in the screen as it tries to find an alternate path. It sounds like this isn't the case. When you said 'supply side' did you mean at the drive end or at the plc end?

Do the plc analog cards have a common have ground rail or negative return, or are the inputs and outputs galvanically isolated from each other?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[ozmosis]

It sounds like equipotential bonding between the VFD and the PLC is the issue. I agree with Scotty on the HF interference finding an alternative path but if the chassis grounding of the PLC is a distance away from the drive or on a different ground plane then it could create this problem.
I also agree with the simple suggestion from Mike: copy the design of the other one that works. There's generally no black art here if all parameters are the same, it's HF interference that is the cause.
If this does not work, a quick check (but not a permanent one!) is to run a reasonable sized ground cable from the chassis of the VFD and the PLC and see if this will create a common ground. If it does then you then need to look at ensuring a sturdy method of equal bonding of the ground of VFD and PLC.
The HF interference can create this problem easily (I've had it countless times)and the only remedy is to ensure the HF goes to ground. However, when you connect external devices like your PLC then the HF noise travels down the signal and may not get to ground on the other side so it stays in the control signal and induces its own signal, we call noise.
Break the loop and get the HF noise to ground at all ends.

 

[Thedroid]

The problem doesn't occur using the calibrator and a meter, keeping the cable in the loop. Our plant is the best grounded. I measured .120 ohms back to the transformer, but i'm not sure of the grounding of the plc. I'm going to try another cable today. Whats has me puzzled is how everything works fine until i let the plc take over. oh yeah the shield is connected at the plc side.

 

[DickDV]

The way I read this, a 4-20ma speed signal goes into the drive and the drive responds appropriately to the speed command. The 4-20ma analog output represents the signal properly until certain speed ranges are encountered and then the analog output spikes or jumps around unexpectedly.

First, does the drive output speed or hz jump around consistent with the analog output spiking. If yes, the drive is receiving some signal to spike its output and the analog output is faithfully reproducing that glitch. If no, the problem is confined to the analog output and its connected receiver.

Second, if the answer above is no, disconnect the analog output from the receiver (plc) and place a 250 ohm resistor across the drive analog output terminals. Measure the analog output current again. If the spiking goes away, you need a signal isolator between the drive and the plc. If the spiking does not go away, the drive is not generating the signal properly and is defective. Just be sure that the drive software maps the analog output to drive running speed and not something else.

 

[roydm]

It sounds like a signal isolator is required between the PLC and drive. These are often required when two ground referenced devices are a long way apart.
It's not unusual to get a device that's outputs are not referenced to the same point as it's inputs.
One thing to check, put a mA meter in series with the positive of each pair and drop off the negative. If you still get current flow or it changes that may indicate the problem.
If you send a 20% signal the drive's frequency should be 20% (no offset) unless somewhere in the configuration you have set up 4mA = other than zero. I have never heard of allowing for slip.
Some drives allow you to display the speed reference, check that against the PLC.
Give us the model of drive and Input /Output modules.
I don't think the unshielded pairs is causing the problem, 4-20 is quite immune to noise.
Regards
Roy

 

[controlsdude]

I would also check to see if you have some programming issue in the PLC where some other function is writing some errroneous value in the analog out. Look at the destructive points in the cross reference, compare them to the good working drive.

I have had problems where the 24vdc common was not the same as the plc 24vdc common.

Cloning the drive is a good idea, cover your base and clone the good drive to make sure someone did not fat finger a default parameter into a bad setting on the bad working drive. This has got me on more than one ocassion.

check the 4-20mA wire routing to make sure the bad drive wiring is correct and installed correctly.

 

[SteveWag]

Phoenix Controls sells a $1000 xmitr/rcvr set, with antennas, you supply 24 volts DC at each end. This will replacate(sp) a 4-20 ma signal over up to 5-8 miles and of course provide isolation. The rcvr could be powered using the control power from the drive. I have used quite a few in power and wastewater plants. Seems you can't run much conduit and wire for $1000.
Steve

 

[ScottyUK]

Steve, you must plenty money! I'd be reaching for a £100 galvanic isolator from MTL, P&F or the like. There'll be 24V at the PLC so power won't be a problem.

These ones are haz area rated but the idea is basically the same:

http://www.mtl-inst.com/products/isolators/5500/index.htm

----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Thedroid]

This morning I took two meters out to the drive and hooked one up to the input and one to the output and had the control room start the drive. I programmed both drives and double checked all parameters, and all were good. I had the drive display the analog input so that I could compare the readings. At 25% I had 8mA in and also 8mA at the drive and out. I repeated this at 50%, 75% and 100%. Once I reached 100% the meter on the Analog in read 6mA but the drive displayed 20mA and the output meter read 20mA also. I tried lowering the speed back down to 25%, but the drive stayed at 100% and the readings did also. I disconnected the output and repeated everything with perfect results. Now back at the plc cabinet I decided to put a jumper from the shield i hooked up at the output over to the empty terminal for input, and everything worked.

Noise can be a real issue in this plant. Everything is well maintained, but the grounding system is somewhat suspect. When the plant was built, only motors over 150hp were externaly grounded. since then all of the motors have been externaly grounded, all tray and conduit bonded and everything tied back to the MCC it's fed from. All MCC's are tied to the transformer that feeds it, all transformers are connected, and everything is tied to a grid connected to the service. Annual ground testing is definately fun, with 600+ motors needing to be tested. I'm not sure how the plc's tie into the plant grounding. There supply power is regulated, but fed from common supplies. I have to get out the meter and check. Also at least 50+ VFD's from fractional up to 1500hp.

 

[Thedroid]

We use a phoenix contacts wireless xmtr/rcvr for a well down the road. From what I heard it was a nightmare to get to work properly

 

[roydm]

So you are telling us as the signal from PLC stepped from 75% to 100% the mA went from 16 down to 6 but the drive stuck at 100%. The signal may have started to oscilate. When you grounded out the shield it stopped. It would be interesting to look at it with an osciliscope if you have one.
At any rate I'm sure you have some sort of a grounding problem and that a signal isolator would have solved, I suggest you pick up a couple for future.
Sounds like you are OK for now.
Good Luck
Roy

 

[ozmosis]

thedroid
Sounds like progress is being made and thanks for the feedback.
Just as a footnote:
HF noise is not 50/60Hz, it is anything upto 30MHz and this behaves very different. A VFD will generate HF noise, there is no way of getting away from this. The key factor is to ensure the HF noise has a low impedance path to ground and this requires a view slightly different to 'safety' ground/earth.

 

[Thedroid]

I do have a few action instruments signal isolators, but they require a power supply don't they. My focus was trying to get everything to work properly without having to pull anymore wire. The shield was grounded the whole time to the plc output, when i jumpered the ground up to the plc input everything straightened out. Before I grounded the shiel at the input, when i gave the drive 20mA the both meters and the drive diagnostics showed 20. when I lowered the signal the input read 6 but the drive and output stayed at 20mA. They would not drop until I shut down the drive. I have access to a fluke scope meter, I'd like to see what it shows.

 

[ScottyUK]

You can break the ground loop at the drive end or at the PLC end. The PLC end must have power doesn't it? Try putting the loop isolator there initially. If that fails rig temporary power to the other end and repeat the test.

The cable doesn't have a joint (Scotchcast) or an intermediate JB where someone might have done something screwy with the screen by any chance?


----------------------------------

If we learn from our mistakes I'm getting a great education!