Baldor vfd signal problems
Baldor vfd signal problems
(OP)
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?
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?





RE: Baldor vfd signal problems
Mike Halloran
Pembroke Pines, FL, USA
RE: Baldor vfd signal problems
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?
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RE: Baldor vfd signal problems
RE: Baldor vfd signal problems
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?
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If we learn from our mistakes I'm getting a great education!
RE: Baldor vfd signal problems
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.
RE: Baldor vfd signal problems
RE: Baldor vfd signal problems
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.
RE: Baldor vfd signal problems
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
RE: Baldor vfd signal problems
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.
RE: Baldor vfd signal problems
Steve
RE: Baldor vfd signal problems
These ones are haz area rated but the idea is basically the same: http://
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If we learn from our mistakes I'm getting a great education!
RE: Baldor vfd signal problems
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.
RE: Baldor vfd signal problems
RE: Baldor vfd signal problems
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
RE: Baldor vfd signal problems
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.
RE: Baldor vfd signal problems
RE: Baldor vfd signal problems
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?
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If we learn from our mistakes I'm getting a great education!