Servo Drive burning out encoder input.

[Chris1234]

Evening, we have a machine (being deliberately vague) it has a main motor, which is driven by a VFD, on the spindle of the shaft driven by the motor is a rotary incremental encoder, something like 1000 pulse / revolution. This encoder controls a Servo Drive which drives a small motor for another part of the process.

Three times now we have had the Servo Drive fail, it happens we think either when the machine is turned on or off, never when it’s running. It could happen if the emergency stop is pressed when it’s running, as seems to happen. But no definite proof.

The fault is burnt tracks on the PCB on the encoder input (the encoder on the main motor shaft, not the servo motor encoder). Both channels A+B from memory. It is a surface mount board so is a pain to repair.

First time it happened, the encoder was replaced, as was the 24v DC PSU powering it. It has happened again twice I think after that. Servo Drive was replaced twice.
My solution was to put current limiting resistors in the encoder A+B channel and 100mA fast blow fuses. It hasn’t happened since so I think this has worked, but it would be nice to find out what was causing it.

Presumably a magnetic field breaking down could cause a spike like a relay or the drive is doing something. The encoder channels are shared with the HSC in the Allen Bradley PLC, no damage has been caused to this.

It is an Invertek Drive controlled by standard digital inputs, only one contactor in the machine, safety relay, HMI, Allen Bradley PLC with 2x HSC add-ons.

Thanks.

 

[marconi]

Chris 1234: Good morning. Burnt tracks on the main motor encoder 'suggests' a relatively high energy event which leads me to wonder about the type of 24DC power supply, from where it is powered and whether there is any surge suppression/over voltage protection on either side of its windings. The indication that current limiting has perhaps prevented repeated damage 'sort of' supports this hypothesis - but does not prove it by any means. Are you able to connect a 'scope up and observe the voltage waveforms during start/stop events

As a precautionary measure then what about fitting a Type 3 surge suppressor from the mains ac protection range on the primary side - cirprotec make them:

Industry / Applications / Home - Cirprotec - http://www.cirprotec.com/Applications/Industry

and

something like one of these CITEL data SPs in the 24V supply to the encoder:

DLAH-06D3 - 1-pair DIN rail plug-in Data surge protector, CITEL - https://citel.fr/en/telecom/din-rail-telecomdata-surge-protector/dlah-06d3

MSB6-24/LD - Ultra Compact Type 2 hard wired surge protector, CITEL - https://citel.fr/en/ac-power/hard-wired-ac-surge-protector/msb6-24ld

DIN Rail Telecom/Data surge protector - Telecom, CITEL - https://citel.fr/en/telecom/din-rail-telecomdata-surge-protector

Cirprotec also do the data SPs but I cannot quickly find them.

First Power and Surge Protection in Sheffield were helpful when I had to source a 12V SP to protect some electronics fed by a mains to 12VDC power supply. Here is the contact:

Rhys Seagrave

Technical Commercial Manager

T: 0114 322 3455 | first-power.co.uk

DD: 0114 322 3454

Unit 8 Riverside Court,

Don Road, Sheffield, S9 2TJ

 

[James]

this encoder is shared between the drive and a plc high speed counter?

are the 0v ref connections joined correctly?
if you have say psu 1 powering pcl and encoder, you should ensure that the 0v connection on the drive is joined to the 0v of the psu.

I am wondering if you are getting an unusual current flow through the encoder when the power is applied or removed from 1 device slightly before another.

 

[pc1966]

Stuff being burned out suggests more than just an induced transient.

Usually the encoder's internal electronics would be floating (i.e. not grounded at the encoder, only at its interface) and only the overall screen grounded to avoid the risk of major earth surge current flowing in the signal wires. That might be worth a check.

Series resistors would help in that case as you seem to have found, but it is a work-around for what ever the original cause is.

Also as Marconi suggests it is always a good idea to have some surge suppression around anything that has a high current inductor (like motors, etc).

 

[Lucien Nunes]

The way I read the OP is that the main motor is sensorless, or at least its drive does not involve the encoder mentioned which is merely for the servo drive to follow it. He distinguishes between the servo system's master encoder on the main motor shaft, the input for which is where the problem lies, and the encoder on the slave motor which does not suffer the same problem although may have an electrically similar configuration.

Burnt tracks suggest, as mentioned, currents in the order of amps, which would be difficult to induce differentially but could occur common-mode. What type of output does the encoder have, TTL or differential? Is the DC output of the external power supply unit earthed or floating? Does it obtain its supply from the same source as the servo drive and does the cable follow the same route as the encoder signal cable?

 

[Chris1234]

Thanks for the replies.

To answer questions:

The PSU is A din rail mounted Meanwell.
I forgot to mention we have an identical machine that doesn’t suffer the same problem.
We have no working oscilloscope.
I tried to record voltage when the e-stop is pressed on a Fluke 289 data logging multimeter, I’ve only seen a surge once as the 289 only takes a reading every 0.25 seconds so it doesn’t always catch what you need it to.

I believe the negative of the 24v PSU is connected to ground but it’s been a few month since I’ve been near it and I’m not definite. The encoder is connected to the Allen Bradley 2071-AP4 along with the negative from the encoder (will also need to check!)

Surge Protection is a good idea but it shouldn’t need it as the other machine doesn’t have this fault.

The encoder screen is connected to earth and I believe (again, need to check as it’s been months!) the supply from the PSU to the encoder is grounded.

Main motor is a standard AC induction with no encoder. There is an encoder connected to the motor shaft which tells the servo drive how fast to run.
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The encoder is:

8.3720.5621.1024 | Kübler Incremental Encoder, 1024 ppr, Push Pull Signal, Hollow Type, 8mm Shaft | RS

uk.rs-online.com

 

[marconi]

Chris1234 - no doubt teaching you to suck eggs - a careful compare and contrast between the two identical(?) systems may be fruitful. The smallest deliberate or accidental difference may be the cause. Can you spot any inadvertent earth loops - say through damaged sheathing around screened cable now touching earthed metalwork? Ideally we want earths starred together.

I looked up the Mean Well PSU. There is capacitive linkage within the PSU between the -V dc rail and the functional earth (FG). There is also as expected capacitive linkage between the EMI mains input filter and FG. So for ac currents of increasing frequency a reducing impedance path between the -V rail and earth. This might be relevant since you describe the failure happening during episodes of fast electrical changes viz start up and stop. See Block Diagram in:

https://www.meanwell-web.com/content/files/pdfs/productPdfs/MW/Mdr-20/MDR-20-spec.pdf

Did you fit the resistors and fuses at the encoder end of the A and B signals cable or the Servo end? For information the encoder has for A and B a push-pull type output stage. I muse - but find it hard to believe - that the output transistors both conduct at the same time for some reason during high frequency/transient events effectively connecting the dc rails together.

I'd check the insulation resistance between the A and B signal conductors and to earth and screening when completely disconnected at the encoder and servo ends.

Early morning thoughts which might or might not be useful.

 

[James]

Try something like this.

Encoder Signal Splitter Master Encoder Signal Splitter

Encoder Signal Splitter - SY057 provides a convenient way of splitting a master encoder signal across multiple drives or motion controllers.

synectic.co.uk

 

[timbobelfast]

Can you connect the encoder through a relay so that it is only energised when the VFD is running to prove?

 

[Chris1234]

Hi, both machines work the same but have obviously been wired up by different people.
It was several months ago that this problem last appeared. As far as I can remember the actual wiring connections were the same, as was the earthing, but I will check when I get a minute.
The encoder goes to a junction block where it is split between the HSC mounted on the Allen Bradley PLC and the Allen Bradley Servo Drive.

Thanks for looking up the PSU, however it was this one (link below) that was put in, it doesn´t have the capacitor link to earth, nor the capacitor link from the EMI filter, but it has an extra opto-coupler (?) on the detection circuit.

The resistors and fuses were fitted (not sure why) after they split on the Servo Drive side.
I will check insulation resistance between channels and to earth, thanks.
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Try something like this.

Encoder Signal Splitter Master Encoder Signal Splitter

Encoder Signal Splitter - SY057 provides a convenient way of splitting a master encoder signal across multiple drives or motion controllers.

synectic.co.uk

I didnt´t know those existed, I did look at putting an opto-coupler on, but the frequency was too fast for it.
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Can you connect the encoder through a relay so that it is only energised when the VFD is running to prove?

Possibly, as it is a single phase VFD, there will be a spare set of contacts on the contactor that, so it would be easy enough to connect another relay for all connections on the encoder.