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<== Date ==> <== Thread ==>

Subject: RE: motor cable assembly questions
From: <[email protected]>
To: <[email protected]>, <[email protected]>
Date: Tue, 8 Jan 2013 13:15:35 +0000
Hi Pete,

See below for a response from our motion guys here at Diamond:

* How does your facility standardize motor cables
   and connectors?

For Motors, we use 8 way UTO aka Souriau "Trim Trio" Metal shells, rubber inserts and gold plated contacts. The style of these connectors means that the inner insulation can touch the outer shell, if the wire's insulation is compromised, a fault-to-earth can develop, but is often apparent at time of installation.
The pinout matrix is arranged per motor type to negate issues with miss-connections. I.e. Steppers use one pinout pattern and Servos use a different pattern.

* How does your standard distinguish between
   systems with drivers that only operate below 50 VDC
   from systems with drivers that may operate above 50 VDC?

All motor cables outer sleeve are Orange in colour ( DESINA Standard), to identify them as power carrying.
Motors above 50vdc (Sometimes 60vdc depending on which standard!) Also bear a label, the black on yellow "Lightning bolt" hazard warning sticker.
We do not currently use a different connector type, but this is under review by the electrical engineering group. The SERIOU connector we use is sufficiently rated for higher voltages , but the female pins come to within 1mm of the face of the insulating insert.

* How does your facility handle grounding of the connector?

All motion cables have a woven screen sheath between the outer insulation and the signal cores.
The UTO Souriau connectors have a ~360degree metalized "claw" which grips the Screen of the cable, at both ends.
For servo motors, a signal core is also employed as the earth conductor.
Where an unusual Motor amplifier is used, we would try to clamp the screen 360degrees as close to the amp as possible.

* Are limit, home, and encoder signal conductors in a separate cable?

For ~~90% of beamline axes, separates are used. The original machine standard has the limits and motor power in the same cable and uses the SERIOU 12 pin connectors, and this is certainly true for front ends. However ID's use proprietary motor connections so separates are also used.

* How does your facility test motor cable assemblies?

Point to point continuity and each core to screen for open circuit. Screen continuity should also be tested as it forms an integral part of our EMC strategy.

* Do you test motor cable assemblies for insulation breakdowns?  How?

Not unless a fault is suspected, in which case an insulation "Megger" test at 500v is usually sufficient for fault finding. For higher powered amplifiers which rectify incoming mains voltages we should test at 1500V

* Is there a standard to switch off the motor driver
   if the end-of-travel limit switches have been passed?

Regular EOT limits, - No.
Some axes also have "Over Travel" (Beyond EOR additional limits)  - Yes these kill motor power.
A risk assessment is performed for each axis, and if the consequences of continued motion beyond EOT are significant in terms of cost or time , then extra switches are recommended. This is now true for both in house designs and externally supplied equipment.

* Do you have other comments?

The Amplifiers in the Delta Tau hardware have proven to "trip" when a phase-fault-to-earth has occurred, this clamps the output. Stoegra amplifiers show a recoverable fault when this occurs, removing motor power from its outputs. Smart drive SM and SD amplifiers have a permanent fail if this occurs. So far these have failed safe ( ie with no motor power at their outputs).
HOWEVER:
Servo motor cables MUST be earthed at both ends or "Touch-voltages" can develop on the metalwork. E.g you can find 70-110vac touch voltage on the screen/chassis if one end of the cable's screen is not terminated properly. This can be especially true where a core is used as earth, and the screen is used as shield. Some kind of "Crosstalk" happens and the phase voltage is superimposed onto the screen/shield. The magnitude may depend on the length of cable and the nature of the earth fault. This capacitative coupling is well described elsewhere and can lead also to corrosion and pitting of bearing surfaces as the current follows many paths to ground.

Iain Johnson, Motion Technician at Diamond Light Source UK.

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References:
motor cable assembly questions Pete Jemian

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