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Anonymous Poster

Power Cable and Incorrect Inlet Pressure

12/07/2009 2:33 AM

Dear Engineers,

In one of our plants we've had the following problem: a NH3-cooling plant did not start because of an 'incorrect inlet pressure' (as indicated on the control panel). It turned out to be a problem caused by the power cable of the motors (2x 200 kW) being too close to the control cabinet.

Can anyone explain how this happened?

Piet

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#1

Re: machine problem

12/07/2009 3:27 AM

Electromagnetic induction.

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Guru
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#2

Re: machine problem

12/07/2009 3:29 AM

A typical refrigerant screw compressor control panel will have a number of low-voltage, low-current sensing devices such as pressure transducers and RTDs. The installation manual may specify that line voltage connections come in on the side of the panel away from sensing devices. Also, the panel will usually be located some distance from the motor connections. If high-power wiring is too close to sensor wiring, the sensor readings may be off.

It may be that in your installation, wiring conduit to the motor passed near to the control panel. Review your layout, and correct if necessary. If the manual did not caution against this possibility, notify the manufacturer about the problem, so that it can be prevented in the future.

If the installation manual did address this, it may be that the electrical installers overlooked the guidelines, in which case they should fix it by rerouting the motor feed(s).

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Anonymous Poster
#3

Re: machine problem

12/07/2009 11:20 AM

Thanks a lot. Actually, that's exactly what happened. The supplier notified us of the need to put the power cable further away (it was a skid mounted installation, so delivered readily cabled like that), and for future installations they will modify this.

I understand electromagnetic induction, however, I was surprised that sensors can be that 'sensible'.

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Guru

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#4
In reply to #3

Re: machine problem

12/08/2009 1:19 AM

Similar to your case, but different- I experienced a condition where control and monitor wiring was not tied down in a panel and vibration of the equipment that it was connected to caused the wires to vibrate/move. This action triggered secondary voltage in the wires as they passed through a field around some power cables connected to main disconnect. The stray voltage in the light gage control wires triggered a "fault" condition similar to yours.

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#7
In reply to #3

Re: machine problem

12/08/2009 11:06 AM

Some pressure sensors use a magnetic or capacitive pickup to sense movement of a bellows or bourdon tube. The magnetic field from the power cable likely overwhelmed the very small signal from the pickup, causing a false output from the transducer.

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#5

Re: Power Cable and Incorrect Inlet Pressure

12/08/2009 2:46 AM

Among the recognized good practices are these:

Separate conduits for power and signal cables.

Proper shielding/grounding of signal cables.

Segregated zones in panels for higher versus lower voltage connections.

If power/signal cables must approach each other, run them crosswise, not parallel.

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Anonymous Poster
#6

Re: Power Cable and Incorrect Inlet Pressure

12/08/2009 3:42 AM

Hello

To answer your question, it sounds like your pressure sensore has suffered electromagnetic interference and therefore may have given the incorrect output to the control panel which opens the valve for the cooling media NH3. the direction of the coolant is very important here...

Regards

David Linzer

Engineer

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#8

Re: Power Cable and Incorrect Inlet Pressure

12/09/2009 3:24 PM

There was a magnetic flux from the power cables of the motors layed alongside the control(pressure switch) cables.The flux from the power cables permeats the control cables and thus cause mal-operation of the pressure switch to deliver accurate signal which could not activate the power cct components of the cooling plant.

Power cables and control(signal) cables are routed seperately to avoid interference of flux especially where we have higher and smaller voltages and even AC power voltages and DC control or signal voltages.

Patrick Whowha

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