IAS Blog

Electrical Noise , Busted Laptop, New thought process

Posted on 10/29/2009 at 12:14 pm by IAS
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This story starts out while having lunch with Stefano from Enerdoor Inc at Subway. I enjoy taking Stefano there because he has only been in the US for a while and is still learning english. Come to think of it so am I. Anyhow.. the people at subway talk fast and when they ask “White or Wheat” it comes out like one word 'whitaweet'. Its fun watching Stefano's puzzled expression as he thinks of what 'whitaweet'. could possibly mean.

During lunch I mentioned a planned trip to help a customer with a noise issue on a machine. The AB power quality monitor is reporting massive fluctuations in voltage that are not really there. This machine is a 300HP Motor-Generator set. It was a 300HP VFD (variable frequency drive) turning a multi tap generator. This machine gives the customer the ability to control voltage and frequency of the output. I was discussing my typical strategy of shielding the leads, perhaps installing a high frequency filter on the leads to the meter. Stefano decided he had best come with me. I welcomed the help.

Stefano and I arrive at the customers and setup our equipment. For me it was setting my laptop on top of the 4' tall enclosure around the 300HP generator. If your thinking that doesn't sound smart you are correct. Now to give myself some credit I recognized this may not be a good place for my computer and planned to keep an eye on it. Stefano setup his EMI/RFI/EMC Receiver, LISN (Line impedance stabilization network), and laptop on a secure table near the equipment. After a brief look around we started the genset to take a test.

How long does it take a laptop to vibrate off a 300HP genset? About 11 seconds. The laptop landed on the corner of the cover and the display broke into more pieces that I bothered to count. I went into, what do I do now, mode while Stefano continued testing.

The VFD turned out to be the source of the electrical noise. Due to the close proximity of the motor and generator electrical noise generated naturally in the VFD output was being passed to the generator and to the power quality monitoring equipment.

Before this experience I would have tried to filter the noise at the power monitoring device. Stefano carefully explained that the end user of this machine could have electrical noise problems from the power it generates. The correct action was to install filtering on the main lines. This eliminates our issue and any potential problems the end customer may have. Our customer strongly agreed. It was a little more complicated due to the multiple taps from the generator to the electrical cabinet but in the end the problem was solved.

I got thinking about the many times since working at Industrial Automation Supply I have been asked to help with electrical noise issues. The most significant issue was the destruction of a large drive caused by a smaller one, which was repeatable and expensive. Other more common issues are with communications lines, encoder signals, and analog values. I was not solving the problem but treating a symptom. I guess both have their place.

As for the laptop. It was totaled. The repairs were ~$300 and the laptop was a 3 year old dell. But the new one sure is nice.