I have an electrical noise problem that I need help with. Please bear with the rambling:
Our lab is measuring the heat transfer performance of automotive diesel fuel heat exchangers. The equipment consists of a diesel fuel supply cart and a wind tunnel in the explosion safe test room, while the data acquisition equipment and supply cart/wind tunnel control box is located in a separate control room. For the data acquisition / control hardware we are using National Instruments Field Point with an RS232 link to the computer.
On the DAQ side, we are reading inlet fuel and outlet fuel Temperatures at our heat exchanger with Laboratory grade, shielded RTD's, as well as the diesel mass flow rate with a Coriolis mass flow meter. We are also measuring the wind speed in our tunnel with a 4-20 mA differential pressure transducer. There are other various measurements being made with thermocouples and other 4-20 mA devices. The sensor data is fed through the wall, away from any and all power lines, into the Field Point DAQ.
On the control side, we are using the Field Point to output signals from the DAQ box to the control box. Control level signals are passed from the DAQ box to the Control box to control the Diesel Flow (100 Vdc motor controlled by a Saftronic DC Motor controller), the Diesel Temperature (600 Vac heaters controller by a 0-10 V controlled 3phase SSR), and the air speed in the wind tunnel (Leeson VFD driver 600 Vac,3 HP motor). These power lines are fed from the control box, through the wall, into the appropriate equipment.
Note the sensor lines are fed through a separate hole in the wall than the power lines, a good 10 feet apart.
Our facility is fed from the 27kV outside into a 480V delta output transformer. This 480 V delta is then enters our facility to the main switch gear / distribution panels. Power for the Diesel test equipment is derived from this 480 V delta supply from a distribution bus bar, into a 480 Wye input , 600 delta output transformer. Note that this transformer is hooked up in reverse, i.e. the label stated that the 600 V Delta is the primary, and the 480 V Wye is the secondary.
When ever the VFD to control the air speed is turned on, the temperature reading from the RTDs drop by about 5 °C, and become very noisy. The other 4-20 mA sensors do not drop in value, but they also become noisy. To my surprise, the temperatures being measured by thermocouple are stable like a rock. We have tried every thing to solve this problem, but I am at my wits end. All the sensors are shielded well, and as far as I can tell, properly. We have tried to power the DAQ by battery, to remove any influence of power line noise, but this didn't help. We have tried different shielding points, and this either has no effect, or a worsening effect. I scoped the ground with a battery powered (thus isolated) oscilloscope, with the +ve and
-ve leads connected to measure the common mode noise. It is noisy at the control box, but clean at the ground entering out building. This tells me that the noise may not be sinking to earth, but when traced, the integrity of the ground seems to be intact.
Again, sorry if I've been rambling, but I am very frustrated by this, and I would love some input from any of you people out there. There is a lot of knowledge among this group, and if anyone can help, this group can.
Thanks for the time, Dan Wright, Dana Corp., Long Mfg.Div.