POWER QUALITY

Power Quality Q&A

Our Expert Offers Answers to Frequently Asked Questions About Power Quality

Question:

We are specifying a drive system that will have a process loop to control drive speed. We are being asked if we want a 4-20 mA or 0-10 VDC signal input. What is the difference?

Answer:

The choice for your application is dependent primarily on the type of control you are looking for, wiring distance, and the susceptibility to noise.

For many years, the typical production line has used 0-10 volts VDC as the primary control voltage. It provides an easily measured signal that can be modified by ratio, adding, subtracting, offsetting or other arithmetic function. This is very useful for complex tensioning and torque control. However, since it is dependent on a steady voltage level, transients, sags, swells and electrical noise can modify the signal giving undesirable variation in the signal and the drive system.

The process industry has used 4-20 mA for many years because it is a current controlled variable. A steady current is required for control and the voltage varies to maintain it.

Hence, voltage drop or fluctuations have little or no effect in normal applications. The signal is zeroed at 4 mA instead of 0 mA to distinguish between a true zero and a faulty transmitter or wiring.

A basic rule of thumb is to use a 0-10 VDC voltage signal for short distances where the control requires complex interaction with other devices. This is typically a factory line application.

In process applications, where single loop controls extend over long distances or where electrical noise is a factor, the 4-20 mA signal is the preferred choice.

Either type of signal should be isolated from power conductors in its own conduit from a safety standpoint and for electrical noise considerations.

Preferably, the signal would be carried on a shielded, twisted pair cable. The rules for grounding the shield are the same. Usually this means grounding the source end and isolating the cable shield from ground everywhere else.

Always minimize the number of cable breaks or terminations.

Question:

A while ago, we had a problem with a drive generating transients that affected other sensitive loads. What can we do to avoid this in the future?

Answer:

Normally, drives and other electronic equipment are pretty much maintenance free and don't get much attention until something isn't working correctly. I have seen a case where the drive was still performing its required function and yet was sending damaging transients into the power system. The only symptom was the periodic failure of an electronic board in equipment on the other side of the plant.

The best preventative maintenance (or in this case, predictive) is to use an oscilloscope or other instrument to periodically look at the voltage and current waveforms of the drive or electronic device. Use a constant load so that the waveform can be compared from time to time. In fact, when the equipment is first commissioned take a snapshot to establish a base condition. You now can compare the output waveform with what is expected from the equipment manual.

Generally, there should be symmetry. If not, a problem may be present. This may result in a catastrophic drive failure in the near future. A heads-up in this regard may save a couple of days production.

The other way to head off problems of this sort is to periodically monitor the power system looking for transients and other unusual phenomena. This can be accomplished with a permanent or portable instrument that is capable of measuring and capturing very fast transients. Some equipment has built-in analysis tools such as the ITIC curve to evaluate whether the transients are of a concern or not. You should be able to store and upload the data into a PC for analysis. In this way, you can begin to look for the problem before it starts to cause significant damage to sensitive electronic components.

David is the President of Wintek Engineering. You can forward your questions or comments to him at wintek@wintek-eng.com. ET