Chattering Coils and Signal Reference Grids

Ask the Experts is proving to be not only a place to get your questions answered, but also a forum to offer answers to your fellow readers. Marc Taylor, staff electrical engineer, Shell Global Solutions, Inc., has some experience that makes him our expert du jour and qualifies him to chime in on a discussion about chattering starter coils first published in the July issue of EC&M.

Q. (From Ask the Experts, July 2003). One of our customers has a wastewater plant equipped with two 20-hp pumps and three 50-hp centrifugal blowers, all rated at 480V, 3-phase. This system, with no changes or additions, has been working without any problems since it was installed in 1986. But in recent months the customer has experienced intermittent incidents when the third blower motor tries to start when all of the other motors are running. The 120V coils on all of the motor starters also start chattering….The three blower motors alternate every time they are called for. It doesn't matter which specific blower motor is the third one to come on when the incident happens…. I think the problem is caused by a voltage sag, which occurs when the third blower motor goes through its locked rotor at start-up. It then dips the 120V power to the coils low enough to make the coils chatter. It seems to be a global problem with the whole system.

Taylor's response: This voltage sag problem presented in the July 2003 column happens quite often in our older power systems, which are fed from a very long utility line. Our facility is located in the middle of nowhere at the end of the circuit. In fact, I have seen this problem pop up where we never had voltage sag problems initially.

We find the usual cause is that the loads on the utility's system and/or our own system, but located ahead of the problem motors, have continued to increase over time to the point where we start to have this same type of voltage sag problem when starting some of our larger low-voltage motors.

To solve this problem, we only run our low-voltage motor starter coils on 120VAC. When we go to medium-voltage motors, we switch to using 125VDC, since we now have switchgear with 125VDC battery systems available for use with the medium-voltage starters.

As long as the voltage sag is just a starting problem, which appears to be the case for this wastewater plant application, the easiest and cheapest way to stop the starter coil chatter during start-up is to add a small full-wave bridge rectifier to each of the motor starter coils and to run the coils on DC power. Starter coils running on DC voltage will hold on much lower voltage levels than those that operate on AC voltages. Typically, coils running on DC will hold on voltages down to about 30% of nominal, instead of 80% as is the case for those that run on AC voltage. This almost always solves the starter coil chattering problem on low-voltage motors.

Q. We have a signal reference grid beneath the raised floor of our IT equipment room. I realize that this grounding system has impedance at 60 Hz that is low enough to equalize any potential differences. This keeps all metallic enclosures, raceways, and any other grounded metal at the same ground potential. But what about higher frequencies? If I remember my basic electrical theory correctly, I will not be able to get this equalization because of the increased impedance associated with these frequencies.

DeDad's answer: Your memory of basic electrical theory is correct, but your signal reference grid (SRG) will be able to handle higher frequencies if it's installed correctly and uses the right type of bonding straps and conductors.

The impedance of a conductor has three basic components: resistance, capacitive reactance, and inductive reactance. Although the inductance (L), in Henrys, will be constant for a given length and cross sectional area of a conductor, the inductive reactance (XL) will vary with the frequency (f) of the applied voltage, according to the equation XL = 2µfL.

So at 60 Hz, XL = 2 × 3.14 × 60L, or 377L. At 30 MHz, XL = 2 × 3.14 × (30 × 106)L, or 188.5 × 106L, or 188,500,000L. By dividing the value of inductive reactance at 30 MHz by that at 60 Hz, you'll find that the “equivalent” resistive value of a given conductor, at 30 MHz, is about 500,000 times greater than the same conductor when the applied voltage is at 60 Hz.

In addition to increased inductive reactance at higher frequencies, there is stray capacitance and inductance between adjacent conductors or between conductors and adjacent grounded metal. There are also some resonance effects. All of the above will also contribute to an increase of apparent conductor impedance.

If you connect the conductors in a mesh or grid to form a lot of low-impedance loops in parallel, you should see little voltage difference between any two points in the grid from 60 Hz up to a frequency where the length of one side of the square represents about 1/10 wavelength. For example, a grid made up of 2-ft squares will provide, at any point, an effective equipotential ground reference point for signals greater than 30 MHz. The Figure above offers a graphic description of impedance vs. frequency plots for equipment grounding only and equipment grounding plus straps to an SRG.

A note of caution: Make sure you're using the shortest possible lead length for bonding jumpers to connect your IT equipment frames to the SRG. Also, make sure you're using braided bonding straps for these connections and 7-strand No. 6 bare copper bonding jumpers to tie under-floor metal components with steel building columns. Finally, verify that every sixth metallic raised floor pedestal is bonded to your SRG.