New motor/alternator technologies revolutionize the concept of the uninterruptible power supply.
In our last episode of "Power Conditioning," we looked at the old concept of a motor driven alternator being brought up to date with new design configurations. We saw how increasing the number, size, and design of bearings made the old commercial unit more capable of "surviving" a premature bearing problem and continuing to support a sensitive load. We saw the changing from pure induction driven machinesto synchronous, "low-slip," and even composite synchronous/induction designs known as "synduction" equipment.
We saw how this technology is used as a very high-grade "line conditioner," which by definition consists of a voltage stabilizing product combined with a noise rejection technology. While we normally think of a less expensive tool to perform line conditioning, the motor alternator is the very best of this group. It not only rejects common-mode noise, but also, by virtue of the shaft or belt connection, stops any line-to-line noise from entering the output-truly a complete mechanical isolation from the incoming energy source, (or disturbances on the customers' distribution system), complete with voltage regulation and in some cases a period of "ride-through" time.
What more con we expect?
Certainly the list of performance characteristics is a good one, and the technology has served us well over the years, but is there anything more we can count on? The answer here is a resounding "yes."
One of the real concerns for operating engineers is in the maintenance area. What can we expect in cost and other troubles possibly awaiting the user? In our experience over the past 16 years, we find the maintainability of this technology far more user friendly than other technologies. For example, most solid-state products usually require service from the original manufacturer, while many of the motor/alternator (M/A) products can be serviced by local staff maintenance personnel or a local motor shop with a relatively small amount of specialized parts. Conversely, the static UPS usually has very distinctly designed parts for each unit, and the proviso that factory service should be used rather than "third-party" offerings. Parts upgrading takes place frequently, and the changing of components over the life of a unit has been a problem in the past. In the case of the static units, a factory service policy complete with a spare parts kit is recommended to be purchased with the original order placement.
On one occasion, where an M/A application was on the top floor of a facility, our client experienced a premature winding failure in the motor, certainly the worst of all possible failures in the worst of all possible locations. Yet, when the service man from the local motor repair shop came to the site, he gave an estimate of three days for a total disassembly of the unit. This meant taking the parts out piecemeal, completely rewinding and rebearing, retesting the component parts, and returning them to make an operating system. We were challenged to think of how to accomplish such an overhaul for a static system. Just as he promised, all went back together in three days, and this unit has continued to perform as specified over the ensuing seven years.
One might look at this technology for its ability to assist in "swallowing up" harmonic currents coming from the devices on the output circuits. While the alternator is not designed as a harmonic trap, it does manage to serve like any other "coil" inductance. When the output impedance remains low, and the harmonic current loading is a small percent of the total current, the unit can perform well in handling the harmonic current without significant voltage distortion and without allowing transfer of the harmonics to the incoming circuit.
A bonus for the facility power factor improvement can be achieved when using the synchronous motor as the input driver. By adjusting the field control on the synchronous motor, you can change from lagging to leading power factor, now supplying VARs (volt-ampere reactive energy) to the system at no additional expense. This method of assisting power factor improvement may be one of the strongest available, and one not subject to the possible damaging effects that harm capacitors.
Maintainability and repairability seem to give the M/A concept the possibility of a long life. The ruggedness of construction combined with the additional features listed above may make this a good tool, especially in harsh factory conditions. There is even more to report on the "packaging" of this tool with other rotary elements.
Combinations or hybrid assembles
In order to extend the usefulness of the motor alternator, different manufacturers have designed innovations to be added to the basic product. In the past, the time of "riding through" a power disturbance was extended with the use of a large flywheel. The rotational energy storage extended the time for satisfactory output from the milli-second range to several seconds. Permitting the motor alternator to keep the load running when the power system experienced a series of momentary breaker operations lasting beyond 30 electrical cycles.
A product improvement came through the special design efforts of John Roesel when he developed his patented system for "written pole" technology. This development brought a large "ride-through" improvement by recognizing and designing a way to provide a constant frequency output, even while the prime mover (motor) is slowing down when incoming energy from the power line is not available. Remember, as the power is removed from the input to the motor, the motor will lose speed, and the whole unit will sacrifice stable frequency. In this breakthrough, the designer found a way to electronically "reposition"the poles of the alternator to compensate for the slowing speed of the motor. In application, this design can produce full rate output for 15 to 20 see, without battery support! Sounds too good to be true, but the technology has been working for many years and is coming to the attention of designers more and more. An example of the "pole writing" is seen in Fig. 1. In a two-pole, 3600-rpm application, the poles of the alternator are successively repositioned "backward" to provide a constant 60-Hz output as the input loses rotation. Thus, making a constant frequency, variable pole machine.
[Figure 1 ILLUSTRATION OMITTED]
In addition to extending the "ride-through" time, the Roesel development provides a new combination for continuous power: The coupling of an extended time motor alternator with a conventional engine generator. In this application, shown in block diagram in Fig. 2 (on page 22), an engine generator and automatic transfer switch (ATS) are located as the alternate source to the utility feed, upstream of the Roesel unit. The ride-through of 15 to 20 sec provides sufficient time to start, synchronize, and transfer online the engine driven unit without any disturbance to the sensitive load. Other loads are connected in a conventional manner as shown.
[Figure 2 ILLUSTRATION OMITTED]
A further extension of the "coupling" of technologies is seen in Fig. 3 where another manufacturer has combined an induction coupling between an engine and its generator. This "hybrid" design is intended to provide continuous power, first as the electric utility energy is used to drive the output, and then as the stored rotational energy is used to start the engine to continue driving the assembly.
[Figure 3 ILLUSTRATION OMITTED]
Field convertibility--motor alternator to continuous power system
In a combination of perhaps the "best of both worlds," we have seen the assembly of dual feed motor alternators connected in parallel on the load side in a fashion shown in Fig. 4. Here, we have seen the advantage of utilizing two continuous utility feeds where they are available to eliminate the need for battery energy in providing uninterruptible power. The layout begins with the use of the synduction technology, in its simplest form acting only as motor driving alternator on each of two units, equally sized, and connected to separate incoming power feeds. Since the units are synchronous, they may be paralleled without costly synchronizing equipment and can handle the 50% load step that occurs if one line should experience an interruption. The units are constructed with a shaft extension for future field mounting of a DC rotating product in the event of the need for conversion to self-contained battery-supported units.
[Figure 4 ILLUSTRATION OMITTED]
The idea of this "field conversion capability" is to provide only the level of performance needed in the initial installation, but have the full flexibility available to convert a unit in the field without loss of the initial investment. In two large telephone support installations, this conversion was a saving feature for the client. The total cost of adding the conversion is greater than the cost to purchase the finished assembly all at one time, but permits flexibility of application and the opportunity to operate at the lesser level as long as possible.
