Older motor-generator technology has been updated so that the MG set is still a reliable and viable source of conditioned power.

Don't discount old power conditioning concepts such as the use of motor generators. Yes, you may be saying, "I thought that stuff went out with the end of World War II." And, you may be right; we could need to "be done" with old-style equipment, as tar as power conditioning is concerned. But, the MG set, with a few modifications and improvements, has developed into a rather significant and reliable power conditioning device. Let's see what's been done to achieve this performance.

The old versus the new

The most-cited reason for frustration with the, "old-style" MG set is bearing failure. Looking back on conventional MG set constructions, we find that the majority of "standard" units uses a two-bearing construction on a common shaft: One bearing on the motor side and one on the generator side. Certainly, this "balanced" design performs well in most common applications, such as changing frequency (60 Hz to 50 Hz), changing from AC to DC or DC to AC, or even changing from single phase to 3 phase. Thus, the "old-style" MG set is used primarily in industrial environments and those not normally associated with sensitive logic systems.

With the coming of age of the computer, this same "old-style" MG set is sometimes used as a power conditioning device for these new sensitive loads. Yes, there are some good performance traits. For example, the MG provides a true mechanical separation between input and output, via the shaft or belt coupling, as shown in Fig. 1. As a result, both normal mode and common mode noise are eliminated. In fact, an MG set located on a computer room raised floor or nearby to the loads it serves virtually makes the need for a shielded isolation transformer unnecessary. In addition, the voltage regulation of the MG set is built-in to the output. Finally, the rotating mass of the device allows some stored energy for riding through short power disturbances.

As time has gone by, various changes have been made to the "old-style" MG set to enhance its performance and reliability. For one thing, the bearings have been beefed up. However, no additional bearings have been added to the design to lessen the possibility of a bearing failure shutdown.

One chronic problem occurs when an induction motor is used as the prime mover. The resulting characteristic, known as slip, is when this type of motor rotates at a speed somewhat slower than the synchronous rotation of the generator; for example, 1740 rpm versus 1800 rpm. Thus, the generator has a difficult time keeping 60 Hz frequency under varying input conditions. Also, this slippage increases if the input power is removed, as in a voltage decay or temporary loss of power. Here, the output frequency diminishes further. The solution to holding frequency steady under these conditions is the addition of a flywheel, which stores rotational energy and extends the time for riding through the disturbance.

One alternative design includes the use of a synchronous motor as the prime mover. This avoids the above mentioned slip problem. Now, there is a motor rotating at the same synchronous speed as the generator, and the input-to-output frequency is exactly maintained ([+ or -]0 % error) since both input and output are locked at the same speed of rotation. The one flaw to this design is the lack of starting torque and/or recovery capability for a pure synchronous motor. Normally, the start is assisted by a pony motor, which brings the unit up to synchronous speed. When the power is removed from the input, the synchronous motor breaks out of locked speed at about 100 milliseconds into the disturbance. If the outage is within that time frame, all goes well and the machine continues to run. If the time of outage exceeds 100 milliseconds, the unit then shuts down and has to be restarted. Field experience shows that this good/poor performance can go either way.

The synduction motor

One development, the synduction motor, goes a long way toward making the "old-style" MG set a computer-conditioning-capable product. This hybrid technology, if you will, consists of updating the prime mover so that it is an induction and synchronous motor all in one. This motor now has the necessary torque capability as an induction device, remains in the induction mode until almost at the synchronous speed, and then internally switches windings to become a synchronous running device that is locked at the synchronous speed, thus assuring constant frequency.

To complete the mechanical part of the redesign, manufacturers have made provisions for multiple sets of bearings: Two on either side of the motor and another two on either side of the generator. Even with a bearing failure, the machine can continue to operate on the three remaining bearings. (We actually witnessed such a condition [one set of failed bearings] at a control manufacturing plant. The facility's electrical foreman heard a loud sound coming from a 100kVA MG set and called us to announce, "You've got a screamer up here!" When we asked about the power to the computer room, he said everything was running fine. We were given a time for the required MG set repairs, and two men from a local motor repair shop took the unit down, first powering the computer room via bypass. They then changed the bearings, balanced the machine, and put it back on line. The good bearings had kept the MG set in operation until the repair could be made.)

One valuable attribute of a synchronous running machine is its ability, by design, to continue running on just two of three phases. This characteristic is very similar to the capability of three single-phase transformers connected in delta: This transformer system will continue to provide 3-phase power with one of the transformers inoperative or failed. We call this an open delta connection, and the transformer bank must be derated by 43% to avoid overloading. Nevertheless, you can continue to have 3-phase service with the one failed unit. The synchronous motor is the rotating equivalent of this circuit, but continues to operate not on the loss of a coil, but on the loss of a line connection, as shown in Fig. 2 (on page 20).

Now, you might think that this capability is of value only when a utility single phasing disturbance occurs. However, when thinking about this worst case scenario (losing a "line"), you should realize that this happens all the time in lightning storms. First Phase A goes, then Phase B, then Phase C, and then back and forth during the storm. As you can see, this capability of running on two of three phases allows the synchronous machine to take power from any two phases at any given instant, thus permitting a ride through of storm disruptions with no time limitations.

Storm protection case history

We have seen instances where the above performance trait has provided a high level of protection in many lightning-prone areas of the country. Probably the most significant of these instances was in the Southeast portion of the U.S., where a company, in the past, had experienced severe damage from lightning. The repeated "ON-OFF-ON-OFF" cycling of the power during these lightning strikes had caused so much damage to computers and resulted in so much cost for equipment replacement and repair labor that the facility's computer service company canceled its annual agreement. We were called in to recommend a solution to the problem. Our suggestion: A composite induction/synchronous MG set.

After the installation, several pieces of computer equipment were allowed to remain powered by "unconditioned" utility power, while the balance was conditioned through the composite induction/synchronous MG set. The next storm arrived, and all the equipment running on the MG set stayed up and suffered no damage. However, the equipment on unconditioned utility power was extensively damaged. Here was a very dramatic, side-by-side comparison of the protecting power of the composite induction/synchronous MG set.