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Originally featured on BroadcastEngineering.com
Jun 25

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6/25/2009 6:00 AM  RssIcon

3516c-generator-set-model-view-2.jpgIn the last column, some issues of selecting and installing a generator were discussed. If you remember anything from that column, it should be to turn to the experts for advice and help. Don't feel the need for a DIY facility-wide backup power system. Let’s continue on by looking at key generator components, starting with the alternator.

Generator has an alternator

For any engineer who’s toyed with cars, the idea that a “generator” has an “alternator” is an oxymoron. Generators by definition produce DC power. Alternators by definition produce AC power. So, the obvious question becomes, "Why would a generator have an alternator?"

It’s the language. Today, generators refer to inclusive systems that consist of the driving power source (typically a motor) and the producer of electrical power (typically the alternator). In this article, if the term alternator is used, it means the internal component that produces AC power. If the term generator is used, it refers to the entire system of motor plus alternator.

Engineers will recognize that electrical motors can actually produce electricity; all you have to do is turn it. So, an alternator is similar to a motor in design in that it has a loop of wire that moves through a magnetic field produced by magnets. As the wire breaks the magnetic field, electrical potential is created in the wire. With sufficient wire, magnetism and rotations, pretty soon you get enough power out of the alternator to do some real work. So, the key to building an alternator is to have sufficient windings, magnetism and rotations to get the power you need.

Fortunately, backup power systems are typically self-contained, there are only a couple of things to consider about the alternator when purchasing a generator. First, you must purchase a brushless system. Small generators use brushes to move the electrical power from the armature to the outside world. Brushes have two key drawbacks. First, they wear out, and there’s no proactive way to check them. And second, they create electrical noise. This probably won’t be an issue with the sizes of generators a TV station or video house would use, but check anyway.

Large alternators produce current by instead moving a magnet near stationary coils, hence brushless output. While designs vary widely, one design consideration is that by adding poles to the magnetic field, it’s possible to reduce the required rotational speed of the drive shaft. When the shaft speed is lowered, so is noise and engine wear.

Output AC specifications

An important aspect in addition to actual shaft speed is the stability of the output AC frequency. As the generator is loaded, it will slow down, which lowers the output frequency. Hence, some method of adjusting the motor speed must be provided. This is where the governor comes in.

Using an electronic governor, it’s possible to provide +.25 percent accuracy to loads, with response times under three seconds. That’s sufficient for most of today’s video equipment.

While the actual output voltage is controllable, it works best only with linear loads. If the generator has to power such divergent devices as elevators and switch-mode power supplies, the output voltage becomes more difficult to stabilize. This makes it important to analyze the loads before selecting an appropriate generator. Nonlinear loads can also create spikes, voltage sags and noise that can negatively affect more sensitive devices. Fortunately, most of today’s power supplies and UPS systems operate near a power factor of 1.0 so reactivity isn’t an issue. However, if your facility does have spike-producing equipment or other reactive devices, seek expert help.

Sizing a generator

Defining the size generator needed ought to be easy. Just total up all the load, and pick a generator bigger than the sum. Uh, no. Let’s look more closely at the decision process.

First, your load may be too large for one generator to handle. Or, you may want additional backup protection by providing N+1 capability. If so, you’ll need to combine generators. While it’s possible to effectively parallel generators, the required additional switch gear, monitoring systems and redundant fuel systems can quickly balloon the budget.

The upside is that it’s easier to scale the backup power system. If you are in the early stages of developing your facility, it may be more cost-effective to design for multiple generators, adding them as needed, rather than buying an oversized generator now. You will have to initially install adequately-sized automatic transfer switches even if it may be some years before they are fully loaded.

Also, buying a huge generator now may be a less attractive option. Generators must be adequately loaded. In other words, you don’t want a 200kW generator powering a 50kW load. In this case, size is important, but not necessarily because bigger is better.

One vendor recommends the following generalized sizing specifications:

1.5 times the nominal UPS capacity for transformerless UPS systems;

2.0 times the nominal UPS capacity when tied to transformer-based UPS systems; and

3.0 times the nominal HVAC running capacity.

Noise

Noise may end up being the second toughest problem for you to solve (after fuel issues). While the exact permitted noise level is always location-dependent, typical maximum permitted generator noise levels range from 45dB(A) to 72dB(A). Because the typical generator can produce noise levels ranging from 80dB(A) to 105dB(A), appropriate noise abatement must be incorporated into the overall installation.

There are generally two types of regulations that must be met: local/state and OSHA. The OSHA regulations pertain primarily to operators. Those regulations can often be met with the use of appropriate hearing protection when workers are in the vicinity of the generator.

062509_table1.jpgRepresentative permitted noise levels are shown in Table 1 to the right. With maximum permitted noise levels ranging from 52 dB(A) to 72 dB(A) and an untreated generator producing 100 dB(A) or more of noise, it’s obvious that more noise reduction must be added.

Just putting the generator in a box won’t be sufficient. The standard enclosure reduces noise by only about 10 dB, so further steps need to be taken. Experts say there are seven areas to consider.

062509_figure1.jpgUsing Figure 1 to the left as an example, let’s walk through some simple steps that can be taken to lower the ambient noise from a generator.

Generators are often housed in a steel, concrete block or wood-framed buildings. Use rigid materials whenever possible. If the enclosure is built of hollow concrete blocks, fill the holes with sand. Be sure all ingress and egress holes and conduits are fully sealed and caulked.

Build the structure sufficiently large so there is room to add internal acoustic materials. Be sure these materials are rugged and oil-/gas-resistant. Studio-quality acoustic panels or unprotected fiberglass will not survive this environment.

The generator will be supplied on rails. The rails should be insulated from the concrete pad with isolation mounts. These mounts may be rubber or steel springs. Any generator components that need external support, like exhaust systems or fuel and cooling lines must also be isolated with springs or rubber mounts. Also, be sure the pad is not connected to part of any studio or control room floor. The generator pad should be a separate, self-contained area.

Because generators consume lots of air and produce plenty of heat, the exhaust system needs careful consideration. Be sure the building’s ventilation design accommodates your particular environment. If the outside ambient temperature in the summer reaches 90 degrees or more, you’ll need a higher volume of air to cool the generator than if the maximum outside temperature is 50 degrees.

The typical 200kW generator requires 3000 cu-ft/s of air to operate properly. If the housing cannot accommodate a sufficiently large inlet/outlet, consider mounting the radiator on a higher building. While this increases installation and maintenance costs, it moves a high-noise generator component above ground level.

Mufflers are standard equipment on generators, and the manufacturer will specify what noise criteria the generator will meet. Combined with additional shielding from an enclosure, this is may be sufficient to meet local zoning regulations. If not, then additional or higher-quality silences can be added. Up to 35 dB(A) of additional noise attenuation is available from what are termed critical silencers.

Beware the bureaucrat

Finally, just because you’ve installed a compliant generator system, don’t assume your work is done. Those pesky legislators, the EPA and OSHA bureaucrats are always looking for new ways to ... (add your own conclusion).

At this point, it is likely that further regulations will be imposed on auxiliary power plants. While you won't likely have to worry about “cap and trade,” you may have to modify a current generator to meet future requirements. Don’t expect to receive grandfather compliance when it comes to environmental regulations.

For this reason, it pays to work with reliable generator vendors. These peoples' careers depend on their products meeting local and federal regulations. You can expect them to provide any needed retrofits to older units. That might not be the case if you bought that old MARS genset I mentioned in a previous column.

Additional resources:

Uninterruptible Power Supplies

Cummins Power

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