Types of Generators And How To Select One

The topic of generators stirs up heated debates within the industry. In this article we will discuss the basics of generators. We will also go over some of the best practices to use when sizing generators. The task of properly sizing the power generation system for a building falls to the designer and/or manufacturer, not the NEC. To understand generator sizing we will need to look at some basic electrical theory concepts that come into play. 

What type of generator do I need?

Choosing the right generator for your needs involves understanding the various types and specifications available. Generators come in different sizes, fuel types, and duty ratings, each suited for specific applications. Whether you need a generator for home backup, industrial use, or outdoor activities, it’s crucial to ask the right questions to ensure you select the most suitable model. Key considerations include the type of fuel the generator will use, such as natural gas, propane, or diesel. The voltage it needs to supply, whether it will provide single-phase or three-phase power, and its duty rating (standby or prime). Prime-rated generators can handle overloads beyond their capacity for a limited time, making them ideal for continuous use. Answering these questions will help you identify the generator that best meets your power requirements and usage conditions.

Power Factor

Power factor comes into play in most instances when sizing a generator. The power factor calculates the ratio between the real power absorbed by the load (watts) and the apparent power flowing in the circuit (VA). Power factor possesses no units and presents a value between 0 and 1, with 1 representing “unity power factor;” a purely resistive circuit that doesn’t draw extra current. The important thing to remember is that the lower a load’s power factor, the more current it will draw. Let’s look at an example. 

Before we start doing an example calculation we need to understand a few mathematical relationships which exist between these values:

VA (Volt-Amps) represents the product of the voltage and the current in a circuit. This measurement is particularly useful in AC (alternating current) circuits where the voltage and current may not be perfectly in phase.

This variation of the power formula is used to calculate the real power (W) in watts from the apparent power (VA) and the power factor (PF) in an electrical circuit. In this variation we are solving for the true power consumed by the load. We do this by measuring the ratio of apparent power to power factor.

This formula is used to calculate the current (I) in amperes in a three-phase electrical system, and accounts for power factor as well as which equals roughly 1.732; a numeric value representing the phase relationship in three-phase circuits.

EXAMPLE: Calculate the current for a 480V 3 phase 45kW load at 0.8 Power Factor, versus the same load at 0.5 Power Factor:

We can observe that the lower power factor requires more current to deliver the same amount of power at a given voltage. This is an important concept to understand when sizing generators. It is also important to understand the difference between apparent power and real power. We want to make sure we are using the correct units for sizing. As an example, calculate all loads in apparent power first, and then convert them to real power using the correct power factor. We rate generators based on their ability to produce power in watts or kilowatts at a defined power factor. Three-phase generators will often have a power factor of 0.8. While most single-phase generators will have a power factor of 1.   

What type of loads will the generator serve?

Motor loads are hardest on the generator, due to the large inrush current from the startup. Soft-start motor starters and frequency drives help prevent this, to an extent. The use of across-the-line motor starters allows the extraction of up to six times the motor’s running current from the generator.

Other loads to consider are nonlinear loads. Such as variable frequency drives (VFD), large uninterruptible power supplies (UPS) or even battery charging systems.  Nonlinear loads can induce unwanted harmonic currents into the system, which can lead to voltage distortion and instability.

In situations where a generator is serving large quantities of motors or nonlinear loads, the generator should be oversized. This is to prevent voltage and frequency distortion. There are many free generator sizing tools out there that can be used for this. But a good rule of thumb is to provide 50% spare capacity. 

Generator Fuel type

Generators can be either diesel fuel, or natural gas/propane fired. Diesel generators are much more robust, and are known for their durability and ability to start up quickly. This is crucial for emergency power systems. They can provide a high power output in a relatively small engine size compared to natural gas or propane generators. Making them ideal for applications requiring reliable and immediate power. Diesel engines are also typically more fuel-efficient than their gas counterparts, translating to lower operating costs over time. Additionally, diesel engines tend to have a longer lifespan due to their robust construction, making them a preferred choice for heavy-duty and continuous-use applications.

Natural gas and propane generators offer an alternative to diesel. Onsite fuel storage is not required as they can be connected to existing gas lines, and they generally run quieter and burn cleaner than diesel. Oftentimes generators are supplying life safety systems in buildings, which require a maximum of 10 seconds to startup. Natural gas generators can have difficulty achieving this in some of the large-size generators. For this reason, diesel generators are generally recommended for sizes 500kW and larger. 

Natural gas and propane generators are often more suitable for residential and smaller commercial applications where the demands for immediate power are less stringent. Their clean and quiet operation, along with the convenience of not needing on-site fuel storage, makes them an attractive option for residential neighborhood settings.

Load Shedding/Load Stepping

Load “stepping” allows the generator to progressively add loads over a certain amount of time. It’s always best to progressively load the generator up, rather than connect all loads instantly at the same time. Adding more load at once leads to larger dips in voltage and frequency. The lower the inrush current incurred on startup, the more stable the system will be. The inverse operation, called “load shedding“, systematically disconnect loads to reduce the total load on the system. Load shedding is accomplished the same way as load stepping – via transfer switches. The generator controller is programmed to switch off certain loads at given time intervals. 

As an example, say we have three transfer switches in a building. One transfer switch is for life safety loads (such as emergency lighting), one is for optional standby loads (servers in a data center), and one is for a fire pump. Under normal operation if the power goes out, the generator will energize both the life safety and optional standby loads and not the fire pump, as there is no fire condition. However, during a fire condition, the generator will provide power to the life safety loads and the fire pump, but not the optional standby loads. This allows the sizing of the generator to be based only on the largest two loads, rather than all three. 

Conclusion

We’ve laid a solid foundation on generator basics, highlighting the critical role of power factors, real and apparent power, generator parameters, and load shedding. As we’ve seen, understanding the type of loads a generator will serve is key to an optimized system. Next, in “Generator Selection Calculator: Generator Sizing Guide,” we will delve deeper into how to actually calculate the size of generator you need, as well as some code considerations. Stay tuned as we continue our exploration into the complex and essential world of generators.