Engineering of Water Systems

Standby and Backup Systems: Generators

By Ed Butts, PE, CPI

Happy New Year! I hope everyone had a safe holiday season, but now it’s time to get back to work.

In October, we concluded a three-part series on internal combustion engines used for standby and backup water service. This month, as an appropriate start to a new year and as a continuation to this series on standby and backup equipment, we will begin a four-part series on generators.

Introduction and General Properties

Generators are widely used in many water wells and water works for primary, supplementary, standby, or emergency backup service for providing AC or DC electrical power to operate raw water, well, and booster pumps in addition to water treatment equipment and chemical feed facilities.

Legal requirements for generators and installation methods are included in applicable NEC and NFPA codes and standards. NFPA 110, Standard for Emergency and Standby Power Systems, provides details far beyond available NEC information regarding the installation, operation, and maintenance as well as defining emergency and legally required power systems.

Proper sizing of a generator is crucial to the long-term success of installing any generator and requires a good working knowledge of electricity and engine and motor load characteristics as well as the varying requirements of motors and other electrical equipment comprising the total load.

Although most generators under 1000 kilowatts (kW) are directly powered by an internal combustion engine as the prime mover or driver, there are other applications where generators are driven by tractors using a power take-off (PTO) connection, steam, wind, or hydraulic-driven engines or turbines—and in certain cases, even another electric motor as the driver.

Prime movers under 1000 equivalent HP generally use fossil fuels such as diesel, gasoline, propane, natural gas, or biofuels as the fuel source. Larger generators used for utility hydroelectric power generation are almost always driven by inline, high efficiency, hydraulic turbines, or Pelton wheels on dams, flumes for open channel flow, and penstocks for conduit flow, reservoirs, and other water containment structures.

Stationary generator installations must consider the combustion and cooling air required by the generator and prime mover as well as the provisions for the removal of exhaust gases. In most cases, noise generation and abatement must also be considered.

These considerations increase the installation costs, especially for reciprocating-engine units such as diesel or gasoline engines. Further, the fuel supply and storage must be considered. Building code and insurance considerations may force the fuel storage tank to be removed from the location of the generator, usually forcing adding a fuel transfer pump and tank (day-tank) near the generator.

Fundamental AC and DC Generator Theory

There are three basic types of electrical generators: DC generators, AC synchronous generators, and AC asynchronous (induction) generators.

A simple generator consists of:

• A strong north and south magnetic field that is developed in a stator
• Conductors that rotate through that magnetic field as part of a rotor
• A means by which a continuous connection is provided to the conductors as they are rotating using slip rings and brushes from an excitation source.

A basic DC generator has four basic parts: a magnetic field, a single conductor or loop, a commutator, and brushes (Figure 1).

The magnetic field may be supplied by either a permanent magnet or an electromagnet. The commutator converts the AC voltage generated in the rotating loop into a DC voltage and also serves as a means of connecting the brushes to the rotating loop.

The purpose of the brushes is to connect the generated voltage to an external circuit. To do this, each brush must develop contact with one of the ends of the loop. Since the loop or armature rotates, a direct connection is impractical. Therefore, the brushes are connected to the ends of the loop through the commutator to transfer the voltage.

DC generators, while limited in scope, are nonetheless used for most automobile applications and many types and sizes of DC motors.

AC synchronous generators are frequently used because they offer precise control of voltage, frequency, and watts. A synchronous machine consists of a stationary armature winding (stator) with many wires connected in series or parallel to obtain the desired terminal voltage. The armature winding is placed into a slotted laminated steel core.

A synchronous machine also consists of a revolving field, the rotor. A mutual flux is developed across the air gap between the rotor and stator which causes the interaction necessary to produce an electromotive force. As the magnetic flux developed by the field poles crosses the air gap of the stator windings, a sinusoidal voltage is developed at the generator output terminals. This process is called electromagnetic induction.

The magnitude of the AC voltage generated is controlled by the amount of DC exciting current supplied to the field. This DC voltage can be provided from an intertied AC exciter generator and then rectified to DC for exciter voltage to the synchronous AC generator (Figure 2a).

If a fixed voltage excitation were applied, the voltage magnitude would be controlled by the speed of the rotor, but this would necessitate a changing frequency. Since the frequency component of the power system is held constant, solid state voltage regulators or static exciters are commonly used to control the field current and thereby accurately control the generator terminal voltage.

The strong magnetic field is produced by a current flow through the field coil of the rotor. The field coil in the rotor receives excitation using slip rings and brushes (Figure 2b). Two brushes are spring-held in contact with the slip rings to provide the continuous connection required between the field coil and the external excitation circuit. The armature is contained within the windings of the stator and is connected to the output.

Each time the rotor makes one complete revolution, one complete cycle of AC output power is developed. A generator has many turns of wire wound into the slots of the rotor. The magnitude of voltage generated by an AC generator is dependent on the field strength and speed of the rotor.

Most generators are operated at a constant speed. Therefore, the generated voltage depends on field excitation or strength. The frequency of the generated voltage is dependent on the number of field poles and the speed which the generator is operated, as shown in Equation 1a and 1b:

Output frequency (f, in Hz) = (RPM × P) ÷ 120 (1a)

RPM = (f × 120) ÷ P (1b)

where:

f = Frequency in hertz (Hz)

RPM = Synchronous rotational speed in revolutions per minute

P = Total number of poles in generator.

Basic Electrical Generation Principles

By definition, an electrical machine that converts mechanical energy into electrical energy is called an electric generator whereas an electrical machine that converts electrical energy into mechanical energy is called an electric motor.

Regardless of size, all electrical generators, whether DC or AC, depend upon the principle of magnetism. Basically, an electrical value, known as an electromotive force (Figure 3a) creates this magnetism. Denoted by EMF or E, it is the flow of electrons either from a storage battery for DC or mechanically induced in a winding because of a coil cutting through a magnetic field or a magnetic field cutting through a coil.

If there is relative motion between a conductor and a magnetic field, a voltage will be induced in the conductor. Despite its name, electromotive force is not actually a force; it is a commonly measured electrical value of voltage using units of volts equivalent to one joule per coulomb of an electric charge.

A generator (i.e., a continually changing magnetic field) or battery (electrochemical cell) is used for the conversion of energy from one form to another. In these devices, one terminal becomes positively charged while the other becomes negatively charged. Therefore, an electromotive force is work done on a per unit electrical charge.

The part of a generator that produces the magnetic field is called the field, while the part in which the voltage is induced is called the armature. For relative motion to take place between the conductor and the magnetic field, all generators must possess two minimum mechanical parts: a rotor and a stator.

The rotor is the part that rotates while the stator remains stationary. In a DC generator, the armature is always the rotor. In alternators, the armature may be either the rotor or stator.

A DC generator is built using the basic principle that an electrical force of repulsion/attraction is induced in a conductor when it cuts across magnetic lines of force. Conversely, a DC motor works on the basic principle that a current-carrying conductor placed in a magnetic field will experience an output or torque force.

Generators, motors, transformers, and solenoids each use the principle of electromagnetism. All generators work on the principle of a dynamically induced electromotive force. EMF is analogous to voltage as the principal force that drives current through an electrical circuit. An electric flux (Figure 3b) is the measure of the electric field through a given surface, even though an electric field by itself cannot flow.

The flux is a way of describing the electric field strength at any distance from the charge causing the field. The change in flux associated with the conductor can exist only when there is a relative motion between the conductor and the flux. The relative motion can be achieved by rotating a conductor with respect to a flux, or conversely, by rotating a flux with respect to a conductor.

A voltage and current will generate in a conductor so long as there is relative motion between a conductor and a flux. Such an induced EMF, which is due to this physical movement of a coil or conductor with respect to a flux or movement of a flux with respect to a coil or conductor, is called a dynamically induced EMF. Thus, an electrical generating action requires the following elemental components to occur:

• A conductor or coil
• A flux
• Relative motion between the conductor and the flux.

For current to be obtained from an electrical circuit, an EMF (i.e., voltage) must be established and maintained between the two ends of the circuit. This electromotive force may be established in several ways, one by means of an electromagnetic generator.

Michael Faraday, who developed what became known as  Faraday’s Law, discovered that an electric potential can be established between ends of a conductor in the following three ways:

• By a conductor moving or cutting across a stationary magnetic field (a DC generator)
• By a moving magnetic field cutting across a stationary conductor (an AC generator)
• By a change in the number of magnetic lines enclosed by a stationary loop or coil (a transformer).

Faraday’s Law states:

The electromotive force induced between the ends of a loop or coil is proportional to the rate of change of the magnetic flux enclosed by the coil or the EMF induced between the ends of a bar conductor is proportional to the time rate at which the magnetic flux is cut by the conductor.

This law emphasizes a rate of change or rate of flux cutting rather than the density or extent of a magnetic field. The second valid law to generators, Lenz’s Law, further states:

A change in the magnetic flux passing through or linking with a loop or coil causes an EMF to be induced in a direction to oppose any change in circuit conditions, this opposition being produced magnetically when current flows in response to the induced EMF.

Thus, whenever there is a change in current that occurs in a magnetizing coil, which changes the flux in the coil, a voltage is induced which tends to buck and prevent the change. If an attempt is made to diminish the current flowing in a magnetizing coil, a voltage will be developed that will tend to keep the current unchanged. Likewise, if another attempt to establish a current in a magnetizing coil is conducted, a voltage will be developed that tends to keep the current from increasing.

Voltage and Flux Generation

There is a definite relationship between the direction of the magnetic flux, the direction of motion of the conductor, and the direction of the induced EMF or voltage. The voltage and current output are perpendicular to both the motion of the conductor and the magnetic field. The voltage produced at any instant point of time is proportional to the number of turns in the coil times the rate of change of flux.

For example, referring to Figure 4a, the coil shown in position AB encloses the maximum amount of flux. The flux decreases as the coil moves toward position CD, and then becomes zero at position CD since the plane of the coil is now parallel to the magnetic field. Then, the flux increases in the opposite direction, reaching a maximum negative value at position BA, diminishing again and returning to zero at position DC.

The flux reverses and increases again in the original direction to reach a maximum value of flux at AB, and the cycle repeats during each rotation.

Although the flux is maximum at positions AB and BA and zero at positions CD and DC, the induced voltage will be maximum at positions CD and DC and zero at positions AB and BA. This is because the voltage depends upon the rate of change of flux or the rate of cutting the flux lines and not upon the quantity enclosed.

If the coil shown in Figure 4a were rotated at a constant speed in a uniform magnetic field, a sinewave output of AC voltage would be obtained where both the amount of flux enclosed and the induced voltage are plotted against time, as indicated in Figure 4b.

Induction Generators

The most common type, an induction or asynchronous generator (Figure 5a), is nothing more than an induction motor driven above its synchronous speed by an amount not exceeding the full load slip the unit would have seen as a motor. Assuming a full load slip of 3%, a motor with a synchronous speed of 1800 RPM would have a full load speed of 1746 RPM. As an induction generator, this unit could also be driven by an external prime mover at 1854 RPM (1800 RPM × 1.03).

The performance characteristics of a generator will vary slightly from those of a motor. In general, the slip RPM and power factor will be lower, and the efficiency will be higher. The differences may be so slight as to be undetectable by normal field measuring methods.

An ordinary AC induction generator produces sinewave power in both three-phase and single-phase configurations in accordance with the number of poles and rotor speed (Figure 5b). They can usually be used as a generator without any internal modifications. In many isolated systems, squirrel cage induction generators with capacitor excitation, known as self-excited induction generators, are very popular.

Because they can recover energy with relatively simple controls, induction generators are useful in applications such as small hydro power plants’ wind turbines, or portable generators. Induction generators are generally not self-exciting electrical generating machines, so they require a source of magnetizing current and reactive power before they can produce power. Thus, a source of leading volt-amperes-reactive (VARs) power is used for excitation. The needed VARs may be supplied by either self-excitation or directly from the utility grid.

Induction generators are inexpensive and simple types of machines, but they offer little control over their output. They require no separate DC excitation source, regulator controls, frequency control, or governor as the synchronous generator.

An induction generator cannot work in isolation because it continuously requires a source of reactive power from the generator or supply system. However, a self-excited or isolated induction generator will work if a capacitor bank is used for providing a reactive power supply instead of an AC supply system (Figure 6).

The function of the capacitor bank is to provide the lagging reactive power to the induction generator as well as load. Therefore, mathematically, the total reactive power provided by the capacitor bank is equal to the summation of the reactive power consumed by the induction generator as well as the load.

There is generation of a small terminal voltage across the stator terminal due to the residual magnetism when the rotor of the induction machine runs at the required speed. Due to this voltage, the capacitor current is produced, the current then sends the current, which then generates the needed excitation voltage.

Generator Configurations

As illustrated in Figure 7, many combined engine and generator plants or sets, commonly called gensets, consist of an assembled group of several individual components including the prime mover, the generator, the excitation system consisting of a power system stabilizer (PSS) and automatic voltage regulator (AVR), and governor, while the entire power train system includes the generator, powered electric motor, motor controls, and the driven load.

The dynamic stability of a system, particularly those with a variable frequency drive (VFD) or submersible pump motor, can be improved by providing a suitably tuned power system stabilizer on certain generators to provide damping to critical oscillatory modes.

A suitably tuned power system stabilizer is a device that measures improvements in system stability when added to a generator’s AVR. It will introduce a component of electrical torque in phase with the generator’s rotor speed deviations, resulting in damping of low frequency power oscillations that the generator may be creating.

The input to the stabilization signal may be one of the locally available signals from the generator or engine such as changes in rotor speed, rotor frequency, accelerating power, or any other suitable signal. This stabilizing signal is compensated for phase and gain to result in an adequate component of electrical torque that results in damping of rotor oscillations and enhances and balances power generation capabilities.

This series is generally oriented towards the application and design of generators, although generalities are included for installing generators in enclosed spaces. Specific design criteria for engines and generator sets in enclosed spaces will be detailed later in this series.

______________________________________________

This concludes our first column on the theory and application of generators for standby and backup service. We will continue this series in April with an overview on basic generator sizing techniques.

Until then, keep them pumping!

---

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.