Nigel Calder Stator connectors for a six-phase generator.
Selecting the Right Onboard Generator Technology
I am not a fan of installing stand-alone generators on boats, but there are situations where they make sense and some where they’re the only option. This is most commonly the case on boats with high electrical loads—energy requirements that exceed the reasonable capabilities of alternators, batteries, and solar panels. Given these needs, let’s explore some of the key characteristics of independent generators and what it takes to optimize an installation.
Output Frequency
Alternating current (AC) power, comprising amps and volts, cycles back and forth from positive to negative with respect to the earth. In the U.S. and parts of the world under its influence, power grids require a stable frequency of 60 cycles a second—60 Hertz (Hz). In Europe and European-aligned regions, this frequency is 50 Hz.
Conventionally, the output frequency of a generator is established by the speed with which a magnet is rotated within a coil of wire, or vice versa. Thus, the desired frequency of 60 or 50 Hz is maintained by running the engine that drives the electric generator at a more-or-less constant speed regardless of the load that’s placed on the generator. This is the underlying principle for all conventional generators.
More recently, inverter-style power electronics have been used to convert the variable voltage and frequency output of a variable-speed generator to a stable output voltage and frequency. In this case, the engine speed can be controlled based on load rather than frequency.
Phase Shifts
Voltage and current in an AC circuit each form a sinusoidal waveform. If the two are aligned (they peak at the same time and cross from positive to negative at the same time), the power in the circuit is calculated by multiplying the two together to give watts (W) or kilowatts (kW). However, with some loads, notably inductive loads such as transformers and electric motors, the two waveforms may not be aligned. In these cases, the current waveform typically lags, crossing from positive to negative after the voltage waveform. This is known as a phase shift. It means that not all the power being supplied (total power) is used by the load; some of it is reflected back to the power source (reactive power).
The total power flowing, known as apparent power, is measured as the product of voltage and current (VA, or kVA where k = 1,000). It includes the power used by the load plus any reactive power. The portion of total power that is put to useful work is known as active power or working power (sometimes real power or true power). It is measured in terms of watts (W) and kilowatts (kW). The difference between apparent power and working power is expressed in terms of a power factor, which is the ratio of working power to apparent power.
Power Factor = kW/kVA
Working power is never greater than apparent power so the power factor is a number between 0 and 1, which can be multiplied by 100 to express it as a percentage.
The greater the difference between the voltage and current waveforms (the more the phase shift), the lower the power factor and the less efficient the circuit. A power factor of 1 (100%), which is typical with purely resistive loads such as an electric toaster, means there is no phase shift, and 100% of the apparent power is put to useful work. In short, apparent power (kVA) equals working power (kW). A power factor of 0.8 (80%), which is not uncommon with electric motors, indicates that only 80% of the apparent power is being put to useful work.
Nigel Calder Top—The voltage and current waveforms are aligned for a power factor of 1. Bottom—The current waveform lags behind the voltage waveform, which means the power factor will be well below 1.
The lower a device’s power factor, the less efficient it is and the higher the supply current (apparent power) needed to operate it as compared to the amperage you might derive by dividing the device’s nameplate rating in kW by its nameplate voltage.
For example, let’s consider a 2 kW, 240V electric motor. If the power factor is 1, the required supply amperage is 2,000W/240V = 8.33A. However, with a power factor of 0.75, the phase shift between the voltage and amperage waveforms means the supply amperage must include the reactive amperage reflected back to the generator. In this circumstance, our 2-kW device will need 8.33A/0.75 = 11.1A out of the generator. In which case the generator powering the motor must now deliver 2 kW/0.75 = 2.67 kVA to the circuit. In other words:
Required kVA = load kW/power factor
If we were to buy a 2-kW generator, to satisfy the 2-kW electric-motor load with a power factor of 0.75, the generator will be seriously undersized.
Power factor has three important implications:
- First, when looking at inductive loads, the closer the power factor is to 1, the more efficient the device.
- Second, the lower the power factor of loads, the more powerful a generator needs to be to handle the inefficiencies. For example, the 2-kW electric motor with a power factor of 0.75 requires a 2.67-kVA generator.
- Third, when sizing conductors for circuits where power factor is a consideration, ampacity calculations should be based on apparent power (kVA) and not working power (kW). If not, you risk installing undersized conductors.
In terms of generator operation, we can draw two important conclusions:
1. If the kW rating of the loads connected to a generator exceeds the rated power of the engine driving the generator, it will overload the engine, which may stall;
2. If the apparent power, taking into account the power factor, exceeds the generator’s kVA rating, its windings will be overloaded but not necessarily the engine, which, in this scenario, may not stall.
Let’s say we have a 10-kVA, 240V generator. This gives us a rated current of 10,000VA/240V = 41.67A. Let’s assume 10 kVA fully loads the engine driving the generator. We switch on devices with a combined working power of 11 kW. The engine will be overloaded. We switch off a device and reduce the working power to 9 kW. But because these devices have a low power factor of 0.75, the apparent current will be (9kW/240V)/0.75 = 50A. Even though the 9 kW does not overload the engine, the 50A draw will overload the generator’s windings.
Nigel Calder | Professional BoatBuilder Magazine It’s especially important in sizing a small generator to be sure to account for power factor when calculating the loads it will be supplying.
Synchronous or Asynchronous
Fundamentally, there are two types of generators—synchronous and asynchronous—with most small marine generators being of the former variety. (Note that some of those using inverters on the output are asynchronous.)
Frequency is controlled by regulating engine speed. This is done via a governor on the engine, which works either mechanically or, in newer models, electronically, which is faster and more precise.
In a synchronous generator, the speed of the engine-driven rotor corresponds exactly to the frequency of the alternating voltage produced. Thus, the term synchronous.
As an example, let’s take a 60-Hz synchronous generator in which the output cycles from negative to positive 60 times per second, or 3,600 times every minute. If the generator is driven by a small, high-speed engine running at 3,600 RPM, the generator will have a rotor with two magnetic poles—a positive and a negative one. Rotating the magnets within a set of coils creates that 60 Hz output. But if it could be driven by a larger, slower-turning engine running at 1,800 RPM, the rotor would need to have four magnetic poles consisting of two positive and negative pairs to yield the same 60 Hz output. There are also engines running at 1,200 RPM fitted to rotors with six magnetic poles. But these are uncommon on small generators.
For a 50-Hz output, the engine speeds for synchronous generators are either 3,000, 1,500, or 1,000 RPM, depending on the number of magnetic poles. These slower speeds explain why the rated AC output from the same engine is almost always lower when it is powering a 50-Hz generator instead of a 60 Hz generator. The engine is running slower for 50 Hz, and therefore develops less power.
Some manufacturers put a gear- or belt-drive between the engine and generator, allowing the engine to run at a different speed, for example 2,400 RPM. This can be a better fit with the engine’s power rating or its peak fuel efficiency. It also may help to minimize noise and vibration.
Asynchronous generators also spin a magnet inside a set of coils. However, for reasons we do not need to detail here, the speed of the rotor is close to, but does not correspond exactly with, the frequency of the alternating voltage. For example, a four-pole synchronous generator with a 60-Hz output runs at a fixed speed of 1,800 RPM, whereas an asynchronous version will run at a variable speed slightly exceeding 1,800 RPM.
Magnetic Fields
Synchronous and asynchronous generators differ principally in how they generate magnetic fields in the rotor.
In most synchronous generators, the rotor is an assembly of laminated iron cores inside copper windings, which works the same way as an alternator. A direct current (the excitation current) is fed to the windings creating a magnetic field that is then rotated by the engine. Another set of windings, the stator windings, surround the rotor and are fixed to the generator case.
The rotating magnetic field induces a voltage in the stator windings. The generator’s voltage is regulated by the excitation current, which has commonly been fed to the excitation windings via slip rings and carbon brushes. Nowadays, we increasingly see brushless synchronous generators in which the excitation current is generated inductively by an auxiliary winding, eliminating the need for brushes and slip rings.
Some older synchronous generators are constructed in reverse. They create the magnetic field in the stator windings and deliver the generator’s output from the rotor via slip rings and brushes. In this configuration, the rotor is called an armature, but the components and principles are fundamentally the same. I will focus on a rotor-style generator and not an armature-type.
In an asynchronous generator, the rotor consists of several relatively thick copper or aluminum rods short-circuited at the ends by a copper or aluminum plate. This assembly, called a cage, is embedded in a laminated steel core. Interaction between an auxiliary stator winding and the rotor induces a magnetic field in the rotor.
Startup Energy
For either generator type to work, there must be some means of creating an initial magnetic field in the rotor, without which there would be no stator output. But as soon as there is stator output, it can be tapped to supply the necessary excitation energy to the rotor.
When a synchronous generator is shut down, the rotor retains a degree of residual magnetism. This is sufficient to induce a low-level output in the stator windings when the generator is restarted. The stator output is rectified to DC and used to supply the field current necessary to produce full generator output. Such generators are said to be self-exciting.
In a synchronous generator with brushes, an arrangement of diodes called a bridge rectifier or full-wave bridge rectifier converts part of the stator’s AC output to DC, which is used to power the field windings. This DC current is fed to the rotor via the brushes.
In a brushless generator, the DC current from the bridge rectifier is fed to a separate exciter winding in the stator to create a stationary magnetic field. This induces AC output in a second exciter winding on the rotor. Additional diodes built into the rotor rectify the output of this second exciter winding to DC, which is then used to power the rotor’s field windings. This complex arrangement supplies the DC field current required by the rotor without the need for brushes or slip rings.
With an asynchronous generator, in many applications the initial excitation current comes from the grid. But the times when we need a generator on boats are exactly when we don’t have a connection to the grid. So, for asynchronous generators in marine use, the initial excitation current is usually provided by residual magnetism in the rotor combined with a set of capacitors that feed stored energy into the stator windings when the generator is started.
Output Control
Once a generator is running, excitation voltage is regulated in one of two ways: typically, with an electronic controller called an automatic voltage regulator (AVR), on synchronous generators, or with a capacitor bank connected in parallel to an auxiliary stator winding on asynchronous generators.
An AVR works like an alternator’s voltage regulator. It senses the AC output voltage at the main stator windings and adjusts the field current in a brushed generator, or the stator-exciter-winding field current in a brushless model, to maintain the desired generator AC output voltage.
Note that the regulation of output voltage and frequency with brushless generators is not as precise as with brushed generators because it’s harder to establish precise control of the field current created in the rotor through the indirect (induced) brushless mechanism of the two sets of exciter windings compared to the direct mechanism of a voltage regulator wired into the field circuit via brushes. The key benefit of a brushless generator is reduced maintenance as compared to a brushed generator.
In addition to the exciter windings and the main stator windings, many synchronous brushless generators include an auxiliary winding in the stator, the output from which is rectified to DC. On larger generators of 8 or more kW, this output is commonly used to power and control the exciter windings once the generator is running. In this case, a failure of the auxiliary winding, its rectifier, or the regulation circuit will disable the generator output. On smaller generators, the DC output is used to charge the generator engine’s starting battery. In this case, a failure of the auxiliary winding circuit will have no effect on the main AC output, meaning it can be ignored when troubleshooting a generator’s AC output.
In an asynchronous generator, where the rotor’s residual magnetism is used in conjunction with capacitors to kickstart the generator, voltage is subsequently controlled through a combination of capacitors and changes in engine speed. Asynchronous generators are cheaper and simpler to build than synchronous ones, but there are disadvantages. Most importantly, generator voltage decreases with increasing load. This can be compensated for by increasing engine speed, but because frequency is tied to speed, this only works to a limited extent. Given variable loads—the norm on boats—voltage and frequency will fluctuate more than with an AVR.
Because of control limitations, the overload capability of an asynchronous generator is significantly less than that of a synchronous model. This is particularly noticeable in the case of inductive loads with high starting or inrush currents common on many older AC electric motors. In some cases, the voltage of an asynchronous generator can drop to a level insufficient to start the connected motor. Synchronous generators on the other hand can briefly deliver several times their nominal current, making them much more resilient handling high inrush currents.
Nigel Calder | Professional BoatBuilder Magazine A conventional diesel-driven synchronous AC generator will run at a steady speed (usually 1,800 RPM or 1,500 RPM) to deliver 60-Hz or 50-Hz current, regardless of the load.
High Speed vs. Low Speed
Until a few decades ago, the maximum speed of the diesel engines commonly used to drive generators was around 2,800 RPM. If you wanted to generate a 50-Hz or 60-Hz alternating voltage, you had to use double magnetic pole pairs at 1,800 RPM or 1,500 RPM. Gasoline engines commonly ran at speeds high enough for single pole pairs—3,600 RPM or 3,000 RPM. Today, there are numerous higher-speed diesels that can also be run with single pole pairs.
What’s the better choice? A common argument for slow-running diesels is their robustness and longevity. The rated service life of the slower speed generator engine can be twice that of a generator driven by the same engine at higher speed. However, if you assume an annual usage period of 10 weeks for a recreational boat, and an average of five operating hours for the generator per day, the theoretical service life for the high-speed generator is as much as 20 years. And double that for the low-speed version. But typically, generators fail far shy of 20 years for reasons other than the engine wearing out, in which case the extended theoretical slow-speed service life is irrelevant.
The situation is different with a commercial operation such as a charter or fishing boat, where the generator often runs around the clock. If you assume an annual usage of 200 days, the nominal service life of a high-speed generator engine can be reached in as little as 18 months. The slower running engine, on the other hand, will last for three years before it needs a major overhaul. Even if the generator with the slow running engine is 30% to 50% more expensive than a comparable high-speed generator, the additional investment is worthwhile.
Apart from the engine lifetime, there are additional selection criteria. For example, weight, size, and installation effort. High-speed generators win in all three categories. For a given output they are around 20 to 30 percent lighter and more compact, and they usually require smaller cooling-water and exhaust pipes.
When comparing low- and high-speed generators of similar electrical output, noise levels are often similar. Subjectively, however, some people perceive the sound from slow-speed engines to be more pleasant than the noise of high-speed models. In either case, the sound shield is the more important factor in noise mitigation. Gasoline-powered generators are often quieter than diesel-powered.
In the noise data from generator manufacturers, you should find two items of information: the sound power level (SWL) and the sound pressure level (SPL). The former indicates how much noise the generator emits independent of the installation site. The latter quantifies the noise at a certain distance from the sound source. The unit of measure in both cases is decibels (dB). A doubling or halving of the sound power or sound pressure results in a difference of 3 dB. Doubling the distance from the sound source also results in a sound pressure level change of 3 dB. When comparing different sound pressure levels, the information must be based on the same distance from the sound source.
One other key selection criterion is the fuel required for the boat’s propulsion engine(s). If this is gasoline, it typically makes more sense to install a marine gasoline-powered generator than a diesel-powered generator, and vice versa for a boat with diesel-fueled propulsion.
Variable-Speed Generators
There are two principal limitations with a conventional generator:
1. To achieve the desired output frequency, it must be run at a fixed speed—about 1,800 or 3,600 RPM in the U.S and 1,500 or 3,000 RPM in Europe. These speeds do not coincide with the peak power ratings of most engines and rarely coincide with peak operating efficiency. Consequently, most generator engines are oversized; even at full load, they run at less than 100% of rated output and almost never at peak efficiency.
2. The engine must be run at the required generating speed, regardless of load on the generator, even on standby. Aside from being inefficient and unnecessarily noisy, low-load operation can damage engines, particularly diesels, running up maintenance bills and shortening engine life.
These problems can be solved with variable-speed technology (VST), in which a three-phase AC alternator is driven at a variable speed to produce a variable voltage and frequency. (Note that some VST is built around a six-phase alternator, because the number of phases can increase efficiency.) This output is fed through an inverter-type device to produce AC at the desired output frequency and voltage. In older VST generators, output was often in the form of a modified sine wave; today it’s typically a true sine wave.
VST generators are very compact, considerably smaller and lighter than a traditional generator with a similar rating. If the output is a true sine wave, it is not only cleaner and more stable than that of any traditional generator, but also cleaner and more stable than the AC output produced by most electric utilities.
The controller can be programmed so that most of the time the generator engine runs at near peak efficiency for the specified output power, making VST generators more efficient than most traditional models, especially at light loads.
Another advantage is most VST generators have a permanent magnet rotor with no brushes or maintenance requirements of any kind. From a troubleshooting perspective, apart from the electronics associated with the inverter, a VST generator has the same core components as a conventional generator.
The VST’s principal challenge is the difficulty of designing an engine-control circuit that will respond quickly enough to accommodate high inrush loads. Where those are expected, a conventional synchronous generator might be the better choice. Other drawbacks are the complexity of the electronics, and the price (which is generally about 30% more than that of a comparable traditional generator).
Nigel Calder | Professional BoatBuilder Magazine Permanent magnet DC generators are becoming less exotic as more boats supply onboard AC demands through stacked inverters.
Nigel Calder | Professional BoatBuilder Magazine The author has had some challenges refining the rectifier technology for his permanent magnet DC generator.
DC Generators
DC generators represent a small minority of generators on boats, but their number is growing for two reasons: Electric propulsion installations are becoming more commonplace, and more AC systems are running entirely off stacked inverters.
On such boats, the primary electrical loads are DC-based, and the boats typically come with a large battery bank. A DC generator is a more effective way than a traditional generator and charger to meet the energy needs and charge batteries. Especially if the generator has an automatic generator start (AGS) that’s triggered by a low-battery state of charge or high sustained DC and AC loads.
However, for a given level of output, DC generators are in general significantly more expensive than AC generators. If you have a propulsion engine on board, it is usually more economical to cover DC loads as high as 10 kWh a day (maybe double this on a catamaran with two propulsion engines) from one or more latest-generation high-output alternators mounted on the propulsion engine(s).
From a troubleshooting perspective, other than the rectification device, a DC generator has the same core components as a conventional generator.
In Part 2 we’ll address practicalities of generator installation and more considerations that will determine the type of generator to best fit a boat’s intended use profile.
About the Author: Nigel Calder is the author of Boatowner’s Mechanical and Electrical Manual and other marine titles. He is a member of the American Boat & Yacht Council’s Electrical Project Technical Committee. He recently teamed with OceanPlanet Energy to continue pursuing his passion for improving the efficiency of boat energy systems, and became one of the co-founders of BoatHowTo, a platform for accurate online marine electrical education.
