A generator set consists of a series of components, including key components such as motors and governors, and non-essential components such as control equipment. The various components that make up the generator set will be described below.
The prime mover
Common generator sets in Australia generally use gasoline, diesel or liquefied petroleum gas, and there are also some steam engines, and biodiesel engines are also expected to be popular in the future. The prime mover rotates the generator to generate electricity, therefore, the prime mover must meet the alternator’s rated power. Similar to the prime mover that is common in motor vehicles, the prime mover in a generator set needs to run at a constant speed to ensure that the frequency of the output voltage is stable, which is the same function as a governor.
The prime mover consists of the following components:
(1) Engine mount. Engine mounts contain cylinders.
(2) Piston. The piston is located in the cylinder. The burning fuel expands into a gas that pushes the piston to move. The piston rings expand against the cylinder wall to provide a combustion and oil seal space. Oil helps reduce friction and wear losses on cylinders.
(3) Cylinder head. The cylinder head seals the top of the cylinder to maintain the integrity of the combustion process. The cylinder head also features air valves that control air inflow, exhaust emissions and fuel injection.
(4) Camshaft. The camshaft rotates at half crankshaft speed and controls the intake and exhaust valves via pushrods or direct contact.
(5) Crankcase. The crankcase contains the main bearings that support the crankshaft, as well as front and rear main seals that prevent oil from leaking under pressure.
(6) Connecting rod. Connecting rods or connecting rods that connect the piston to the crankshaft through trunnions on the piston and big end bearings on the crankshaft.
(7) Camshaft. The camshaft rotates about its centerline due to the action of the piston on its offset journal bearing.
(8) Oil pan. The oil pan is located at the bottom of the prime mover and stores lubricating oil to lubricate the prime mover.
(9) Oil supply system. The fuel supplied to the cylinder can be either by injection or by the carburetor.
(10) Turbocharger (optional). A turbocharger is an air pump that is driven by exhaust gas to increase the amount of air in the cylinder. At the same time, the fuel will increase accordingly, and its power will also increase, thus exceeding the output of the same engine in its natural state. With the increased reliability of turbochargers, they are becoming more and more popular now. A naturally aspirated prime mover draws air into the cylinder at ordinary atmospheric pressure. In an internal combustion engine using a turbocharger, the air pressure drawn into the cylinders is greater than atmospheric pressure. In the case of outputting a given power, the cost of an internal combustion engine using a turbocharger is lower than that of a naturally aspirated prime mover.
Additionally, larger prime movers can also include an aftercooler, which will also increase the prime mover’s rated power output. This recooling of the air through the turbocharger makes the air denser, so more air can be pushed into the cylinders.
(11) Governor. The governor stabilizes the speed of the prime mover around a preset value by adjusting the fuel supply according to the power required to drive the load. The accuracy of the governor in the output power application can generally be divided into three gears: A0, A1 and A2. All governors include speed sensors, controllers and actuators. The A0 class is the strictest and must maintain a constant speed (also known as synchronous speed regulation). Usually this level of speed regulation is done by an electronic governor. The A1-level governor controls the output frequency of the prime mover at 52~53Hz under no-load conditions. From no-load to full load, the frequency drops by 4%~5%. Therefore, the output frequency of the prime mover is controlled at 50Hz at full load. High-level mechanical speed regulation generally takes this form.
(12) FLYWHEEL. The flywheel, located behind the camshaft, provides an inertial mass to overcome the recurring irregularities of the combustion process, as well as a means of controlling power output. This allows the generator set to supply the motor load with the surge power it needs to start. Usually gasoline engines and small high-speed diesel engines do not have flywheels, while the installation on low-speed diesel engines is more typical.
One difference between a genset prime mover and a car engine is that the car engine is water cooled, while the genset prime mover can be either water cooled or air cooled. While many prime movers contain electric starting mechanisms (starter motors for diesel engines, ignition systems for gasoline units), some small gasoline generator sets are designed to be started manually. A genset that includes a starter motor also includes a DC-charged alternator. The following components are also common in prime movers, including fuel and oil filters, fuel pumps, and air purifiers. And if the prime mover is water-cooled, there will also be a fan and a water pump.
Additionally, some generator sets have control systems that monitor the operation of the prime mover. Monitoring includes oil level, temperature and prime mover speed. If the generator set includes this monitoring function, various sensors will be installed in the prime mover.
Alternator
An alternator is also commonly referred to as a generator, but to be precise it would be an alternator, including a stator, rotor and excitation system.
(1) Stator. The stator refers to the external stationary part of the alternator and contains the main winding that generates the induced voltage, the windings are divided into 2 poles and 4 poles. The voltage of the stator is applied to the output terminal, and at the same time, it is connected to the automatic voltage regulation device (AVR) to ensure the stability of the output voltage. The stator also includes stator field coils.
(2) Rotor. The rotor is the rotating part of the alternator and can be divided into single-phase and two-phase. The single-phase rotor and prime mover flywheel are uniformly designed and eliminate the need for additional bearings at the end of the drive shaft. The two-phase alternator must be connected to the prime mover via an elastic coupling.
The rotor is wound with a rotor main coil, or “field coil,” which acts as an electromagnet to provide a magnetic field. When the magnetic field rotates through the stator coil, an induced voltage is generated in the stator coil, which is the output voltage of the alternator. The magnetic field thus produces an excitation function that enables the alternator to operate.
The speed of the rotor depends on the number of poles of the stator. The rotor speed of the 4-pole generator is 1500r/min, which produces an AC output voltage of 50Hz; the 2-pole generator needs to generate a voltage of 50Hz, and the rotor speed must reach 3000r/min. The rotor field is powered by a DC power source (exciter). An exciter is an external device that is mechanically coupled to the alternator, or is itself part of the alternator. For example, the rotor part of the brushless alternator includes the rotor coil excitation part.
Excitation system. The field current comes from the exciter, which itself is a small alternator. Modern alternators in generator sets typically use brushless excitation systems, ie exciter systems that no longer use the carbon brushes and slip rings commonly used in older or larger generator sets. The alternator has both main coils and field coils (smaller in size).
The assembly method of the field winding is opposite to that of the main winding, that is, the field winding is arranged on the stator, and its output winding is on the rotor. current. The excitation magnetic field is supplied by an automatic voltage regulator, and an AC signal is generated in the rotor excitation winding, which is rectified and applied to the rotor main winding.
Self-excitation refers to the ability of the alternator excitation system to use the residual magnetism of the main magnetic field to establish the excitation voltage. During the start-up phase of the motor, these residual magnetisms can build up a small voltage, supplying current to the AVR and exciter, resulting in a strong magnetic field that further enhances the output voltage until the AVR adjusts the output to a normal level.
Automatic voltage regulation device
An automatic voltage regulator that measures the alternator output at the stator windings, compares it to a reference voltage, and adjusts its output voltage applied to the exciter to maintain the main stator output voltage at a reasonable level. Temperature, load type and motor speed all affect voltage regulation.
(1) Base. The genset base is used to support the prime mover, alternator and all parts of the engine. Usually consists of 2 parallel rails, also called slide rails. The prime mover and alternator are usually connected to the base by means of elastic vibration devices.
(2) Fuel tank. Gasoline generators have a fuel tank above the prime mover. Gasoline can be supplied from a similar tank or a fuel tank mounted along the base. Users often install auxiliary wall-mounted or stand-up tanks to increase storage capacity. Natural gas generator sets do not have a fuel tank, but require a prime mover to be connected to the cylinder (or cylinder).
(3) Batteries. If the prime mover is started by the starter motor, the genset should also include a starter battery. The starting battery or battery pack (static voltage 24V) is usually installed on the base of the generator set. The battery can be charged either directly by the AC motor or via a battery charger powered by the generator set output. In an independent power supply system, the alternator should not be the only power source for the battery, unless the unit is powered at least once a week for 4~6h. The standard alternator should be designed to run for 4~6h after starting. After a short-time running test, the output power will decrease with the adjustment of the control system, and the charging power provided by the alternator is not enough to recharge the battery. At this time, an auxiliary battery charger should be set to connect to the output terminal of the generator set. Charge the battery. If you are also interested in the knowledge of generator batteries, you can visit Tycorun Battery to read more related articles.
(4) Muffler. Mufflers are usually applied to small generator sets (less than 20kVA) of independent systems, and some form of flexible connection is usually used to connect the exhaust pipe to the muffler to ensure that the exhaust gas is discharged.
Alternator parameters
(1) Output power. The rated power of the generator set is the apparent power, which is usually based on the output under the condition that the power factor is 0.8. The power factor of 0.8 is the power factor of general loads in most occasions, but in fact there are many loads whose power factor is not 0.8. Without considering the power factor, such as the inverter, the capacity of the motor (kVA) is the maximum safe output power that can be provided. For three-phase equipment, each phase can provide 1/3 of the maximum output power.
If the load power factor is very low (eg less than 0.5), the alternator may not be able to provide the full rated capacity. The upper limit of the AVR may be reached because the excitation current increases at low power factor.
Under relatively high power factor (such as unity power factor, PF=1), the output power of the alternator does not exceed the rated capacity. However, the actual mechanical power driving the generator will be higher, requiring more power from the prime mover.
Depending on the design of the generator set, this requirement may exceed the capacity of the prime mover, in which case a higher rated capacity generator should be replaced.
The rated capacity of a generator of a certain physical size varies with the type of insulation, and is classified according to the allowable temperature rise when the ambient temperature is above 40°C.
(2) Voltage. In Australia, the output voltage of the alternator is usually 415V (3 phase) and 240V (single phase). Most of the backup generator sets in the independent power supply system are single-phase, and three-phase generators are only required in large power stations that drive certain equipment.
In a three-phase power supply system, 415V is the line voltage and 240v is the phase voltage. In a reconfigurable alternator system, it is possible to switch from one mode to another, but the rated capacity changes accordingly. For example, a 10kVA three-phase alternator system has a single-phase rated capacity of 6.7kVA.
(3) Full load current. Full load current refers to the maximum current that can be output at rated output. That is, the rated capacity of the generator is related to the output current characteristics, and this is the maximum safe output current that can be provided without considering the power factor.
The generator is allowed to be overloaded for a short time. For example, the generator is generally allowed to run for 1 hour with 10% overload within 12 hours, which depends on the characteristics of the generator factory specifications.
(4) Instantaneous current. The instantaneous current refers to the instantaneous current value that the AC motor can withstand at the moment of driving the load, which represents the high starting current characteristics of the motor, such as the starting current value when the motor is directly started. Typical ratings are 300% of full load current in 10s.
(5) Speed. The motor speed is generally 1500r/s (4 poles) or 3000r/s (2 poles). From a design standpoint, the importance of speed is that the motor speed needs to be determined at the time of design. Generally speaking, a motor with a slower speed has a longer life, less noise, and also saves fuel, and is of course heavier and more expensive. The selection of the motor depends on the user’s budget and operating environment. For example, belt drive is an option to consider. When the prime mover is running at a medium speed of 1500r/s, different types of belt drive will generate 5%~20% of the power. loss.
(6) Derating. The rated parameters of the motor are given in the case of a power factor of 0.8, in kVA, as well as motor overload capacity parameters and instantaneous current parameters.
The rated value of motor output is related to temperature, altitude and humidity: when the temperature exceeds 40℃, the rated value decreases by 3% for every 5℃ increase; when the altitude exceeds 1000m, the rated value decreases by 3% for every 500m increase; in the case of high humidity , usually with an alternator that can adapt to tropical conditions.