Generator Designs - Permanent Magnet (PM)
Micropower systems currently in the market use the generator
designs based on the PM technology. The generator itself has two
electromagnetic components: the rotating magnetic field
constructed using permanent magnets; and the stationary armature
constructed using electrical windings located in a slotted iron
core. Figure 2 shows the construction of a typical PM generator
in a cross sectional view.
The PM’s are made using high-energy rare earth materials such as Neodymium Iron Boron or Samarium Cobalt. Retention of the PM”S on the shaft is provided by high strength metallic or composite containment ring. The stationary iron core is made of laminated electrical grade steel. Electrical windings are made from high purity copper conductors insulated from one another and from the iron core. The entire armature assembly is impregnated using high temperature resin or epoxy.
The voltage output from the generator is unregulated, multiple phase ac. This voltage varies as a function of the speed and load. This voltage output is connected to a solid state power conditioning system. Typically, the solid state power conditioning system uses buck/boost techniques and regulates the entire power output.
The technology of induction generator is based on the relatively mature electric motor technology. Induction motors are perhaps the most common types of electric motors used throughout the industry. Early developments in induction generators were made using fixed capacitors for excitation, since suitable active power devices were not available. This resulted in unstable power output since the excitation could not be adjusted as the load or speed deviated from the nominal values. This approach became possible only where a large power system with infinite bus was available, such as in a utility power system. In this case the excitation was provided from the infinite bus. With the availability of high power switching devices, induction generator can be provided with adjustable excitation and operate in isolation in a stable manner with appropriate controls.
Induction generator also has two electromagnetic components: the rotating magnetic field constructed using high conductivity, high strength bars located in a slotted iron core to form a squirrel cage; and the stationary armature similar to the one described in the previous paragraph for PM technology. Figure 3 shows the construction of a typical induction generator in a cross sectional view.
The voltage output from the generator is regulated, multiple phase ac. The control of the voltage is accomplished in a closed loop operation where the excitation current is adjusted to generate constant output voltage regardless of the variations of speed and load current. The excitation current, its magnitude and frequency is determined by the control system. The excitation current is supplied to the stationary armature winding from which it is induced into the short circuited squirrel cage secondary winding in the rotor.
Switched Reluctance (SR)
The technology of SR generator is based on the concepts that magnetically charged opposite poles attracts. Typically, there are unequal number of salient poles on the stator and rotor. Both are constructed of laminated electrical grade steel. Figure 4 shows a cross sectional view of the construction of the SR generator. The number of poles shown on the stator is 6. The number of poles shown the rotor is 4. Other pole combinations such as 8/6, 10/8 are possible. There is no winding on the rotor. Armature coils located on stator poles are concentric and are isolated from one another. When the coils on opposite poles such as 1 and 1 shown in Figure 4, are excited the corresponding stator poles are magnetized. The rotor poles A-A are closest to the stator poles 1 and 1. These are magnetized to opposite polarity by induction and are attracted to
the stator poles. If the prime mover drives the rotor in the opposite direction, voltage is generated in the stator coil to produce power.
The voltage output from the SR generator is DC and has high ripple content. The voltage output can be filtered, and is regulated by adjusting the duration of the excitation current. The commutation of the stator coil is accomplished by the controller.
INDUCTION GENERATOR OPERATION
Figure 5 shows the speed torque characteristics of an induction motor operating from a constant frequency power source. Most readers are familiar with this characteristic of the induction motor operation. The operation of the induction motor occurs in a stable manner in the region of the speed torque curve indicated in Figure 5. The torque output as well as the power delivered by the motor varies as the motor speed changes. At synchronous speed no power is delivered at all. The difference between the synchronous speed and the operating speed is called the slip. The output torque and power vary linearly with the slip.
If the induction motor is driven to a speed higher than the synchronous speed, the speed torque curve reverses as shown in Figure 6. In the stable region of this curve, electric power is generated utilizing the mechanical input power from the prime mover. Once again the generated power is a function of the slip, and varies with the slip itself.
In the generator mode, if the slip is controlled in accordance with the load requirements, the induction generator will deliver the necessary power. It must be remembered that the synchronous speed is a function of the electrical frequency applied to the generator terminals. On the other hand, the operating shaft speed is determined by the prime mover. Therefore to generate power, the electrical frequency must be adjusted as the changes in the load and the prime mover speed occur.
In addition to the requirement stated above, the excitation current must be provided to the generator stator windings for induction into the rotor. The magnitude of the excitation current will determine the voltage at the bus. Thus the excitation current must be regulated at specific levels to obtain a constant bus voltage. The controller for the induction generator has the dual function as follows:
i) Adjust the electrical frequency to produce the slip corresponding to the load requirement.
ii) Adjust the magnitude of the excitation current to provide the desirable bus voltage.
Figure 7 depicts the region of generator mode operation for a typical induction generator. A number of torque speed characteristic curves in the stable region of operation are shown to explain the operation. As an example, consider the situation when the prime mover is at the nominal or 100% speed. The electrical frequency must be adjusted to cater for load changes from 0 to 100% of the load. If a vertical line is drawn along the speed of 100%, it can be observed that the electrical frequency must be changed from 100% at no load to about 95% at full load if the prime mover speed is held at 100%.
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