The system of Fig. 2.16a can be augmented (see Fig. 2.17) with a position control feedback loop so that the rotor angular displacement can be controlled. Although there are different types of motors (induction, synchronous, reluctance, permanent – magnet, DC, etc.), all these motors can – in principle – be equipped with inner current control and outer speed and position feedback loops. It is up to the designer to select the best motor for a specific drive application with respect to size, costs, and safety issues related to explosive environments. The latter point disqualifies most DC motors because of their mechanical commutation and their associated arcs between brushes and commutator segments. For this reason DC motors will not be covered in this text in great detail, however, DC machines will be used to demonstrate control principles – indeed they serve as a role model for the control of AC machines.
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Some of the most recently investigated systems relate to renewable energy, such as drive trains for electric (hybrid) automotive applications, the generation of electricity from photovoltaic sources, and from wind. Figure 2.18 depicts a closed-loop control system for an electric car drive consisting of either battery or fuel cell as
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Fig. 2.17 Closed-loop current, speed, and position control
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solar cell generator
Fig. 2.19 (a) Photovoltaic power generation with maximum power tracker and inverter. (b) Photovoltaic system supplying power to DC motor and pump energy source, a six-step or pulse-width-modulated (PWM) inverter, and either a permanent-magnet motor (where the combination of an inverter and a permanent – magnet machine/motor/generator is called a brushless DC machine/motor/generator) or an induction motor as well as an internal combustion (IC) engine.
A commonly employed solar power generation system is illustrated in Fig. 2.19. It consists of an array of solar cells, a peak-power tracking component, a DC-to-DC converter which is able to operate either in a step-down or step-up mode, and an inverter feeding AC power into the utility system. Less sophisticated photovoltaic drive applications also exist, where a solar array directly feeds a DC motor driving a pump [4], as shown in Fig. 2.19b; in this case the costs of the entire drive are reduced. Figure 2.20 illustrates the block diagram of a variable-speed, direct-drive permanent magnet 20 kW wind power plant [5].
