Electric vehicle
Voltage rangeStep-up converter kicks inhere of fuel cellOutput of step-up converterFigure 7.16
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| 165 Voltage range Step-up converter kicks inhere of fuel cell Output of step-up converter Figure 7.16 Graph of voltage against current fora fuel cell with a step-up chopper circuit that regulates to a voltage a little less than the maximum stack voltage It should be pointed out that, of course, the current out from a step-up converter is less than the current in. In Figure 7.16, if the fuel cell is operating at point A, the output will beat point A a higher voltage but a lower current. Also, the system is not entirely ‘loss-free’ while the converter is not working. The current would allow through the inductor and the diode, resulting in some loss of energy. These step-up and step-down switcher or chopper circuits are called DC/DC converters. Complete units, ready made and ruggedly packaged, are available as ‘off-the-shelf’ units in a wide range of powers and input and output voltages. However, when they are used as motor controller circuits, as in the case of electric vehicles, the requirements of having to produce a variable voltage, or axed output voltage fora variable input voltage (as in the case when braking a motor using regenerative braking, then such off-the-shelf units will not often be suitable. In such cases special circuits must be designed, and most motors can be supplied with suitable controllers. As we have seen, the circuits required are, in principle, quite simple. The key is to control properly the switching of an electronic switch. This control is usually provided by a microprocessor. 7.2.4 Single-Phase Inverters The circuits of the previous two sections are the basis of controlling the classical DC motor. However, the motors to be considered in the next section require alternating current. The circuit that produces AC from DC sources such as batteries and fuel cells is known as an inverter. We will begin with the single-phase inverter. The arrangement of the key components of a single-phase inverter is shown in Figure 7.17. There are four electronic switches, labelled ABC and D, connected in what is called an ‘H-bridge’. Across each switch is a diode, whose purpose will become 166 Electric Vehicle Technology Explained, Second Edition Fuel cell D A C B LOAD + − → + − → + − → + − → Download 3.49 Mb. Share with your friends:
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