Abstract
Rectifiers convert AC into DC. With a controlled rectifier, energy can also be transferred from the DC-side to the AC-side. However, the AC source (i.e. the grid) always remains necessary as it is responsible for the commutation of the switches (using the emf of the source or, put differently, using the reactive power of the source, to switch off the thyristors). Controlled rectifiers can therefore not be used to convert DC into AC with variable frequency. Nevertheless, the only energy-efficient way to obtain variable speed operation of rotating field machines (i.e. induction and synchronous machines) is by feeding them from a variable frequency source. Inverters are able to convert DC into AC with variable frequency and, in most cases, also variable amplitude. Contrary to controlled rectifiers, inverters require switches that can be turned on and off at will at any instant. Nowadays, the switches used in inverters are mainly Mosfets (for lower power), IGBTs or IGCTs (for very high power).
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- 1.
In most figures the symbol used for these switches is that of a GTO, but this may also represent any other switch.
- 2.
In reality, pure DC current sources do not exist and the current value will depend on the load; however, the shape is rectangular if the inductance is large enough.
- 3.
In most figures, these switches are depicted as GTOs.
- 4.
As explained further on, these are not pure harmonics.
- 5.
This can be considered as a special case of ‘optimal PWM’.
- 6.
This can be calculated as follows: with a third harmonic added to the sine reference, the combined reference is \(v_{ref}=E\cdot M[\cos \omega _{o}t+\gamma \cos 3\omega _{o}t]\). Deriving this expression yields that the maximum of this voltage occurs for \(\cos \omega _{o}t_{o}=\sqrt{\frac{3\gamma -1}{12\gamma }}\). Substituting this in the reference then yields for the maximum value \(v_{ref, max}=E\cdot M[\frac{1}{3}(1-3\gamma )\sqrt{1-\frac{1}{3\gamma }}]\). This maximum value has a minimum for \(\gamma =-\frac{1}{6}\) and the minimum is (\(\sqrt{3}/2)\cdot E\cdot M\). Thus without losing pulses, M may increase to 2/\(\sqrt{3}\).
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Melkebeek, J.A. (2018). Inverter. In: Electrical Machines and Drives. Power Systems. Springer, Cham. https://doi.org/10.1007/978-3-319-72730-1_11
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DOI: https://doi.org/10.1007/978-3-319-72730-1_11
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