Abstract
This chapter discusses the means for the speed change of induction machines. The speed regulation is required in both generators and motors. Induction machines that serve as generators in wind power stations revolve at variable speed. Therefore, the machine and the associated equipment must ensure conversion of mechanical work in electrical energy at variable speed. The machines used as motors often serve in motion control applications, where the speed changes in continuous manner.
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Notes
- 1.
Slip rings’ access to the rotor winding can be used to take the slip power out of the rotor circuit, as shown in Fig. 17.4. It is also possible to use the static power converter of different topology and to use it to supply the power to the rotor circuit. In this case, slip power and slip speed are negative, and the rotor revolves at the speed Ω m  > Ω e . The two considered topologies are called subsynchronous cascade and supersynchronous cascade. Over the past century, there were also applications of wound rotor induction machines with slip rings and a four-quadrant (reversible) static power converter in the rotor circuit. With four-quadrant rotor converter, wound rotor machine can operate with Ω m  > Ω e as well as with Ω e  > Ω m . Some early wind power solutions were conceived with wound rotor induction generators and static power converter in the rotor circuit. The advantage of this approach is relatively low slip power which results in relatively low voltage and current ratings of semiconductor power switches. More recent wind power generators are based on squirrel cage induction machines and full-power transistor-based static power converters that provide the interface between the constant frequency mains and variable frequency stator voltages.
- 2.
When the machine is supplied from an inverter with power transistors, the frequency of the supply can be varied. Consequently, the rotor speed can be varied too.
- 3.
Electromagnetic torque depends on the fourth power of linear dimensions. Hence, T em ~ V 4/3.
- 4.
Due to flux decrease in the field weakening regime, the magnetizing current I m is lower than the rated magnetizing current. This allows for a slight increase in the rotor current liable for the torque generation.
- 5.
Magnetic field creates forces acting on surfaces delimiting different domains. These forces can be described by introducing equivalent pressure p(N/m2). The force acting on surface S is equal to F = pS. The energy density of the magnetic field in the first domain, next to the boundary, is w 1 = μ 1 H 21 /2. Across the boundary, in the second domain, the energy density is w 2 = μ 2 H 22 /2. Equivalent pressure is equal to w 1−w 2.
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© 2013 Springer Science+Business Media New York
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Vukosavic, S.N. (2013). Variable Speed Induction Machines. In: Electrical Machines. Power Electronics and Power Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-0400-2_17
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DOI: https://doi.org/10.1007/978-1-4614-0400-2_17
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