Chapter 4: introduction to rotating machines

Chapter 4: Introduction to Rotating Machines

This lecture note is based on the textbook # 1. Electric Machinery - A.E. Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans- 6th edition- Mc Graw Hill series in Electrical Engineering. Power and Energy

• The objective of this chapter is to introduce and discuss some of the principles underlying the performance of electric machinery, both ac and dc machines.

§4.1 Elementary Concepts

• Voltages can be induced by time-varying magnetic fields. In rotating machines, voltages are generated in windings or groups of coils by rotating these windings mechanically through a magnetic field, by mechanically rotating a magnetic field past the winding, or by designing the magnetic circuit so that the reluctance varies with rotation of the rotor.

• The flux linking a specific coil is changed cyclically, and a time-varying voltage is generated.
• Electromagnetic energy conversion occurs when changes in the flux linkage result from mechanical motion.
• A set of such coils connected together is typically referred to as an armature winding, a winding or a set of windings carrying ac currents.
  • In ac machines such as synchronous or induction machines, the armature winding is typically on the stator. (the stator winding)
  • In dc machines, the armature winding is found on the rotor. (the rotor winding)
• Synchronous and dc machines typically include a second winding (or set of windings), referred to as the field winding, which carrys dc current and which are used to produce the main operating flux in the machine.
  • In dc machines, the field winding is found on the stator.
  • In synchronous machines, the field winding is found on the rotor.
  • Permanent magnets can be used in the place of field windings.
• In most rotating machines, the stator and rotor are made of electrical steel, and the windings are installed in slots on these structures.The stator and rotor structures are typically built from thin laminations of electrical steel, insulated from each other, to reduce eddy-current losses.

§4.2 Introduction to AC And DC Machines

§4.2.1 AC Machines

• Traditional ac machines fall into one of two categories: synchronous and induction.

• In synchronous machines, rotor-winding currents are supplied directly from the stationary frame through a rotating contact.
• In induction machines, rotor currents are induced in the rotor windings by a combination of the time-variation of the stator currents and the motion of the rotor relative to the stator.

• Synchronous Machines

Fig. 4.1: a simplified salient-pole ac synchronous generator with two poles.

• The armature winding is on the stator, and the field winding is on the rotor.
• The field winding is excited by direct current conducted to it by means of stationary carbon brushes that contact rotating slip rings or collector rings.
• It is advantages to have the single, low-power field winding on the rotor while having the high-power, typically multiple-phase, armature winding on the stator.
• Armature winding (a,a) consists of a single coil of N turns.
• Conductors forming these coil sides are connected in series by end connections.
• The rotor is turned at a constant speed by a source of mechanical power connected to its shaft. Flux paths are shown schematically by dashed lines.