Transcript Lecture 7
Power System Fundamentals EE 317 Lecture 7 20 October 2010 Chapter 4 AC Machinery Types and Highlights The Rotating Magnetic Field Relationship between Frequency and Magnetic Field Rotation Induced Voltage in AC Machines Induced Torque in an AC Machine AC Machinery Types AC machines (generators and motors) Two major classes: Synchronous machines Induction machines Two major parts of machines Stator Rotor Stators and Rotors STATOR S ROTOR N Simple Way to Remember Stator is STATIONARY part of machine Rotor is ROTATING part of the machine Where’s the Field? In most AC machines, the magnetic field circuits and field currents are in the rotors… THIS MEANS the induced (or supplied) AC field and currents are in the stator: If POWER comes out torque goes in and this is an AC generator If POWER is supplied torque comes out and this is an AC motor Synchronous Machines Motors and generators whose magnetic field current for the rotor is supplied by a separate DC power source Synchronous generators are used to produce nearly all the electric power produced in the world Induction Machines Motors and generators whose magnetic field current is supplied by magnetic induction (transformer action) into the field windings of the rotor (a DC power source is not required) Although induction machines can be motors or generators they have many disadvantages as generators. Thus, they are referred to typically as induction motors. Most popular type of AC motor The Rotating Magnetic Field STATOR S N ROTOR N CCW Rotation S How we create these fields… The fundamental principle of AC machine operation is that: If a threephase set of currents, each of equal magnitude and differing in phase by 120o, flows in a three-phase winding, it will produce a rotating magnetic field of constant magnitude 3-phase stator winding 3 2 STATOR S B-Field 1 1 N 3 2 http://www.shermanlab.com/xmwang/javappl/ac Motor1.html How we create these fields… In a synchronous generator: DC currents create N-S poles in the rotor, which drive a rotating magnetic field flux through the stationary coils of the stator In an induction motor: AC currents in the stator create a rotating magnetic flux which the internal rotor (with N-S poles) constantly chases Magnitude of B-field B33 B11 B22 Relationship between Frequency and Magnetic Field Rotation IN A 3-PHASE 2 3 S STATOR 1 1 3 N 2 What is speed of field rotation? What is relationship? Of electrical frequency and speed of the magnetic field within the stator? Multiple Pole AC machines A 3-phase, two pole machine has 6 stator windings If you want to have a four pole machine how many stator windings will it have? Winding orientation is as follows: a-c’-b-a’-c-b’ (a-c’-b-a’-c-b’) Four pole – 3-phase machines When 3-phase currents move through the 4 pole stator – 2 N and 2 S poles are created The governing equations then change for angle, frequency and rotation speed: In general, for multi-pole machines If magnetic poles on the AC stator number P, then there are P/2 repetitions of the winding sequence ( a-c’-b-a’-c-b’ ) around its inner surface P And… e m 2 P fe fm 2 P e m 2 Reversing Direction of Rotation If the direction of the current in any two of the three coils in a 3-phase stator is swapped, the direction of the magnetic field rotation will be reversed Take the winding sequence when you start your HW and prove it to yourself… 3-ph voltages Reversal of any phase Induced Voltage in AC Machines The same way 3-phase currents in the stator produce a rotating magnetic field, a rotating magnetic field can produce a 3-phase set of voltages in the stator. Equations RMS Voltage in a 3-phase Stator Wye vs. Delta Connection Rms value depends upon connection type When Y connected it is …? When delta connected it is just ephase example Question 1: Label where each of the following electrical quantities would be found in both the "Y" and "Delta" three-phase configurations: • Phase voltage • Line voltage • Phase current • Line current Examples Question 2: Explain the difference between a balanced polyphase system and an unbalanced polyphase system. What conditions typically cause a polyphase system to become unbalanced AC Machine Power Flows Electrical or copper losses Core losses Mechanical losses Stray load losses