Lecture 6

Lecture 6

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  IE243 ELECTRICAL MACHINES–I [Cr.Hrs = 3+1] [Marks: 100+50] By Asif Ahmed Memon
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  Q1. The lineardc machine has a battery voltage of (Last Three digits of your roll No.)V. an internal resistance of 0.(Second and third digit of your Roll No.) ohm. and a magnetic flux density of 0.1 T into the page (a) What is this machine's maximum starting current? What is its steady-state velocity at no load? (b) Suppose that a 30-N force pointing to the right were applied to the bar. What would the steady- state speed be? How much power would the bar be producing or consuming? How much power would the battery be producing or consuming? Explain the difference between these two figures. Is this machine acting as a motor or as a generator (e) Assume that the bar is unloaded and that it suddenly runs into a region where the magnetic field is weakened to 0.06 T. How fast will the bar go now? Q2 A simple rotating loop between curved pole faces connected to a battery and a resistor through a switch. The resistor models the total resistance of the battery and the wire in the machine. The physical dimensions and characteristics of this machine are r = O.(last two digits)m R = 0.(second and third digit)Ω VB =First three digit V I = 1.0m B = O.25T (a) What is the machine's maximum starting current? What is its steady-state angular velocity at no load? (b) Suppose a load is attached to the loop, and the resulting load torque is 10 N· m. What would the new steady-state speed be? How much power is supplied to the shaft of the machine? How much power is being supplied by the battery? Is this machine a motor or a generator? (c) Suppose the machine is again unloaded, and a torque of 7.5 N • m is applied to the shaft in the direction of rotation. What is the new steady-state speed? Is this machine now a motor or a generator?
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  • COMMUTATION ANDARMATURE CONSTRUCTION IN REAL DC MACHINES – The Rotor Coils – Connections to the Commutator Segments • The Lap Winding • The Wave Winding • The Frog-Leg Winding • PROBLEMS WITH COMMUTATION IN REAL MACHINES – Armature Reaction – L di/dt Voltages • Solutions to the Problems with Commutation – BRUSH SHIFTING – COMMUTATING POLES OR INTERPOLES – COMPENSATING WINDINGS • THE INTERNAL GENERATED VOLTAGE AND INDUCED TORQUE EQUATIONS OF REAL DC MACHINES • THE CONSTRUCTION OF DC MACHINES – Pole and Frame Construction – Rotor or Armature Construction – Commutator and Brushes – Winding Insulation • POWER FLOW AND LOSSES IN DC MACHINES
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  COMMUTATION AND ARMATURECONSTRUCTION IN REAL DC MACHINES The Rotor Coils The number of conductors on a machine 's armature is given by
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  COMMUTATION AND ARMATURECONSTRUCTION IN REAL DC MACHINES The Rotor Coils Normally, a coil spans 180 electrical degrees. This means that when one side is under the center of a given magnetic pole, the other side is under the center of a pole of opposite polarity. The physical poles may not be located 180 mechanical degrees apart, but the magnetic field has completely reversed its polarity in traveling from under one pole to the next. The relationship between the electrical angle and mechanical angle in a given machine is given by If a coil spans 180 electrical degrees, the voltages in the conductors on either side of the coil will be exactly the same in magnitude and opposite in direction at all times. Such a coil is called a full-pitch coil. Sometimes a coil is built that spans less than 180 electrical degrees. Such a coil is called a fractional-pitch coil, and a rotor winding wound with fractional pitch coils is called a chorded winding. The amount of chording in a winding is described by a pitch factor p, which is defined by the equation Sometimes a small amount of chording will be used in dc rotor windings to improve commutation
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  COMMUTATION AND ARMATURECONSTRUCTION IN REAL DC MACHINES The Rotor Coils Most rotor windings are two-layer windings, meaning that sides from two different coils are inserted into each slot. One side of each coil will be at the bottom of its slot, and the other side will be at the top of its slot. Such a construction requires the individual coils to be placed in the rotor slots by a very elaborate procedure (see Figure 8-1 2). One side of each of the coils is placed in the bottom of its slot, and then after all the bottom sides are in place, the other side of each coil is placed in the top of its slot. In this fashion, all the windings are woven together, increasing the mechanical strength and uniformity of the final structure.
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  COMMUTATION AND ARMATURECONSTRUCTION IN REAL DC MACHINES Connections to the Commutator Segments Once the windings are installed in the rotor slots, they must be connected to the commutator segments. There are a number of ways in which these connections can be made, and the different winding arrangements which result have different advantages and disadvantages. The distance (in number of segments) between the commutator segments to which the two ends of a coil are connected is called the commutator pitch Yc. If the end of a coil (or a set number of coils, for wave construction) is connected to a commutator segment ahead of the one its beginning is connected to, the winding is called a progressive winding. If the end of a coil is connected to a commutator segment behind the one its beginning is connected to, the winding is called a retrogressive winding. If everything else is identical, the direction of rotation of a progressive-wound rotor will be opposite to the direction of rotation of a retrogressive-wound rotor.
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  COMMUTATION AND ARMATURECONSTRUCTION IN REAL DC MACHINES Connections to the Commutator Segments Rotor (armature) windings are further classified according to the plex of their windings. A simplex rotor winding is a single, complete, closed winding wound on a rotor. A duplex rotor winding is a rotor with two complete and independent sets of rotor windings. If a rotor has a duplex winding, then each of the windings will be associated with every other commutator segment: One winding will be connected to segments I, 3, 5, etc., and the other winding will be connected to segments 2, 4, 6, etc Similarly, a triplex winding will have three complete and independent sets of windings, each winding connected to every third commutator segment on the rotor.Collectively, all armatures with more than one set of windings are said to have multiplex windings Finally, armature windings are classified according to the sequence of their connections to the commutator segments. There are two basic sequences of arnature winding connections lap windings and wave windings. In addition, there is a third type of winding, called a frog-leg winding, which combines lap and wave windings on a single rotor.
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