ECEg439:-Electrical Machine II 2.1.General Arrangement of DC Machine
Objecties To instill an understanding of the underlying electromagnetic effects permitting electric machine operation and introduce basic DC machine types To describe the construction of these machines To examine the main types of DC machine
DC Machines The direct current (dc) machine can be used as a motor or as a generator. DC Generator conert Mechanical Energy Input at their shafts into to Electrical Energy in the form of Voltage or Current. A DC motor conert Electrical energy into rotary (or linear) mechanical energy at the output shaft. The major adantages of dc machines are the easy speed and torque regulation. 3
Principle of DC machines The working principle of the DC generator is Faraday s Law, which states that emf and electric current if the circuit is closed, is produced when a conductor cuts through magnetic force lines. The opposite of the law applies for the DC motor. Motion is produced when a current carrying wire is put in a magnetic field.
Conersation of Energy Electrical Energy ( BlV E. I. t ). I. t ( B. I. l ).( V ( BIl. t ) ). d Mechanical F. d ( BIl ( BIl ). d ( BIl ). d ). d Energy 1. A Generator conerts Mechanical Energy into Electrical Energy, Where as a Motor conerts Electrical Energy into Mechanical Energy. 2. The Energy conersion from Electrical to mechanical or ice-ersa take place ia the magnetic field proided by the field system. 3. A single rotating dc machine can either be operated as a generator or as a motor.?????? d? ( V. t )
commutation In DC machines the current in each wire of the armature is actually alternating, and hence a deice is required to conert the alternating generated current into the DC by a mechanical deice is called a commutator.
Contd. Fig. 1( a) DC generator: Induced AC emf is conerted to DC oltage using commutator ; Fig. 1(b) DC motor: input direct current is conerted to alternating current in the armature at to produce a unidirectional torque.
DC Generator Operation 8
DC Generator Operation The N-S poles of a dc machine produces constant magnetic field and the rotor coil turns in this field. The conductors in the rotor slots cut the magnetic flux lines, which induce oltage in the rotor coils. The coil has two sides: one is placed in slot a, the other in slot b. a B S 1 2 30 N V dc b I r_dc (a) Rotor current flow from segment 1 to 2 (slot a to b) 9
DC Generator Operation In Figure below, the conductors in slot a are cutting the field lines entering into the rotor from the north pole, The conductors in slot b are cutting the field lines exiting from the rotor to the south pole. The cutting of the field lines generates oltage in the conductors. 1. The induced oltage is connected to the generator terminals through the commutators (1 & 2) and brushes. 2. The induced oltage in b is positie, and in a is negatie. 3. The positie terminal is connected to commutator segment 2 and to the conductors in slot b. 4. The negatie terminal is connected to segment 1 and to the conductors in slot a. S b 1 2 a B 30 N V dc I r_dc (a) Rotor current flow from segment 1 to 2 (slot a to b)
DC Generator Operation When the coil passes the neutral zone: Conductors in slot a are then moing toward the south pole and cut flux lines exiting from the rotor Conductors in slot b cut the flux lines entering the in slot b. This changes the polarity of the induced oltage in the coil. The oltage induced in a is now positie, and in b is negatie. The simultaneously the commutator reerses its terminals, which assures that the output oltage (V dc ) polarity is unchanged. In Figure B the positie terminal is connected to commutator segment 1 and to the conductors in slot a. The negatie terminal is connected to segment 2 and to the conductors in slot b. S a 1 30 N I r_dc (b) Rotor current flow from segment 2 to 1 (slot b to a) 2 b B 11 V dc
DC Motor Operation 12
DC Motor Operation In a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field. The rotor is supplied by dc current through the brushes, commutator and coils. The interaction of the magnetic field and rotor current generates a force that dries the motor 13
DC Motor Operation The magnetic field lines enter into the rotor from the north pole (N) and exit toward the south pole (S). The poles generate a magnetic field that is perpendicular to the current carrying conductors. The interaction between the field and the current produces a Lorentz force, The force is perpendicular to both the magnetic field and conductor S 30 N V dc (a) Rotor current flow from segment 1 to 2 (slot a to b) S B b a 1 2 a B I r_dc 30 N 1 2 b I r_dc V dc (b) Rotor current flow from segment 2 to 1 (slot b to a) 14
DC Motor Operation The generated force turns the rotor until the coil reaches the neutral point between the poles. At this point, the magnetic field becomes practically zero together with the force. Howeer, inertia dries the motor beyond the neutral zone where the direction of the magnetic field reerses. To aoid the reersal of the force direction, the commutator changes the current direction, which maintains the counterclockwise rotation. S 30 N V dc (a) Rotor current flow from segment 1 to 2 (slot a to b) B S a b 1 2 a B 30 N 1 2 b I r_dc I r_dc V dc (b) Rotor current flow from segment 2 to 1 (slot b to a) 15
DC Motor Operation Before reaching the neutral zone, the current enters in S N 30 V dc segment 1 and exits from segment 2, b Therefore, current enters the coil end at slot a and exits from I r_dc slot b during this stage. After passing the neutral zone, B the current enters segment 2 a and exits from segment 1, This reerses the current S N 30 V dc direction through the rotor coil, when the coil passes the b neutral zone. I The result of this current r_dc reersal is the maintenance of the rotation. (b) Rotor current flow from segment 2 to 1 (slot b to a) 1 2 (a) Rotor current flow from segment 1 to 2 (slot a to b) 1 2 a Neutral Zone B 16
Basic DC Motor Operation Consider the illustration below With the current flowing the wire as shown, and the magnetic field in the direction indicated, it is clear there is a force on the conductors acting as shown below If the wire is free to rotate around the ends (terminals) then the wire would rotate - beginnings of motor action.
Contd. Consider the situation shown on the last slide, The currents in the wire are, taking a cross- section, in opposite directions The magnetic field across this crosssection clearly illustrates the areas where the magnetic flux is increased and decreased due to the magnetic flux from the wires This illustration demonstrates the elastic band nature of lines of magnetic flux, which will always act in a way to try to shorten themseles
Contd.
Contd. Consider the situation if the wire rotates through 90 0
Contd. The forces on the conductor remain acting in the same directions, With the piot point at the wire ends (terminals) there is now zero torque acting on the wire forcing it to rotate (Neutral zone) Consider the situation after a further 90 0 rotation of the wire (assume the wire has sufficient momentum to rotate) The torque acting on the conductors now rotates the wire in the opposite sense. How can this be aoided? The answer would be to reerse either the magnetic field or the direction of current
Contd.
Contd. It is easier to reerse the current flow in the wire this is managed by employing a commutator A commutator ensures that the current is reersed in the armature (turning conductor) eery half rotation thereby allowing the forces on the wire to aid rotation Take the preious example and add a commutator to the supply end of the wires.
Contd.
Contd. By including a basic commutator it is possible to obtain the forces on the conductors in the same sense for a 360 0 rotation. Howeer there are still periods of zero torque for the simple two piece commutator considered This would lead to a ery uneen drie and could, dependent on the load, seriously effect either the motor, the load or both To compensate for this effect utilize a commutator with more segments
Contd.
Improed Commutator Haing a commutator with more segments means that there are no zero torque parts of the rotation cycle. This significantly improes the drie of the motor I r_dc /2 Shaft Brush Rotation I r_dc I r_dc /2 Pole winding 8 1 2 N 7 3 S 6 5 4 Insulation Rotor Winding I r_dc Copper segment
Example of many segment Commutator & Motor A common application of a DC motor is a battery powered hand drill. The commutator has many segments and deliers relatiely smooth output torque
Commutator Action As the commutator passes a brush, the direction of current flow reerses, ensuring constant drie torque in the direction of rotation