DC Machine

DC Machine - DC Machines Sattar Hussain...

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Unformatted text preview: DC Machines Sattar Hussain [email protected] 1 1- Rotating electric machines The two major parts of electrical machine are the stator (the stationary part) and the rotor (the rotating part) . The stator supports the main pole cores on which field windings are wounded. The field windings carry the magnetizing or excitation currents. The rotor holds the armature windings (placed in slots) which receives or delivers electric power depending on the operation mode (motor or generator). 2 2- Generator and motor actions Any modern dc machine can run as a motor or as a generator. 3 2.1- Generator action Consider a single-coil two-pole machine. When the coil rotates at a uniform angular velocity ω m , the generated (induced) voltage will be an alternating voltage. acac a simple AC generator 4 For half a revolution (for a two-pole generator) the emf is directed around the coil from a to d; the other half it is directed from d to a. The current through the load resistor, which is connected to the stationary brushes that are in contact with the slip rings A and B will then be alternating current (ac current). The waveform is not quite sinusoidal, since this would be true only under uniform field. Commutator Action If the dc machine to work as a DC generator , the slip rings are replaced by a single slip ring that is cut in half to form two sections. Both sections are insulated from each other and from the machine shaft. Each coil end is connected to a section (segment). These segments are referred to as commutator segments . 5 The result is that the connection to the coil via the brushes change every have cycle so that the load voltage is unidirectional . Thus, as the emf in the coil reverses its polarity, the segments of the split ring change position under the stationary brushes simultaneously. a simple DC generator 6 The generated voltage can be improved and the waveform improved by placing many more multi-turn coils and commutator segments. In the two-coil armature shown, the coil sides are placed in slots on a rotor cylinder. 7 2.2- Motor Action When the dc machine operates as a motor, it is the developed torque that is of prime interest. The armature current is supplied by the dc source connected to the motor terminals. Thus, the power flows into the machine as opposed to the power flow out of the machine in the generator. 8 3- DC machine configurations 9 V t V t V t V t V t 4- DC machine models When a field winding is excited, a magnetic flux φ is generated by the field current I f . Then, the torque acting on the rotor is proportional to the product of the magnetic flux and the current in armature I a or, where K a is the machine constant given by: where, p = number of magnetic poles N =number of conductors per coil M =number of parallel paths in armature winding N.m a a I K T φ = M pN K a π 2 = 10 The mechanical power generated (or absorbed) is equal to the product of the machine torque and the mechanical...
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This note was uploaded on 02/20/2011 for the course EES 612 taught by Professor Karim during the Fall '10 term at Ryerson.

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DC Machine - DC Machines Sattar Hussain...

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