Friday, 17 October 2014

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A  DC motor  is a mechanically commutated  electric motor  powered from  direct current  (DC). The stator is stationary in space by definition and therefore the current in the rotor is switched by the commutator  to also be stationary in space. This is how the relative angle between the stator and rotor magnetic flux is maintained near 90 degrees, which generates the maximum torque.
DC motors have a rotating armature winding (winding in which a voltage is induced) but non-rotating armature magnetic field and a static field winding (winding that produce the main magnetic flux) or permanent magnet. Different connections of the field and armature winding provide different inherent speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the voltage applied to the armature or by changing the field current.

Principle of DC Motor

This DC or  Direct Current Motor  works on the principal, when a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. This is known as motoring action. If the direction of   electric current   in the wire is reversed, the direction of rotation also reverses. When magnetic field and electric field interact they produce a mechanical force, and based on that the working principle of  dc motor  established.   The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of   electric current then the thumb represents the direction in which force is experienced by the shaft of the  dc motor . Structurally and construction wise a  Direct Current Motor  is exactly similar to a D.C. Generator, but electrically it is just the opposite. Here we unlike a generator we supply electrical energy to the input port and derive mechanical energy from the output port
  • Shunt motor

The field coil and the armature windings are connected in shunt or parallel across the power source. The armature winding consists of relatively few turns of heavy gauge wire. The voltage across two windings is the same but the armature draws considerably more current than the field coil. Torque is caused by the interaction of the current caring armature winding with the magnetic field produced by the field coil. If the DC line voltage is constant, the armature voltage and the field strength will be constant. The speed regulation is quite good; the speed is a function of armature current and is not precisely constant. As the armature rotates within the magnetic field, an EMF is induced in its wining. This EMF is in the direction opposite to the source EMF and is called the counter EMF (CEMF), which varies with rotational speed. Finally, the current flow through the armature winding is a result of the difference between source EMF and CEMF. When the load increases, the motor tends to slow down and less CEMF is induced, which in turn increases the armature current providing more torque for the increased load

Motor speed is increased by inserting resistance into the field coil circuit, which weakens the magnetic field. Therefore, the speed can be increased from “basic” or full-load, full-field value to some maximum speed set by the electrical and mechanical limitations of the motor.

Series motor

The field coil and armature windings are connected in series to the power source. The field coil is wound with a few turns of heavy gauge wire. In this motor, the magnetic field is produced by the current flowing through the armature winding; with the result that the magnetic field is weak when the motor load is light (the armature winding draws a minimum current). The magnetic field is strong when the load is heavy (the armature winding draws a maximum current). The armature voltage is nearly equal to the PS line voltage (just as in the shunt wound motor if we neglect the small drop in the series field)

Consequently , the speed of the series wound motor is entirely determined by the load current. The speed is low at heavy loads, and very high at no load. In fact, many series motors will , if operated at no load , run so fast that they destroy themselves . The high forces , associated with high speeds , cause the rotor to fly apart , often with disastrous results to people and property nearby . The torque of any DC motor depends upon the product of the armature current and the magnetic field. For the series wound motor this relationship implies that the torque will be very large for high armature currents, such as occur during start-up. The series wound motor is, therefore, well adapted to start large heavy-inertia loads, and is particularly useful as a drive motor in electric buses, trains and heavy duty traction applications. Compared to the shunt motor, the series DC motor has high starting torque and poor speed regulation.

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