DC Motor Principle of operation
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DC Motor Principle of operation
DC Motor Principle of operation
A current-carrying conductor experiences a mechanical force when it is placed in a magnetic field, which is the DC Motor principle of operation. Field winding creates the necessary magnetic field in a practical d.c. motor while armature conductors act as current-carrying conductors and consequently experience force. A torque is a force that twists or turns the armature as conductors are inserted into slots that are on the periphery of the device. The torque is the result of multiplying the force by the radius at which it acts. As a result, the entire armature is subject to a torque and begins to rotate. Let’s examine this driving behaviour in more detail.
Consider a single conductor placed in a magnetic field as shown in the Fig. 8.1(a). The magnetic field is produced by a permanent magnet but in a practical d.c. motor it is produced by the field winding when it carries a current.
The conductor is now excited by a different supply to carry a current in a specific direction. As seen in Fig. 8.1, it carries a current away from the observer (b). Any conductor that carries current creates a magnetic field around it, which causes the conductor to also create a flux around it. By using the right-hand thumb rule, one can determine the flux’s direction. Flux moves clockwise around a conductor when current direction is taken into account. The permanent magnet’s primary flux is not depicted in Fig. 8.1 for clarity’s sake (b).
Now there are two fluxes present,
1. The flux produced by the permanent magnet called main flux.
2. The flux produced by the current carrying conductor.
These are shown in the fig.8.2(a). From this, it is clear that on one side of the conductor, both the fluxes are in the same direction. In this case, on the left of the conductor there is gathering of the flux lines as two fluxes help each other. As against this, on the right of the conductor, the two fluxes are in opposite direction and hence try to cancel each other. Due to this, the density of the flux lines in this area gets weakened. So on the left, there exists high flux density area while on the right of the conductor there exists low flux density area as shown in the Fig. 8.2(b).
This flux distribution around the conductor acts like a stretched rubber band under tension. This exerts a mechanical force on the conductor which acts from high flux density area towards low flux density area, i.e, from left to right or the case considered as shown in the fig.8.2(b)
The main flux is produced by a field winding in a practical dc motor in place of the permanent magnet, and all the armature conductors mounted around the armature drum are subjected to mechanical force. Because of this, the entire armature experiences a torque—a twisting force—and the motor’s armature begins to rotate.
Direction of Rotation of Motor
The magnetic force experienced by the conductor in a motor is given by,
F = BlI newtons (N)
B = Flux density due to the flux produced by the field winding
l = Active length of the conductor
I = Magnitude of the current passing through the conductor
The direction of such force i.e., the direction of rotation of a motor can be determined by Fleming’s left hand rule. So Fleming’s right hand rule is to determine direction of induced e.m.f. i.e., for generating action while Fleming’s left hand rule is to determine direction of force experienced i.e. for motoring action.
DC Motor Principle
A dc motor is a device that changes d.c. power into mechanical power. Its operation is based on the idea that a magnetic field causes a current-carrying conductor to experience a mechanical force.
The direction of this force is given by Fleming’s left hand rule and magnitude is given by;
F = BlI newtons (N)
B = Flux density due to the flux produced by the field winding
l = Active length of the conductor
I = Magnitude of the current passing through the conductor
Basically, there is no constructional difference between a d.c. motor and a d.c. generator. The same d.c. machine can be run as a generator or motor.