There are different types of motors used in the industr […]
There are different types of motors used in the industries today:
All these motors have more or less same principle of operation.
When a current carrying coil is placed in the magnetic field, it is acted upon by a force whose direction is given by Fleming's left hand rule.
This force (Torque) is given by the equation,
T = B*I*L*w*cos x
B = Flux density of the magnetic field in weber/m^2
I = Current flowing through the coil in amperes
L = Length of the coil in meters
w = width of armature turn
cos x = Cosine of the angel between the magnetic field and direction of rotation
Initially considering the armature is in its starting point or reference position where the angle x = 0.
Since x = 0, the term cos x = 1, or the maximum value, hence torque at this position is maximum given by T = BILw. This high starting torque helps in overcoming the initial inertia of rest of the armature and sets it into rotation.
Once the armature is set in motion, the angle x between the actual position of the armature and its reference initial position goes on increasing in the path of its rotation until it becomes 90° from its initial position. Consequently the term cosα decreases and also the value of torque.
The torque in this case is given by T = BILwcos x which is less than BILw when x is greater than 0°.
In the path of the rotation of the armature a point is reached where the actual position of the rotor is exactly perpendicular to its initial position, i.e. x = 90°, and as a result the term cos x = 0.
The torque acting on the conductor at this position is given by,
i.e. virtually no rotating torque acts on the armature at this instance. But still the armature does not come to a standstill, this is because of the fact that the operation of DC motor has been engineered in such a way that the inertia of motion at this point is just enough to overcome this point of null torque. Once the rotor crosses over this position the angle between the actual position of the armature and the initial plane again decreases and torque starts acting on it again.