You can find DC motors in many portable home appliances, automobiles and types of industrial equipments.
In this text, we will logically understand the operation and construction of commercials DC motors.
Let`s first start with the simplest DC motor possible, it looks like this:
The stator provides a constant magnetic field and the armature, which is the rotating part, is a simple coil, the armature is connected to a DC power source through a pair of comutator rings. When the current flows through the coil, an electromagnetic force is induced on it according to the Lorentz law, so the coil will start to rotate.
You`ll notice that as the coil rotates, the commutator rings connected with the power source of opposite polarity. As it result on the left side of the coil the electricity will always flow [away" and on the right side electricity will always flow [towards".
This ensures that the torque action is also in the same direction throughout the motion, so the coil will continue rotating. But, if you observe the torque action on the coil closely, you`ll notice that, when the coil is nearly perpendicular to the magnetic flux, the torque action nears zero. As a result there will be irregular motion of the rotor, if you run such a DC motor, here is the trick to overcome this problem, add one more loop to the rotor, with a separate comutator pair for it.
In this arrangement when the first loop is in the vertical position, the second loop will be connected to the power source, so the motor force is always present in the system. Moreover, the more such loops, the smoother will be the motor rotation, in a practical motor the armature loops are fitted inside slots with highly permeable steel layers. This will enhance magnetic flux interaction, spring loaded comutator brushes help to maintain contact with the power source, a permanent magnetic stator pole is used only for very small DC motors. Most often, an electromagnetic is used the field coil of the electromagnetic is powered from the same DC source.
The field coils can be connected to the rotor windings in two different ways:
Parallel or series
The result is two different kinds of DC motor constructions, a shunt and a series motors.
The series wound motor has good starting torque, but speed drops drastically with the load, the shunt motor has a low starting torque but it is able to run almost at a constant speed, irrespective of the load acting on the motor.
Unlike the other electrical machines, DC motors exhibits a unique characteristic, the production of BACK EMF. A rotating loop in a magnetic field will produce an EMF according to the principle of the electric magnetic induction. The case of the rotating armature loops is also the same and internal EMF will be induced that opposes to the applied input voltage.
The BACK EMF reduces armature current by a large amount, BACK EMF is proportional to the speed of the rotor, at the starting of the motor BACK EMF is too low, thus the armature current becomes too high, leading to the burnout of the rotor, thus a proper starting mechanism that controls the applied input voltage is necessary in large DC motors.
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