Basic knowledge of DC motor
In the field of electric power drag, with the advent of the inverter, AC speed control technology is increasingly mature and low cost in the continuous erosion of the position of DC speed technology, but until today, DC speed technology is still sticking to it’s predominant position. A DC motor in simple words is a device that converts direct current into mechanical energy. It’s of vital importance for the industry today. In order to understand the operating principle of DC motor we need to first look into its constructional feature.
1. The construction of DC motor
It’s construction can be divided into two parts: stator and rotor. Note: Do not confuse the commutation pole with the commutator.
The stator includes: main pole, base, commutator pole, brush device and so on.
The rotor includes: armature core, armature windings, commutator, shaft and fan.
The basic construction of a DC motor contains a current carrying armature which is connected to the supply end through commutator segments and brushes it is placed within the north south poles of a permanent or an electro-magnet as shown in the diagram below
2. Operating principle of DC motor
Now to go into the details of the operating principle of DC motor. The working of DC motor is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. The direction of mechanical force is given by Fleming’s Left-hand Rule and its magnitude is given by F = BIl Newton.
Consider a part of a multipolar DC motor as shown in the picture below. When the terminals of the motor are connected to an external source of DC supply:
(1) the field magnets are excited developing alternate N and S poles;
(2) the armature conductors carry currents. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction.
Suppose the conductors under N-pole carry currents into the plane of the paper and those under S-pole carry currents out of the plane of the paper as shown in the picture. Since each armature conductor is carrying current and is placed in the magnetic field, mechanical force acts on it. On applying Fleming’s left hand rule, it is clear that force on each conductor is tending to rotate the armature in anticlockwise direction. All these forces add together to produce a driving torque which sets the armature rotating.
When the conductor moves from one side of a brush to the other, the current in that conductor is reversed and at the same time it comes under the influence of next pole which is of opposite polarity. Consequently, the direction of force on the conductor remains the same.
It should be noted that the function of a commutator in the motor is the same as in a generator. By reversing current in each conductor as it passes from one pole to another, it helps to develop a continuous and unidirectional torque.