- Electrical Machines Tutorial
- Electrical Machines - Home
- Basic Concepts
- Electromechanical Energy Conversion
- Energy Stored in a Magnetic Field
- Singly-Excited and Doubly Excited Systems
- Rotating Electrical Machines
- Faraday’s Laws of Electromagnetic Induction
- Concept of Induced EMF
- Fleming’s Left Hand and Right Hand Rules
- Transformers
- Electrical Transformer
- Construction of Transformer
- EMF Equation of Transformer
- Turns Ratio and Voltage Transformation Ratio
- Ideal and Practical Transformers
- Transformer on DC
- Losses in a Transformer
- Efficiency of Transformer
- Three-Phase Transformer
- Types of Transformers
- DC Machines
- Construction of DC Machines
- Types of DC Machines
- Working Principle of DC Generator
- EMF Equation of DC Generator
- Types of DC Generators
- Working Principle of DC Motor
- Back EMF in DC Motor
- Types of DC Motors
- Losses in DC Machines
- Applications of DC Machines
- Induction Motors
- Introduction to Induction Motor
- Single-Phase Induction Motor
- Three-Phase Induction Motor
- Construction of Three-Phase Induction Motor
- Three-Phase Induction Motor on Load
- Characteristics of 3-Phase Induction Motor
- Speed Regulation and Speed Control
- Methods of Starting 3-Phase Induction Motors
- Synchronous Machines
- Introduction to 3-Phase Synchronous Machines
- Construction of Synchronous Machine
- Working of 3-Phase Alternator
- Armature Reaction in Synchronous Machines
- Output Power of 3-Phase Alternator
- Losses and Efficiency of 3-Phase Alternator
- Working of 3-Phase Synchronous Motor
- Equivalent Circuit and Power Factor of Synchronous Motor
- Power Developed by Synchronous Motor
- Electrical Machines Resources
- Electrical Machines - Quick Guide
- Electrical Machines - Resources
- Electrical Machines - Discussion
Working Principle of DC Motor
The working principle of a DC motor is based on the law of electromagnetic interaction. According to this law, whenever a current carrying conductor or coil is placed in a magnetic field, the conductor or coil experiences an electromagnetic force.
The magnitude of this force is given by,
$$\mathrm{\mathit{F=BIL}}$$
Where,
$\mathit{B}$ is the magnetic flux density,
$\mathit{I}$ is the current flowing in the conductor or coil, and
$\mathit{l}$ is length of the conductor.
The direction of this force can be determined by Fleming’s left-hand rule (FLHR) which we discussed in Module 1 (Basic Concepts) of this tutorial.
In order to understand the working principle of dc motor, consider a two pole DC motor as shown in Figure-1.
When terminals of this DC motor are connected to an external source of DC supply, the following two phenomenon happen inside the machine −
The field electromagnets are excited developing alternate N and S poles.
The armature conductors carry electric currents. Where, conductors under N-pole carry currents in one direction (say inside of the plane of the paper), while conductors under S-pole carry currents in the opposite direction (say outward of the plane of the paper).
Since, in this case, each conductor is carrying a current and is placed in a magnetic field. Due to the interaction between the current and magnetic field, a mechanical force acts on the conductor.
By applying Fleming’s left hand rule, it is clear that the mechanical force on each conductor is tending to move the conductor in the anticlockwise direction. The mechanical forces on all the conductors add together to produce a driving torque that sets the armature rotating.
When the conductor moves from one pole side to the other, the current in that conductor is reversed due to commutation action, and at the same time, it comes under the influence of the next pole of opposite polarity. As a result, the direction of the force on the conductor remains the same. In this way, the armature of a DC motor rotates continuously in one direction.
Armature Torque of DC Motor
The armature of the dc motor rotates about its axis. Thus, the mechanical force acting on the armature is known as armature torque. It is defined as the turning moment of a force acting on the armature conductors, and is given by,
$$\mathrm{\mathit{\tau _{a}}/conductor\:=\:\mathit{F\times r}}$$
Where, F is the force on each conductor and r is the average radius of the armature.
If Z is the number of conductors in the armature, then the total armature torque is given by,
$$\mathrm{\therefore \mathit{\tau _{a}}\:=\:\mathit{ZF\times r}\:=\:\mathit{ZBIL\times r}}$$
Since,
$$\mathrm{\mathit{B}\:=\:\frac{\mathit{\phi }}{\mathit{a}};\:\mathit{I\:=\:\frac{I_{a}}{A}};\mathit{a\:=\:\frac{\mathrm{2}\pi rl}{P}}}$$
Where, $\phi$ is flux per pole,$\mathit{I_{a}}$ is armature current,l is the effective length of each armature conductor, A is the number of parallel paths, and P is the number of poles. Then,
$$\mathrm{\mathit{\tau _{a}}\:=\:\frac{\mathit{Z\phi I_{a}}P}{\mathrm{2}\pi A}}$$
Since for a given dc motor, Z, P and A are fixed.
$$\mathrm{\therefore \mathit{\tau _{a}}\propto \mathit{\phi I_{a}}}$$
Hence, the torque in a DC motor is directly proportional to flux per pole and armature current.