- DC Motor Definition: A DC motor is defined as a device that converts DC electrical energy into mechanical energy.
- Principle of Operation: It operates on the principle that a current-carrying conductor in a magnetic field experiences a force, producing motion.
- Fleming’s Left Hand Rule: This rule helps determine the direction of the force in a DC motor using the left hand’s thumb, index, and middle fingers.
- Back Emf: The back Emf in a DC motor opposes the supply voltage and is proportional to the motor’s speed, helping regulate its performance under varying loads.
- Types of DC Motors: The main types are shunt-wound, series-wound, and compound-wound, each classified by how the field winding connects to the armature.
What is a DC Motor?
Principle of DC Motor
A DC motor produces torque when a current-carrying conductor sits in a magnetic field and tends to move.
That force is the Lorentz force on a current in a magnetic field. The DC motor or direct current motor uses that motoring action to turn the shaft. Faraday induction still appears as back emf once the armature rotates.
Fleming’s left hand rule gives the force direction in a DC motor. Hold the thumb, first finger and middle finger of the left hand at right angles. The first finger follows the field, the middle finger follows the current and the thumb follows the force on the conductor.
In construction a direct current motor is the same machine as a DC generator. Electrically the ports are reversed: electrical input and mechanical output for motoring.
Unlike a generator, electrical energy is fed to the input port of a DC motor and mechanical energy is taken from the output port. The block diagram shows supply voltage (E) and current (I) at the electrical port, and torque T and speed ω at the mechanical port.
Parameter K links the input and output port variables of the direct current motor.
The same machine can motor or generate if the electrical and mechanical ports are swapped.
Detailed Description of a DC Motor
The diagram below is a simple electrical model of the DC motor.
The circle is the armature. Brushes on that circle are the external terminals where supply voltage is applied.
A shaft from the armature centre drives the mechanical load. Armature resistance Ra is shown in series with the supply terminals.
Supply voltage E is applied across the brushes. Electric current in the armature, in the stator magnetic field, produces torque Tg. That torque Tg turns the armature.
Those current-carrying armature conductors also cut the stator field as they rotate, so they generate emf Eb in the same way as a generator.
Generated emf Eb opposes the supply voltage. It is called back emf.
The generator form of that emf is
Where, P = no of poles
φ = flux per pole
Z= No. of conductors
A = No. of parallel paths
and N is the speed of the DC Motor.
From that equation Eb is proportional to speed N. Any rotation of a direct current motor therefore produces back emf. With rotor speed ω in rad/s, Eb is also proportional to ω.
If load slows the motor, Eb falls. The voltage difference E − Eb then rises.
The larger E − Eb raises armature current and torque, which limits the speed drop. A shunt DC motor therefore has fairly stiff speed regulation. It does not hold exactly the same speed at every load.
Armature current Ia is
At start, speed ω = 0 so Eb = 0.
The armature winding electrical resistance (Ra) is small, so starting current is high until back emf builds. A starter is used to limit that current. As speed rises, back emf rises and current falls.
Types of DC Motors
Direct motors are grouped by how the field winding connects to the armature.
There are 3 basic types of DC motor:
Thank you. Clear and informative.
No worries at all Jess – happy to hear you found it helpful.