Explain the working of a dc motor. What is the significance of back e.M.F in dc motors?
Answers
Let’s make up an example of a fictitious DC motor that has permanent magnets (i.e. fixed strength field).
Let’s say a voltage of 12 V is applied to the motor. The armature winding has a resistance of 0.2 Ohms (neglecting other resistances in the path e.g. brush contacts).
If there is no back EMF, the armature willdraw a current of 60 A and produce a heating of 720 Watts in just the (I^2)R loss. Outrageous ! Right ?
However, as the armature’s current interacts with the field winding’s magnetic field, the armature experiences the torque, and it starts moving. The movement induces a voltage in the armature winding that opposes the external voltage. This opposing voltage is what we call the back EMF. The back EMF is proportional to field, and it is also proportional to the angular speed of the armature.
Now, the external voltage drops across the armature due to the sum of the back EMF and the armature I-R drop.
Vs - Vb = Ia * Ra
Where
Vs is supply voltage,
Vb is back EMF,
Ra is resistance of armature winding, and
Ia is the armature current
In practice, the resistance of the armature is small enough that the IR drop (at reasonable currents) is a very small fraction of the external voltage.
Thus, Vs = Vb (approximately)
That is, in practical terms, the back EMF almost balances out the external voltage.
At the start, the armature is not moving, and the back EMF is ZERO. The armature current is very high. The torque is the product of the field and the armature current. The initial torque is high. As the armature moves, the back EMF appears, and grows higher. The armature current drops gradually. The torque drops gradually, and the speed approaches the stead state speed.
Thus, the back EMF plays two roles
It limits the currentIt plays a self-regulating role on speed and helps run the motor at a close to constant speed (a small percentage of variation) from no load to full load.
Due to back EMF, you can take an ideal voltage source, and a near-ZERO resistance armature winding, and the motor would still limit itself to a finite speed, current, and torque. That would not be possible without back EMF.
Hope this helps.
Answer:
The DC motor is the motor which converts the direct current into the mechanical work. It works on the principle of Lorentz Law, which states that “the current carrying conductor placed in a magnetic and electric field experience a force”. And that force is the Lorentz force.
Working Principle of DC Motor
A DC motor is an electrical machine which converts electrical energy into mechanical energy. The basic working principle of the DC motor is that whenever a current carrying conductor places in the magnetic field, it experiences a mechanical force.
Fleming’s left-hand rule and its magnitude decide the direction of this force.
Fleming’s Left Hand Rule:
If we stretch the first finger, second finger and thumb of our left hand to be perpendicular to each other, and first finger represents the direction of the magnetic field, the second finger represents the direction of the current, then the thumb represents the direction of the force experienced by the current carrying conductor.
F = BIL Newtons
Where,
B = magnetic flux density,
I = current and
L = length of the conductor within the magnetic field.
When armature winding is connected to a DC supply, an electric current sets up in the winding. Permanent magnets or field winding (electromagnetism) provides the magnetic field. In this case, current carrying armature conductors experience a force due to the magnetic field, according to the principle stated above.
The Commutator is made segmented to achieve unidirectional torque. Otherwise, the direction of force would have reversed every time when the direction of movement of the conductor is reversed in the magnetic field. This is how a DC motor works!