The inverse electromagnetic field is caused by the coil rotation inside the stator magnetic field. Its unit is the same as the voltage, the volt. Let us briefly discuss the characteristics of the anti-electromagnetic fields in dc motors.
How does a DC motor generate electromagnetic fields?
A DC motor has a magnetic field due to a permanent magnet or electromagnet. When supplied to the armature, the voltage generated across the armature windings is the opposite of the armature current. Because of the interacting magnetic field, the armature is forced to rotate. The voltage generated at both ends of the armature in a DC motor is called the anti-electromagnetic field or anti-electromagnetic field in a DC motor. The reverse electromagnetic field is always opposite to the supply voltage. To keep the armature of a DC motor rotating, the supply voltage must force the current through the armature winding to reverse the electromagnetic field.
Here is the mathematical expression of the inverse electromagnetic field:
Eb = Φ NZ / 60 * P/A
Among them, the magnetic flux/pole = Φ, armature speed = N, total number of armature conductors = Z, armature winding = A.
The figure below shows the DC motor, all parts labeled to understand how the emf generates, and the internal structure of the DC motor.
Therefore, the energy conversion in a DC motor is only due to the creation of this anti-electromagnetic field. When the DC motor runs without load, the voltage provided is almost equal to the anti-electromagnetic field generated by the DC motor. That is because dc motors require a small torque (Ta=KIa) to overcome friction and winding resistance.
It can be concluded from the above discussion that anti-electromagnetic fields play a dynamic role in the operation of DC motors. The anti-electromagnetic fields have another function, which is to make the motors adjust automatically. Then, how it makes the dc motor automatic regulation, let us through the following study.
The counter electromagnetic field makes the DC motor adjust automatically.
Assuming that the motor operates under no-load conditions, at this stage, the DC motor only needs very little torque to adjust the friction and winding loss. As a result, the DC motor extracts less current. The inverse electromagnetic field and the current in a DC motor are mutually dependent, so the value of the inverse electromagnetic field will decrease with the decrease of the electric current. At this stage, the magnitude of the counter electromagnetic field is approximately equal to the voltage provided.
When a sudden load is applied on the shaft of the DC motor, the DC motor slows down. As the speed of the DC motor decreases, the amplitude of the anti-electromagnetic field will also decrease. A small inverse electromagnetic field will extract a large current from the power supply. The armature current induces a greater torque in the armature, which is needed in a DC motor. As a result, the DC motor runs continuously at a new speed.
The load on the DC motor is suddenly reduced. In this case, the drive torque on the DC motor is greater than the load torque. The driving torque increases the speed of the DC motor, which also increases the anti-electromagnetic field. The armature current is reduced by the high value of the inverse electromagnetic field. Thus, the armature current value produces a drive torque, which is equal to the load torque. As a result, the DC motor will rotate uniformly at the new speed.
Therefore, the generated counter emf operates the DC motor by adjusting the armature current to meet the load requirements. Here are some of the advantages of anti-emf.
The advantage of the anti-electromagnetic field in DC motor
The inverse electromagnetic field enables the DC motor to be automatically regulated, which means that it operates at a constant speed from no-load to maximum load. A DC motor generates an armature current based on the load applied to it. We calculate the armature current by the following formula. The figure below also shows the armature current in the DC motor. I=(V-Eb)Ra
The inverse electromagnetic field is vital for running a DC motor without generating fluctuations and effectively responding to the output load.