What is Back EMF ?
As the armature of a DC motor spins within a magnetic field, electromagnetic induction occurs, inducing an electromotive force (emf) in the armature’s conductors, similar to a generator’s operation. This induced emf opposes the applied voltage (following Lenz’s law) and is referred to as back emf or counter emf. Denoted by Eb, it’s determined by the equation:
$$E_{b}=\cfrac{NP\phi{}Z}{{60A}}$$
The magnitude of the back or counter EMF depends on the following as per the above equation:
- The field flux per pole (φ).
- Number of poles (P)
- The Speed of the Motor (N).
- Number of conductors in the armature winding (Z).
Electrical model of a DC motor
In Figure 1, if we apply KVL we get:
$$V-I_aR_a-E_b=0$$
Initially, with zero speed (N) and zero back electromotive force (Eb), the motor experiences a notably high armature current (Ia). This substantial current, resulting from the interplay between the field flux and armature current, generates the torque required for operation:
$$\tau{}\propto{}I_a$$
The armature experiences the torque, initiating the motor’s acceleration. As the motor gains speed, the emergence of back electromotive force (Eb) occurs since this force is directly proportional to the motor’s speed. The counter EMF’s magnitude remains consistently lower than the applied DC voltage due to the armature’s IaRa drop.
Significance of Back EMF in DC Motor
A DC motor is effectively a spinning inductor in a magnetic field. If a voltage is applied across an inductor, the current keeps on rising to infinite. What stops the current rising in the DC motor to infinite? It is the Back EMF of the DC motor.
The presence of back EMF in a DC motor results in its self-regulation, enabling the motor to draw an appropriate level of armature current necessary to generate the required torque for the mechanical load.
Measuring RPM of a DC motor from Back EMF
In some DC motor applications, precise speed control is required. It can be achieved by the use of external sensors like Hall sensors but it has space and cost limitations. A cost-effective solution is using the motor’s inherent back EMF for RPM measurement, which is essential in applications like haptic feedback systems. It introduces a sensorless method utilizing the microcontroller’s onboard Analog Digital Converter for PWM signal generation, eliminating the need for additional measurement chips or components.