Back EMF stands for Back Electromotive Force. In simple terms, the Back EMF is an electromotive force which occurs as the motor turns and acts like a generator. Critically the Back EMF can be measured and is directly proportional to the speed of the motor. An intelligent motor controller such as the ZDBL15 can then read this force and use it to measure the actual speed of the motor. This speed can then be either maintained using Back EMF as a reference to measure and adjust the speed.
A significant disadvantage of using this method is that no back EMF is generated when the motor is stationary, meaning that start up is affected by operating in open loop. As a result of this, the motor can take a small amount of time to settle and run efficiently. A second disadvantage is that at low speeds the back EMF is small and therefore can be quite difficult to measure accurately, which can result in inefficient operation. For situations where low speed is required it is worth considering either sensored motors or stepper motors.