I read in an online book at https://www.learncse.com/images/Lessons/Lesson_14.pdf following reason for using a voltage regulator to power DC motors. While I understand the importance of using a regulator in certain situations, I am not convinced of the reason given. Please let me know if the following is correct?
The lesson as posted at https://www.learncse.com/images/Lessons/Lesson_14.pdf is to control a DC motor using an H-bridge and interfaced with an Arduino.
One characteristic of the DC motor that is very important for the
purposes of this lesson is the motor's electrical resistance. The
slower the motor turns the lower its electrical resistance
and, therefore, the greater its demand for electrical current.Reason
The reason this is important is that in these lessons the motors are
powered by batteries. Batteries, in turn, have an internal resistance.
As the resistance of a motor goes down, the voltage across the motor
also drops as the proportion of the battery's internal resistance to
total resistance increases. The result is all motors being driven by
the batteries slow. To the observer, the DC motors just seem to be
running rather poorly.Outcome
To minimize that effect, this lesson uses a battery with a higher
voltage than what was intended for the motor and a voltage regulator
to maintain the voltage delivered to the motor at a lower but constant
value. This approach works because the internal resistance of the
battery impacts the voltage being delivered to the regulator, not to
the motor.
Best Answer
It is important to consider that the first line of the quoted material contains the words "for the purposes of this lesson." The explanation that follows may be adequate for that purpose, but it is overly simplified for a general understanding. The phrase "As the resistance of a motor goes down" uses the term "resistance" to describe the combination of the motor's resistance and reverse electromagnetic force or "back EMF." The reason for motor speed decrease is not mentioned. The reason would probably be increased load torque. Also not covered is the performance of the motor from the time it is switched on until it reaches a stable operating speed,
In general, rather than a voltage regulator, it is more desirable to use and electronic speed controller (ESC) with a DC motor, and brushless DC motors can not function without an ESC. If an ESC is used, the ESC performs the voltage regulation in a manner that is more suitable for a motor, and an additional voltage regulator is not necessary. The quoted source may cover those points later in the lesson or in subsequent lessons.
Re: Comment & Link to Lesson
The lesson is a crude oversimplification. The motor used is very tiny when viewed in the universe of motors that includes motors propelling electric vehicles, ocean-going ships and industrial machinery. A brushed DC motor should be modeled as a resistor in series with a generator with polarity opposing the supply voltage. The voltage (back EMF) generated by that generator is proportional to motor speed. After accelerating to the maximum speed with no load, the back EMF is close to matching the supply voltage. Any load tends to reduce the motor speed and the EMF allowing increased current so that the current multiplied by EMF is equip to the mechanical power produced. The series resistance of an efficient motor may dissipate only 10 or 20 percent of the power taken from the supply with the remainder converted to mechanical power.
Find a lesson that uses the resistor - back EMF model. The one you have found does a terrible injustice to electrical engineering.