Chapter 8: DC motors
Chapter Overview
Chapter 8 will cover DC motors. DC motor’s main advantages are that they can produce more torque than a comparable AC motor and can operate on batteries, which makes them ideal for use in portable power tools and equipment. It is important to become familiar with the different types of DC motors because of their wide range of uses and the fact that they are starting to become more prevalent in the electrical industry—worksheet information link.
Key points to remember:
- The advantages of DC motors are: High torque potential and the ability to operate on batteries for use with portable tools and equipment.
- The disadvantages of DC motors are: Relatively high maintenance requirements, and relatively short life span.
DC Motors Basics
A motor is defined as a machine that converts electrical energy into mechanical motion through the principle of electromagnetic induction. Motors are constructed very similar to generators with the difference stemming from the fact that they convert electrical energy into rotating motion instead of converting mechanical motion into electrical energy as is the case with generators. All motors are sized and separated by their respective horsepower (hp) and voltage rating. Horsepower is the rate of work that can be done over time. Work is defined as the application of force over a distance (x the cosine of the angle). The “torque” the motor produces is a measure of the rotational force that the motor is capable of producing, and unlike work it is produced even in the absence of movement.
Image source: https://commons.wikimedia.org/wiki/File:Animation_einer_Gleichstrommaschine_Flux_Linien.gif
Dc motors are comprised of a stator or the “stationary” outer part of the motor responsible for producing the magnetic field usually constructed with permanent magnets that can be more compact but limits the size and strength of the motor itself. The armature or rotor that “rotates” through the associated stator’s magnetic field creating current flow as it spins. The commutator (root word “commute”) that is segmented to reverse the flow of current to match the DC input voltage polarity and transmit the current to the brushes. The brushes which are the first to wear as they deliver the input voltage directly to the fast-moving commutator.
Types of DC Motors
The four basic types of DC motors are series wound, shunt, compound, and permanent-magnet motors. DC “series” motors have their stator connected at the factory in series with the armature. These types of motors produce high starting torque but struggle with speed control. DC “shunt” motors have their stator connected at the factory in parallel with the armature. Shunt connected motors exhibit better speed control than series connected DC motors but lack the high torque. Compound motors are connected in both series and parallel and offer a compromise of decent torque and speed regulation. DC permanent-magnet motors use magnets on the outside of the motor (stator) to produce the necessary magnetic field but are limited in size and used predominately in smaller applications e.g. automobile power windows and windshield wipers.
Editing note: insert flash cards for four types of DC motors here:
Looking Deeper
The varying types of DC motor construction is due to the large amount of applications that DC motors can serve. DC motors have the advantage of being able to run off of batteries and are widely used with portable tools and equipment. DC motors will continue to see use in many “off-grid” electrical systems as they are advantageous for use with the direct current (DC) produced by photovoltaic (PV) generation systems.
Keep in mind:
The fact that series-wound motors produce more starting torque than shunt-wound DC motors is understandable when we consider how current behaves in these configurations. In series circuits, the current remains the same, which results in greater linear torque potential. In contrast, in parallel circuits, the voltage remains constant, allowing for better speed control. This distinction is why there are different construction types of DC motors.
A device that converts electrical energy into rotational movement through the principle that current carrying conductor moving through a magnetic field will tend to be forced out of the field at right angles.