Understanding DC Motors

Introduction

DC motors are found in a wide variety of household items such as appliances, cooling fans, children’s toys, electric toothbrushes, and so on.

A DC motor is an electromagnetic machine that converts electrical energy into mechanical (rotational) energy. It does so by creating magnetic fields while drawing current from a direct current power source.

NOTE: The “stator” refers to the stationary part, the “rotor” refers to the rotating part.

The major parts of a DC motor (see above graphic):

1. Two Electromagnets Magnets (also called the “stator”) - Fixed curve-shaped electromagnets (shown above in black) are mounted directly across from each other 180 degrees apart inside the motor housing, each having a north and south pole. With electromagnets, they only generate lines of flux when power is applied. Permanent stator magnets are used only in very small DC motors.

   The field coils of the electromagnet are powered from the same DC source as the rotor. Therefore, the stator field windings can be wired either in “series” or “parallel” with the incoming rotor brush power wires.

2. Armature (also called the “rotor”) - A rotating assembly with a central shaft and is suspended by bearings in the motor housing on each end of the shaft. The load is connected to one end of the shaft through mechanical means, such as a pulley, gear, or direct coupling. The armature is made up of several copper wires wound through slots in a stack of iron laminated layers to form a rotating cylinder wrapped in copper wires between the two fixed magnets. Having “several” copper wires in each armature loop increases the amount of flux that is cut by each conductor moving through the magnetic field; thus, increasing the amount of torque the motor can generate. The slots in the armature are parallel to the shaft around the cylinder, which causes the copper wires in each slot to cut perpendicular to the lines of flux while rotating.

3. Commutator Ring (connected to the wire loops on the rotor) - A rotating ring of segmentsis (shown above as a copper-colored ring of segments) attached to the ends of the rotating armature loops. Each armature loop has two commutator segments. Each commutator segment is separated by slots which serve as a means of constantly reversing the polarity on each armature loop as they pass in front of the brushes. This reversing of polarity is what keeps the motor rotating. In theory, the simplest form of a commutator ring would consist of only one single armature loop and one pair of commutator rings connected to each end of the armature loop. However, this single-loop concept would cause irregular movement of the motor and could even stall while the slots between the two commutator segments are in front of the brushes, since the opposite sides of the loop are 180 degrees apart. To prevent this irregular movement, several additional armature loops and commutator segments are added around the circle to decrease the angular distance between each loop; thus, smoothing out the operation of the motor.

4. Brushes - A pair of fixed, spring-loaded, electrically conductive mechanisms that maintain contact between the DC power source and the rotating commutator ring. Over time, this pair of conductive brushes will wear and eventually lose contact with the commutator ring and cause the motor to stop running or run intermittently.

Types of DC Motors

The 5 main types of DC motors are:

1. Brushed - as described above, equipped with two spring-loaded brush assemblies applying constant pressure between the power connections and the commutator ring.

2. Brushless - The rotor has rotating permanent magnets around a stator…the stator is a set of stationary energized electromagnetic coils and windings (see graphic below).

3. Series Wound - The stator field windings are wired in series with the incoming rotor brush power wires. Has good starting torque, but its speed drops drastically with the load.

4. Shunt (Parallel) Wound - The stator field windings are wired in parallel with the incoming rotor brush power wires. Has a low starting torque, but is able to run almost at a constant speed regardless of the load on the motor.

5. Compound Windings - a combination of both series and shunt windings to improve starting torque while maintaining constant torque under increased loads. The total net flux is the result of the sum (cumulative) or difference (differential) of the shunt flux and the series flux.

   Compound Cumulative: Fnet = Fshunt + Fseries

   Compound Differential: Fnet = Fshunt - Fseries

   Compound DC motors are suitable for almost all applications within their range of capabilities.

Conclusion

One of the unique features of a DC motor is its ability to produce “back-EMF” that opposes the applied input voltage. Back-EMF is proportional to the speed of the rotor and greatly reduces the amount of armature current. At the starting of the motor, back-EMF is too low to reduce the armature current. Consequently, this high armature current would cause armature burnout. Therefore, a proper starting mechanism is needed to control the applied input voltage in large DC motors.

For information on motors that run on AC and DC power, see the post on “Universal Motors.”