DC motors are ubiquitous machines found in a variety of electronic equipment used in various applications.

Typically, these motors are deployed in equipment that requires some form of rotary or motion-producing control. Direct current motors are essential components in many electrical engineering projects. Having a good understanding of DC motor operation and motor speed regulation enables engineers to design applications that achieve more efficient motion control.

This article will take a close look at the types of DC motors available, their mode of operation, and how to achieve speed control.

What are DC Motors?

Like AC motors, DC motors also convert electrical energy into mechanical energy. Their operation is the reverse of a DC generator which produces an electric current. Unlike AC motors, DC motors operate on DC power–non-sinusoidal, unidirectional power.

Basic Construction

Although DC motors are designed in various ways, they all contain the following basic parts:

• Rotor (the part of the machine that rotates; also known as the “armature”)
• Stator (the field windings, or “stationary” part of the motor)
• Commutator (can be brushed or brushless, depending on the motor type)
• Field magnets (provide the magnetic field that turns an axle connected to the rotor)

In practice, DC motors work based on interactions between magnetic fields produced by a rotating armature and that of the stator or fixed component.

A sensorless DC brushless motor controller. Image used courtesy of Kenzi Mudge. 

Operating Principle

DC motors operate on Faraday’s principle of electromagnetism which states that a current-carrying conductor experiences a force when placed in a magnetic field. According to Fleming’s “Left-hand rule for electric motors,” the motion of this conductor is always in a direction perpendicular to the current and the magnetic field.

Mathematically, we can express this force as F = BIL (where F is force, B is the magnetic field, I stand for current, and L is the length of the conductor).

Types of DC Motors

DC motors fall into different categories, depending on their construction. The most common types include brushed or brushless, permanent magnet, series, and parallel.

Brushed and Brushless Motors

A brushed DC motor utilizes a pair of graphite or carbon brushes which are for conducting or delivering current from the armature. These brushes are usually kept in close proximity to the commutator. Other useful functions of brushes in dc motors include ensuring sparkless operation, controlling the direction of current during rotation, and keeping the commutator clean.

Brushless DC motors do not contain carbon or graphite brushes. They usually contain one or more permanent magnets that spin around a fixed armature. In place of brushes, brushless DC motors utilize electronic circuits to control the direction of rotation and speed.

Permanent Magnet Motors

Permanent magnet motors consist of a rotor surrounded by two opposing permanent magnets. The magnets supply a magnetic field flux when dc is passed, which causes the rotor to spin in a clockwise or anti-clockwise direction, depending on the polarity. A major benefit of this type of motor is that it can operate at synchronous speed with a constant frequency, allowing for optimal speed regulation.

Series-wound DC Motors

Series motors have their stator (usually made of copper bars) windings and field windings (copper coils) connected in series. Consequently, the armature current and field currents are equal. High current flows directly from the supply into the field windings which are thicker and fewer than in shunt motors. The thickness of the field windings increases the load-carrying capacity of the motor and also produces powerful magnetic fields that give series DC motors a very high torque.

Shunt DC Motors

A shunt DC motor has its armature and field windings connected in parallel. Owing to the parallel connection, both windings receive the same supply voltage, although they are excited separately. Shunt motors typically have more turns on the windings than series motors which creates powerful magnetic fields during operation. Shunt motors can have excellent speed regulation, even with varying loads. However, they usually lack the high starting torque of series motors.

A motor and speed control circuit installed in a mini drill. Image used courtesy of Dilshan R. Jayakody 

DC Motor Speed Control

There are three main ways to achieve speed regulation in series DC motors–flux control, voltage control, and armature resistance control.

1. Flux Control Method

In the flux control method, a rheostat (a type of variable resistor) is connected in series with the field windings. The purpose of this component is to increase the series resistance in the windings which will reduce the flux, consequently increasing the motor’s speed.

2. Voltage Regulation Method

The variable regulation method is typically used in shunt dc motors. There are, again, two ways to achieve voltage regulation control:

• Connecting the shunt field to a fixed exciting voltage while supplying the armature with different voltages (aka multiple voltage control)
• Varying the voltage supplied to the armature (aka the Ward Leonard method)

3. Armature Resistance Control Method

The armature resistance control is based on the principle that the speed of the motor is directly proportional to the back EMF. So, if the supply voltage and the armature resistance are kept at a constant value, the speed of the motor will be directly proportional to the armature current.