DC Motor: Types & Operation

The substantial evolution of power electronics has made possible the construction of static converters employing reliable, low-cost and simple maintenance thyristors. With this, DC motors, despite their high cost, have become an alternative in a series of applications that need this fine tuning of speed and high torque. Figure 1 shows a typical DC motor.

Figure 1. DC motor

DC Motor Operation

The DC motors can be divided into two main magnetic structures:

Stator: Composed of a ferromagnetic structure with salient poles where there are coils placed that form the magnetic field. Typically, two windings are placed in the stator: the series field, consisting of a small number of turns with larger cross section coil wire, and the shunt field, made by a large number of coils with smaller cross section, as shown in Figure 2.

The magnetic field can also be produced by permanent magnets with this type of machine application restricted to applications of small power, such as toys, hot blowers, computer disc drives, etc.

Rotor: Is an electromagnet composed of an iron core with windings connected to a mechanical system called a commutator. The commutator is along with the rotor shaft and has a cylindrical surface with blades that are connected to the rotor windings and with brushes that are pressed together with these blades and connected to the power supply. The commutator has the function of transforming DC into AC in a suitable way for motor torque development.

Figure 2. Series and shunt field in a stator.

Figure 3. Stator and rotor integrated in a 2 poles DC motor.

Figure 3 shows the stator and rotor integrated in a two-pole DC motor.

The DC motor can be divided into two distinct circuits: armature (rotor) and field (stator). In this analysis, it will be considered a motor powered by two voltage supplies: one for the armature circuit (Ua) and one for the field circuit (Uf), as shown in Figure 4.

The DC motor operation is based on the produced force from the interaction between the magnetic field and the armature current in the rotor making the rotor moves.

Applying Kirchhoff’s law in the armature circuit we have:

$$U_a=I_aR_a+E\ \ \ \ \ \left(1\right)$$

Where:

• U_a is the armature voltage
• R_a is the armature resistance
• I_a is the armature current
• E is the induced electromotive force

Figure 4. Armature and field circuit in a DC motor.

The air gap flux (ø) is proportional to the field current (If):

$$\phi=k_2.I_f\ \ \ \ \ \ \ \ (2)$$

where k2  is the field constant.

The motor torque is given by:

$$C=k_3\times\phi\times\ I_a\ \ \ \ \left(3\right)$$

where k3 is the torque constant.

Considering Faraday’s Law, the induced electromotive force (E) is proportional to the magnetic air gap flux (ø) and the rotation (n), thus:

$$E=k_1\times\phi\times\ n\ \ \ \ \left(4\right)$$

where:

• n is the speed
• k1 is the constant considering rotor dimensions, number and pole connection

Arranging Equations (1) and (4), we have the motor speed expressed by:

$$n=k_1\frac{U_a-\ I_aR_a}{\phi}\ \ \ \ \ \left(5\right)$$

In practice, the armature resistance is very small, causing a small voltage drop in the armature (Ra.Ia≅0), so we will have the following equation to for speed:

$$n=k_1\frac{U_a}{\phi}\ \ \ \ \ \ (6)$$

We can conclude that the speed is directly proportional to the armature voltage and inversely proportional to the air gap flux. In this configuration, we have the field winding independent of the armature winding, and it is named separately excited DC motor, which we could see in Figure 4. The speed adjusted by the voltage variation in this field, being this type of motor, is the most applied in the industry.

According to the type of excitation, the motors can be divided as follows.

Series DC Motor

In this configuration, the field coils are in series with the armature winding. As the field winding is connected in series with the armature, it should be built with few turns of wire having a cross section sufficiently large to withstand the high current that flows through armature windings.

In the series, excited motor, the magnetic air gap flux (ø) per pole depends on the armature current (Ia), which depends on the load applied to the motor. This causes the machine to have a high torque at low speeds and the speed can be very high when the motor is in an unload condition. This occurred due to the low field current that causes low magnetic air gap flux (ø), which is in the denominator of Equation (5), resulting in high velocities for small field values. It is recommended to always keep load on the shaft, since the speed can increase to very high values, resulting in high centrifugal forces that can damage the machine. Figure 5 shows a series excited motor configuration.

Shunt Excited DC Motor

In this type of machine, the field coils are in parallel with the armature windings considered a type of self-excited motor. As the windings are connected in parallel (subjected to the same voltage supply) the field winding coils are made with a large number of turns of small cross sections.

Figure 5. Series excited motor configuration.

Figure 6. Shunt excited motor configuration.

In this configuration, there are separate branches: one for the armature current (Ia) and the other one for the field current (If), which we can see in Figure 6.

The overall current is divided in two parts: armature (Ia) and field (If), and could be obtained by the following equations:

$$I_f=\frac{U}{R_f}\ \ \ \ \ (7)$$

$$I\ =\ I_a+I_f\ \ \ \ \ (8)$$

So, when this motor is in the running condition, the supply voltage is constant and the current shunt is obtained by Equation (7), making the field current and, consequently, the flow constant, so this motor is considered constant flux or constant speed. This characteristic is desired in industrial applications.

Compound Excited DC Motor

This motor has the constructive characteristics of the series and the shunt excited motors. Figure 7 shows the wiring diagram for this type of motor.

The magnetic field created by the shunt winding is always greater than that generated by the series winding. When the motor operates at low load, the current in the armature Ia that flows in the series winding is small, causing the magnetic field to be negligible. However, the shunt winding can be completely energized, keeping the operating characteristics of the machine.

Figure 7. Compound excited motor configuration.

When the load is applied on the shaft, the current in the series field increases and, consequently, the magnetic field increases, but the magnetic field in the shunt winding remains constant. This causes the speed (which depends on the flow ø) to fall from no-load to full-load conditions between 10% and 30%.

There are two connection types for this cumulative compound motor where the magnetic field of the shunt fields and series are summed and the differential compound where the series field is connected in order to oppose the magnetic field is generated by the shunt winding.

NOTE: The constant development of power electronics should lead to a progressive reduction in the use of DC motors. This is because variable frequency drives developed for induction motors, especially the squirrel cage, are already becoming more attractive options in terms of speed control due to association with the low cost and maintenance of this type of motor.

Comparative Analysis of DC Motor Types

Below is a comparative analysis of all major types of DC motors based on specific technical parameters such as torque characteristics, speed regulation, starting current, field excitation, load handling, applications, and maintenance.

Key Takeaways

• DC motors are widely used for applications requiring precise speed control and high torque, despite advancements in AC motor drives.
• The motor consists of a stator (stationary magnetic field source) and a rotor (rotating armature), often integrated with a commutator and brushes for current direction control.
• Stator magnetic fields can be generated by windings (series or shunt) or permanent magnets for low-power applications.
• Rotor windings receive current through brushes and commutator segments, producing torque via electromagnetic interaction.
• The motor’s operation follows Kirchhoff’s Law and Faraday’s Law, with key equations for electromotive force (E) and torque (C) derived from flux (ø), armature current (Ia), and speed (n).
• Speed control is achieved by adjusting armature voltage (Ua) or field flux, with speed inversely proportional to flux and directly proportional to armature voltage.
• Separately excited DC motors have independent armature and field circuits, offering precise speed control and are commonly used in industrial settings.
• Series excited motors deliver high torque at low speed but can reach dangerously high speeds under no load; best used under consistent load conditions.
• Shunt excited motors maintain constant speed due to parallel field connection, suitable for stable load applications.
• Compound excited motors combine series and shunt windings, providing balanced performance across varying loads.
• Cumulative compound motors enhance magnetic flux under load, while differential compound motors reduce it, impacting speed regulation.