Direct Current (DC) motors are widely used in engineering designs due to the torque-speed characteristics they possess with different electrical or mechanical configurations.

A great advantage of DC motors is that it is possible to control them smoothly and in most cases they are reversible, they respond quickly thanks to the fact that they have a great torque ratio to the inertia of the rotor.

Another advantage is the implementation of dynamic braking, where the energy generated by the motor is fed to a dissipating resistor, and regenerative braking where the energy generated by the motor feeds back to the DC power supply, this is widely used in applications where they are desired fast braking and high efficiency.


Direct current motors can be classified according to the way they create the magnetic fields of the stator.

  • Permanent magnet
  • Shunt winding
  • Winding series
  • Composite winding


The following graph is a representation of the torques that a motor can provide at different speeds at nominal voltages.

For a given torque provided by the motor, the current-torque curve can be used to determine the current required when the rated voltage is applied to the motor.

As a general rule, the motors generate large torques at low speed, and large torques imply a greater current demand by the motor.

The starting torque or critical torque (Ts): It is the maximum torque that a motor can provide at zero speed, associated with the start or overcharge of the motor.

The speed of no charge Wmax: It is the maximum sustained speed that the motor can achieve. This speed can only be achieved when no charge or torque is applied to the motor.


In this type of motors, the fields of the stator are generated by permanent magnets that do not require external power supply and therefore do no produce heating. PM motors are lighter and smaller compared to other DC motors with some equivalent characteristics since the field strength of the permanent magnet is high. It is also easy to reverse the direction of rotation by switching the direction of the applied voltage, since the current and field change direction only in the rotor.

The permanent magnet motor is ideal in computer control applications due to its torque-speed linearity, although they are only used in low power applications since their rated power is usually limited to 5 hp (3278 W) or less.

Permanent magnet DC can be with brushes, brushless or stepper motors.


Conformed by an armature and field windings connected in parallel that are activated by the same source.

Shunt motors have almost constant speed over a large charge range, have a starting torque of approximately 1.5 times the nominal operating torque, have lower starting torque than any of the DC motors and can be economically converted to allow an adjustable speed when placing a potentiometer in series with the field windings. The total charge current is the sum of the armature and field currents.


They have armature and field windings connected in series, so armor and field currents are equal. The series motors generate very high starting torques, extremely variable speed depending on the charge, and high speed when the charge is small.

Large series motors can fail catastrophically when discharged suddenly due to dynamic force at high speeds, this is called uncontrolled.

The torque-speed curve for a series motor has a hyperbolic shape, which implies an inverse relationship between torque and speed, with almost constant power.


These motors include both bypass field and series windings, resulting in combined characteristics of bypass and series motors.

Part of the charge current passes through the armature and series windings, the remaining charge current passes only through the bypass windings. The maximum speed of a compound motor is limited; its speed regulation is not as good as that of a shunt motor. The torque produced by the compound motors is a little lower than that of similar-sized motors. 

NOTE: Unlike the permanent magnet motor, when the polarity of the power for a bypass, series or compound DC motor changes, the direction of rotation does not change (meaning that these motors are not reversible). The reason for this is the polarity of the stator and the rotor changes because the field and armature windings are excited by the same source.

Erick Méndezelectronic, motor, dc