DC Motor

DC motor rear and front view
DC motor rear and front view
DC motors are also called direct cur­rent motors, their direct coun­ter­part are AC motors or also called alter­nat­ing cur­rent motors. As the name implies, a DC motor can be pow­ered by DC cur­rent from a DC volt­age source, such as a 12V bat­tery. The pic­ture above shows the back and front view of a DC motor. To bet­ter explain the struc­ture and func­tion, a very sim­ple motor design was chosen. 

Design DC Motor

Like any elec­tric motor, the DC motor con­sists of a rotor and a sta­tor. Unlike AC motors, the non-rotat­ing sta­tor of the DC motor con­sists of per­ma­nent mag­nets. The per­ma­nent mag­nets are attached to a cylin­dri­cal shroud made of sheet met­al. The rotat­ing rotor con­sists of indi­vid­ual thin elec­tri­cal sheets. The rotor is wound with the wind­ings made of cop­per wire. The indi­vid­ual wind­ings are con­nect­ed to the com­mu­ta­tor. Two brush­es for the plus and minus pole loop on the rotat­ing com­mu­ta­tor. The brush­es can be made of car­bon pins or met­al, for exam­ple. To improve the con­tact, the car­bon pins are pressed onto the com­mu­ta­tor by small springs. The brush­es are con­nect­ed to the plus and minus poles for the DC volt­age. Unlike an AC motor, which usu­al­ly has three ter­mi­nals for the pow­er sup­ply, the DC motor has only two ter­mi­nals for a DC voltage.

Function DC Motor

Like any elec­tric motor, the DC motor requires a chang­ing mag­net­ic field, which changes depend­ing on the posi­tion. The mag­net­ic field is gen­er­at­ed by a cur­rent flow in the wind­ings. In order for the mag­net­ic field to change depend­ing on the posi­tion, the direc­tion of the cur­rent flow in the wind­ings must change. This is exact­ly what the so-called com­mu­ta­tor is need­ed for, which changes the polar­i­ty of the volt­age applied to the wind­ings. So the com­mu­ta­tor ensures that either a pos­i­tive volt­age or a neg­a­tive volt­age is applied to wind­ings. There­fore, the direc­tion of cur­rent flow in the wind­ing also changes and so does the mag­net­ic field. The com­mu­ta­tor can there­fore be regard­ed as a mechan­i­cal invert­er which turns the DC cur­rent in the motor into an AC cur­rent for the wind­ings. This is why the com­mu­ta­tor is also called a mechan­i­cal cur­rent inverter.

DC motor commutator
DC motor com­mu­ta­tor front and top view

DC Motor Types

There are three dif­fer­ent types of DC motors:

  • self-excit­ed DC motor
  • sep­a­rate­ly excit­ed DC motor
  • per­ma­nent mag­net DC motor

The per­ma­nent mag­net DC motor is the most pop­u­lar due to its sim­ple design. The sep­a­rate­ly excit­ed DC motor and self-excit­ed DC motor do not have per­ma­nent mag­nets in the sta­tor; instead, these DC motors use elec­tro­mag­nets in the sta­tor. For self-excit­ed DC motors, the volt­age for the elec­tro­mag­net is used direct­ly from the DC volt­age. Instead, sep­a­rate­ly excit­ed DC motors use an addi­tion­al volt­age that can be set inde­pen­dent­ly of the DC volt­age of the DC motor. The direct coun­ter­part to DC motors are brush­less DC motors, or BLDC motors. As the name implies, BLDC motors do not have brush­es, so they do not have a com­mu­ta­tor. Brush­less motors belong to the AC motors. Iron­less or slot­less DC motors have no slots in the rotor, i.e. no teeth around which wind­ings are wound. The wind­ings are there­fore wound around air, so to speak. This is why the term “air gap wind­ing” is used.

Control DC Motor

The direc­tion of rota­tion can be eas­i­ly set via the polar­i­ty at the two ter­mi­nals of the DC motor. So for exam­ple +12V for clock­wise rota­tion and ‑12V for coun­ter­clock­wise rota­tion. The exact sup­ply volt­age and direc­tion of rota­tion should be tak­en from the data sheet of the man­u­fac­tur­er of the DC motor. The speed can be changed via the volt­age of the DC motor. How­ev­er, in order to con­trol the speed cor­rect­ly, a sen­sor or encoder is need­ed to detect the speed. There are also spe­cial meth­ods to cal­cu­late the speed from the motor cur­rent. A PI con­troller can be used as a reg­u­la­tor, which adjusts the volt­age depend­ing on the devi­a­tion between the set­point and actu­al speed. The volt­age lev­el adjust­ed in this way changes the DC cur­rent flow­ing through the motor. In this way, the speed of the DC motor is con­trolled to the set speed.

DC motor commutator with windings
DC motor com­mu­ta­tor with windings

DC Motors Applications

DC motors are most­ly used for sim­ple actu­a­tors. A large area of appli­ca­tion for actu­a­tors with DC motors is the auto­mo­tive indus­try. There, DC motors are used for flaps and valves. Also in mod­el­ling, many DC motors are used for ser­vo dri­ves to steer mod­el air­planes and for remote-con­trolled cars and boats.

Advantages and Disadvantages

Because DC motors are man­u­fac­tured in very high quan­ti­ties in a ful­ly auto­mat­ed process, they are very cost-effec­tive. In most cas­es, low-cost fer­rites are used instead of neodymi­um-iron-boron mag­nets to keep man­u­fac­tur­ing costs low. The weak­er mag­nets pro­vide low­er effi­cien­cy and less pow­er den­si­ty. How­ev­er, there are also pre­mi­um DC motors that have good effi­cien­cies. How­ev­er, these are then com­pa­ra­ble to AC motors in terms of cost. A major dis­ad­van­tage is the lim­it­ed life­time due to fric­tion wear of the brush­es. These also lim­it the speed of DC motors. The so-called brush fire, which occurs between the brush­es and the com­mu­ta­tor, also lim­its the appli­ca­tions of DC motors.