Direct-current (DC) motors are often used in variable speed applications. The DC motor can be designed to run at any speed within the limits imposed by centrifugal forces and commutation considerations. You are probably already familiar with vehicular motors which are almost always DC types. Many machine tools also use DC motors because of the ease with which speed can be adjusted.
There are three main types of DC motors. These are:
The shunt wound motor can adjust speed over a wide range, but once the speed
is set, speed is nearly constant with changes in load. The series wound motor's
speed, on the other hand, drops as the load increases. 
This series wound
motor's characteristic is useful when the motor must respond to large changes of
torque, including starting large loads. This is why this motor design is popular
for vehicles such as earth-moving equipment and railroad locomotives. The
compound-wound motor combines the characteristics of both the shunt and series
motors to best match the motor to its load.
All DC motors other than the relatively small brushless types use a commutator assembly on the rotor. This requires periodic maintenance and is partly responsible for the added cost of a DC motor when compared to a squirrel-cage induction motor of the same power. The speed adjustment flexibility often justifies the extra cost.
