Types of DC Motors

Shunt Wound DC Motors

Shunt wound DC motors provide medium starting torque, 125% to 200% full load, and are capable of delivering 300% of full load torque for short periods. With excellent speed control, shunt wound motors generally drive loads requiring speed control and low starting torque.
Some applications include fans, blowers, centrifugal pumps, conveyors, elevators, printing presses, woodworking machines, and metalworking machines.
There are two basic types of shunt wound DC motors. Self-excited shunt wound motors have a shunt field and armature connected to the same power supply.
In separately excited shunt wound motors, shunt field and armature connect to separate power supplies.

Series Wound DC Motors

A series wound DC motor has its armature and field connected in a series circuit. These type motors normally drive loads that require high torque and do not require precise speed regulation. Series DC motors are ideal for traction work where the load requires a high breakaway torque. Such are locomotives, hoists, cranes, automobile starters, or oil drilling rig applications.

Starting torque developed in series motors normally ranges between 300% and 375% of full load, but attain 500% of full load torque. These motors deliver this high starting torque because their magnetic field operates below saturation.
An increase in load results in an increase in both armature and field current. As a result, the armature flux and field flux increase simultaneously. Since the torque developed in DC motors is dependent upon the interaction of armature and field flux, torque increases by the square of current increase.
Speed regulation in series motors is inherently less precise than in shunt motors. If motor load diminishes, current flowing in both the armature field circuits reduces as well, effecting a reduction in flux density.
This results in a greater increase in speed than realized in shunt motors. Removal of mechanical load from series motors results in indefinite speed increase whereby centrifugal forces generated by the armature eventually destroy the motor.

Compound Wound DC Motors
Whenever an operation requires speed regulation characteristics unavailable in series or shunt motors, compound wound motors perform well. With medial starting torque capability, between 180 and 260% of full load, they deliver constant operating speeds under any percentage of full load.

This characteristic is a result of placing part of the field circuit in series with that of the armature. When under load, increased series winding current raises the level of field flux. Enlarged field flux in compound wound motors yields greater reduction in speed than in a shunt motor.
The compound wound DC motor comprises both series and shunt windings. The shunt winding connects in parallel with armature and series windings. Some associated applications include punch presses, shears, crushers, and reciprocating compressors.
Permanent Magnet DC Motors

Permanent magnet motors are well fit for use where response time is a factor. Their speed characteristics are similar to those of shunt wound motors. Built with a conventional armature, they use permanent magnets rather than windings in the field section. DC power is supplied only to the armature.
Permanent magnet motors are not expensive to operate since they require no field supply. The magnets, however, lose their magnetic properties over time, and this effects less than rated torque production. Some motors have windings built into the field magnets that re-magnetize the cores and prevent this from happening.
Automobiles have installed DC permanent magnet motors that control power seats, windows, and windshield wipers. DC permanent magnet motors produce high torque at low speed, and are self-braking upon disconnection of electrical power. Permanent magnet motors cannot endure continuous operation because they overheat rapidly, destroying the permanent magnets.

Universal DC Motors
Universal motors seldom exceed one horsepower, and do not run at constant speeds. The speed of a universal motor varies with its load. Among the applications using these motors are vacuum cleaners, food mixers, portable drills, portable power saws, and sewing machines.
In most cases, little more than a few hundred rpm is reached with heavy loads. When the motor operates with no load, the speed may attain 15,000 rpm.
The universal series motor differs in design from a true induction motor. The rotor of a universal motor is made of laminated iron wound with wire coils. The ends of the coils, or loops, connect to a commutator. Electric current in the motor flows through a complete circuit formed by the stator winding and rotor winding. Brushes ride on the commutator and conduct the current through the rotor from one stator coil to the other. Directed by these brushes the rotor current interacts with the magnetic field of the stator causing the rotor to turn. When the direction of current flow changes in the stator, it changes in the rotor. Since the magnetic field is reversed, the rotor continues to turn.
Universal motors have series wound rotor circuitry similar to that of DC motors. They have high starting torque and high starting current. The name universal derives from the motor's capability of operating on either AC or DC power sources.
Universal, variable speed motors slow down with increased loads. High horsepower-to-size ratio is characteristic of their design. Due to the brush/commutator setup, universal motors require more maintenance than other motor designs.