DC Motors: Transforming Theory into Application

June 2, 2011  |  Articles

Direct current (DC) electric motors convert electrical energy into mechanical energy. 1 How they accomplish this energy conversion and under what conditions is the investigative journey every motor engineer must process while designing a machine drive. The task is not as complex as it seems. But it requires an understanding of magnetic principles 2, mastering the types of D.C motors 3 and their operational characteristics 4 to understanding their proper application 5. This article will touch on the high points of motor theory through real-world applications.

Magnetic Principles and Motor Theory

All machine designs involving rotating equipment ultimately rely on theory to guide the engineer’s application choices. Hence, a very brief review of magnetic principles and motor theory is always a convenient starting point for any discussion of DC motor applications. The laws of physics 6 have blessed the world of machine design with the existence of magnetism, which is the foundation of motor theory 7. In essence, magnets, permanent or electromagnetic, produce fields of magnetic flux. These magnetic fields can produce an induced EMF through a coil of wire when relative movement between the field and a current carrying conductor occurs; and if this movement is reversed, so is the direction of the magnetic field, according to Faraday’s Law 8. Thus, in theory, motor action or torque is produced when electrical energy is applied to conductor in a changing magnetic field, causing current flow in the conductor, generating both an induced EMF and a CEMF (Lenz’s Law) 9 resulting in rotational or mechanical energy. 10.

DC Motors: Physical and Functional Descriptions

DC motors are commonly used in industrial machinery because of their inherent advantages 11—good speed control, high starting torque, reliable control methodology—which generally outweigh the increased maintenance costs 12 associated with them. Let us overview the major elements and functions of DC motors:

Construction

The generic DC motor is constructed 13 with armature and field windings, interpoles, a frame or stator, a segmented commutator, a brush assembly and end bells. The rotating armature winding is wound on a laminated core, mounted on a steel shaft, supported by shaft bearings, and is connected to the segmented commutator that receives external DC power through the brush assembly. Brushes conduct the current from external DC power circuit to the commutator and finally to the armature windings. The frame or stator supports the field windings and interpoles. The end bells encase all the parts of the motor into one unit.

Operation

DC motors 14 produce torque and mechanical motion due to the interaction of the magnetic fields of the rotating armature coil and the stationary field coil mounted on the frame. The changing magnetic field of the armature is possible through the use of electrically conductive carbon brushes 15, which ride on the segmented, commutator ring 16; external DC power is applied to the brushes through the commutator to the armature windings. As current flows through the armature coil, a magnetic field results. The field windings mounted on the frame, also set up a magnetic field. After the rotating armature passes through half of a complete rotation, the commutator switches the direction of the current flow, thereby changing the direction of the magnetic field in the armature winding. This change produces opposing magnetic fields and sustains torque and rotation through the next half cycle of rotation until the commutator changes the direction of current flow and the magnetic field again.

Types

The field and armature windings of DC motors can be connected in series, shunt (parallel) or series-shunt to achieve different kinds of speed-torque characteristics. Hence, the three general categories of wound field DC motors are shunt-wound, series-wound and compound-wound. In series-wound motors, the armature is connected in series with the field to provide high starting torque; however, they do not operate at no-load: when speed decreases, torque increases, which can create a possibly unsafe runaway condition. In shunt wound motors, the armature and field are connected in parallel. This wiring arrangement produces an inverse speed-torque relationship: as speed increases, torque decreases. The compound-wound is a combination of a series- and shunt-wound motor by placing the field winding in series with the armature in addition to a shunt field. This type offers a combination of good starting torque and speed control.

Brushless motors 17 are a hybrid type of DC motor that does not use a commutator. Rather, it is constructed 18 with a permanent magnet rotor, optical shaft encoder that gives positional feedback information, a DC controller that excites the the phase of stator windings required to develop torque based upon the encoder’s feedback. Brushless motors characteristically have high maximum operating speeds, high torque to weight ratios and are compact in design (fractional horsepower). They are typically used in robotic arm applications. 19

Controls

DC motors are controlled manually or remotely through switches, relays or motor starters 20, which contain overload protection and reversing capabilities. For variable speed applications, they are controlled by thyristor power converters, called a DC drives 21, which provide not only start/stop and motor protection capabilities, but also control accel/decel ramps, speed control and response, reversing, dynamic braking features, etc. Thyristors create undesirable line harmonics that cause reactive heating and reduced efficiency; however; harmonic problems can be mitigated by applying harmonic filters and premium, SCR-rated motors.

Application Considerations

Applying DC motors 22 in industrial applications, e.g., cranes, mills, pumps, presses, machine tools, etc., requires a careful consideration of the torque-speed requirements of the load and matching them to the motor’s capabilities for power (kw), torque, speed control 23, thermal range, duty rating, and start-stop frequency to ensure the motor can operate within safe parameters. For instance, crane applications require high breakaway torque (even at zero speed), fast reversing and dynamic (regenerative) braking. Series DC motors are typically used in these applications but are also used in elevators and conveyor applications. Shunt DC motors have moderate starting torque but good speed regulation and typically operate as constant speed prime movers for applications such as belt-driven machines, lathes, machine tools and fans. Compound DC motors have high starting torque and constant speed under load. 24 They are typically used in rolling and press applications.

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