Direct Current Motor
The term "direct-current motor" is frequently shortened to "DC motor" for ease of understanding and brevity within the content.
In a DC motor, the torque produced is determined by the multiplication of two key elements: the intensity of the magnetic field formed by the stator and the electrical current flowing within the rotor. The magnetic flux generated by the stator's field is known as field flux, which holds immense significance in driving the motor's functionality.
Additionally, it is important to note that the armature of a DC motor is also known as the rotor. The flow of electric current within the armature is specifically referred to as armature current. It is crucial to differentiate between the term "armature," which plays a key role in producing torque, and the word "amateur," despite their visual similarity.
DC motors can be broadly categorized into two main groups: permanent-magnet motors and winding-field motors, each serving specific purposes and applications in the realm of electronic devices and machinery.
Permanent-magnet motors utilize permanent magnets to produce the essential magnetic fields for their functioning. These magnets play a crucial role in ensuring a steady and dependable supply of magnetic flux, ultimately enhancing the motor's overall efficiency and performance.
On the other hand, winding-field motors do not rely on permanent magnets for operation. Rather, they create magnetic fields by utilizing coils of wire, called windings, which receive power from the flow of electricity. This unique setup provides increased versatility in managing the motor's magnetic field intensity and controlling its rotational movement.
Fig. 1.4 Permanent-magnet motor and DC motor
DC motors are broadly categorized into two main types: permanent-magnet motors, which rely on fixed magnets for operation, and winding-field motors, which use coils for generating magnetic fields.
Permanent-magnet motors, known for their reliance on permanent magnets to establish essential magnetic fields for operation, benefit from a continuous and consistent source of magnetic flux. This characteristic not only enhances the efficiency of the motor but also boosts its overall reliability.
In contrast, permanent magnet motors utilize permanent magnets to produce magnetic fields, whereas winding-field motors do not rely on them. The latter generate magnetic fields by passing electric current through coils of wire, called windings. This method provides flexibility in adjusting the motor's magnetic field intensity and rotation direction.
The decision on which motor type to choose depends on various factors like application specifications, preferred performance attributes, and budget constraints. Permanent-magnet motors offer straightforwardness and steady performance, whereas winding-field motors deliver adaptability and the ability to control the magnetic field as needed for particular operational requirements.
Permanent-magnet DC motors are classified into the following three types by
armature (rotor) type:
1. Slotted type
2. Slot less type
3.Coreless type
The coreless type of 3 is also called the moving-coil type.
Winding-field motors, also known as field wound motors, rely on coils of wire, referred to as windings, to generate magnetic fields upon the passage of electric current. Differing from permanent-magnet motors, these motors offer flexibility as the strength and orientation of the magnetic field can be modified by changing the magnitude and path of the electrical current that is sent to the windings.
This distinction in motor types offers engineers and designers a range of options tailored to various applications and performance needs. Permanent-magnet motors prioritize simplicity and reliability, whereas winding-field motors deliver increased flexibility and precise control over motor functionality.
Winding-field DC Motor
Fig. 1.5 Disassembly photography of winding-field motor
Figure 1.5 illustrates a motor designed to produce field flux by utilizing electromagnets. Winding-field DC motors have historically been the preferred choice for medium to large-scale uses, commonly providing around 1 horsepower, roughly equivalent to 750 watts, in terms of output power capacity.
This specific category of electric motors is subdivided into three separate types depending on how the field winding and armature winding are connected. Historically, DC motors with winding fields were predominantly employed in tasks needing moderate to high power levels, often achieving up to approximately 1 horsepower (around 750 watts).
Within this precise classification system, winding-field DC motors are meticulously divided into three distinct subcategories depending on how the field winding and armature winding are interconnected. These specific differences in connectivity play a pivotal role in influencing the overall motor performance, efficiency, and appropriateness for particular industrial or commercial purposes.
(see Fig. 1.6).
Shunt Motor
The shunt motor is designed with a stator that contains a concentrated winding and a rotor that is furnished with a commutator, as shown in Figure 1.5. This configuration involves connecting the field winding (located in the stator) and the armature winding (found in the rotor) in parallel, as demonstrated in Figure 1.6(1).
One notable attribute of the shunt motor is its innate capacity to uphold a fairly steady rotational speed, irrespective of variations in the workload exerted on the motor shaft. Referred to as shunt characteristics, this trait guarantees uniform and dependable functionality under diverse operational circumstances, rendering the shunt motor ideal for tasks demanding consistent and trustworthy performance.
Series Motor
In a series motor setup, both the field winding and armature winding are interconnected in series, creating a connection that is depicted in Figure 1.6<2>. This configuration is crucial for the efficient operation of the motor, ensuring that the magnetic field and the current flow work in harmony to produce the desired output.
One of the distinctive features of this type of motor is its notable fluctuation in rotational speed based on the applied load. It delivers substantial torque when starting up or operating at low speeds, with an increase in rotational speeds as the load decreases. This unique attribute, referred to as series characteristics, makes the series motor particularly suitable for specialized uses like cranes, electric trains, and elevators.
However, with the advent of induction motors or synchronous motors equipped with inverters for variable speed control, series motors are gradually being replaced in many applications due to their improved efficiency and performance. This shift towards more advanced motor technologies highlights the continuous evolution and innovation in the field of electric motors, bringing about enhanced energy savings and operational flexibility for various industries.
It is crucial to highlight that series motors have the capability to function on alternating current as well, a matter that will be examined more closely in relation to commutator motors. Nevertheless, using a motor specifically built for direct current with alternating current can lead to higher core loss and additional types of inefficiencies, ultimately causing abnormal heat production.
Regarding the feasibility of converting a shunt motor to a series motor or vice versa by altering the connection of the field winding and armature winding, it is not practically achievable due to fundamental differences in the construction of the two motor types. The distinct design components of shunt and series motors prevent a simple interchanging of the winding connections to switch between the two motor configurations effectively.
Shunt motors are designed with a field winding that consists of a fine wire wound in a large number of turns, thus creating higher resistance. On the other hand, series motors are equipped with a field winding made up of thick wire wound in a small number of turns, resulting in lower resistance levels.
Attempting to connect the field winding and armature winding of a series motor in a shunt configuration may lead to a sudden increase in field current, which could cause harm to the field winding. Similarly, if the field winding and armature winding of a shunt motor are connected in a series setup, it would decrease the field current flow (equivalent to armature current), hindering the motor's functionality and causing it to not work as designed.
Separate-field Motor
The separate-field motor is known for having distinct field and armature windings connected to individual power sources, as shown in Figure 1.6<3>. This design enhances efficiency and performance in electrical systems.
An exceptional attribute of this motor setup is its capacity to cover a wide spectrum of speed adjustments. It accomplishes this by autonomously managing the current sent to the field and armature windings. Through individual adjustments to these windings, users can meticulously adjust the motor's speed, enabling accurate control customized to the unique requirements of the current task.
Brushless DC Motor
The brushless DC motor signifies a notable progression from conventional DC motors as it does away with the necessity for brushes and commutators. Let's delve into its prominent characteristics:
In a brushless DC motor, a significant departure from conventional permanent-magnet DC motors lies in the arrangement of the permanent magnet and armature winding components. In contrast to traditional setups, brushless DC motors feature the permanent magnet located on the rotor side and the armature winding positioned on the stator side. This design configuration is a distinguishing feature of brushless DC motors, contributing to their efficiency and performance.
Brushless operation: In contrast to conventional DC motors that depend on brushes for power switching as commutators shift position, brushless DC motors utilize Hall elements. These Hall elements identify the rotor's position signal and offer feedback to the inverter, allowing accurate control of energization without requiring brushes.
Figure 1.7 System Configuration of a Brushless DC Motor
The illustration in Figure 1.7 provides a detailed insight into the system configuration of a brushless DC motor. This motor, although it receives its drive voltage in alternating current from the inverter, is specifically known as a brushless DC motor with an independent field. This distinction sets it apart from the conventional categorization under AC motors.
While operating without brushes, the brushless DC motor still adheres to the rotational concept of a traditional DC motor, upholding a comparable link between torque and speed.
The brushless DC motor stands out for its exceptional controllability inherited from traditional DC motors, coupled with the brushless advantage for reduced electromagnetic noise, longer product lifespan, improved energy efficiency, and increased design flexibility. These features make brushless DC motors highly sought-after in diverse industries like information-processing equipment (e.g., HDDs, CD-ROM drives) and household appliances (e.g., refrigerators, washing machines), among others.
During the initial phases of brushless DC motor advancement, distributed winding was widely utilized for the stator winding. Nevertheless, concentrated winding has now emerged as the dominant technique, delivering enhanced performance and efficiency. For more detailed information on distributed and concentrated windings, please consult the section labeled "Distributed Winding and Concentrated Winding."
This motor is also classified into the following according to the
difference in the method used to install permanent magnets to the rotor:
(1) SPM (Surface Permanent Magnet)
(2) IPM (Interior Permanent Magnet)
The brushless DC motor is available in two main configurations known as Surface Permanent Magnet (SPM) and Interior Permanent Magnet (IPM), each offering unique characteristics and benefits in various applications.
Surface Permanent Magnet (SPM): Within the SPM category, permanent magnets are securely attached to the external surface of the rotor, as illustrated in Figure 1.8. This particular configuration provides numerous benefits, such as simplified production and upkeep processes, along with enhanced cooling capabilities thanks to the magnets being in direct contact with circulating air.
Interior Permanent Magnet (IPM): The IPM motor type, illustrated on the right side of Figure 1.8, incorporates permanent magnets integrated within the rotor assembly. These magnets are incorporated using various techniques, such as molding or placement within slots in the rotor core. The main objective of the IPM configuration is to reduce the likelihood of magnet detachment caused by centrifugal force, a concern often present in SPM motors operating at high speeds. Furthermore, IPM motors can capitalize on reluctance torque, a phenomenon where the rotor's magnetic reluctance changes concerning its position concerning the stator magnets. This additional torque can optimize the motor's overall performance and efficiency, especially when operating at low speeds or when subjected to heavy loads.
Another important classification criterion for brushless DC motors is the inclusion or absence of sensors like Hall elements or rotary encoders. These sensors help detect the position of SPM or IPM poles. Alternatively, some motors operate using a sensorless drive system, which relies on algorithms to estimate rotor position using feedback signals like back electromotive force (EMF).
When employed as actuators, brushless DC motors are commonly compactly packaged, incorporating the primary motor, drive inverter, and control circuitry. This seamless integration can be observed in Figure 1.9, demonstrating how all necessary elements are contained in a single enclosure, streamlining the setup and use.
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