This article takes an in depth look at AC Motors and their use.
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An AC motor or alternating current motor is an electric motor that consists of a stator with a coil that is supplied with alternating current to convert electric current into mechanical power. The stator is the stationary part of the motor while the rotor is the rotating part. AC motors can be single or three phase with three phase motors mainly used for bulk power conversion. Single phase AC motors are used for small power conversions.
AC motors can be classified into two main types: synchronous and induction. In a synchronous motor, the shaft rotates at a speed that matches the frequency of the applied current. This is achieved through multiphase AC electromagnets on the stator, which create a rotating magnetic field. In contrast, an induction motor, also known as an asynchronous motor, operates with only the stator excited. The stator's magnetic flux induces current in the rotor's short-circuited coil, generating torque that causes the rotor to turn.
AC motors are a versatile power source used in a broad range of applications due to their flexibility, efficiency, and quiet operation. They can be found in pumps, water heaters, garden equipment, ovens, off-road vehicles, and numerous other appliances, tools, and equipment. Their adaptability makes them a fascinating choice for many different uses.
AC motor design is relatively simple, featuring a copper-wound stator that generates a rotating magnetic field. AC induction motors are designed to meet IE3 and IE4 standards, which are international benchmarks for motor efficiency.
An AC motor consists of two main components: the stator, which is the stationary outer part, and the rotor, which is the rotating inner part attached to the motor shaft. Both components produce rotating magnetic fields. The rotating magnetic field in the stator is generated by the alternating current flowing through its windings.
In an AC motor, the winding functions as both the armature and the field winding. When the stator is supplied with AC voltage, it creates a rotating magnetic field that moves at a synchronous speed. This rotating field induces voltages in both the stator and rotor windings, enabling the motor to operate.
The term "AC motor" encompasses several types, including single-phase, three-phase, brake, synchronous, asynchronous, customized, two-speed, and three-speed single-phase motors. The distinctions among these versions are based on their intended applications. Some AC motors are designed for simpler, smaller tasks, while others are built to handle larger, more demanding jobs. A key difference among them is the phase of the electrical supply, which varies between residential and industrial uses.
Residential electricity typically uses single-phase or double-phase power, while industrial applications often use three-phase power. This difference in electrical supply is a major factor distinguishing industrial AC motors from their residential counterparts.
AC motors are often referred to as induction motors because they generate torque through electromagnetic induction, which occurs as the magnetic field from the stator induces current in the rotor.
An AC motor can be started using either a contactor or a manual starter. A contactor facilitates the control of power to the motor by toggling it on or off. A manual starter, on the other hand, includes a manual switch that allows the operator to manually control or adjust the power. This type of starter is referred to as "across-the-line," meaning the motor is directly connected to the power source. As a result, the motor receives the full voltage supply, which is typically six to eight times the rated current.
Star-delta starters are another common type, designed to reduce the initial voltage supplied to the motor during startup. In this configuration, the stator windings are initially connected in a star (Y) configuration. Once the motor reaches a certain speed, the windings switch to a delta (Δ) configuration. This method effectively reduces the starting line current.
An auto transformer starter uses a similar method as a delta starter. Again, the initial current is limited to reduced voltage being applied to the stator. The advantage of an auto transformer starter is that the torque and current can be adjusted by the correct tapping.
A rotor impedance starter connects directly to the rotor through slip rings and brushes. Initially, it sets the rotor resistance to its maximum, which decreases gradually as the motor accelerates. While effective for controlling motor start-up, rotor impedance starters are typically bulky and costly.
Single-phase motors generate a pulsating magnetic field, which prevents them from starting on their own. This is because the pulsating field does not produce sufficient torque to initiate motor operation.
Soft starters offer advanced control over motor acceleration and deceleration, allowing for smooth and gradual starting and stopping. Unlike across-the-line starters, soft starters help reduce mechanical stress on the motor and connected equipment, leading to less wear and tear.
The stator generates a rotating magnetic field and comprises several key components: a solid metal core, a coil of wire, and interconnections. While not all AC motors use a squirrel cage, it is a common design feature. In AC motors, electricity is supplied directly to the stator's outer coils. The stator consists of multiple laminated plates extending from its core, which are wound with copper wire to create the magnetic field.
For a three phase AC motor, it has three phase windings with a core and housing. The windings are 120° apart, which can be six or twelve windings . The windings are placed on a laminated iron core. The construction of the core can be seen in the diagram below.
Unlike a DC motor, the rotor on an AC motor does not have any connection with the external power source. It receives its power from the stator. In a three phase induction motor, the rotor can be a squirrel cage or wound version.
In the squirrel cage rotor design, the rotor comprises bars with end rings at both ends. There are various versions of the squirrel cage rotor, including split phase, capacitor start, capacitor start and run, permanent split phase capacitor run, and shaded pole types. These versions are classified into categories A, B, C, D, and E. Typically, the squirrel cage is constructed from aluminum or copper.
In a squirrel cage motor, the rotor bars interact with the stator's electromagnetic field (EMF). As the current in the stator fluctuates, the EMF changes accordingly, causing the rotor to rotate and produce motion. A crucial aspect of this operation is that the rotor does not rotate at the same frequency as the AC current; it continuously tries to catch up, which generates rotational motion. If the rotor were to match the frequency of the AC current, it would stop moving, and no rotation would occur.
A wound or slip ring AC motor is a specialized type of AC motor that is always three-phase. It has the same basic components as other AC motors, but its rotor features a cylindrical laminated core wound with wire, similar to the stator windings. The ends of these wires are connected to slip rings mounted on the output shaft. These slip rings link to brushes and a variable speed resistor, allowing precise control of the motor's speed and torque. This capability for fine-tuning speed and torque is a significant advantage of wound rotor motors.
Wound motors are asynchronous, meaning there is a speed difference between the stator and the rotor. This slippage occurs as current is induced in the rotor, creating a disparity between the rotating field of the stator and the rotor's speed. As the motor runs, this interaction reduces the stator's effective field strength, allowing for precise control over rotation, torque, and overall motor performance.
The AC motor, invented by Nikola Tesla, is utilized in a wide range of applications worldwide. Tesla's foundational work involved discovering the principle of rotating magnetic fields (RMF), which is fundamental to alternators. He was instrumental in developing the rotating field and electromagnetic induction techniques that generate torque in rotating machinery.
Since its inception over a century ago, the AC motor has evolved into various types tailored for different functions. A key distinction among AC motors lies in the rotor design, which can be either a squirrel cage or wound type. This primary difference leads to the diverse range of AC motor types available today.
Single-phase AC motors are designed for use with a single-phase power supply. These motors are typically smaller and more cost-effective, with fractional kilowatt capacities. They operate with a single-phase AC electrical supply, featuring one main winding and an auxiliary winding that is positioned perpendicular to the main winding.
The rotor in a single-phase AC motor operates based on the double revolving field theory, which involves two opposing rotating magnetic fields. The resulting torque produced is balanced and counteracts itself, ensuring efficient motor operation.
Polyphase motors, which can be either two-phase or three-phase, are a type of AC motor similar in operation to single-phase motors. In a polyphase motor, the stator poles are not aligned, causing the rotor to pass by the stator poles at different times. A polyphase system consists of a set of equal voltages with the same frequency, arranged to have a uniform phase difference between adjacent electromagnetic fields (EMF). While polyphase systems can be two-phase, three-phase, or even six-phase, three-phase systems are the most common.
Three-phase systems, often referred to simply as polyphase systems, deliver approximately 1.5 times more output than single-phase systems. Additionally, the current in a polyphase system remains constant, in contrast to the pulsating current of a single-phase system.
A synchronous AC motor operates with its shaft rotating at the same frequency as the current supply, with the rotation period corresponding to an integer multiple of AC cycles. The synchronous speed is constant, and this is the speed at which the motor generates electromotive force.
The speed of a synchronous motor remains unaffected by variations in load, meaning that changes in load do not influence the motor's speed. Unlike self-starting motors, synchronous motors are not self-starting; they require an external method to reach synchronous speed before they can function properly.
Reluctance motors are single-phase motors that operate with a precisely defined rotating magnetic field, but they do not have a synchronous speed. These motors utilize reluctance torque, which is a type of torque found in iron-based devices. The motor generates torque by creating an internal magnetic field through the interaction with an external field. For reluctance torque to be effective, the rotor must align with the magnetic field at specific angles relative to the outer field's poles.
The distinctive feature of a hysteresis motor lies in its rotor, which incorporates semi-permanent magnetic material. Torque is generated by the magnetic flux lagging behind the external magnetizing force, with the eddy currents contributing to the motor's torque. Hysteresis motors are known for their precise speed control, minimal vibration, and quiet operation.
A hysteresis motor features a core made of non-magnetic material, coated with a special magnetic layer. The rotor is a smooth cylinder without windings. The hysteresis ring, typically made of chrome or steel, exhibits a hysteresis loop, contributing to the motor's unique characteristics.
A repulsion motor is a type of single-phase motor that operates based on the repulsion between similar magnetic poles. In addition to the rotor and stator, it features a commutator brush assembly. The rotor is equipped with a distributed DC winding connected to the commutator, similar to a DC motor, with carbon brushes short-circuited on themselves.
As the rotor circuit is short-circuited, it receives power from the stator through transformer action. The fundamental operating principle of a repulsion motor involves the repulsion between like poles, where north poles repel each other, as do south poles.
An asynchronous motor generates rotational motion through an induced current in its rotor. It is the most common type of AC motor, relying on an AC current supplied to the stator for its operation. All the power required for the motor is supplied to the stator; the rotor receives its power through induction.
The rotor's induction occurs due to its proximity to the stator's electromagnetic field, which induces a magnetic field in the rotor, causing it to spin. Asynchronous motors do not use brushes or slip rings, making them highly efficient and reliable. Their simplicity and rugged design make them suitable for heavy-duty applications.
The National Electrical Manufacturers Association (NEMA) establishes standards for motors, as outlined in NEMA Standard Publication No. MG 1. These standards are based on best practices and manufacturing guidelines for electrical equipment. AC motors designed for specialized applications are not covered by NEMA classifications and are referred to as "above NEMA" motors.
Induction motors are categorized according to their electrical design. NEMA has defined five classifications for AC motors: A, B, C, D, and E. Each classification describes specific characteristics and performance attributes:
The table below provides a general description of the various NEMA classifications and their typical uses.
| NEMA Classifications | |
|---|---|
| Motor A | A motors are commonly used for fans, pumps, and blowers where large starting torques aren't necessary and the motor doesn't need to support a large load. |
| Motor B | B motors are commonly used for fans, pumps, and blowers where large starting torques aren't necessary and the motor doesn't need to support a large load. |
| Motor C | C motors are best used in machines that require the motor start under a load such as conveyors, compressors, crushers, stirring motors, agitators, and reciprocating pumps. |
| Motor D | D motors are used for machinery with high peak loads such as elevators, hoists, oil-well pumping, wire drawing motors, and punch presses. |
| Motor E | E motors can be used in similar applications to A and B motors like fans, pumps, motor-generator sets, and blowers with low starting torque. |
AC motors have a wide range of applications, from powering household appliances to driving large machinery. Their low cost and high efficiency make them suitable for numerous uses. Wherever electrical motors are required, AC motors are often central to the application.
AC motors are more powerful than many other types of motors because they can generate greater torque with a strong current. They are available in various sizes, configurations, and strengths to meet the diverse power needs of different industries.
AC motors are versatile and adaptable, making them suitable for a wide range of applications due to their efficiency and quiet operation. Common uses for AC motors include pumps, water heaters, lawn and garden equipment, ovens, and off-road motorized equipment.
AC induction motors are the most prevalent and widely used type of AC motor.
Three-phase AC motors are predominantly used in industrial settings. They are comprised of three main parts: the rotor, the stator, and the enclosure. The rotor and stator are the primary working components, while the enclosure protects the motor and houses its internal parts.
AC motors are widely used in industrial applications due to their strength, adaptability, durability, and simple design, which makes them easy to maintain. They are capable of efficiently powering a range of equipment, from industrial pumps to home appliances, and can easily adapt to different functions.
The stator is the stationary component of an AC motor and serves as the motor's electromagnetic circuit. It is constructed from laminations—thin metal sheets stacked to form a hollow cylinder. Using laminations helps minimize energy loss by reducing eddy currents.
Stator windings consist of copper wire wound around the stator's slots. The number of slots is determined by the number of phases in the power supply. For example, a three-phase motor typically features six slots with three pairs of coil windings, each pair offset by 120 degrees. The term "winding" refers to the complete electromagnetic circuit formed by multiple identical coils. Generally, more coils result in smoother motor operation.
The phase of a motor indicates how many electric currents energize the coils. In a three-phase motor, the coil count can be three, six, or twelve.
When the motor is powered on, the stator connects directly to the power source, creating an electromagnet from the coils and stator.
The rotor is the rotating part of an AC motor. The squirrel cage rotor is the most common type. Similar to the stator, it is constructed by stacking laminations to create a cylindrical shape. The squirrel cage is formed by inserting evenly spaced conductor bars into the rotor's slots. These bars are typically made of aluminum or copper.
After the laminations are stacked and the conductor bars are in place, a steel shaft is pressed into the center of the assembly.
Bearings in an AC motor serve to support and position the rotor, maintain a small air gap, and transfer loads to the motor. They are designed to operate efficiently at various speeds while minimizing friction.
AC motors use several types of bearings, including ball bearings and roller bearings. The lifespan of a bearing is influenced by the number of revolutions or operating hours it can withstand, as well as operating conditions and lubrication.
The air gap is the space between the rotor and stator in an AC motor, and it is a crucial element of the motor's design. This gap must be large enough to prevent contact between the rotor and stator surfaces, accounting for dimensional tolerances, bearing looseness, and movement. To optimize motor efficiency, the air gap should be as small as possible, as larger gaps require more power to achieve adequate magnetization.
In AC motors, heat accumulates in the windings, necessitating an internal cooling system. Within the motor's enclosure, a fan is mounted on the rotor shaft at the opposite end from the axle that drives the connected machine. This fan draws in cool air and directs it across the windings, while hot air is expelled out the rear of the enclosure.
The enclosure of an AC motor safeguards the internal components from dust, liquids, and other contaminants, provides convective cooling, and ensures electrical safety. The level of protection offered largely depends on the quality of materials used in the enclosure's construction. Both NEMA and IEC set specifications for enclosure designs. An ingress protection (IP) code, such as IP65, classifies the level of protection provided; a higher IP code indicates better protection.
Some enclosures are designed with heat fins on the sides and do not include a fan for cooling, while totally enclosed fan-cooled enclosures feature a fan mounted on the rotor shaft.
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