Induction Motor and Alternator Explored
Electrical motors today have become a crucial part of our modern world. Today, the study and advancement of these devices have diversified like never before, creating huge opportunities for aspiring engineers. Some expert reading about AC/DC motors is presented here through many linked articles.
Everything You Wanted Know About AC Induction Motors
Motors are devices able to convert electrical energy to mechanical energy. Our industries primarily rely heavily on these devices, especially the AC motors. In fact, AC motors were probably the main machines that brought about the industrial revolution, without which even the modern world would feel completely handicapped. The subject and the concept of AC motors is rather complex and definitely needs a thorough and a comprehensive study.
The following links take you through a systematic learning course, beginning with the entire classification of AC motors, and proceeds with an elaborate discourse regarding various parameters associated with AC induction motors.
What are AC Induction Motors?
Basically an induction motor, also referred to as asynchronous motor, is a kind of AC motor in which the rotor acquires the power through the process of magnetic induction without incorporating slip rings. The main application of this category of motors can be seen in industrial drive mechanisms due to their more versatile operating principles.
Looking at Synchronous Motors
Unlike induction motors where the toque is acquired through the interaction of the induced current in the rotor and the rotating magnetic field, in synchronous motors it is achieved through the rotating magnetic fields from the stator enforced over the rotor. This results in a steady rate of rotation, and hence the name. Fundamentally, synchronous motors employ three types of rotors: squirrel cage rotors, slip ring rotors, and solid core rotors.
What’s the Difference between Synchronous and Induction Motors
The fundamental difference between these two motors is that the speed of the rotor relative to the speed of the stator is equal for synchronous motors, while the rotor speed in induction motors is less than its synchronous speed. This is why induction motors are also known as asynchronous motors.
How Synchronous AC Motors Work.
A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain multiphase AC electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field of any AC motor. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.
How to Start Synchronous Motors Smoothly Through Electronics.
Soft starting, as the name indicates, is a smooth starting of a motor without any moving contactors. It employs a electronic circuit for starting having thyristors on a common circuit board. Soft starting is considered an efficient electronic switching technique.
Why We Need Soft Starting for Synchronous Motors?
Let us take a case where the synchronous motor is started initially as an induction motor and brought to synchronous condition later. In this, when a synchronous motor is connected directly to a three phase AC supply without any starter, a very large stator current flows through the stator winding. The stator current drawn by the motor during startup will be up to five to eight times the full load current. This is due to the fact that the rotor cuts a maximum rate of flux, creating a large induced current in the rotor of the motor due to large induced EMF. Because of this increased stator and rotor current, the supply power factor will be very low typically about 0.2 lagging.
This rises to 0.5 lagging on no load then to about 0.85 lagging on full load. This starting surge current reduces as the motor accelerates up to its running speed.
The motor operating at low power factor (0.5 lagging) at the light load condition reduces the efficiency as the supply current is relatively high causing significant I2R resistive losses (copper losses). In order to attain good efficiency from the motor, the power factor has to be improved.
The only way to improve the power factor of the motor at light load is to reduce the supply voltage given to the motor. This can be achieved with the applicable voltage controller. A type of voltage controller working on electronic circuits employed to control the supply voltage of the motor is called a soft starter.
The soft starter, which is an electronic voltage controller, is also known as an energy manager. It can match the supply voltage to the startup and load conditions. Such a controller aims to maintain the operating power factor as high as possible to minimize supply current and power losses. An important thing to note is that this type of voltage controller does not control the speed of the motor; that can be controlled by frequency of the supply. However this speed control is only possible in induction motor whereas the synchronous motor is a constant speed motor that runs at its synchronous speed.
Soft Starting of Synchronous Motor
The soft starters employed for starting synchronous motor have back to back thyristors or triacs in each three phase AC supply line to the motor. These thyristors are gated to delay turn-on with each AC half cycle, and the delayed switching applies a reduced average AC voltage to the motor, which means it supplies gradually increasing AC voltage during start up. The applied motor voltage is gradually ramped up by the starter software program until the full voltage level is reached. Once the motor achieves the rated speed, the thyristor allows the rated current to flow in the lines to the motor and hence the full voltage is applied there by achieving maximum efficiency. Strictly speaking, the electronic switching circuit is bypassed for normal running. It also employs a gate control circuit which controls the thyristor function.
Strictly speaking, the electronic switching circuit is bypassed for normal running. It also employs a gate control circuit which controls the thyristor function. Start and stop commands can be given from this control circuit. The thyristor operates according to the command given to the gate control circuit.
How Squirrel Cage Induction Motors Operate.
The field windings in the stator of an induction motor set up a rotating magnetic field through the rotor. The relative motion between this field and the rotation of the rotor induces electric current in the conductive bars.
Where is the Squirrel in the Motor?
A 3 phase squirrel cage induction motor is a type of three phase induction motor which functions based on the principle of electromagnetism. It is called a 'squirrel cage' motor because the rotor inside of it – known as a 'squirrel cage rotor' – looks like a squirrel cage.
How do Slip Ring Motors Function?
The main function of a slip ring is to simplify the operation of a system or improve mechanical performance by transferring the electrical power and/or signals from the stationary part of the machine out to where it is needed on the rotating part.
How do Slip Ring Motors Start?
Slip-ring motors are invariably started by rotor resistance starting. Slip-ring motors are invariably started by rotor resistance starting. In this method, a variable star-connected rheostat is connected in the rotor circuit through slip rings and full voltage is applied to the stator winding.
Controlling Speed of Slip Ring Motors.
There are two mechanisms to control speed, the first is through rotor rheostat control and the second is through cascade control. The objective of speed control can be achieved through the rheostat which in the first place is used to induce resistance in the system and set up the starting phase for motor slip ring.
Other Forms of AC Motors
AC motors can be further classified into single phase motors, which may consist of magnetic repulsion motors widely used in locomotives, and other traction based engines. Our residential ceiling fans or the motors involving a capacitor start mechanism also come under AC motors.
How do Single Phase Motor Work?
Single-phase motors use both stators and rotors like other AC motors, though they work much differently. In three-phase motors, the 120 degrees of phase separation between the three AC currents running through the stator windings produces a rotating magnetic field; however, the magnetic field made by only a single phase “pulsates” between 2 motor poles, as there is only one AC current producing two possible magnetic field states (the AC current has two sinusoidal peaks, where the magnetic fields will be equal but opposite in orientation, or “up-down”). This approximates a rotating field, but not completely. These motors must be given an initial “shove”, or feel a force “out-of-phase” with the stator phase in order for initial movement of the rotor to occur. The stationary rotor will not feel any effects from this pulsating, “up-down” magnetic field if it not already moving, as the up-down magnetic forces cancel each other out perfectly. Motor starters solve this issue by adding an out-of-phase influence (auxiliary windings, capacitors, etc.), which then creates a simulated rotating magnetic field to start the motor. More information on these starters can be found in our article on motor starters.
How do Magnetic Repulsion Motors Work?
The basic principle behind the working of repulsion motor is that “similar poles repel each other.” This means two North poles will repel each other. Similarly, two South poles will repel each other. When the repulsion motor winding is supplied with single-phase AC, it produces a magnetic flux along the direct axis.
How do Universal Motors Work?
A universal motor is a special type of motor that runs on both AC and DC power supplies. Universal motors are series-wound (the armature and field windings are in series). The series connection allows them to generate high torque; hence the universal motors are generally built into the device they are meant to drive.
Controlling Speed in Universal Motors.
Speed control of Universal Motors can be done by:
magnitude control.
Field flux control.
Frequency control.
Phase angle control.
How Capacitor Start Motors Work.
A start capacitor stays in the circuit long enough to rapidly bring the motor up to a predetermined speed, which is usually about 75% of the full speed, and is then taken out of the circuit, often by a centrifugal switch that releases at that speed. Afterward the motor works more efficiently with a run capacitor.
How Shaded-Pole Single Phase Motors Work.
A shaded-pole motor is a small squirrel-cage motor in which the auxiliary winding is composed of a copper ring or bar surrounding a portion of each pole. When single phase AC supply is applied to the stator winding, due to shading provided to the poles, a rotating magnetic field is generated.
Understanding AC Motor Speed Controllers and Conditions
All mechanical devices must involve a control feature through which its force can be regulated as desired for the particular application. AC motors accommodate and incorporate this facility through various external electrical and electronic devices, especially designed for the purpose. Also since AC motor efficiency can be critical with the input frequency, they also require strict monitoring through respective devices.
Measuring Motor Speeds.
How to Calculate Motor RPM. To calculate RPM for an AC induction motor, you multiply the frequency in Hertz (Hz) by 60 — for the number of seconds in a minute — by two for the negative and positive pulses in a cycle. You then divide by the number of poles the motor has: (Hz x 60 x 2) / number of poles = no-load RPM.
The Effect of Frequency on AC Motors.
A frequency higher than the rated frequency usually improves the power factor but decreases locked-rotor torque and increases the speed and friction and windage loss. At a frequency lower than the rated frequency, the speed is decreased, locked-rotor torque is increased, and power factor is decreased.
How Variable Frequency Drive Units Work.
A variable frequency drive takes in fixed AC voltage and frequency into a rectifier bridge to convert the voltage to DC, uses a capacitor bank and inductors to filter the DC, then inverts the DC voltage back to AC and sends it out to the motor at the frequency desired.
How Dimmer Switches Work.
The dimmer switch works by simply cutting the amount of electricity that is flowing through the circuit into the lights. That means, instead of drinking electricity, it is sipping it, and sending less to your lights.
How to Build a Dimmer Switch for controlling Ceiling Fans
DC Motors and Their Control Theory Explained
DC motors were once more popular than AC induction motors because of the ease with which their speed could be controlled. However, with the advent of modern electronics, AC motors today can be controlled as efficiently as their DC counterparts, making them the better favorites. Yet still, DC motors can be found applied widely in many different fields. Mainly DC motors are used for lighter applications, like in RC models, solar trackers, toys, computers hard drives, automotive actuators, CD/DVD player drives, etc.
Being smaller in size these motors have become the favorites with the hobbyists for making various robotic models. This has also led to the development of improved PWM controlled circuits for controlling these motors precisely as per the specs of the application.
Importantly DC motors are classified as brushed and brushless types, the latter being more efficient due to the involvement of a frictionless operating principle. A better view of these types of motors is explained through the following links along with a few interesting motor-control circuits that can be built at home:
How do DC Series Motors Work
A series DC motor converts electric energy to mechanic energy based on the electromagnetic principle. In this type of DC motor, a power supply terminal sits at one end of the armature and field coils. By applying the voltage, power starts in these terminals and passes through armature and field windings.
How do Stepper Motors Work
The stepper motor converts a pulsing electrical current, controlled by a stepper motor driver, into precise one-step movements of this gear-like toothed component around a central shaft. Each of these stepper motor pulses moves the rotor through one precise and fixed increment of a full turn.
How do Shunt-Wound DC Motors Work
The field winding in shunt DC motors is made of many windings of thin wire, to both increase the magnetic field's strength and limit the current through the coil. By doing so, the current is reduced through the field coil and thus increases in the armature (remember, the current is shared when in parallel).
Using electric Motors in Hybrid Vehicles
Hybrid electric vehicles are powered by an internal combustion engine and one or more electric motors, which uses energy stored in batteries. A hybrid electric vehicle cannot be plugged in to charge the battery. Instead, the battery is charged through regenerative braking and by the internal combustion engine.
Simple DC Motor Control
We can control the speed of DC motor manually or through an automatic control device.
Thus, the speed of a DC motor can control in three ways:
By varying the flux, and by varying the current through field winding.
By varying the armature voltage, and the armature resistance.
Through the supply voltage.
What is Back EMF in BLDC Motors?
In simpler terms, the EMF resists the motor's natural movement and is referred to as a “back” EMF. For a given motor of fixed magnetic flux and number of windings, the magnitude of the EMF is proportional to the angular velocity of the rotor.
How to Adjust Your PC Fan Speed
Use the arrow keys on your keyboard to scroll through the BIOS menu to the "Monitor," "Status" or other similarly named submenu (this will also vary slightly by manufacturer). Select the "Fan Speed Control" option from the submenu to open the fan controls
How Motors are Extensively Used for Generating Electricity
Very interestingly the definition of an electrical motor completely gets topsy turvy when these are employed or configured as electricity generators. The conversion of electrical energy into mechanical rotational energy through magnetic induction in the earlier mode now transforms and becomes responsible for generating electricity through the conversion of applied mechanical energy into electrical energy.
The device is popularly known as an alternator, since the generated electricity is in the form of AC or alternating current.
Comments
Post a Comment