Difference Between Synchronous And Induction Motors Pdf

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difference between synchronous and induction motors pdf

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All rotary electric motors, ac, and dc, operate because of the interaction of two magnetic fields. Rotation is caused by the interaction between the two fields. In a simple dc motor, there is a rotating magnetic field whose polarity is reversed every half turn by means of a brush-commutator combination. Brushes — basically conductive carbon rods which brush against the conductors on the rotor as they turn — also serve the purpose of getting the electrical current into the spinning armature.

Induction motor

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. Three-phase squirrel-cage induction motors are widely used as industrial drives because they are self-starting, reliable and economical.

Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although traditionally used in fixed-speed service, induction motors are increasingly being used with variable-frequency drives VFD in variable-speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque centrifugal fan, pump and compressor load applications. Squirrel-cage induction motors are very widely used in both fixed-speed and variable-frequency drive applications.

By manually turning switches on and off, Walter Baily demonstrated this in , effectively the first primitive induction motor. The first AC commutator-free three-phase induction motors were independently invented by Galileo Ferraris and Nikola Tesla , a working motor model having been demonstrated by the former in and by the latter in Tesla applied for US patents in October and November and was granted some of these patents in May In April , the Royal Academy of Science of Turin published Ferraris's research on his AC polyphase motor detailing the foundations of motor operation.

George Westinghouse , who was developing an alternating current power system at that time, licensed Tesla's patents in and purchased a US patent option on Ferraris' induction motor concept. Westinghouse employee C. Scott was assigned to assist Tesla and later took over development of the induction motor at Westinghouse.

Lamme developed a rotating bar winding rotor. Induction motor improvements flowing from these inventions and innovations were such that a horsepower induction motor currently has the same mounting dimensions as a 7. In both induction and synchronous motors , the AC power supplied to the motor's stator creates a magnetic field that rotates in synchronism with the AC oscillations. Whereas a synchronous motor's rotor turns at the same rate as the stator field, an induction motor's rotor rotates at a somewhat slower speed than the stator field.

The induction motor stator's magnetic field is therefore changing or rotating relative to the rotor. This induces an opposing current in the induction motor's rotor, in effect the motor's secondary winding, when the latter is short-circuited or closed through an external impedance. The induced currents in the rotor windings in turn create magnetic fields in the rotor that react against the stator field. The direction of the magnetic field created will be such as to oppose the change in current through the rotor windings, in agreement with Lenz's Law.

The cause of induced current in the rotor windings is the rotating stator magnetic field, so to oppose the change in rotor-winding currents the rotor will start to rotate in the direction of the rotating stator magnetic field. The rotor accelerates until the magnitude of induced rotor current and torque balances the applied mechanical load on the rotation of the rotor.

Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates slightly slower than synchronous speed. The difference, or "slip," between actual and synchronous speed varies from about 0.

As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field induced in the rotor and the rotation rate of the stator's rotating field is called "slip".

Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as "asynchronous motors". An induction motor can be used as an induction generator , or it can be unrolled to form a linear induction motor which can directly generate linear motion.

The generating mode for induction motors is complicated by the need to excite the rotor, which begins with only residual magnetization. In some cases, that residual magnetization is enough to self-excite the motor under load.

Therefore, it is necessary to either snap the motor and connect it momentarily to a live grid or to add capacitors charged initially by residual magnetism and providing the required reactive power during operation. Similar is the operation of the induction motor in parallel with a synchronous motor serving as a power factor compensator. A feature in the generator mode in parallel to the grid is that the rotor speed is higher than in the driving mode.

Then active energy is being given to the grid. To determine the number of coil groups per phase in a 3-phase motor, count the number of coils, divide by the number of phases, which is 3.

The coils may span several slots in the stator core, making it tedious to count them. For a 3-phase motor, if you count a total of 12 coil groups, it has 4 magnetic poles. For a pole 3-phase machine, there will be 36 coils. The number of magnetic poles in the rotor is equal to the number of magnetic poles in the stator. Since the short-circuited rotor windings have small resistance, even a small slip induces a large current in the rotor and produces significant torque.

The typical speed-torque relationship of a standard NEMA Design B polyphase induction motor is as shown in the curve at right. Suitable for most low performance loads such as centrifugal pumps and fans, Design B motors are constrained by the following typical torque ranges: [30] [b].

As the load torque increases beyond breakdown torque the motor stalls. There are three basic types of small induction motors: split-phase single-phase, shaded-pole single-phase, and polyphase. A single phase induction motor requires separate starting circuitry to provide a rotating field to the motor. The normal running windings within such a single-phase motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.

In certain smaller single-phase motors, starting is done by means of a copper wire turn around part of a pole; such a pole is referred to as a shaded pole. The current induced in this turn lags behind the supply current, creating a delayed magnetic field around the shaded part of the pole face.

This imparts sufficient rotational field energy to start the motor. These motors are typically used in applications such as desk fans and record players, as the required starting torque is low, and the low efficiency is tolerable relative to the reduced cost of the motor and starting method compared to other AC motor designs. Larger single phase motors are split-phase motors and have a second stator winding fed with out-of-phase current; such currents may be created by feeding the winding through a capacitor or having it receive different values of inductance and resistance from the main winding.

In capacitor-start designs, the second winding is disconnected once the motor is up to speed, usually either by a centrifugal switch acting on weights on the motor shaft or a thermistor which heats up and increases its resistance, reducing the current through the second winding to an insignificant level.

The capacitor-run designs keep the second winding on when running, improving torque. A resistance start design uses a starter inserted in series with the startup winding, creating reactance.

Self-starting polyphase induction motors produce torque even at standstill. Available squirrel-cage induction motor starting methods include direct-on-line starting, reduced-voltage reactor or auto-transformer starting, star-delta starting or, increasingly, new solid-state soft assemblies and, of course, variable frequency drives VFDs. Polyphase motors have rotor bars shaped to give different speed-torque characteristics.

The current distribution within the rotor bars varies depending on the frequency of the induced current. At standstill, the rotor current is the same frequency as the stator current, and tends to travel at the outermost parts of the cage rotor bars by skin effect. The different bar shapes can give usefully different speed-torque characteristics as well as some control over the inrush current at startup. Although polyphase motors are inherently self-starting, their starting and pull-up torque design limits must be high enough to overcome actual load conditions.

In wound rotor motors, rotor circuit connection through slip rings to external resistances allows change of speed-torque characteristics for acceleration control and speed control purposes. Before the development of semiconductor power electronics , it was difficult to vary the frequency, and cage induction motors were mainly used in fixed speed applications. Applications such as electric overhead cranes used DC drives or wound rotor motors WRIM with slip rings for rotor circuit connection to variable external resistance allowing considerable range of speed control.

However, resistor losses associated with low speed operation of WRIMs is a major cost disadvantage, especially for constant loads. The speed of a pair of slip-ring motors can be controlled by a cascade connection, or concatenation. The rotor of one motor is connected to the stator of the other. The most common efficient way to control asynchronous motor speed of many loads is with VFDs. With scalar control , only the magnitude and frequency of the supply voltage are controlled without phase control absent feedback by rotor position.

Scalar control is suitable for application where the load is constant. Vector control allows independent control of the speed and torque of the motor, making it possible to maintain a constant rotation speed at varying load torque. But vector control is more expensive because of the cost of the sensor not always and the requirement for a more powerful controller.

The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles.

Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Single-phase motors require some mechanism to produce a rotating field on startup.

Cage induction motor rotor's conductor bars are typically skewed to avoid magnetic locking. Since an open, drip proof ODP motor design allows a free air exchange from outside to the inner stator windings, this style of motor tends to be slightly more efficient because the windings are cooler. At a given power rating, lower speed requires a larger frame.

The method of changing the direction of rotation of an induction motor depends on whether it is a three-phase or single-phase machine. In the case of three-phase, reversal is straightforwardly implemented by swapping connection of any two phase conductors.

In a single-phase split-phase motor, reversal is achieved by changing the connection between the primary winding and the start circuit. Some single-phase split-phase motors that are designed for specific applications may have the connection between the primary winding and the start circuit connected internally so that the rotation cannot be changed.

Also, single-phase shaded-pole motors have a fixed rotation, and the direction cannot be changed except by disassembly of the motor and reversing the stator to face opposite relative to the original rotor direction. The power factor of induction motors varies with load, typically from around 0. For economic and other considerations, power systems are rarely power factor corrected to unity power factor. Various regulatory authorities in many countries have introduced and implemented legislation to encourage the manufacture and use of higher efficiency electric motors.

There is existing and forthcoming legislation regarding the future mandatory use of premium-efficiency induction-type motors in defined equipment. For more information, see: Premium efficiency. Many useful motor relationships between time, current, voltage, speed, power factor, and torque can be obtained from analysis of the Steinmetz equivalent circuit also termed T-equivalent circuit or IEEE recommended equivalent circuit , a mathematical model used to describe how an induction motor's electrical input is transformed into useful mechanical energy output.

The equivalent circuit is a single-phase representation of a multiphase induction motor that is valid in steady-state balanced-load conditions. Paraphrasing from Alger in Knowlton, an induction motor is simply an electrical transformer the magnetic circuit of which is separated by an air gap between the stator winding and the moving rotor winding. The following rule-of-thumb approximations apply to the circuit: [53] [54] [55].

Linear induction motors, which work on the same general principles as rotary induction motors frequently three-phase , are designed to produce straight line motion. Uses include magnetic levitation , linear propulsion, linear actuators , and liquid metal pumping. From Wikipedia, the free encyclopedia. For the electric car company, see Tesla, Inc.

Difference between Synchronous Motor and Induction Motor

In the electrical systems, we use either in industries, power stations or domestic needs, motors and generators have become a common thing. With the demand for high energy efficient and less power consuming systems, the invention of new models of these electrical devices is seen. The basic calculating factor for motors and generators reliable operation is the Power factor. It is the ratio of applied power over the required power. Usually, the total powered consumed at the industries and factories are calculated based on the power factor. So, power factor should always be maintained at unity.


Difference in working · Synchronous motor: Stator poles rotate at the synchronous speed (Ns) when fed with a three phase supply. The rotor is fed with a DC.


Synchronous Motors vs. Induction Motors - What's the Difference?

AC motors are divided into two types, synchronous motors and asynchronous motors which are also called induction motors. The biggest difference between synchronous motors and asynchronous motors induction motors is whether the speed of rotor is consistent with the speed of the rotating magnetic field in the stator. Furthermore, there are big differences specific to the performance parameters and applications between the two.

Christian Cavallo. Electric motors come in hundreds of sizes, shapes, and varieties, and the sheer amount of choices can be paralyzing when looking for the best option. The first step in finding any motor is determining its power source; is it powered by AC current, or DC? However, both of these categories still contain many kinds of machines, so this article will help further differentiate the AC motor class our article on brushless vs.

Skip to Main Content. A not-for-profit organization, IEEE is the world's largest technical professional organization dedicated to advancing technology for the benefit of humanity. Use of this web site signifies your agreement to the terms and conditions. Experimental comparison of induction and synchronous reluctance motors performance Abstract: The aim of this paper is to investigate and compare the torque behavior of induction motors and transverse laminated synchronous reluctance motors. Each induction motor is compared with a synchronous reluctance motor, that has the same stator lamination and winding but, obviously, different rotor.

Difference between Synchronous motor and Induction motor

Synchronous Motor : Types and Applications

Difference Between Induction and Synchronous Motor is explained with the help of various factors, like the type of excitation used for the machine. The speed of the motor, starting and operation, the efficiency of both the motors, its cost, usage, applications, and frequency. An Induction Motor is also known as Asynchronous Motor. It is so called because it never runs at synchronous speed.

Recieve free updates Via Email! Home Electrical machines Power system Ask a question Contact electricaleasy. Share: Facebook Twitter Linkedin. AC motors can be divided into two main categories - i Synchronous motor and ii Asynchronous motor. An asynchronous motor is popularly called as Induction motor.

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. Three-phase squirrel-cage induction motors are widely used as industrial drives because they are self-starting, reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although traditionally used in fixed-speed service, induction motors are increasingly being used with variable-frequency drives VFD in variable-speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque centrifugal fan, pump and compressor load applications. Squirrel-cage induction motors are very widely used in both fixed-speed and variable-frequency drive applications.

Difference between Synchronous Motor and Induction Motor

Reader Interactions

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