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applications of single phase induction motor

arabische frauen in berlin kennenlernen Single phase frauen aus thailand kennenlernen induction motors are used in a wide range of sie sucht ihn markt düsseldorf applications where only single phase single freizeit treff dresden supply is available.

These are manufactured in fractional kilowatt range to meet the requirements of various applications such as ceiling fans, food mixers, refrigerators, vacuum cleaners, portable drills, hair driers, etc.

Let us discuss various types of single phase induction motors in brief.

Table of Contents

Introduction to Single Phase Induction Motors

As the name suggests, these motors operate on single phase AC supply. Single phase induction motors are extensively used in low power applications such as domestic appliances as mentioned above.

These are small in size and less expensive to manufacture. These motors are also called as fractional KW motors, because most of these motors are constructed in fractional kilo-watt capacity.

Single phase induction motors consist of two main single freizeit treff dresden parts; stator and rotor. The construction of these motors is more or less similar to a three-phase squirrel-cage induction motor.

The stator is a stationary part and it has laminated construction, which is made up of stampings. These stampings consists of slots on its periphery to carry the stator winding. This winding is excited with a single phase AC supply.

The rotor is a rotating part and its construction is of squirrel cage type. The rotor consists of uninsulated aluminum or copper single freizeit treff dresden bars which are placed in the slots.

These rotor bars are permanently shorted at both ends with the help of end rings as shown in figure.

Squirrel cage rotar of a single phase induction motor
There is no physical connection between the stator and rotor, but there is a small and uniform gap between them.

The rotor acts as a conductor which when placed in the stator magnetic field, an emf is induced in it, produces its own magnetic field which further interacts with stator field to produce the torque.

Single phase induction motor constructionWhenever a single phase AC supply is given to the stator winding, an alternating magnetic field is produced around the stator.

Due to the pulsating nature of the field which reverses for every half-cycle, cannot produce rotation in a stationary squirrel cage rotor.

In case of three phase induction motor, the field produced by the supply is of rotating type and hence they are self starting motors.

But in case of single phase motors, the field produced by the stator is not rotating (but alternating only) and hence single phase motors are not self starting.

But, if the rotor is rotated by any other means (by hand or any tool), the induced currents in the rotor will assist with stator currents to produce revolving field. This field causes the motor to run in the direction it is started even with a single winding.

However, it is not possible to give initial rotation every time externally if the motors are attached to loads. This problem can be avoided by converting single phase motor into a two-phase motor temporarily in order produce revolving flux. This is achieved by providing a starting winding in addition to main or running winding.

The auxiliary or starting winding is made highly resistive whereas the main or running winding is made highly inductive.

Due to the large phase difference between these two, the torque produced by the rotor is high enough to start it. Once the motor reaches 75 percent of its speed, the auxiliary winding may be disconnected by a centrifugal switch and the motor able to run on a single main winding.

Single phase induction motors are used primarily for domestic and light-industrial applications where three-phase supply is generally not available.

Types of Single Phase Induction Motors

As mentioned above that, due to the rotating magnetic field of the stator, the induction motor becomes self starting. There are many methods of making a single phase induction motor as self starting one.

Based on the starting method, single phase induction motors are basically classified into the following types.

  1. Split-phase motor
  2. Capacitor start motor
  3. Permanent capacitor run motor
  4. Capacitor start capacitor run motor
  5. Shaded pole motor

The rotating magnetic field is produced when there are minimum two alternating fluxes, having a phase difference between them.

The resultant of these two fluxes produces a rotating flux which rotates in space in one particular direction. So in all the above methods or say types of induction motors, the additional flux other than main flux should have a certain phase difference with respect to main or stator flux.

If the phase difference is more, starting torque will be more. So the starting torque of the motor depends on the rotating magnetic field and thereby, additional means (whether it is an auxiliary winding or anything).

Once the motor picks up the speed, this additional winding is removed from the supply. This is the basic principle followed by all these types of single phase induction motors.

Let us discuss these types of motors in brief.

Split Phase Induction Motor

Split phase motors

This is one of the most widely used types of single phase induction motors. The essential parts of the split phase motor include main winding, auxiliary winding and a centrifugal switch.

This is the simplest arrangement to set up a rotating magnetic field by providing two winding on the same stator core as shown in figure.

Split phase induction motor circuit diagram

The auxiliary or starting winding carries a series resistance such that its impedance becomes highly resistive in nature.

It is not wound identical to the main winding but contains fewer turns of much smaller diameter as compared to main winding.

This will reduce the amount of start current arabische frauen in berlin kennenlernen lags the voltage. The main winding is inductive in nature in such that current lags the voltage by some angle. This winding is designed for the operation of 75 % of synchronous speed and above.

These two windings are connected in parallel across the supply. Due to the inductive nature, current through main winding lags the supply voltage by a large angle while the current through starting winding is almost in phase with voltage due to resistive nature.

Hence there exists a phase difference between these currents and thereby phase difference between the fluxes produced by these currents. The resultant of these two fluxes produce rotating magnetic field and hence the starting torque.

The centrifugal switch is connected in series with the starting winding. When the motor reaches 75 to 80 percent of synchronous speed, the centrifugal switch is opened mechanically and thereby auxiliary winding is out of the circuit. Therefore, the motor runs sueddeutsche zeitung bekanntschaften er sucht sie only with main winding.

Split phase motors give poor starting torque due to small phase difference between main and auxiliary currents. Also, the power factor of these motors is poor. These are mainly used for easily started loads such as blowers, fans, washing machines, grinders, etc.

Capacitor Start Induction Motor

This motor is similar to the split phase motor, but in addition a capacitor is connected in series to auxiliary winding. This is a modified version of split phase motor.

Since the capacitor draws a leading current, the use of a capacitor increases the phase angle between the two currents (main and auxiliary) and hence the starting torque. This is the main reason for using a capacitor in single phase induction motors.

Here the capacitor is of dry-type electrolytic one which is designed only for alternating current use. Due to the inexpensive type of capacitors, these motors become more popular in wide applications.

These capacitors are designed for definite duty cycle, but not for continuous use. The schematic diagram of capacitor start motor is shown in figure below.

Capacitor Start Induction Motor circuit diagram

The operation of this motor is similar to the split phase motor where the starting torque is provided by additional winding.

Once the speed is picked up, the additional winding along with capacitor is removed from the circuit with the help of centrifugal switch. But, the difference is that the torque produced by this motor is higher than split phase motor due to the use of capacitor.

Due to the presence of a capacitor, the current through auxiliary winding will leads the applied voltage by some angle which is more than that of split case type.

Thus, the phase difference between main and auxiliary currents is increased and thereby starting torque.

The performance of this motor is identical to the split phase motor when it runs near full load RPM. Due to the capacitor, the inrush currents are reduced in this motor.

These motors have very high starting torque up to 300% full load torque. However the power factor is low at rated load and rated speed.

Owing to the high starting torque, these motors are used in domestic as well as industrial applications such as water pumps, grinders, lathe machines, compressors, drilling machines, etc.

Permanent Capacitor Induction Motor

This motor is also called as a capacitor run motor in which a low capacitor is connected in series with the starting winding and is not removed from the circuit even in running condition. Due to this arrangement, centrifugal switch is not required.
Here the capacitor is capable of running continuously. The low value capacitor produces more leading phase shift bur less total starting current as shown in phasor diagram.

Hence, the starting torque produced by these motors will be considerably lower than that of capacitor start motor. The schematic circuit of this motor is shown in figure below.

Permanent Capacitor Induction Motor circuit diagram

 

In this, the auxiliary winding and capacitor remains in circuit permanently and produce an approximate two phase operation at rated load point. This is the key strength of these motors.

This will result better power factor and efficiency. However, the starting torque is much lower in these motors, typically about 80 percent of full load torque.

Due to the continuous duty of auxiliary winding and capacitor, the rating of these components should withstand running conditions and hence permanent capacitor motor is more than equivalent split phase or capacitor start motors. These motors are used in exhaust and intake fans, unit heaters, blowers, etc.

Capacitor Start and Capacitor Run Induction Motor

These motors are also called as two-value capacitor motors. It combines the advantages of capacitor start type and permanent capacitor type induction motors.

This motor consists of two capacitors of different value of capacitance for starting and running. A high value capacitor is used for starting conditions while a low value is used for running conditions.

Capacitor start and capacitor run motorIt is to be noted that this motor uses same winding arrangement as capacitor-start motor during startup and permanent capacitor motor during running conditions. The schematic arrangement of this motor is shown in figure below.

Capacitor start and capacitor run motor circuit diagram

At starting, both starting and running capacitors are connected in series with the auxiliary winding. Thus the motor starting torque is more compared with other types of motors.

Once the motor reaches some speed, the centrifugal switch disconnects the starting capacitor and leaves the running capacitor in series with auxiliary winding.

Thus, both running and auxiliary windings remain during running condition, thereby improved power factor and efficiency of the motor.

These are the most commonly used single phase motors due to high starting torque and better power factor. These are used in compressors, refrigerators, air conditioners, conveyors, ceiling fans, air circulators, etc.

Shaded Pole Induction Motor

This motor uses entirely different technique to start the motor as compared with other motors so far we have discussed now.

This motor doesn’t use any auxiliary winding or even it doesn’t have a rotating field, but a field that sweeps across the pole faces is enough to drive the motor. So the field moves from one side of the pole to another side of the pole.

Although these motors are of small ratings, inefficient and have low starting torque, these are used in a variety of applications due to its outstanding features like ruggedness, low initial cost, small size and simple construction.

A shaded pole motor consists of a stator having salient poles (or projected poles), and a rotor of squirrel cage type. In this, stator is constructed in a special way to produce moving magnetic field.

Stator poles are excited with its own exciting coils by taking the supply from a single phase supply. A 4-pole shaded pole motor construction is given in below figure.

Shaded pole induction motor constructionEach salient pole is divided into two parts; shaded and un-shaded. A shading portion is a slot cut across the laminations at about one third distance from one edge, and around this a heavy copper ring (also called as shading coil or copper shading band) is placed.

This part where shading coil is placed is generally termed as shaded part of the pole and remaining portion is called as un-shaded part as shown in figure.

Let us discuss how the sweeping action of the field takes place.

When an alternating supply is given to the stator coils, an alternating flux will be produced. The distribution of flux in the pole face area is influenced by the presence of copper shading band.

Let us consider the three instants, t1, t2, and t3 of alternating flux for an half cycle of the flux as shown in figure.

Shaded pole motor working

  1. At instant t= t1, the rate of change of flux (rising) is very high. Due to this flux, an emf is induced in the copper shading band and as the copper shading band is shorted, current circulates through it. This causes current to create its own field.According to Lenz’s law, the current through copper shading band opposes the cause, i.e., rise of supply current (and hence rise of main flux). Therefore the flux produced by shading ring opposes the main flux. So there is weakening of flux in the shaded part while crowding of flux in un-shaded part. So the axis of overall flux shifts to non-shaded part of the pole as shown in the figure.
  2. At instant t=t2, the rate of rise of flux is almost zero, and hence very little emf is induced in the shaded band. It results negligible shaded ring flux and hence there is no much affect on distribution of main flux. Therefore, the distribution of flux is uniform and the overall flux axis lies at the center of the pole as shown in figure.
  3. At instant t=t3, the rate of change of flux (decreasing) is very high, and induces emf in copper shading band. The flux produced by the shading ring is now opposes the cause according to Lennz’s law. Here, the cause is decreasing flux, and opposing means its direction is same as that of main flux. Hence, this flux strengthens the main flux. So there will be crowding of flux in the shaded part compared to the non shaded part. Due to this overall flux axis shifts to the middle of shaded part.This sequence will repeat for negative cycle too and consequently it produce moving magnetic field for every cycle from non shaded part of the pole to shaded part of the pole. Due to this field, motor produces the starting torque. This starting torque is low about 40 to 50 percent of full load torque. Therefore, these motors are used in low starting torque applications such as fans, toy motors, blowers, hair dryers, photocopy machines, film projectors, advertising displays, etc.

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Electric Drives - AC Motors

(Description and Applications)

 

AC Motors

 

Polyphase Induction Motors

One third of the world's electricity consumption is used for running induction motors driving pumps, fans, compressors, elevators and machinery of various types. The AC induction motor is a common form of asynchronous motor whose operation depends on three electromagnetic phenomena:

  • Motor Action - When an iron rod (or other magnetic material) is suspended in a magnetic field so that it is free to rotate, it will align itself with the field. If the magnetic field is moving or rotating, the iron rod will move with the moving field so as to maintain alignment.
  • Rotating Field - A rotating magnetic field can be created from fixed stator poles by driving each pole-pair from a different phase of the alternating current supply.
  • Transformer Action - The current in the rotor windings is induced from the current in the stator windings, avoiding the need for a direct connection from the power source to the rotating windings.

The induction motor can be considered as an AC transformer with a rotating secondary winding.

 

  • Rotating Fields

Rotating magnetic fields are created by polyphase excitation of the stator windings. In the example below of a 3 phase motor, as the current applied to the winding of pole pair A (phase 1) passes its peak and begins to fall, the flux associated with the winding also begins to weaken, but at the same time the current in the winding of the next pole pair B (phase 2) and its associated flux is rising. Simultaneously the current through the winding of the previous pole pair C ( phase 3) and its associated flux will be negative and rising (towards positive). The net effect is that a magnetic flux wave is set up as the flux created by the stator poles rotates from one pole to the next, about the axis of the machine, at the frequency of the applied voltage. In other words, the rotating flux field appears to the stator as the north and south poles of a magnet rotating about the stator.

 

 

The magnitude of the rotating flux wave is proportional applied. Ignoring the effect of the back EMF set up by the induced currents in the rotor windings, the flux density B will be proportional to the applied voltage.

 

 

  • Transformer Action

The stator carries the motor primary windings and is connected to the power source. There are normally no external connections to the rotor which carries the secondary windings. Instead the rotor windings are shorted.

When a current flows in the stator windings a current is induced in the shorted secondary windings by transformer action. The magnitude of the rotor current will be proportional to the flux density B in the air gap (and the relative motion, called the, of the rotor with respect to the rotating field as we shall see below).

Torque is produced by the reaction between the induced rotor currents and the air-gap flux created by the stator currents.

Many rotor types are used. The most popular AC motors use "squirrel cage" rotors which are constructed from copper or aluminium bars fixed between conducting end rings which provide the short circuit path for the currents induced in the bars.

 

 

Since there are no connections to the rotating windings, the costly commutator can be eliminated and with it, a potential source of unreliability.

 

 

  • Torque Generation (Motor Action)

When the motor is first switched on and the rotor is at rest, a current is induced in the rotor windings (conductors) by. Another way of seeing this is that the relative motion of the rotating flux passing over the slower moving (initially static) rotor windings causes a current to flow in the windings by.

Once current is flowing in the rotor windings, the due to the Lorentz force on the conductors comes into effect. The reaction between the current flowing in the rotor conductors and the magnetic flux in the air gap causes the rotor to rotate in the same direction as the rotating flux as if it was being dragged along by the flux wave.

 

Similar to the DC machine, the torque in an induction motor T is proportional to the flux density B and the induced rotor current I. Thus

T = k1 BI

Where k1 is a constant depending on the number of stator turns, the number of phases and the configuration of the magnetic circuit.

 

The rotor speed builds up due to the motor action described above, but as it does so, the relative motion between the rotating stator field and the rotating rotor conductors is reduced. This in turn reduces the generator action and thus the current in the rotor conductors and the torque on the rotor. As the speed of the rotor approaches the speed of the rotating field, known as the, the torque on the rotor drops to zero. Thus the speed of an induction motor can never reach the synchronous speed.

 

An induction motor is therefore an asynchronous machine

 

  • Slip

The relative motion between the rotating field and the rotating rotor is called the slip and is given by:

S = Ns- Nr
         Ns

Where S is the slip, Ns is the synchronous speed in RPM, and Nr is the rotor speed.

 

Since the rotor current is proportional to the relative motion between the rotating field and the rotor speed, the rotor current and hence the torque are both directly proportional to the slip.

 

The rotor current is proportional to the rotor resistance. Increasing the rotor resistance will reduce the current and increase the slip; hence a form of speed and torque control is possible with. Increased rotor resistance also has the added benefit of reducing the input surge current and increasing starting torque on switch on, but all of these benefits are at the expense of more complex rotor designs and unreliable slip rings to give access to the rotor windings.

 

  • Speed

Synchronous speed in RPM is given by:

Ns = 120 (f)
           p

Where f is the powerline frequency in Hz and p is the number of poles per phase. p must be an even integer since for every north pole there is a corresponding south pole.

The following table shows motors speeds for motors with different numbers of poles working with different AC supply frequencies.

 

Corner

Rotor Speed (rpm)

Corner

Number of poles

2

4

6

8

10

12

Frequency 50 Hz

3000

1500

1000

750

600

500

Frequency 60 Hz

3600

1800

1200

900

720

600

 

The actual speed of the motor depends on the load it must drive. Increasing the load on the motor causes it to slow down increasing the slip. The motor speed will settle at an equilibrium speed when the motor torque equals the load torque. This occurs when the slip provides just enough current to deliver the required torque.

 

  • Speed Control
  • Pole Changing

Early machines were designed with multiple poles to facilitate speed control by pole changing. By switching in different numbers or combinations of poles a limited number of fixed speeds could be obtained.

 

  • Variable Rotor Resistance

The speed of induction motors can however be varied over a limited range by varying the rotor resistance as noted in the section on but only by using wound rotor designs negating many of the advantages of the induction motor.

 

  • Variable Frequency

Since motor speed depends on the speed of the rotating field, speed control can be effected by changing the frequency of the AC power supplied to the motor.

 

As in most machines the induction motor is designed to work with the flux density just below the over most of its operating range to achieve optimum efficiency.

The flux density B is given by:

B = k2V

         f

Where V is the applied voltage, f is the supply frequency and k2 is a constant depending on the shape and configuration of the stator poles.

In other words if the flux density is constant, the Volts per Hertz is also a constant. This is an important relationship and it has the following consequences.

  • For speed control, the supply voltage must increase in step with the frequency, otherwise the flux in the machine will deviate from the desired optimum operating point. Practical motor controllers based on frequency control must therefore have a means of simultaneously controlling the motor supply voltage. This is known as Volts/Hertz control.
  • Increasing the frequency without increasing the voltage will cause a reduction of the flux in the magnetic circuit thus reducing the motor's output torque. The reduced motor torque will tend to increase the slip with respect to the new supply frequency. This in turn causes a greater current to flow in the stator, increasing the IR volt drop across the windings as well as the I2R copper losses in the windings. The result is a major drop in the motor efficiency. Increasing the frequency still further will ultimately cause the motor to stall.
  • Increasing the voltage without increasing the frequency will cause the material in the magnetic circuit to saturate. Excessive current will flow giving rise to high heat dissipation due to I2R losses in the windings and high eddy current losses in the magnetic circuit and ultimately failure of the motor due to overheating. Increasing the voltage will not force the motor to exceed the synchronous speed because as it approaches the synchronous speed the torque drops to zero.

    The variable frequency is normally provided by an. See more about

 

Note also that since the in the rotor is proportional to the and the flux density in turn is proportional to the line voltage, the torque, which depends on the product of the flux density and the rotor current, is proportional to the square of the line voltage V.

 

  • Generator Action

If an induction motor is forced to run at speeds in excess of the synchronous speed, the load torque exceeds the machine torque and the slip is negative, reversing the rotor induced EMF and rotor current. In this situation the machine will act as a generator with energy being returned to the supply.

If the AC supply voltage to the stator excitation is simply removed, no generation is possible because there can be no induced current in the rotor.

  • Regenerative braking

Thus in traction applications, regenerative braking is not possible below synchronous speed in a machine fed with a fixed frequency supply. If however the motor is fed by a variable frequency inverter then regenerative braking is possible by reducing the supply frequency so that the synchronous speed becomes less than the motor speed.

AC motors can be microprocessor controlled to a fine degree and can regenerate current down to almost a stop whereas DC regeneration fades quickly at low speeds.

  • Dynamic Braking

Induction motors can be brought rapidly to a stop (and / or reversed) by reversing one pair of leads which has the effect of reversing the rotating wave. This is known as "plugging". The motor can also be stopped quickly by cutting the AC supply and feeding the stator windings instead with a DC (zero frequency) supply. With both of these methods, energy is not returned to the supply but is dissipated as heat in the motor. These techniques are known as dynamic braking.

 

See more about.

 

  • Starting

Three phase induction motors and some synchronous motors are not self starting but design modifications such as auxiliary or "damper" windings on the rotor are incorporated to overcome this problem.

 

Usually an induction motor draws 5 to 7 times its rated current during starting before the speed builds up and the current is modified by the back EMF. In wound rotor motors the starting current can be limited by increasing the resistance in series with the rotor windings.

In squirrel cage designs, are used to control the current to prevent damage to the motor or to its power supply.

Even with current control the motor can still overheat because, although the current can be limited, the speed build up is slower and the inrush current, though reduced, is maintained for a longer period.

 

  • Power Factor

The current drawn by an induction motor has two components, the current in phase with the voltage which governs the power transfer to the load and the inductive component, representing the magnetising current in the magnetic circuit, which lags 90° behind the load current.

The is defined as cosΦ where Φ is the net lag of the current behind the applied voltage due to the in phase and out of phase current components. The net power delivered to the load is VAcosΦ where V is the applied voltage, A is the current which flows.

 

Various methods of power factor correction are used to reduce the current lag in order to avoid losses due to poor power factor. The simplest is to connect a capacitor of suitable size across the motor terminals. Since the current through a capacitor leads the voltage, the effect of the capacitor is to counter-balance the inductive element in the motor canceling out the current lag.

Power factor correction can also be accomplished in the.

 

  • Characteristics

One of the major advantages of the induction motor is that it does not need a commutator. Induction motors are therefore simple, robust, reliable, maintenance single freizeit treff dresden free and relatively low cost.

They are normally constant speed devices whose speed is proportional to the mains frequency.

Variable speed motors are also possible by using motor controllers which provide a variable frequency output.

 

  • Applications

Three phase induction motors are used wherever the application depends on AC power from the national grid. Because they don't need commutators they are particularly suitable for high power applications.

They are available with power handling capacities ranging from a few Watts to more than 10 MegaWatts.

They are mainly used for heavy industrial applications and for machine tools.

The availability of solid state inverters in recent years means that induction motors can now be run from a DC source. They are now finding use in automotive applications for electric and hybrid electric vehicles. Induction motors are seen as more rugged for these applications than permanent magnet motors which are vulnerable to possible degradation or demagnetization of the magnets due to over-temperature or accidental over-current at power levels over about 5 kW. Nevertheless, the induction motor can be ill-suited for some automotive applications because of the difficulties associated with extracting heat from the rotor, efficiency problems over wide speed and power ranges, and a more expensive manufacturing process due to distributed windings. and motors may offer better solutions for these applications.

 

Wound Rotor Induction Motor

Now of historic interest only, these motors were designed to permit characteristics of the machine. They used conventional windings on the rotor which were accessible through slip rings. The rotor windings were not connected to the supply line but current through the windings could be controlled by external rheostats connected in series with the windings. Modern electronic controls have made these designs obsolete.

 

Single Phase Induction Motors

At first sight it might be assumed that it would be impossible to create a rotating field using only a single phase supply. With the aid of an auxiliary stator winding displaced from the main winding it is however possible to create a second in the auxiliary winding, out of phase with the MMF in the main winding, and this is sufficient to generate the rotating field.

  • Capacitor-Run Motors

The necessary phase difference between the main and auxiliary windings can be provided by connecting a high value capacitor in series with the auxiliary winding. These motors are commonly used in household washing machines, refrigerators and shower pumps and can easily be identified by the large electrolytic capacitor strapped to the motor body.

As an alternative to using an external capacitor, the split-phase method uses a high resistance auxiliary winding. The difference in the impedance of the two windings is sufficient to create the necessary phase difference between the currents in the two windings.

  • Shaded Pole Motors

The shaded pole motor uses another, rather crude, method of inducing a second stator MMF, out of phase with the main MMF in order to create the desired rotating field from a single phase AC supply. A short circuited turn of thick copper, known as the shading ring, is mounted in a slot in the pole piece. Some of the magnetic flux produced by the main winding induces a current in the shading ring which produces its own weak flux which opposes and retards the main flux through the ring so that the resulting flux through the ring is out of phase with the main flux. Thus there is a phase difference between one side of the pole and the other. Though inefficient, this method is once more sufficient to set up a rotating field.


 

  • Characteristics

Single phase induction motors are less efficient than polyphase machines and were developed mainly for domestic use since most dwellings are only supplied with single phase power.

No control of speed.

  • Applications

All kinds of household appliances and light industrial applications.

 

Synchronous AC Motors

The synchronous motor is similar to the induction motor in that it is a polyphase machine in which the stator produces a rotating field, however the rotor is constructed from either permanent magnets or electromagnets energised by direct current supplied through slip rings.

  • Torque

The torque depends on the attraction of the rotor magnets to the rotating magnetic poles and not on relative motion between the windings in the rotor and the rotating magnetic field. It can therefore lock on to the rotating field. See. Unlike squirrel cage induction motors, synchronous motors can run and produce torque at synchronous speed.

They are difficult to start on mains frequency because the rotating field is too fast so they need to start at lower frequency or they need unexcited auxiliary windings or a rudimentary squirrel cage to bring the rotor up to synchronous speed. As the motor approaches synchronous speed it will suddenly snap in to synchronisation.

  • Pull In Torque

To achieve synchronisation the motor torque must be greater then the load torque. The torque developed when the motor locks on to synchronous speed is called the pull in torque. If the load is greater than the pull in torque the motor will not reach synchronous speed.

  • Pull Out Torque

As the load on the motor is increased, the motor torque and the will also increase. However if the torque angle exceeds 90 degrees the torque will begin to fall and the motor will lose synchronisation and eventually stop. The pull out torque is typically 1.5 times the continuously rated torque.

 

  • Characteristics

Synchronous operation.

  • Applications

Fixed speed applications such as clocks and timers

 

Synchronous Reluctance Motors

The operating principle of the basic reluctance motors is described in the section about.

The so called "synchronous" reluctance motor is designed to run on mains frequency alternating current and it uses distributed stator windings similar to those used in squirrel cage induction motors. The rotor however needs salient poles to create a variable reluctance in the motor's magnetic circuit which depends on the angular position of the rotor. These salient poles can be created by milling axial slots along the length of a squirrel cage rotor. See diagram below.

 

 

  • Characteristics

The synchronous reluctance motor is not self starting without the squirrel cage. During run up it behaves as an induction motor but as it approaches synchronous speed, the reluctance torque takes over and the motor locks into synchronous speed.

  • Applications

Used where regulated speed control is required in applications suc as metering pumps and industrial process equipment.

 

Hysteresis Motor

The Hysteresis Synchronous motor consists of a wound stator producing a rotating field and a rotor in the form of a cylindrical shell with crossbars all made from hard steel with relatively high.

 

 

At start up, the combined effects of eddy currents in the steel causing induction motor action and remanent magnetism in the steel causing the magnetic poles to follow the rotating field, together cause the motor speed to build up. As the motor approaches synchronous speed the magnetic effect of the crossbars behaving like a permanent magnet causes the motor to lock on to synchronous speed. The net result is that the torque is roughly constant at all speeds.

 

  • Characteristics

Simple design

Starts as an induction motor and locks in as a synchronous motor.

Having a smooth rotor of homogenous material, the noise and vibration produced is inherently low. Since there are no pole faces or saliencies, the magnetic path is of constant permeability, thus eliminating the magnetic pulsations which are the major cause of noise in the salient pole type.

  • Applications

Their efficiency is low, and applications are restricted to small power ratings.

Used extensively in tape recorders and clocks.

Now mostly replaced by permanent magnet motors.

 

Universal Motors

An AC motor which uses separately excited rotor windings using a commutator to feed current to rotor coils behaves in much the same way as a brushed DC motor and can in fact be used as a universal motor taking its supply either from an AC or a DC source.

Unlike induction and synchronous motors, the speed of universal motors is not limited by the electric mains supply frequency and can easily exceed one revolution per cycle. This makes them useful for household appliances such as blenders, vacuum cleaners and hair dryers which need high-speed operation. Speeds of up to 30,000 RPM are possible but the current carrying capacity is limited by the commutator and brushes which restricts their use to low power applications of about 1 kilowatt or less. See more about.

 

See also

 

 

 

 

Split Phase Induction Motor

The Split Phase Motor is also known as a Resistance Start Motor. It has a single cage rotor, and its stator has two windings known as main winding and starting winding. Both the windings are displaced 90 degrees in space. The main winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance.The Connection Diagram of the motor is shown below.

Split-Phase-Indcution-Motor-fig-1A resistor is connected in series with the auxiliary winding. The current in the two windings is not equal as a result the rotating field is not uniform. Hence, the starting torque is small, of the order of 1.5 to 2 times of the started running torque. At the starting of the motor both the windings are connected in parallel.

As soon as the motor reaches the speed of about 70 to 80 % of the synchronous speed the starting winding is disconnected automatically from the supply mains. If the motors are rated about 100 Watt or more, a centrifugal switch is used to disconnect the starting winding and for the smaller rating motors relay is used for the disconnecting of the winding.

A relay is connected in series with the main winding. At the starting, the heavy current flows in the circuit, and the contact of the relay gets closed. Thus, the starting winding is in the circuit, and as the motor attains the predetermined speed, the current in the relay starts decreasing. Therefore, the relay opens and disconnects the auxiliary winding from the supply, making the motor runs on the main winding only.

The phasor diagram of the Split Phase Induction Motor is shown below.

Split-Phase-Indcution-Motor-fig-2 The current in the main winding (IM) lag behind the supply voltage V almost by the 90-degree angle. The current in the auxiliary winding IA is approximately in phase with the line voltage. Thus, there exists the time difference between the currents of the two windings. The time phase difference ϕ is not 90 degrees, but of the order of 30 degrees. This phase difference is enough to produce a rotating magnetic field.

The Torque Speed Characteristic of the Split Phase motor is shown below.

Split Phase Induction Motor figure

Here, n0 is the point at which the centrifugal switch operates. The starting torque of the resistance start motor is about 1.5 times of the full load torque. The maximum torque is about 2.5 times of the full load torque at about 75% of the synchronous speed. The starting current of the motor is high about 7 to 8 times of the full load value.

The direction of the Resistance Start motor can be reversed by reversing the line connection of either the main winding or the starting winding. The reversal of the motor is possible at the standstill condition only.

Applications of Split Phase Induction Motor

This type of motors are cheap and are suitable for easily starting loads where the frequency of starting is limited. This type of motor is not used for drives which require more than 1 KW because of the low starting torque. The various applications are as follows:-

  • Used in the washing machine, and air conditioning fans.
  • The motors are used in mixer grinder, floor polishers.
  • Blowers, Centrifugal pumps
  • Drilling and lathe machine.

1 Comment

Zahra Doe Morbi gravida, sem non egestas ullamcorper, tellus ante laoreet nisl, id iaculis urna eros vel turpis curabitur.

3 Comments

Zahra Doejune 2, 2017
Morbi gravida, sem non egestas ullamcorper, tellus ante laoreet nisl, id iaculis urna eros vel turpis curabitur.
Zahra Doejune 2, 2017
Morbi gravida, sem non egestas ullamcorper, tellus ante laoreet nisl, id iaculis urna eros vel turpis curabitur.
Zahra Doejune 2, 2017
Morbi gravida, sem non egestas ullamcorper, tellus ante laoreet nisl, id iaculis urna eros vel turpis curabitur.

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