the difference between asynchronous and synchronous motors

7/21/2019 The Difference Between Asynchronous and Synchronous Motors

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The difference between asynchronous and synchronous motors

The increasing importance of energy efficiency has brought electric motor makers to promote a

variety of schemes that improve motor performance. Unfortunately the terminology associatedwith motor technologies can be confusing, partly because multiple terms can sometimes be

used interchangeably to refer to the same basic motor configuration. Among the classicexamples of this phenomenon is that of induction motorsand asynchronous motors.

All induction motors are asynchronous motors. The asynchronous nature of induction-motor

operation comes from the slip between the rotational speed of the stator field and somewhat

slower speed of the rotor. A more-specific explanation of how this slip arises gets into details of

the motor internals.

Most induction motors today contain a rotational element (the rotor) dubbed a squirrel cage. The

cylindrical squirrel cage consists of heavy copper, aluminum, or brass bars set into grooves andconnected at both ends by conductive rings that electrically short the bars together. The solid

core of the rotor is built with stacks of electrical steel laminations. The rotor contains fewer slots

than the stator. The number of rotor slots must also be a nonintegral multiple of stator slots so as

to prevent magnetic interlocking of rotor and stator teeth when the motor starts.

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It is also possible to find induction motors containing rotors made up of windings rather than asquirrel cage. The point of this wound-rotor configuration is to provide a means of reducing the

rotor current as the motor first begins to spin. This is generally accomplished by connecting each

rotor winding to a resistor in series. The windings receive current through some kind of slip-ring

arrangement. Once the rotor reaches final speed, the rotor poles get switched to a short circuit,thus becoming electrically the same as a squirrel cage rotor.

The stationary part of the motor windings is called the armature or the stator. The stator windings

connect to the ac supply. Applying a voltage to the stator causes a current to flow in the statorwindings. The current flow induces a magnetic field which affects the rotor, setting up voltage

and current flow in the rotor elements.

A north pole in the stator induces a south pole in rotor. But the stator pole rotates as the ac

voltage varies in amplitude and polarity. The induced pole attempts to follow the rotating statorpole. However, Faradays law says that an electromotive force is generated when a loop of wire

moves from a region of low magnetic-field strength to one of high magnetic-field strength, and
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vice versa. If the rotor exactly followed the moving stator pole, there would be no change in

magnetic-field strength. Thus, the rotor always lags behind the stator field rotation. The rotor

field always lags behind the stator field by some amount so it rotates at a speed that is somewhatslower than that of the stator. The difference between the two is called the slip.

The amount of slip can vary. It depends principally on the load the motor drives, but also isaffected by the resistance of the rotor circuit and the strength of the field that the stator flux

induces.

A few simple equations make the basic relationships clear.

When ac is initially applied to the stator, the rotor is stationary. The voltage induced in the rotor

has the same frequency as that of the stator. As the rotor starts spinning, the frequency of the

voltage induced in it,fr, drops. Iffis the stator voltage frequency, then slip,s,relates the two via

fr=sf. Heresis expressed as a decimal.

When the rotor is standing still, the rotor and stator effectively form a transformer. So the voltageEinduced in the rotor is given by the transformer equation

E= 4.44 f Nm

whereN= the number of conductors under one stator pole (typically small for a squirrel-cage

motor) and m= maximum magnetic flux, Webers. Thus, the voltageErinduced while the rotorspins depends on the slip:

Er= 4.44 s f Nm= s E

Explanation of synchronous motors

A synchronous motor has a special rotor construction that lets it rotate at the same speedthatis, in synchronizationwith the stator field. One example of a synchronous motor is the

stepping motor, widely used in applications that involve position control. However, recent

advances in power-control circuitry have given rise to synchronous-motor designs optimized foruse in such higher power situations as fans, blowers, and to drive axles in off-road vehicles.

There are basically two types of synchronous motors:

Self-excitedUsing principles similar to those of induction motors, and

Directly excited usually with permanent magnets, but not always

The self-excited synchronous motor, also called a switched-reluctance motor, contains a rotor

cast of steel that includes notches or teeth, dubbed salient poles. It is the notches that let the rotor

lock in and run at the same speed as the rotating magnetic field.


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To move the rotor from one position to the next, circuitry must sequentially switch power to

consecutive stator windings/phases in a manner analogous to that of a stepping motor. The

directly excited synchronous motor may be called by various names. Usual monikers includeECPM (electronically commutated permanent magnet), BLDC (brushless dc), or just a brushless

permanent-magnet motor. This design uses a rotor that contains permanent magnets. The

magnets may mount on the rotor surface or be inserted within the rotor assembly (in which casethe motor is called an interior permanent-magnet motor).

The permanent magnets are the salient poles of this design and prevent slip. A microprocessorcontrols sequential switching of power on the stator windings at the proper time using solid-state

switches, minimizing torque ripple. The principle of operation of all these synchronous-motor

types is basically the same. Power is applied to coils wound on stator teeth that cause a

substantial amount of magnetic flux to cross the air gap between the rotor and stator. The fluxflows perpendicular to the air gap. If a salient pole of the rotor is aligned perfectly with the stator

tooth, there is no torque produced. If the rotor tooth is at some angle to the stator tooth, at least

some of the flux crosses the gap at an angle that is not perpendicular to the tooth surfaces. The

result is a torque on the rotor.Thus, switching power to stator windings at the right time causes aflux pattern that results in either clockwise or counterclockwise motion.

One other type of synchronous motor is called a switched reluctance (SR) motor.

Its rotor consists of stacked steel laminations with a series of teeth. The teeth are magnetically

permeable, and the areas surrounding them are weakly permeable by virtue of slots cut intothem. Thus the rotor needs no windings, rare-earth materials, or magnets.

Unlike induction motors, there are no rotor bars and consequently no torque-producing currentflow in the rotor. The absence of any form of conductor on the SR rotor means that overall rotor

losses are considerably lower than in other motors incorporating rotors carrying conductors.

Torque produced by the SR motor is controlled by adjusting the magnitude of current in the

stator electromagnets. Speed is then controlled by modulating the torque (via winding current).


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The technique is analogous to the same way speed is controlled via armature current in a

traditional brush-dc motor.

An SR motor produces torque proportional to the amount of current put into its windings. Torque

production is unaffected by motor speed. This is unlike ac-induction motors where, at high

rotational speeds in the field-weakening region, rotor current increasingly lags behind therotating field as motor rpm rises.

1.

Synchronous motors operate at synchronous speed (RPM=120f/p) while induction motors

operate at less than synchronous speed (RPM=120f/pslip). Slip is nearly zero at zero load

torque and increases as load torque increases.

2. Synchronous motors require DC excitation to be supplied to the rotor windings; induction

motors dont.

3. Synchronous motors require a DC power source for the rotor excitation.

4. Synchronous motors require slip rings and brushes to supply rotor excitation. Induction

motors dont require slip rings, but some induction motors have them for soft starting or speed

control.

5. Synchronous motors require rotor windings while induction motors are most often

constructed with conduction bars in the rotor that are shorted together at the ends to form a

squirrel cage.

6. Synchronous motors require a starting mechanism in addition to the mode of operation that

is in effect once they reach synchronous speed. Three phase induction motors can start by

simply applying power, but single phase motors require an additional starting circuit.

7. The power factor of a synchronous motor can be adjusted to be lagging, unity or leading while

induction motors must always operate with a lagging power factor.

8. Synchronous motors are generally more efficient that induction motors.

9. Synchronous motors can be constructed with permanent magnets in the rotor eliminating the

slip rings, rotor windings, DC excitation system and power factor adjustability.

10. Synchronous motors are usually built only is sizes larger than about 1000 Hp (750 kW)

because of their cost and complexity. However, permanent magnet synchronous motors and

electronically controlled permanent synchronous motors called brushless DC motors areavailable in smaller sizes.

Salient Pole vs Non Salient Pole Synchronous Generator Difference

Salient Pole Synchronous Alternator:


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Salient pole Generators will have large diameter and short axial length

Pole shoes cover 2/3 of the pitch

Salient Poles are laminated in order to reduce eddy currents

They are used in hydraulic turbines or diesel engines

Salient pole generators will have typical speed about 100 to 375 rpm.

As the speed of the water turbine is slow hence more number of poles are required to attain

the frequency. Therefore Salient pole machines will have typically number of poles will be

between 4 to 60.

Cheaper compared to cylindrical rotor machines for speeds below 1000rpm.

Causes excessive windage losses

Flux distribution is not uniform due to the presence of salient poles, hence emf waveformgenerated is not good compared to cylindrical machine

Salient Pole Synchronous Generators are employed in Hydro-Power plants.

Non-Salient pole Synchronous Alternator:

Non-Salient pole generators will have smaller diameter and longer axial length

They are used for High speed operation (typically speed will be 1500 and 3000 rpm)

Better in dynamic balancing because of absence of salient poles.

Less windage loss

Robust construction and noiseless operation

Nearly sinusoidal flux distribution around the periphery, therefore gives a better emfwaveform than salient pole machine

No need to provide damper windings (except in special case to assist the synchronising)because the field poles themselves acts as efficient dampers.

Non-Salient pole generators are used in Thermal,Gas and in Nuclear Power plants.


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Read more: http://electricalquestionsguide.blogspot.com/2011/12/salient-pole-vs-non-salient-

pole.html#ixzz392BtqTN3

the difference between asynchronous and synchronous motors

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