Showing posts with label Induction Motor. Show all posts
Showing posts with label Induction Motor. Show all posts

Braking Methods of 3 Phase Induction Motor

Induction Motor Braking Regenerative Plugging Dynamic Braking of Induction Motor

Induction motors are most widely using motors in present days for domestic and industrial purpose.But controlling of AC motor is little bit difficult than DC motor due to alternating nature of voltage.Speed control of induction motors is difficult than dc motors.That’s why induction motor use was restricted.But efficiency wise,usage wise induction motors has lots of advantages.So some speed control,braking methods of induction motor  are invented.

Braking is the most important part of speed control.Braking is two types.They are,mechanical and electrical.In mechanical braking we have disadvantages like heat dissipation,mechanical brakes also depends on the skill of the operator.Electric braking is easy and reliable,it can stop the induction motors very quickly.So we go for electrical braking on induction motor.Though the motor is brought to rest electrically, to maintain its state of rest a mechanical brake is must.

Types Of Electrical Braking Methods On Induction Motor

Dynamic or Rheostatic Braking On Induction Motor

In this braking a high resistance is inserted in the rotor circuit of induction motor, with the help of rheostat.In the below diagram braking on 3 phase induction motor is shown. Whenever we need braking one of the  supply line out of R, Y or B is disconnected from the supply.Depending upon the condition of this disconnected line,we classified two types they are,

1. Two lead connections : In this method, the line disconnected, kept open. This is shown in the Fig(a) and is called two lead connections of rheostatic braking.

2. Three lead connections : In this method, After disconnection of the line,it is connected directly to the other line of the machine. This is shown in the Fig(b).

Note : This method is effective only for slip ring or wound rotor induction motors.

As one of the motor terminal is disconnected from the supply, the motor continues to run as single phase motor. In this case the, breakdown torque i.e. maximum toque decreases to 40% of its original value and motor develops no starting torque at all. And due to high rotor resistance, the net torque produced becomes negative and the braking operation is obtained.

Which is preferred in rheostat braking on induction motor?

The braking torque is small in the case of two lead connections,high in the case of three lead connections.The braking torque is high at high speeds. But in three lead connections there is possibility of inequality between the contact resistances in connections of two paralleled lines. This might reduce the braking torque and even may produce the motoring torque again.So we prefer two lead connection over three lead connection.Even there is low braking torque in two lead system.

The torque-slip characteristics for motoring and braking operation of 3 phase induction motor is shown here,
Uses: We use dynamic or rheostat braking in cranes.

Plugging On Induction Motor

Plugging induction motor braking is applied by just reversing the supply phase interchanging connections of any two phases of stator we can attain plugging braking of induction motor.Due to the reversal of phase sequence, the direction of rotating magnetic field gets reversed. This produces a torque in the reverse direction and the motor tries to rotate in opposite direction. This opposite flux acts as brake and it slows down the motor.During plugging the slip is (2 - s), if the original slip of the running motor is s.
Disadvantages of plugging induction motor braking :
1. During the plugging operation very high I²R losses occur in the form of heat.This heat is more than produced when rotor is normally locked.
2. So we can't apply plugging frequently as due to high heat produced rotor which can damage or melt the rotor bars and even may over heat the stator as well.
Advantages of plugging induction motor braking :
1. It the quickest way.

D.C.Braking On Induction Motor

This is also similar to dynamic braking of induction motor,two connections of stator are disconnected from the supply and connected to a dc source.When d.c. is supplied to the stator, stationary poles N, S are produced in stator. As rotor is rotating, rotor cuts the flux produced by the stationary poles. Thus the a.c. voltage gets induced in the rotor. This voltage produces an a.c. current in the rotor. The motor works as a generator and the R losses are dissipated at the expenditure of kinetic energy stored in the rotating parts. Thus D.C dynamic braking is achieved. This is the quick way to stop induction motor along with high load.

Advantages of d.c. dynamic braking of induction motor are,

1. Quick,Less heat produced as compared to the plugging method.
2. The energy dissipated in the rotor is not dependent on the magnitude of the d.c. current.
3. The braking torque is proportional to the square of the d.c. current.
4. Can be used for wound rotor,squirrel cage rotor induction motors.

Regenerative Braking On Induction Motors

The input power to a three phase induction motor is given by, Pin = 3 Vph Iph cos Φ Where, Φ is the phase angle between stator phase voltage Vph and the stator phase  current  Iph.In motoring action This Φ is less than 90° for the motoring action.Φ > 90° for generating action.
When the induction motor runs above the synchronous speed, relative speed between the conductors and air gap rotating field reverses, as a result the phase angle because greater than 90° rotor produces torque in opposite direction to achieve the braking thus regenerative braking takes place.

Regenerative braking of induction motor can be applied only in case of constant frequency and speed must be above the synchronous speed.In generating action we get some power,we can use this to run other machine.

The torque-slip characteristics for motoring and generating action are shown in the above figure. 

Tags:Induction motor braking methods wikipedia,ppt,advantages of electric braking over mechanical braking,regenerative braking of dc motor pdf

Differences Between Induction Motor and Transformer

Comparison of Induction Motor With A Transformer

An induction motor may be considered to be a transformer with a rotating short circuited secondary. The stator winding corresponds to transformer primary and rotor winding to transformer secondary. However, the following differences between the two are worth noting:

(i) Unlike a transformer, the magnetic circuit of a 3-phase induction motor has an air gap. Therefore, the magnetizing current in a 3-phase induction motor is much larger than that of the transformer. For example, in an induction motor, it may be as high as 30-50 % of rated current whereas it is only 1-5% of rated current in a transformer.

(ii) In an induction motor, there is an air gap and the stator and rotor windings are distributed along the periphery of the air gap rather than concentrated on a core as in a transformer. Therefore, the leakage reactances of stator and rotor windings are quite large compared to that of a transformer.

(iii) In an induction motor, the inputs to the stator and rotor are electrical but the output from the rotor is mechanical. However, in a transformer, input as well as output is electrical.

(iv) The main difference between the induction motor and transformer lies in the fact that the rotor voltage and its frequency are both proportional to slip s. If f is the stator frequency, E2 is the per phase rotor e.m.f. at standstill and X2 is the standstill rotor reactance/phase, then at any slip s, these values are:

Rotor e.m.f./phase, 
E'2 = s E2Rotor reactance/phase,
X'2 = sX2Rotor frequency, f' = sf


Tags:difference between transformer and induction motor,Induction Motor and Transformer Comparisons

3 Phase Induction Motor Working Principle

Working Principle of Three Phase Induction Motor

In the previous post we discussed construction of induction motorIn this article we are going to discuss about "how induction motor works?".Consider a portion of 3-phase induction motor as shown in Fig. The operation of the motor can be explained as under.

(i) When 3-phase stator winding is energized from a 3-phase supply, a rotating magnetic field is set up which rotates round the stator at synchronous speed Ns (= 120 f/P).

(ii) The rotating field passes through the air gap and cuts the rotor conductors, which as yet, are stationary. Due to the relative speed between the rotating flux and the stationary rotor, e.m.f.s are induced in the rotor conductors. Since the rotor circuit is short-circuited, currents start flowing in the rotor conductors.

(iii) The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field.

(iv) The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenz’s law. According to this law, the direction of rotor currents will be such that they tend to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it.

Slip in Three Phase Induction Motor:-

We have seen above that rotor rapidly accelerates in the direction of rotating field. In practice, the rotor can never reach the speed of stator flux. If it did, there would be no relative speed between the stator field and rotor conductors, no induced rotor currents and, therefore, no torque to drive the rotor.
The friction and windage would immediately cause the rotor to slow down. Hence, the rotor speed (N) is always less than the suitor field speed (Ns). This difference in speed depends upon load on the motor. The difference between the synchronous speed Ns of the rotating stator field and the actual rotor speed N is called slip. It is usually expressed as a percentage of synchronous speed i.e.

slip, s= (Ns-N)/Ns

(i) The quantity Ns - N is sometimes called slip speed.
(ii) When the rotor is stationary (i.e., N = 0), slip, s = 1 or 100 %.
(iii) In an induction motor, the change in slip from no-load to full-load is
hardly 0.1% to 3% so that it is essentially a constant-speed motor.

Tags:Working Principle of 3 phase Induction Motor,three phase induction motor working operation theory,types of induction motor induction motor working principle pdf.

Construction of Induction Motor

Induction Motor Construction:

Induction motor is the most commonly used machine in domestic purpose and industrial purpose. Induction motor consists essentially of two main parts : 

a) stator and b) rotor. 

Induction Motor Stator:-

1.The stator of an induction motor is almost same as that of a synchronous motor or generator. The main function of the stator core is to carry the alternating flux.

2.In order to reduce the eddy current  loss, the stator core is laminated. These laminated types of structure are made up of stamping which is about 0.4 to 0.5 mm thick. It is made up of a number of stampings, which are slotted to receive the windings.

3.The stamping is generally made up of silicon steel, which helps to reduce the hysteresis loss occurring in motor. The stator of induction motor carries a 3-phase winding and that is fed from a 3-phase supply.

Stator core with winding

4.It is wound for a definite number of poles, the exact number of poles being detemined by the requirements of speed. Greater the number of poles, lesser the speed and vice versa.

5.When supplied to 3-phase windings, produce a magnetic flux, which is of constant magnitude but which revolves (or rotates) at synchronous speed (given by Ns=120f/P). This revolving magnetic flux also called as rotating magnetic field which induces an e.m.f. in the rotor by mutual induction. 

Induction Motor Rotor:

Induction motors are available with two types of rotors basically.
i) Squirrel-cage rotor : Motors employing this type of rotor are known as squirrel-cage induction motors.
ii)Phase-wound or wound rotor : Motors employing this type of rotor are variously known as 'phase-wound' motors or 'wound' motors or as 'slip-ring' motors. 

Squirrel-cage rotors :

Almost 90 percent of induction motors arc squirrel-cage type, because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible. The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors which, it should be noted clearly, are not wires but consist of heavy bars of copper, aluminium or alloys. One bar is placed in each slot, rather the bars are inserted from the end when semi-closed slots are used. The rotor bars are brazed or electrically welded or bolted to two heavy and stout short-circuiting end-rings, thus giving us. what is so picturesquely called, a squirrel-case construction (Fig.). 

It should be noted that the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes. 

The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew.This is useful in two ways :

i) it helps to make the motor run quietly by reducing the magnetic hum and

ii)it helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two.

In small motors, another method of construction is used. It consists of placing the entire rotor core in a mould and casting all the bars and end-rings in one piece. The metal commonly used is an aluminium alloy.

Another form of rotor consists of a solid cylinder of steel without any conductors or slots at all. The .motor operation depends upon the production of eddy currents in the steel rotor. 

Advantages of squirrel cage induction rotor
  • Its construction is very simple and rugged.
  • As there are no brushes and slip ring, these motors requires less maintenance.

Phase-wound or wound rotor :

This type of rotor is provided with 3-phase, double-layer, distributed winding consisting of coils as used in alternators. The rotor is wound for as many poles as the number of stator poles and is always wound 3-phase even when the stator is wound two-phase.

The three phases are starred internally. The other three winding terminal!: arc brought out and connected to three insulated slip-rings mounted on the shaft with brushes resting on them.

These three brushes arc further externally connected to a 3-phase star-connected rheostat. This makes possible the introduction of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor, as shown in Fig. and for changing its  speed-torque/current characteristics.

Phase-wound Induction Motor Circuit 

When running under normal conditions, the slip-rings arc automatically short-circuited by means of a metal collar, which is pushed along the shaft and connects all the rings together. Next, the brushes are automatically lifted from the slip-rings to reduce the frictional losses and the wear and tear. Hence, it is seen that under normal running conditions, the wound rotor is short-circuited on itself just like the squirrel-case rotor. 

Advantages of slip ring induction motor :
  • It has high starting torque and low starting current.
  • Possibility of adding additional resistance to control speed.
Application induction motor : 
Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc.

*here it is the detailed information about induction motor construction and important part are explained with clean images

Tags:Construction of an Induction Motor,3 phase induction motor construction,induction motor construction,working ppt,induction motor construction video,induction motor construction and working pdf