3 lecture motor note pdf phase

The other three winding terminals are slip-rings mounted on the shaft with brushes resting on them. These three brushes are 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. When running under normal conditions, slip-rings are automatically short circuited by means of a metal collar, which is pushed along the shaft and connects all the rings together.

Made of close-grained alloy cast iron. Built from high quality low loss silicon steel laminations and flash enameled on both sides. Have moisture proof tropical insulation and embodying mica and high quality varnishes.

Are carefully spaced for most effective air circulation and are rigidly braced to withstand centrifugal forces and any short circuit stresses. The stator rabbets and bore are machined carefully to ensure uniformity of air gap.

Ball and roller bearings are used to suit heavy duty, trouble free running and for enhanced service life.

Light aluminium fans are used for adequate circulation of cooling air and are securely keyed onto the Rotor shaft. Slip rings are made of high quality phosphor bronze and are of molded construction. Induction motor works on the principle of electromagnetic induction.

When three phase supply is given to the stator winding, a rotating magnetic field of constant magnetic field is produced. The speed of rotating magnetic field is synchronous speed, N S r.

This rotating field produces an effect of rotating poles around a rotor. Let direction of this magnetic field is clockwise as shown.

Now at this instant rotor is stationary and stator flux R. So its obvious that there exists a relative motion between the R. Now the R. Whenever a conductor cuts the flux, emf.

As rotor forms closed circuit, induced emf. Any current carrying conductor produces its own flux. So rotor produces its flux called rotor flux. For assumed direction of rotor current, the direction of rotor flux is clockwise as shown. This direction can be easily determined using right hand thumb rule. Now there are two fluxes, one R. Both the fluxes interact with each. On left of rotor conductor, two fluxes are in same direction hence added up to get high flux area.

On right side of rotor conductor, two fluxes are in opposite direction hence they cancel each other to produce low flux area. So rotor conductor experiences a force from left to right, due to interaction of the two fluxes. As all rotor conductor experiences a force, overall rotor experiences a torque and starts rotating. So interaction of the two fluxes is very essential for a motoring action. As seen from the figure, the direction of force is same as that of rotating magnetic field.

Hence rotor starts rotating in the same direction as that of R. Developed by Therithal info, Chennai. Toggle navigation BrainKart. Posted On : There are two types of 3-phase induction motor based on the type of rotor used: Squirrel cage induction motor.

Slip-ring induction motor over squirrel cage Induction motor Advantages: It is possible to speed control by regulating rotor resistance. An induction motor essentially consists of two main parts: stator and Rotor. Stator: The stator of an induction motor is in principle, the same as that of a synchronous motor or generator.

Log in with Facebook Log in with Google. Remember me on this computer. Enter the email address you signed up with and we'll email you a reset link. Need an account? Click here to sign up. Download Free PDF. Induction Motors Notes. Prem Ananth. A short summary of this paper. Design of Induction Motors Introduction: Induction motors are the ac motors which are employed as the prime movers in most of the industries. Such motors are widely used in industrial applications from small workshops to large industries.

These motors are employed in applications such as centrifugal pumps, conveyers, compressors crushers, and drilling machines etc. Constructional Details: Similar to DC machines an induction motor consists of a stationary member called stator and a rotating member called rotor. However the induction motor differs from a dc machine in the following aspects. Laminated stator 2. Absence of commutator 3. Uniform and small air gap 4.

The magnetic path is laminated to reduce eddy currents, reducing losses and heating. The cross-sectional area of these windings must be large enough for the power rating of the motor.

Fig 1 shows the cross sectional view of an induction motor. Details of construction of stator are shown in Figs Fig 1: Stator and rotor laminations The rotor Rotor is the rotating part of the induction motor.

The rotor also consists of a set of slotted silicon steel laminations pressed together to form of a cylindrical magnetic circuit and the electrical circuit. The electrical circuit of the rotor is of the following nature Squirrel cage rotor consists of a set of copper or aluminum bars installed into the slots, which are connected to an end-ring at each end of the rotor.

Aluminum rotor bars are usually die-cast into the rotor slots, which results in a very rugged construction. Even though the aluminum rotor bars are in direct contact with the steel laminations, practically all the rotor current flows through the aluminum bars and not in the lamination Wound rotor consists of three sets of insulated windings with connections brought out to three slip rings mounted on one end of the shaft.

The external connections to the rotor are made through brushes onto the slip rings as shown in fig 7. Due to the presence of slip rings such type of motors are called slip ring motors. Sectional view of the full induction motor is shown in Fig. Slip ring rotor Fig 7. Connection to slip rings Fig. The following design details are required. The main dimensions of the stator. Design details of rotor and its windings 4. Performance characteristics. In order to get the above design details the designer needs the customer specifications Rated out put power, rated voltage, number of phases, speed, frequency, connection of stator winding, type of rotor winding, working conditions, shaft extension details etc.

In addition to the above the designer must have the details regarding design equations based on which the design procedure is initiated, information regarding the various choice of various parameters, information regarding the availability of different materials and the limiting values of various performance parameters such as iron and copper losses, no load current, power factor, temperature rise and efficiency Output Equation: output equation is the mathematical expression which gives the relation between the various physical and electrical parameters of the electrical machine.

The suitable values of Bav can be selected from design data hand book. The suitable values of q can be selected from design data hand book. Choice of power factor and efficiency Choice of power factor and efficiency under full load conditions will increase with increase in rating of the machine.

Percentage magnetizing current and losses will be lower for a larger machine than that of a smaller machine. Further the power factor and efficiency will be higher for a high speed machine than the same rated low speed machine because of better cooling conditions. Taking into considerations all these factors the above parameters will vary in a range based on the output of the machine.

Similar to Bav and q, efficiency and power factor values can be selected from Design data hand book. Separation of D and L The output equation gives the relation between D2L product and output of the machine. To separate D and L for this product a relation has to be assumed or established. Following are the various design considerations based on which a suitable ratio between gross length and pole pitch can be assumed. To obtain minimum over all cost 1.

To obtain good efficiency 1. To obtain good over all design 1. To obtain good power factor 1. Hence to obtain the best power factor the following relation will be usually assumed for separation of D and L. However the obtained values of D and L have to satisfy the condition imposed on the value of peripheral speed. Design of Stator Stator of an induction motor consists of stator core and stator slots.

Stator slots: in general two types of stator slots are employed in induction motors viz, open clots and semiclosed slots. Operating performance of the induction motors depends upon the shape of the slots and hence it is important to select suitable slot for the stator slots. In such type of slots assembly and repair of winding are easy.

However such slots will lead to higher air gap contraction factor and hence poor power factor. Hence these types of slots are rarely used in 3 induction motors. Hence in this type of slots assembly of windings is more difficult and takes more time compared to open slots and hence it is costlier. However the air gap characteristics are better compared to open type slots. However the slot width will be varying from top of the slot to bottom of the slot with minimum width at the bottom as shown in Fig.

Though there are no rules for selecting the number of stator slots considering the advantages and disadvantages of selecting higher number slots comprise has to be set for selecting the number of slots. Following are the advantages and disadvantages of selecting higher number of slots. Advantages : i Reduced leakage reactance. So selected number of slots should satisfy the consideration of stator slot pitch at the air gap surface, which should be between1.

Conductor cross section: Area of cross section of stator conductors can be estimated from the stator current per phase and suitably assumed value of current density for the stator windings. Usual value of current density for stator windings is 3 to 5 amps. Based on the sectional area shape and size of the conductor can be decided. If the sectional area of the conductors is below 5 mm2 then usually circular conductors are employed. If it is above 5 mm2 then rectangular conductors will be employed.

Standard bare size of round and rectangular conductors can be selected by referring the tables of conductors given in Design data Hand book. In case of rectangular conductors width to thickness ratio must be between 2. Area of stator slot: Slot area is occupied by the conductors and the insulation.

Once the number of conductors per slot is decided approximate area of the slot can be estimated. The detailed dimension of the slot can be estimated as follows. Size of the slot: Normally different types of slots are employed for carrying stator windings of induction motors. Generally full pitched double layer windings are employed for stator windings. For double layer windings the conductor per slot will be even.

These conductors are suitably arranged along the depth and width of the winding. Stator slots should not be too wide, leading to thin tooth width, which makes the tooth mechanically weak and maximum flux density may exceed the permissible limit. Hence slot width should be so selected such that the flux density in tooth is between 1.

Further the slots should not be too deep also other wise the leakage reactance increases. As a guideline the ratio of slot depth to slot width may assumed as 3 to 5. Slot insulation details along the conductors are shown in Fig. This slot insulation is called the slot liner, thickness of which may be taken as 0. Suitable thickness of insulation called coil separator separates the two layers of coils.

Thickness of coil separator is 0. Wedge of suitable thickness 3. Lip of the slot is taken 1. Figure 13 shows the coils placed in slots.

Fig The flux density in the stator tooth is limited to 1. Depth of stator core below the slots: There will be certain solid portion below the slots in the stator which is called the depth of the stator core. This depth of the stator core can be calculated by assuming suitable value for the flux density Bc in the stator core.

Generally the flux density in the stator core may be assumed varying between 1. Depth of the stator core can be calculated as follows.

Problems Ex. Obtain the following design information for the stator of a 30 kW, V, 3 , 6 pole, 50 Hz delta connected, squirrel cage induction motor, i Main dimension of the stator, ii No.

Assume suitable values for the missing design data. Using data of this machine determine the core dimensions, number of slots and number of stator conductors for a 11kW, volts,6 pole, 50 Hz motor.

The winding factor is 0. July Soln. During the preliminary design of a kW, volts, 3 phase, 8 pole 50 Hz slip ring induction motor the following design data have been obtained. Standard size of the conductor selected satisfying the requirements is 2. Thus sectional area of the conductor