Machine Design Ppt Final

RAHUL PANDEY 08104046

Polyphase induction motor consists two major parts:1 2

STATOR ROTOR

PRINCIPLE ACTIONwinding is excited with a.c voltage rotating magnetic field set up

field produces an e.m.f in the other winding by transformer action

circulates current in the latter if it is short circulated.

currents flowing in the second winding interact with the field produced by the first winding

thereby producing a torque which is responsible for the rotation of the rotor.

STATORStationary part of induction motor.

Cylinderical structure. Build up of dynamo grade lamination. Lamination are 0.35 mm or 0.5 mm thick.

STATOR Larged sized motors are made of

segmental laminations.

usually between 0.3 to 0.6m.

The peripheral length of one segment, For quick assembly of stator core,

maximum chord of segment should not be less than 0.37m. should not be less than 60 mm.The distance between adjacent dovetails Long cores are divided into number of

stacks with radial ventilating ducts. 50 to 60 mm.

Width of stack core should not less than

STATOR FRAMESFrames of electrical machines are structures in which stator core is assembled. They serve four distinct purposes:

They enclose the core and windings. 2. They shield the live and moving machine part from human contact and from injury caused by intruding objects or weather exposure. 3. They transmit the torque to the machine supports, and are designed to withstand twisting forces and shocks. 4. They serve as ventillating housing or means of guiding the coolant into effective channels.1.

DESIGN OF STATOR FRAMESdesign of frames has special significance in the case of induction machines. frames should be strong and rigid both during construction and after the assembly Length of air gap is small and if the frame is not rigid, the rotor will not remain concentric with stator and giving rise to unbalnced magnetic pull

.

LOW TENSION MACHINESMachine upto 50 kW. Frame may be die - cast or fabricated. Die cast in a strong silicon aluminium alloy ADVANTAGE : facilitates the use of thicker cross section at places where greater mechanical strength is required. Die cast frames do not require machining

LOW TENSION MACHINESRadial ventilating ducts, are providing an annular space for air between core and the frame. Frames of totally enclosed machines are provided with axial fins to increase the heat dissipation surface area . Stator core lamination are fixed to the frame with the help of clamping rings.

HIGH TENSION MACHINESThe frames are fabricated by welding

steel plates so to have the adapatability to new design and modification. The frame is a short short cylinder,

or a box, with end end plates and axial ribs. The frame is usually made in the

form of of a box of T shaped cross section in order to increase rigidity. In machine of larger axial length the

box has intermediate walls in addition to the two side walls.

Walls are designed that two consecutive walls are

not more than 0.5 m apart ( gives additional rigidity to the frames).The frames of larger horizontal shaft machines (

with outside diameter greater than 3.5 to 4m) are made with joints located in horizontal plane. Openings are located in the outside surface frame

for providing outlet to cooling air in radial direction.

ROTORMoving part of an induction motor. Rotor cores of small machines are often put on the shaft directly and keyed to it for transfer of torque. Washers or thrust rings are used for axial clamping. The thrust ring is put on the shaft when hot and on cooling the rings grips the shaft ( costly for mass production).

In practise, a thrust ring with a cut is used and this ring is placed in a ring groove in the shaft.

ROTOR DESIGNRotor lamination are punched

as a single unit in the case of small machines while in larger machines the lamination are segmented. Radial and Axial ducts are used to provide paths for ventilating air. The number of radial ventilating ducts provided in the rotor is equal to that in the stator. The single unit lamination are fixed on the shafts by means of armature hub or on ribs.

ROTOR DESIGNIn the case of cast-steel hub

the laminations are pressed together in axial direction with clamping rings.

The core is compressed

between end plates when ribs are used.

The segmental lamination are

fixed to rotor spider comprises of shaft with arms and stiffeners.

Squirrel cage

wound rotor

ROTOR WINDINGSIn present day induction motors are manufactured with squirrel cage rotors, consisting of uninsulated bars of aluminium or copper that are joined together at both ends by end rings of similar conducting material.

CONSTRUCTION OF ROTOR WINDINGEmployed winding of cast aluminium. Assembled rotor lamination are placed in a mould after which molten

aluminium is forced in.Pressure is applied to form bars, end rings and cooling fins as a extensions of

end rings, known as die-cast rotor.It is popular as there is no joints and hence no possibility of high contact

resistance.Silicon alloy with 6

12% Si is used because pure aluminium does not cast

well.

ROTOR WINDING DESIGN Copper or brass bars, of round or rectangular shape to fit the rotor slots

tightly, are driven into slots, projecting a short distance on each end of the core.It is necessary to keep the bars tight in the slots because loose bars can be

damaged quickly by mechanical vibration and thermal cycling.Deep bar rotor are used for high starting torque.

ROTOR DESIGN For high starting torque wound

rotor is used. The wound rotor has completely insulated copper winding very much like the stator winding. Wave winding is used as it has advantage that the number of cross connectors between groups of coil is reduced. Gives compact arrangement of cross connectors which have a good mechanical balance of revolving structure.

The winding can be connected

in star or delta.

The three ends are brought out

of three slip rings.

The current is collected from

slip rings with carbon brushes, from which it led to resistance for starting torque.

Squirrel cage motor has the following advantages as compared with wound rotor.I. II. III. IV. V. VI.

No slip rings, brush gear, short circuiting devices, devices, rotor terminals for starting rheostats are required. It is cheaper and rugged in construction. It has slightly higher efficiency. It has better space factor for rotor slots, a shorter overhang and consequently a smaller copper loss. It has bare end rings, a larger space for fans and thus the cooling condition are better. It has a smaller rotor overhang leakage which gives a better power factor and a greater pull out torque and overload capacity.

Not possible to introduce resistances in the rotor circuit.Low starting torque.

The slip rings ( for wound rotor machines ) are made of either brass or phosphor bronze. Pressed together on a body of reinforced thermo-setting resin carried on a mild steel hub. The slip rings are located either between the core and the bearing or on the shaft extension. In the latter case the shaft is made hollow to allow the three connection from rotor to slip rings to pass through bearings.

SHAFT AND BEARINGAir gap of an induction motor is made as smallShaft is made short and stiff so that rotor may not have any significant deflection,.

Even small deflection would create large irregularities in the air gap. Lead to production of an unbalanced magnetic pull.

ADVANTAGE OF BEARINGWith their use accurate centering is much simpler.

Overall length of the machine is reduced.

For small motors, roller bearing may be used at the driving end and a ball bearing at the non-driving end.

For the large and heavy rotors journal bearing of self aligning spherical seated type are used