ELECTRICAL MACHINES SEMINAR

[born_for seminars]

ABSTRACT
Induction motor design has a major effect on the behavior and performance of an induction motor.
The induction motor can be treated essentially as a transformer for analysis.
In order to perform useful work, the induction motor must be started from rest and both the motor and load accelerated up to full speed.
The starting torque of an induction motor starting with a fixed voltage, will drop a little to the minimum torque known as the pull up torque as the motor accelerates, and then rise to a maximum torque known as the breakdown or pull out torque at almost full speed and then drop to zero at synchronous speed.
Once the motor is up to speed, it operates at low slip, at a speed determined by the number of stator poles.
Motor Characteristics:
 Direct -current motors
 Shunt -wound motors
 Series-wound motors
 Compound wound motors
Motors designation consists of groups of capital letters and figures, which are marked on the motor nameplate.
Motors cannot take over axial or radial loads from the driven equipment.
Motors are designed for direct coupling with the driven equipment by means of flexible coupling.
Synchronous Generators:
All 3-phase generators (or motors) use a rotating magnetic field.
Compass needle will follow the magnetic field exactly, and make the one revolution per cycle.
If you start forcing the magnet around you will discover that it works like a generator, sending alternating current back into the grid.
The rotor electromagnets are connected to the current by using brushes and slip rings on the axle of the generator.
Induction Motor Design
has a major effect on the behaviour and performance of an induction motor. design of a motor are well understood.
Stator design.
The stator is the outer body of the motor which houses the driven windings on an iron core. In a single speed three phase motor design, the standard stator has three windings, while a single phase motor typically has two windings. The stator core is made up of a stack of round pre-punched laminations pressed into a frame which may be made of aluminium or cast iron. The laminations are basically round with a round hole inside through which the rotor is positioned. The inner surface of the stator is made up of a number of deep slots or grooves right around the stator. It is into these slots that the windings are positioned. and an electromagnet also has a North and a South pole. As the induction motor Stator is essentially like one or more electromagnets depending on the stator windings, it also has poles in multiples of two. i.e. 2 pole, 4 pole, 6 pole etc.
the power rating of the motor is determined by the losses which comprise copper loss and iron loss, and the ability of the motor to dissipate the heat generated by these losses.
Rotor Design.
The Rotor comprises a cylinder made up of round laminations pressed onto the motor shaft, and a number of short-circuited windings.The rotor windings are made up of rotor bars passed through the rotor, from one end to the other, around the surface of the rotor. The bars protrude beyond the rotor and are connected together by a shorting ring at each end. The bars are usually made of aluminium or copper made of brass.
. Equivalent Circuit.
The induction motor can be treated essentially as a transformer for analysis. The induction motor has stator leakage reactance, stator copper loss elements as series components, and iron loss and magnetising inductance as shunt elements. The rotor circuit likewise has rotor leakage reactance, rotor copper (aluminium) loss and shaft power as series elements.
. The magnetising current varies depending on the design of the motor. For small motors, the magnetising current may be as high as 60%, but for large two pole motors, the magnetising current is more typically 20 - 25%. At the design voltage, the iron is typically near saturation, so the iron loss and magnetising current do not vary linearly with voltage with small increases in voltage resulting in a high increase in magnetising current and iron loss.
Starting Characteristics.
In order to perform useful work, the induction motor must be started from rest and both the motor and load accelerated up to full speed. Typically, this is done by relying on the high slip characteristics of the motor and enabling it to provide the acceleration torque.
Induction motors at rest, appear just like a short circuited transformer,.