AC Motors & Their Types

Group Members:M Abdullah (p12-6322)Salman Syed (p12-6403)Yasar Hayat (p12-6405)

Topic Introduction

Electromechanical device that converts electrical energy to mechanical energy.

Mechanical energy used to e.g. Rotate pump impeller, fan, blower Drive compressors Lift materials

Motors in industry: 70% of electrical load.

What is an Electric Motor?

Principle of how motors work

Structural Design An electric motor is all about magnets and magnetism: A motor uses magnets to create motion. A motor is consist of two magnets.

Classification of Motors

Electric Motors

Alternating Current (AC) Motors

Direct Current (DC) Motors

Synchronous Induction

Three-PhaseSingle-Phase

Self ExcitedSeparately Excited

Series ShuntCompound

AC Motors

Alternating Current (AC) Motors

Synchronous Induction

Three-PhaseSingle-Phase

Alternating current (AC) motors use an electrical current, which reverses its direction at regular intervals.

An AC motor has two basic electrical parts: a "stator" and a "rotor".

CLASSIFICATION OF AC MOTOR

AC Motors Electrical current reverses direction Two parts: stator and rotor

Stator: stationary electrical component Rotor: rotates the motor shaft

Two types• Synchronous motor• Induction motor

statorrotor

Induction motors

Induction motors can be classified into two main groups: single-phase induction motorsthree-phase induction motorsSingle-phase induction motors: These only have one stator winding, operate with a single-phase power supply...Three –phase induction motors:The use three sets of stator coils the rotating magnetic field drags the rotor around with it.

Induction motors•Induction motors are the most common motors used for various equipments in industry. ComponentsRotor

• Squirrel cage• Wound rotor

Stator

Single Induction motor

•Single phase motors are very widely used in home, offices, workshops etc. as power delivered to most of the houses and offices is single phase. In addition to this, single phase motors are reliable, cheap in cost, simple in construction and easy to repair.

3 phase Induction Motor

• StatorAs its name indicate stator is a stationary part of induction motor. A three phase supply is given to the stator of induction motor.• RotorThe rotor is a rotating part of induction motor. The rotor is connected to the mechanical load through the shaft. The rotor of the three phase induction motor are further classified as• Squirrel cage rotor• Slip ring rotor or wound rotor or phase wound rotor

Squirrel Cage Induction Motor

•Advantages of squirrel cage induction rotor

1. Its construction is very simple and rugged2. as there are no brushes and slip ring, these motors requires less maintenance.

•Applications:Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing machines etc

Slip Ring Induction Motor

• Slip ring or wound three phase induction motor : In this type of three phase induction motor the rotor is wound for the same number of poles as that of stator but it has less number of slots and has less turns per phase of a heavier conductor.The rotor also carries star or delta winding similar to that of stator winding. The rotor consists of numbers of slots and rotor winding are placed inside these slots.

•Advantages of slip ring induction motor

1. It has high starting torque and low starting current.2. Possibility of adding additional resistance to control speed

•Applications:Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc

Synchronous motor As the name suggests

Synchronous motors are capable of running at constant speed irrespective of the load acting on them.

Synchronous motors have got higher efficiency than its counterparts. Its efficiency ranges from 90 – 92%.

AC Synchronous motor

Stator: It carries three phase winding and is fed from 3-phase supply.

Rotor: It carries permanent magnet poles that rotate exactly with same speed as that of the stator magnetic field, hence the name synchronous motor.

It is not self starting.

Synchronous Motor-Principle

22

The rotor acting as a bar magnet will turn to line up with the rotating magnet field. The rotor gets locked to the RMF and rotates unlike induction motor at synchronous speed under all load condition

Synchronous SpeedSpeed at which RMF rotates or Synchronous speed can easily be derived as follows,

speed of synchronous motor can be very accurately controlled. This is the reason why they are suitable for high precision applications.

Why Synchronous motors are not self starting ?

North Pole of the Rotor will obviously get attracted by South Pole of RMF, and will start to move in the same direction. But since the rotor has got some inertia, this starting speed will be very low. So it will give repulsive force. This will make the rotor move backward. As a net effect the rotor won’t be able to start.

Making Synchronous Motor Self Start• To make synchronous motor self start, a squirrel cage

arrangement is cleverly fitted through pole tips. They are also called as damper windings.

At the starting rotor field coils are not energized. So with revolving magnetic field, electricity is induced in squirrel cage bars and rotor starts rotating just like an induction motor starts.

Out of Synchronism

•Synchronous motors will produce constant speed irrespective of motor load only if the load is within the capability of motor. If external torque load is more than torque produced by the motor, it will slip out of synchronism and will come to rest. Low supply voltage and excitation voltage are other reasons of going out of synchronism

Application of Synchronous Motor

• Synchronous motor having no load connected to its shaft is used for power factor improvement. it is used in power system in situations where static capacitors are expensive.

• Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500KW, the size, weight and cost of the corresponding induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc