chapter 8a-ac motor .pptx

AC MOTOR

INTRODUCTION AC motor is more simple and much

greater capability than DC motor in the same physical condition

Rugged/heavy duty

Less maintenance – no brushes

but

difficult to control….??????

TYPES OF AC MOTOR

Depends on the type of rotor used :

1. The induction motor - speed slightly slower than the supply frequency

2. The synchronous motor - rotates exactly at the supply frequency

INDUCTION MOTORS

General The induction machine is used as the most

common motors in different applications. It is the workhorse of industry.

It has a stator and a rotor like other type of motors.

2 different type of rotors:

1. Squirrel-cage winding,2. Wound-rotor (special characteristics)

Majority of the motors used by industry are squirrel-cage induction motors.

INDUCTION MOTORS - cnt’d

Induction motor components

INDUCTION MOTORS - cnt’d

• Induction motor Construction

A typical motor consists of two parts:

1-An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, (STATOR)

2-An inside rotor attached to the output shaft that is given a torque by the rotating field (ROTOR)

Squirrel-Cage Rotor Rotor is from laminated iron core with

slots. Metal (Aluminum) bars are molded in the

slots instead of a winding. Two rings short circuits the bars. One or 2 fans are attached to the shaft in

the sides of rotor to cool the circuit

StatorRotor

Consists of copper bars, which are pushed into the slots in body of the rotor.

The ends are welded to copper end rings, so that all the bars are short circuited.

Most common

Rotor• Two-types of rotor windings:

1) Squirrel-cage windings (most common)

Squirrel-Cage Rotor

Rotor2). Wound Rotor• It is usually for large 3 phase induction motors.

• Rotor has a winding the same as stator and the end of each phase is connected to a slip ring.

• Expensive and require maintenance of the slip rings and brushes,

• Was the standard form for variable speed control before the advent of motor

The stator of an induction motor is laminated iron core with slots similar to a stator of a synchronous machine.

Stator construction

Coils are placed in the slots to form a three or single phase winding.

•Single phase induction motor• Single phase induction motors generally have a construction similar

to that of a three phase motor: an ac windings is placed on the stator, short-circuited conductors are placed in a cylindrical rotor. The significant difference is, of-course, that there is only a single phase supply to the stator.

PRINCIPLES OPERATION OF INDUCTION MOTOR

Rotor bars(cage)

Operating Principle (Induction Motor)

All the following events take place simultaneously:

1.A voltage E = BLV is induced in each conductor while it is being cut by the flux (Faraday’s Law)

2. The induced voltage produces currents which circulate in a loop around the conductors (through the bars).

3. Since the current-carrying conductors lie in a magnetic field, they experience a mechanical force (Lorentz force).

4. The force always acts in a direction to drag the conductor along with the magnetic field.

• Now close the ladder upon itself to form a squirrel cage, and place it in a rotating magnetic field – you have an induction motor!

As the motor starting from rest, it will gain speed. Motor accelerates and try to catch the synchronous speed.Imagine …..!!!If the motor suddenly rotates at the same speed of synchronous speed, what would happen?There’ll be no flux cutting, hence no

induced voltage (current). So no force experienced by the rotor bars. The rotor accelerates until the magnitude of induced rotor current and torque balances the applied load. Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates slower than synchronous speed – “SLIP”.

Consider a simple stator with 6 salient poles (2 poles per phase) - windings AN, BN, CN.

The windings are mechanically spaced at 120° from each other.

The windings are connected to a 3- phase source. AC currents ia, ib and ic will flow in the windings, but will

be displaced in time by 120°. Each winding produces its own MMF, which creates a

flux across the hollow interior of the stator The 3 fluxes combine to produce a magnetic field that

rotates at the same frequency as the supply. Such a magnetic flux produced by balanced three phase

currents flowing in thee-phase windings is called a rotating magnetic flux(RMF). RMF rotates with a constant speed (Synchronous Speed).

• .• .

Rotating Magnetic Field

The synchronous speed of an AC motor is the rotation rate of the rotating magnetic field created by the stator.

               where f is the frequency of the AC supply current in Hz and p is the number of magnetic pole pairs per phase.

This shows, speed of induction motor is controlled mainly by source frequency.

Synchronous speed, NS

‘Slip’ in an Induction Motor

The slip s is defined as 'the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm or in percent or ratio of synchronous speed'

X100%

where ns is stator electrical

speed, nr is rotor mechanical speed

‘Slip’ in an Induction MotorThe slip s is defined as 'the difference between synchronous

speed and operating speed, at the same frequency, expressed in rpm or in percent or ratio of synchronous speed'