Electrical Machines(AC Motors/Induction Motor)

By:-Anil KumarVerma

M.Sc.- III Sem. (ELECTRONICS)PAPER-IV UNIT-III

SOS in Electronics & PhotonicsPt. R.S. U. Raipur (C.G.)

By:-Anil KumarVerma

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Electric Motors

Alternating Current (AC)Motors

Direct Current (DC)Motors

Classification of Motors

Synchronous Induction

Three-PhaseSingle-Phase

Self ExcitedSeparatelyExcited

Series ShuntCompound

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Motors are categorized on the basis of input supply,construction and operation principles

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". The stator is in the stationary electrical component.The rotor is the rotating electrical component, which in turnrotates the motor shaft.

The main advantage of DC motors over AC motors is thatspeed is more difficult to control for AC motors. Tocompensate for this, AC motors can be equipped withvariable frequency drives but the improved speed controlcomes together with a reduced power quality.

There are two types of AC motors: synchronous (see figure)and induction. The main difference between thesynchronous motor and the induction motor is that the rotorof the synchronous motor travels at the same speed as therotating magnetic field.3

Motors are categorized on the basis of input supply,construction and operation principles

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". The stator is in the stationary electrical component.The rotor is the rotating electrical component, which in turnrotates the motor shaft.

The main advantage of DC motors over AC motors is thatspeed is more difficult to control for AC motors. Tocompensate for this, AC motors can be equipped withvariable frequency drives but the improved speed controlcomes together with a reduced power quality.

There are two types of AC motors: synchronous (see figure)and induction. The main difference between thesynchronous motor and the induction motor is that the rotorof the synchronous motor travels at the same speed as therotating magnetic field.

AC Motors• Electrical current reverses direction

• Two parts: stator and rotor• Stator: stationary electrical component• Rotor: rotates the motor shaft

• Speed difficult to control

• Two types• Synchronous motor• Induction motor

• Electrical current reverses direction

• Two parts: stator and rotor• Stator: stationary electrical component• Rotor: rotates the motor shaft

• Speed difficult to control

• Two types• Synchronous motor• Induction motor

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AC Motors – Synchronous motor• Constant speed fixed by system

frequency• DC for excitation and low starting torque:

suited for low load applications• Can improve power factor: suited for high

electricity use systems• Synchronous speed (Ns):

• Constant speed fixed by systemfrequency

• DC for excitation and low starting torque:suited for low load applications

• Can improve power factor: suited for highelectricity use systems

• Synchronous speed (Ns):

Ns = 120 f / P F = supply frequencyP = number of poles

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AC Motors – Induction motor• Most common motors in industry

• Advantages:• Simple design

• Inexpensive

• High power to weight ratio

• Easy to maintain

• Direct connection to AC power source

• Most common motors in industry

• Advantages:• Simple design

• Inexpensive

• High power to weight ratio

• Easy to maintain

• Direct connection to AC power source

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AC Motors – Induction motorComponents

• Rotor• Squirrel cage: conducting bars in parallel slots

• Wound rotor: 3-phase, double-layer, distributedwinding

Components

• Rotor• Squirrel cage: conducting bars in parallel slots

• Wound rotor: 3-phase, double-layer, distributedwinding

• Stator• Stampings with slots to carry 3-phase windings• Wound for definite number of poles

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How induction motors work• Electricity supplied to stator• Magnetic field generated that moves around rotor• Current induced in rotor

• Rotor produces second magnetic fieldthat opposes stator magnetic field

• Rotor begins to rotate

• Rotor produces second magnetic fieldthat opposes stator magnetic field

• Rotor begins to rotate Electromagnetics

Stator

Rotor

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Three-phase induction motor

• Three-phase supply produces magnetic field• Squirrel cage or wound rotor• Self-starting• High power capabilities• 1/3 to hundreds HP applications: pumps,

compressors, conveyor belts, grinders• 70% of motors in industry!

• Three-phase supply produces magnetic field• Squirrel cage or wound rotor• Self-starting• High power capabilities• 1/3 to hundreds HP applications: pumps,

compressors, conveyor belts, grinders• 70% of motors in industry!

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Speed and slip

• Motor never runs at synchronousspeed but lower “base speed”

• Difference is “slip”• Install slip ring to avoid this• Calculate % slip:

• Motor never runs at synchronousspeed but lower “base speed”

• Difference is “slip”• Install slip ring to avoid this• Calculate % slip:

% Slip = Ns – Nb x 100Ns

Ns = synchronous speed in RPMNb = base speed in RPM

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Relationship load, speed and torque

At start: highcurrent andlow “pull-up”torque

At 80% of fullspeed:highest “pull-out” torqueand currentdrops

At start: highcurrent andlow “pull-up”torque

At 80% of fullspeed:highest “pull-out” torqueand currentdrops

At full speed:torque andstator currentare zero

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Efficiency of Electric Motors

Motors loose energy when serving a load

• Fixed loss

• Rotor loss

• Stator loss

• Friction and rewinding

• Stray load loss

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Motors loose energy when serving a load

• Fixed loss

• Rotor loss

• Stator loss

• Friction and rewinding

• Stray load loss

Efficiency of Electric Motors Factors that influence efficiency• Age• Capacity• Speed• Type• Temperature• Rewinding• Load

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Factors that influence efficiency• Age• Capacity• Speed• Type• Temperature• Rewinding• Load

Motor Load

• Motor load is indicator of efficiency

• Equation to determine load:

Load = Pi x HP x 0.7457

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Load = Pi x HP x 0.7457

= Motor operating efficiency in %HP = Nameplate rated horse powerLoad = Output power as a % of rated powerPi = Three phase power in kW

Input power measurement

• Three steps for three-phase motors

Step 1. Determine the input power:

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Input power measurement

• Three steps for three-phase motors

Step 1. Determine the input power:

Input power measurement

• Three steps for three-phase motors

Step 1. Determine the input power:

Stepper Motors Also called Stepping or Step motors.

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