What is induction motor

OAn induction motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.

Working principle of induction

motorO When ac supply is given to the stator

winding of induction motor, the alternating current starts flowing through the stator or main winding. This alternating current produces an alternating flux called main flux. This main flux also links with the rotor conductors and hence cut the rotor conductor.

FARADAY’S LAW OF ELECTROMAGNETIC

INDUCTION

According to the Faraday’s law of electromagnetic induction, emf gets

induced in the rotor. As the rotor circuit is closed one so, the current starts

flowing in the rotor. This currents called the rotor current. This

rotor current produces its own flux called rotor flux. Since this flux is

produced due to induction principle so, the motor working on this

principle got its name as induction motor. Now there are two fluxes one is

main flux and another is called rotor flux. These two fluxes produce the

desired torque which is required by the motor to rotate.

Induction motors are used worldwide inmany residential, commercial, industrial,and utility applications.

Induction Motors transform electricalenergy into mechanical energy.

It can be part of a pump or fan, orconnected to some other form ofmechanical equipment such as a winder,conveyor, or mixer.

It consists of two parts:

1. Stator - It is the stationary part of the motor.

2. Rotor - It is the rotating part of the motor.

STATOR

Stator has three main parts:

Outer Frame – It is the outer body of the of the motor. It protects the inner part of the machine.

Stator Core – Built up of high grade silicon steel. Carries the alternating magnetic field

Stator winding – Has a three phase winding.

WINDINGS

CORESLOTS

Effect of 3 Phase Current Passing Through a Stator Winding:

When a 3 phase AC current passes through the winding It produces a rotating magnetic field (RMF). As shown in the figure below a magnetic field is produced which is rotating in nature. We will see how this is produced in the next section.

The Concept of a Rotating Magnetic Field

To understand a rotating magnetic field, we consider a simplified 3 phase winding with just 3 coils.

A wire carrying current produces a magnetic field around it. Now for this special arrangement, the magnetic field produced by 3 phase A.C current will be as shown at a particular instant.

The components of A.C current will vary with time.

Due to the variation in the A.C current, the magnetic field also varies in orientation and its magnitude remains the same.

The speed of rotation of the magnetic field is known as synchronous speed.

The Effect of RMF on a Closed Conductor

Assume we are putting a closed conductor inside such a rotating magnetic field.

Since the magnetic field is fluctuating an E.M.F will be induced in the loop according to Faraday’s law. The E.M.F will produce a current through the loop.

The situation has become as if a current carrying loop is situated in a magnetic field. This will produce a magnetic force in the loop according to Lorentz law, So the loop will start to rotate.

ROTOR

There are two types of rotors which are employed in 3 – phase induction motor.

Squirrel Cage Rotor.

Phase Wound/ Slip Ring Rotor.

SQUIRREL CAGEROTOR

It consists of a laminated cylindrical corehaving semi closed circular slots at the outerperiphery.Copper or aluminum bar conductors areplaced in these slots and short circuited ateach end by copper or aluminum rings calledshort circuiting rings.The rotor winding is permanently shortcircuited and it is not possible to add anyexternal resistance.

The rotor slots are not parallel to the shaft

but skewed to

Reduce humming .

Provide smoother torque for different positions of rotor.

Reduce magnetic locking of stator and rotor.

PHASE WOUND ROTOR

It is also called SLIP RING ROTOR.

Consists of a laminated core having semiclosed slots at the outer periphery andcarries a 3-phase insulated winding.

The rotor is wound for the same number ofpoles as that of stator.

The three finish terminals are connectedtogether forming a star point and the threestar terminals are connected to three sliprings fixed on the shaft.

Construction (Enclosure)

The enclosure consists of a frame (or yoke) and

two end brackets (or bearing housings). The stator

is mounted inside the frame.

The rotor fits inside the stator with a slight air gap

separating it from the stator. There is NO direct

physical connection between the rotor and the

stator.

The enclosure also protects the electrical and

operating parts of the motor from harmful effects of

the environment in which the motor operates.

Bearings, mounted on the shaft, support the rotor

and allow it to turn. A fan, also mounted on the

shaft, is used on the motor shown below for

cooling.

Stator

Rotor

Air gap

The Working of an Induction MotorA 3-phase AC current passing through a Stator winding produces a rotating magnetic field.

current will be induced in the bars of the squirrel cage and it will start to rotate.

We can note variation of the induced current in squirrel cage bars.

This is due to the rate of change of magnetic flux in one squirrel bar pair which is different from another, due to its different orientation. This variation of current in the bar will change over time.

which is the most commonly used one in induction motors.

RMF produces a torque on rotor as

in the simple winding case.

Poles and speedO Every ac induction motor has poles, just like a magnet. However,

unlike a simple magnet, these poles are formed by bundles of magnet wire (windings) wound together in slots of the stator core.Inmost cases, you can look inside the motor and count the number of poles in the winding; they are distinct bundles of wire evenly spaced around the stator core.

O The number of poles, combined with the ac line frequency (Hertz, Hz), are all that determine the no-load revolutions per minute (rpm) of the motor. So, all four-pole motors will run at the same speed under no-load conditions, all six-pole motors will run at the same speed, and so on.

O The mathematical formula to remember in helping make this calculation is the number of cycles (Hz) times 60 (for seconds in a minute) times two (for the positive and negative pulses in the cycle) divided by the number of poles.

Therefore, for a 60-Hz system, the formula would be:

60 x 60 x 2 = 7,200 no-load rpm ÷ number of poles.

For a 50-Hz system, the formula would be:

50 x 60 x 2 = 6,000 no-load rpm ÷ number of poles.

Using this formula, you can see that a four-pole motor

operating on the bench under no-load conditions runs at 1,800

rpm (7,200 ÷ 4 poles). Note that when an ac motor is loaded,

the spinning magnetic field in the stator does not change

speed. Instead, the rotor or moving part of the motor is

restrained by the load from “catching up” to the field speed.

The difference between the field speed of 1,800 rpm in this

example and the rotor speed of approximately 1,725 rpm is

called the “slip.” Slip varies with the load over a narrow

operating range for each motor design.

Motor Speeds, Both Loaded and Unloaded Our spinning four-pole motor, then, operates at 1,800 rpm in

this example under no-load conditions and approximately 1,725 rpm under load. Motors of this speed are commonly found in belted applications such as blowers, fans, air-handling equipment, compressors, and some conveyors .A two-pole motor operates at 3,600 rpm (7,200 rpm ÷ 2) unloaded, and approximately 3,450 under load. Two-pole motors often are found in pump applications, such as sump pumps, swimming pool pumps, and water re circulating equipment.

One thing for the service technician to keep in mind in the field is that the higher the rpm, the noisier a motor may sound to the untrained ear. It is beneficial to become aware of the different speed-related sounds motors make.

found in ceiling fans.

O Six-pole motors run at 1,200 rpm unloaded (7,200 ÷ 6) and between 1,050 and 1,175 rpm loaded. They are often used for air-handling equipment, direct-drive applications, window fans, furnace blowers, room air conditioners, heat pumps, and other equipment where the relatively slower motor speed makes for quieter operation. All can come in either totally open, totally enclosed, or combination models, adding to their versatility.

O To satisfy consumers’ desires for quieter motors, manufacturers have developed eight-pole motors. These operate at 900 rpm (unloaded) and approximately 800 rpm under load. They are being used in applications where customers expect quieter operation, such as room air conditioners and outdoor heat pump applications.

O Less-common pole configurations include 12-pole motors (600 rpm) that are used in applications requiring slow speeds, such as washing machines, and 16-pole motors (450 rpm unloaded), often found in

electricity is induced in rotor by magnetic induction rather than direct electric connection , That's why the name induction motor is used.To aid such electromagnetic induction, insulated iron core lamina are packed inside the rotor.

Thin layers of iron lamina which are

packed in rotor

The Speed of Rotation of a RotorBoth the magnetic field and rotor are rotating.

To find the speed of the rotor let's consider different cases.

Consider a case where the rotor speed is same as the magnetic field speed.

Since both the magnetic field and the rotor are rotating at same speed, relative to the rotor, the magnetic field is stationary.

The rotor will experience a constant magnetic field, so there won’t be any induced e.m.f and current. This means zero force on the rotor bars, so the rotor will gradually slow down.

But as it slows down, the rotor loops will experience a varying magnetic field, so induced current and force will rise again and the rotor will speed up.

In short, the rotor will never be able to catch up with the speed of the magnetic field. It rotates at a specific speed which is slightly less than synchronous speed.

SlipThe difference between the flux (Ns) and therotor speed (N) is called slip.

% Slip = (Ns – N) × 100

Slip speed = Ns – N

Energy Transfer in the MotorIn an induction motor, electrical energy is enters via the Stator and output from the rotor, the mechanical rotation is received from the rotor.

But between the power input and output, there will be numerous energy losses associated with the motor. Various components of these losses are friction loss, copper loss, eddy current and hysteresis loss.

Such energy loss during the motor operation is dissipated as heat, so a fan at the other end helps in cooling down the motor.

A cooling fan is used to remove heat

liberated by motor

SPEED CONTROL OF AN INDUCTION MOTORThe speed of an induction motor can be easily controlled by varying the frequency of the 3-phase supply.

To maintain a constant (rated) flux density, the applied voltage must also be changed in the same proportion as the frequency (as dictated by Faraday’s law).

This speed control method is known as Volts per Hz.

Above rated speed, the applied voltage is usually kept constant at rated value; this operation is referred to as constant HP. At low frequencies (i.e. speeds), the voltage must be boosted in order to compensate for the effects of the stator resistance.

Advantages and disadvantages of induction motors

“Advantages”

They have only one moving part, the rotor, which makes them low-cost, quiet, long-lasting, and relatively trouble free.

DC motors, by contrast, have a commutator and carbon brushes that wear out and need replacing from time to time.

The friction between the brushes and the commutator also makes DC motors relatively noisy (and sometimes even quite smelly).

“Disadvantages”Since the speed of an induction motor depends on the frequency of the alternating current that drives it, it turns at a constant speed unless you use a variable-frequency drive.

the speed of DC motors is much easier to control simply by turning the supply voltage up or down.

Induction motors can be fairly heavy because of their coil windings.

Unlike DC motors, they can't be driven from batteries or any other source of DC power without using an inverter. That's because they need a changing magnetic field to turn the rotor.