Three Phase Induction Motor: Generation of Rotating Magnetic Fields, Construction and Working of a 3-ph Induction Motor, Squirrel cage IM, Slip-ring IM

Rotating Magnetic Field

Rotating magnetic field is magnetic field which rotates in space about some point or axis. The North and South poles continuously rotates with a specific speed, called synchronous speed. To understand the generation of the magnetic field we will first imagine one stator of an electric motor where three-phase winding is physically distributed in the stator core in such a manner that winding of each phase is separated from other by 120o in space.

stator of rotating machine

Although the vector sum of three currents in a balanced three-phase system is zero at any instant, but the resultant of the magnetic fields produced by the currents is not zero rather it will have a constant non-zero value rotating in space in respect to time.

The magnetic flux produced by the current in each phase can be represented by the equations given below. This is a similar representation of current is a three-phase system as the flux is cophasial with the current.

Where, φR, φY and φB are the instantaneous flux of corresponding Red, Yellow and Blue phase winding, φm amplitude of the flux wave. The flux wave in the space can be represented as shown below.

Now, on the above graphical representation of flux waves, we will first consider the point 0.

Here, the value of φR is
The value of φY is
The value of φB is
The resultant of these fluxes at that instant (φr) is 1.5φm which is shown in the figure below.

Now, on the above graphical representation of flux waves, we will consider the point 1, where ωt = π / 6 or 30o.
Here, the value of φR is

The value of φY is

The value of φB is

The resultant of these fluxes at that instant (φr) is 1.5φm which is shown in the figure below. here it is clear that the resultant flux vector is rotated 30o further clockwise without changing its value.

Now, on the graphical representation of flux waves, we will consider the point 2, where ωt = π / 3 or 60o.
Here, the value of φR is

The value of φY is

The value of φB is

The resultant of these fluxes at that instant (φr) is 1.5φm which is shown in the figure below. here it is clear that the resultant flux vector is rotated 30° further clockwise without changing its value.

Now, on the graphical representation of flux waves, we will consider the point 3, where ωt = π / 2 or 90o.
Here, the value of φR is

The value of φY is

The value of φB is

Induction Motor

Machine:-

Machine is a device which converts one form of energy to another form.

Motor:-

A machine which converts electrical energy to mechanical energy is called as motor.

AC MOTOR:-

AC motor is an electrical device which converts electrical energy into mechanical energy. The conversion of electrical energy to mechanical energy is through a coupling medium of magnetic flux. This magnetic flux is set-up in the air gap of AC Motor. Electrical energy supplied to the stator is first converted into magnetic energy in the air gap, which in turn is converted into mechanical energy. This is the reason, AC Motor is known as electromagnetic device.

Classification of AC Motor:

Based on the working principle, there are mainly three types of AC motors: Induction motor, Synchronous Motor and AC Commutator Motor.

An induction motor can further be of two types: Single Phase Induction Motor and Three Phase Induction Motor. This classification of induction motor is based on the type of power supply required for its operation. 1 phase AC supply is required for single phase induction motor whereas 3 phase AC supply is needed for three phase induction motor.

On the basis of construction, there are two types of induction motor: Squirrel cage Induction Motor and Slip Ring Induction Motor. Squirrel cage induction motor is most used motor in industries. Slip Ring Induction motor is used where speed control is required.

General Principal Of Induction Motor:-

The motor which works on the principle of electromagnetic induction is known as the induction motor. Electromagnetic induction is the phenomenon in which the electromotive force induces across the electrical conductor when it is placed in a rotating magnetic field.

Advantages of Induction Motor:

The most important advantage of an induction motor is that its construction is quite simple in nature. The construction of the Stator is similar in both Synchronous motors as well as induction motors. However, a slip ring is required to feed DC Supply to the Rotor in the case of a Synchronous Generator. These Slip rings are not required in a Squirrel cage induction motor because the windings are permanently short circuited. When compared with a DC Motor, the induction motor does not have Brushes and hence, maintenance required is quite low. This leads to a simple construction.
The working of the motor is independent of the environmental condition. This is because the induction motor is mechanically strong.
A Squirrel cage induction motor does not contain Brushes, Slip rings and Commutators. Due to this reason, the cost of the motor is quite low.
Due to the absence of Brushes, there are no sparks in the motor. It can also be operated in hazardous conditions.
Unlike synchronous motors, a 3 phase induction motor has a high starting torque, good speed regulation and reasonable overload capacity.
An induction motor is a highly efficient machine with full load efficiency varying from 85 to 97 percent.

Disadvantages of Induction Motor:-

A single phase induction motor, unlike a 3 phase induction motor, does not have a self starting torque. Auxiliaries are required to start a single phase motor.
During light load conditions, the power factor of the motor drops to a very low value. This is because during the start, the motor draws a large magnetizing current to overcome the reluctance offered by the air gap between the Stator and the Rotor. Also, the induction motor will take very less current from the supply main. The vector sum of Load current and Magnetizing current lags the voltage by around 75-80 degrees and hence, the power factor is low. Due to high magnetizing current, the copper losses of the motor increase. This in turn leads to decrease in the efficiency of the motor.
Speed control of an induction motor is very difficult to attain. This is because a 3 phase induction motor is a constant speed motor and for the entire loading range, the change in speed of the motor is very low.
Induction motors have high input surge currents, which are referred to as Magnetizing Inrush currents. This causes a reduction in voltage at the time of starting the motor.
Due to poor starting torque, the motor cannot be used for applications which require high starting torque.

Parts of an INDUCTION MOTOR :-

Squirrel Cage Induction Motor:-

A 3 phase squirrel cage induction motor is a type of three phase induction motor which functions based on the principle of electromagnetism. It is called a ‘squirrel cage’ motor because the rotor inside of it – known as a ‘squirrel cage rotor’ – looks like a squirrel cage.

This rotor is a cylinder of steel laminations, with highly conductive metal (typically aluminium or copper) embedded into its surface. When an alternating current is run through the stator windings, a rotating magnetic field is produced.

This induces a current in the rotor winding, which produces its own magnetic field. The interaction of the magnetic fields produced by the stator and rotor windings produces a torque on the squirrel cage rotor.

One big advantage of a squirrel cage motor is how easily you can change its speed-torque characteristics. This can be done by simply adjusting the shape of the bars in the rotor. Squirrel cage induction motors are used a lot in industry – as they are reliable, self-starting, and easy to adjust.

Squirrel Cage Induction Motor Working Principle:-

When a 3 phase supply is given to the stator winding it sets up a rotating magnetic field in space. This rotating magnetic field has a speed which is known as the synchronous speed.

This rotating magnetic field induces the voltage in rotor bars and hence short-circuit currents start flowing in the rotor bars. These rotor currents generate their self-magnetic field which will interact with the field of the stator. Now the rotor field will try to oppose its cause, and hence rotor starts following the rotating magnetic field.

The moment rotor catches the rotating magnetic field the rotor current drops to zero as there is no more relative motion between the rotating magnetic field and rotor. Hence, at that moment the rotor experiences zero tangential force hence the rotor decelerates for the moment.

After deceleration of the rotor, the relative motion between the rotor and the rotating magnetic field re-establishes hence rotor current again being induced. So again, the tangential force for rotation of the rotor is restored, and therefore again the rotor starts following rotating magnetic field, and in this way, the rotor maintains a constant speed which is just less than the speed of rotating magnetic field or synchronous speed.

Slip is a measure of the difference between the speed of the rotating magnetic field and rotor speed. The frequency of the rotor current = slip × supply frequency.

Squirrel Cage Induction Motor Construction:-

A squirrel cage induction motor consists of the following parts:

Stator
Rotor
Fan
Bearings

Squirrel Cage Induction Motor Working Principle

Stator

It consists of a 3 phase winding with a core and metal housing. Windings are such placed that they are electrically and mechanically 120o apart from in space. The winding is mounted on the laminated iron core to provide low reluctance path for generated flux by AC currents.

Rotor:-

It is the part of the motor which will be in a rotation to give mechanical output for a given amount of electrical energy. The rated output of the motor is mentioned on the nameplate in horsepower. It consists of a shaft, short-circuited copper/aluminium bars, and a core.

Application of Squirrel Cage Induction Motor:-

Squirrel cage induction motors are commonly used in many industrial applications. They are particularly suited for applications where the motor must maintain a constant speed, be self-starting, or there is a desire for low maintenance.

These motors are commonly used in:

Centrifugal pumps
Industrial drives (e.g. to run conveyor belts)
Large blowers and fans
Machine tools
Lathes and other turning equipment

Advantages of Squirrel Cage Induction Motor:-

Some advantages of squirrel cage induction motors are:

They are low cost
Require less maintenance (as there are no slip rings or brushes)
Good speed regulation (they are able to maintain a constant speed)
High efficiency in converting electrical energy to mechanical energy (while running, not during startup)
Have better heat regulation (i.e. don’t get as hot)
Small and lightweight
Explosion proof (as there are no brushes which eliminate the risks of sparking)

Disadvantages of Squirrel Cage Induction Motor:-

Very poor speed control
Although they are energy efficient while running at full load current, they consume a lot of energy on startup
They are more sensitive to fluctuations in the supply voltage. When the supply voltage is reduced, induction motor draws more current. During voltage surges, increase in voltage saturates the magnetic components of the squirrel cage induction motor
They have high starting current and poor starting torque (the starting current can be 5-9 times the full load current; the starting torque can be 1.5-2 times the full load torque)

Slip Ring Induction Motor

A slip ring induction motor is referred to as an asynchronous motor as the speed at which it operates is not equal to the synchronous speed of a rotor. The rotor of this type of motor is wound type. It comprises of a cylindrical laminated steel core and a semi-closed groove at the outer boundary to accommodate a 3-phase insulated winding circuit.

Construction

The construction includes two parts: Stator and Rotor.

Stator

The stator of this motor comprises of various slots that are arranged to support the construction of a 3-phase winding circuit connecting to a 3-phase AC source.

Rotor

The rotor of this motor consists of a cylindrical core with steel laminations. Besides this, the rotor has parallel slots to accommodate 3-phase windings. The windings in these slots are arranged at 120 degrees to each other. This arrangement can reduce noise and avoid irregular pausing of a motor.

Working of Slip Ring Induction Motor

This motor runs on the principle of Faraday’s law of electromagnetic induction. When a stator winding is excited with an AC supply, the stator winding produces magnetic flux. Based on faraday’s law of electromagnetic induction, the rotor winding gets induced and generates a current of magnetic flux. This induced EMF develops torque that enables the rotor to rotate.

However, the phase difference between the voltage and current do not meet the requirements to generate high starting torque as torque developed is not unidirectional. The external resistance of high value is connected with the circuit to improve the phase difference of a motor. As a result, inductive reactance and phase difference between I and V is reduced. Consequently, this reduction helps the motor to generate high stating torque. The slip ring induction motor diagram is shown below.

Slip Ring Induction Motor Connection Diagram

Applications:-

Some of the applications of slip ring induction motor are
These motors are used where higher torque and low starting current are required.
Used in applications like elevators, compressors, cranes, conveyors, hoists, and many more

Advantages of Slip Ring Induction Motor

The advantages are
High and excellent starting torque to support high inertia loads.
It has a low starting current due to external resistance
Can take full load current that is 6 to 7 times higher

Disadvantages of Slip Ring Induction Motor

The disadvantages are
Includes higher maintenance costs due to brushes and slip rings compared to squirrel cage motor
Intricate construction
High copper loss
Low efficiency and low power factor
Expensive than 3 phase squirrel cage induction motor.

Difference Between Squirrel Cage and Slip Ring Induction Motor:-

Sr. No.	Squirrel Cage Induction Motor	Slip Ring / Wound Rotor Induction Motor
1)	1. In Squirrel cage induction motors the rotor is simplest and most rugged in construction.	In slip ring induction motors the rotor is wound type. In the motor the slip rings, brushes are provided. Compared to squirrel cage rotor the rotor construction is not simple.
2)	Cylindrical laminated core rotor with heavy bars or copper or Aluminium or alloys are used for conductors.	Cylindrical laminated core rotor is wound like winding on the stator.
3)	Rotor conductors or rotor bars are short circuited with end rings.	At starting the 3 phase windings are connected to a star connected rheostat and during running condition, the windings are short circuited at the slip rings.

4)	Rotor bars are permanently short circuited and hence it is not possible to connect external resistance in the circuit in series with the rotor conductors.	It is possible to insert additional resistance in the rotor circuit. Therefore it is possible to increase the torque; the additional series resistance is used for starting purposes.
5)	Cheaper cost.	Cost is slightly higher.
6)	No moving contacts in the rotor.	Carbon brushes, slip rings etc are provided in the rotor circuit.
7)	Higher efficiency.	Comparatively less efficiency.
8)	Low starting torque. It has 1.5 time full load torque.	High starting torque. It can be obtained by adding external resistance in the rotor circuit .

9)	Speed control by rotor resistance is not possible.	Speed control by rotor resistance is possible.
10)	Starting current is 5 to 7 times the full load current.	Less starting current compared to squirrel cage Induction Motor.

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