Electrical Machines

What is an electric motor?An electric motor converts electrical power to mechanical power in its rotor.

Introduction

What are the two types of motor?AC and DC. They differentiate from the power that is supplied. One uses AC while the other DC.

Introduction

What is a DC motor?A DC is a type of motor where power is supplied to the rotating device by means of electromagnetic induction.

Introduction

What are the main types of AC Motor?There are two main types of AC motors i.e. synchronous and induction motor we will focus on the latter.

Introduction

What is an induction motor?An induction motor is a type of AC motor where power is supplied to the rotating device by means of electromagnetic induction.

Introduction

• These are perhaps the most important form of AC motor• Rather than use slip rings to pass current to the field coils in the rotor,

current is induced in the rotor by transformer action• The stator is similar to that in a synchronous motor• The rotor is simply a set of parallel conductors shorted together at

either end by two conducting rings

Introduction

Who created the induction motor?Nikola Tesla

Introduction

It is the most widely used electrical motor.Almost 80% of the mechanical power used by industries is provided by three phase induction motor because of its simple and rugged construction, low cost, good operating characteristics, absence of commutator and good speed regulation.In three phase induction motor, the power is transferred from stator to rotor winding through induction.

Induction Motor

Construction

Like all motors the Induction Motors consist of two major parts.

Stator : The stator is the stationary electrical component it is built by putting together iron layers forming a group of individual magnets arranged in such a way that they form a hollow cylinder with one pole of each magnet facing the center this is usually made of cold rolled grain oriented silicon steel to reduce hysteresis and eddy current loss the periphery of this cylindrical stator is provided with a number of slots.A Three-Phase copper winding is distributed among these slots that when a Three-Phase power is supplied a rotating magnetic field is produced this magnetic field rotates at a synchronous speed.

Induction Motor

3-Phase

Induction Motor

3-Phase

Slots

InducedRotating Magnetic Field Three-Phase Power

Windings

Stator of Three Phase Induction MotorThe stator of the three phase induction motor consists of three main parts : • Stator frame,• Stator core,• Stator winding or field winding.

Induction Motor

3-Phase

Stator frame• It is the outer most part of the three phase induction motor. Its

main function is to support the stator core and the field winding. It acts as a covering and it provide protection and mechanical strength to all the inner parts of the induction motor. The frame is either made up of die cast or fabricated steel. The frame of three phase induction motor should be very strong and rigid as the air gap length of three phase induction motor is very small, otherwise rotor will not remain concentric with stator, which will give rise to unbalanced magnetic pull.

Induction Motor

3-Phase

Stator core• The main function of the stator core is to carry the alternating

flux. In order to reduce the eddy current loss, the stator core is laminated. These laminated types of structure are made up of stamping which is about 0.4 to 0.5 mm thick. All the stamping are stamped together to form stator core, which is then housed in stator frame. The stamping is generally made up of silicon steel, which helps to reduce the hysteresis loss occurring in motor.

Induction Motor

3-Phase

Stator Winding or Field Winding• The slots on the periphery of stator core of the

three phase induction motor carries three phase windings. This three phase winding is supplied by three phase ac supply. The three phases of the winding are connected either in star or delta depending upon which type of starting method is used. The squirrel cage motor is mostly started by star – delta starter and hence the stator of squirrel cage motor is delta connected. The slip ring three phase induction motor are started by inserting resistances so, the stator winding of slip ring induction motor can be connected either in star or delta. The winding wound on the stator of three phase induction motor is also called field winding and when this winding is excited by three phase ac supply it produces a rotating magnetic field.

Induction Motor

3-Phase

Rotor: is the rotating electrical component of the motor and is connected to the mechanical load through the shaft. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis. The rotor is located inside the stator.

Induction Motor

Rotor

StatorShaft

Conductor

Rotor: is the rotating electrical component of the motor and is connected to the mechanical load through the shaft. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis. The rotor is located inside the stator.

The rotor of the three phase induction motor is further classified as:• Squirrel cage rotor,• Slip ring rotor or wound rotor or phase wound rotor.The other parts, which are required to complete the induction motor, are:• Shaft for transmitting the torque to the load. This shaft is made up of steel.• Bearings for supporting the rotating shaft. • One of the problems with electrical motor is the production of heat during

its rotation. In order to overcome this problem we need fan for cooling.• For receiving external electrical connection Terminal box is needed.• There is a small distance between rotor and stator which usually varies from

0.4 mm to 4 mm. Such a distance is called air gap.

Induction Motor

Squirrel-cage induction motor

Induction Motor

Initially when the rotor is inactive the flux of the rotating magnetic field cuts every solid conductor of the rotor. As the rotor conductors are stationary a sinusoidal stator field is rotating, there will be a change in flux linkage with the rotor conductors. As a result there will be an induced current in the rotor conductors as per Faraday’s Law of electromagnetic induction. Whenever this current starts flowing through the conductor, it reacts with the magnetic field.

A current carrying conductor inside a magnetic field experiences a mechanical force hence the mechanical force will be experienced by the rotor conductors due to this force the rotor will start rotating inside the stator the rotor will try to achieve the speed of the rotating magnetic field once the rotor achieves the speed of rotating magnetic field there will be no relative speed difference between the rotor and the rotating magnetic field hence no current will be induced. That means at that condition no mechanical force acts on the rotor hence it decelerates. As the rotor decelerates there will be a speed difference between the rotor and magnetic field and due to this event will begin accelerating again. (Fig 1-A)

Induction Motor

3-Phase

How it

Works

Due to this phenomenon the rotor will never raise the speed of the rotating magnetic field and the rotor will not stop rotating till the Three-Phase power is there in the stator. The rotor rotation speed is slightly lower than the magnetic field.

Induction Motor

3-Phase

How it

Works

Fig 1 – A

Squirrel cage three phase induction motor: The rotor of the squirrel cage three phase induction motor is cylindrical in shape and have slots on its periphery. The slots are not made parallel to each other but are bit skewed (skewing is not shown in the figure of squirrel cadge rotor beside) as the skewing prevents magnetic locking of stator and rotor teeth and makes the working of motor more smooth and quieter. The squirrel cage rotor consists of aluminum, brass or copper bars (copper bras rotor is shown in the figure beside). These aluminum, brass or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor. The rotor conductors are permanently shorted by the copper or aluminum rings called the end rings. In order to provide mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as "squirrel cage induction motor". The squirrel cage rotor winding is made symmetrical. As the bars are permanently shorted by end rings, the rotor resistance is very small and it is not possible to add external resistance as the bars are permanently shorted. The absence of slip ring and brushes make the construction of Squirrel cage three phase induction motor very simple and robust and hence widely used three phase induction motor. These motors have the advantage of adapting any number of pole pairs. The below diagram shows squirrel cage induction rotor having aluminum bars short circuit by aluminum end rings.

Induction Motor

3-Phase

Types

Advantages of squirrel cage induction rotor-Its construction is very simple and rugged.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.

Induction Motor

3-Phase

Types

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. The three end terminals are connected together to form star connection. As its name indicates three phase slip ring induction motor consists of slip rings connected on same shaft as that of rotor. The three ends of three phase windings are permanently connected to these slip rings. The external resistance can be easily connected through the brushes and slip rings and hence used for speed control and improving the starting torque of three phase induction motor. The brushes are used to carry current to and from the rotor winding. These brushes are further connected to three phase star connected resistances. At starting, the resistance are connected in rotor circuit and is gradually cut out as the rotor pick up its speed. When the motor is running the slip ring are shorted by connecting a metal collar, which connect all slip ring together and the brushes are also removed. This reduces wear and tear of the brushes. Due to presence of slip rings and brushes the rotor construction becomes somewhat complicated therefore it is less used as compare to squirrel cage induction motor.

Induction Motor

3-Phase

Types

Advantages of slip ring induction motor - It has high starting torque and low starting current.Possibility of adding additional resistance to control speed.Application: Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc.

Induction Motor

3-Phase

Types

Single-phase induction motors are used the most when it comes to the usage induction motors today. This type of AC motor is the least expensive and requires the lowest maintenance, and thus is used the most. Single-phase induction motors use concise type of wire loops on a frame and attain their torque from currents stimulated in these loops by the altering magnetic field formed in the stator (stationary) twirls.

The Voltage that is induced in the coil results in a formation of a clockwise torque unlike the 3-phase induction motor, the stator frame in the single –phase motor does not revolve. Instead it merely alternates divergence between poles as the AC voltage changes polarization.

Induction Motor

1-Phase

How it

Works

When single phase ac supply is given to the stator winding of single phase 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 conductors. 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 current is 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. (Fig 2-A)

Induction Motor

1-Phase

How it

Works

When single phase ac supply is given to the stator winding of single phase 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 conductors. 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 current is 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. (Fig 2-A)

Induction Motor

1-Phase

How it

Works

Due to this some iterations of the Single Phase Induction motors are not self-starting by nature because the two fluxes cancel each other out hence there is no net torque. But when it is moved the phases go out of equilibrium and one flux produces more torque than the other hence retaining the speed. (Double Revolving Field Theory)

A simple solution to this is to create an auxiliary winding from the first one to reinforce a specific flux and add a capacitor to cancel out of the flux hence you get one strong big flux that makes the Single-phase Motor a self-starting motor.

Induction Motor

1-Phase

How it

Works

Resulting to a non-self starting motor

Induction Motor

1-Phase

How it

Works

Fig 2-A

Note:Single-phase induction motors are not self-starting without an auxiliary stator winding driven by an out of phase current of near 90o. Once started the auxiliary winding is optional.The auxiliary winding of a permanent-split capacitor motor has a capacitor in series with it during starting and running.A capacitor-start induction motor only has a capacitor in series with the auxiliary winding during starting.A capacitor-run motor typically has a large non-polarized electrolytic capacitor in series with the auxiliary winding for starting, then a smaller non-electrolytic capacitor during running.The auxiliary winding of a resistance split-phase motor develops a phase difference versus the main winding during starting by virtue of the difference in resistance.

Induction Motor

1-Phase

How it

Works

Comparison between Single Phase and Three Phase Induction Motors

• Single phase induction motors are simple in construction, reliable and economical for small power rating as compared to three phase induction motors.

• The electrical power factor of single phase induction motors is low as compared to three phase induction motors.

• For same size, the single phase induction motors develop about 50% of the output as that of three phase induction motors.

• The starting torque is also low for asynchronous motors.

• The efficiency of single phase induction motors is less as compare it to the three phase induction motors.

• Three phase induction motor (self starting in nature)• Single phase induction motor (not self starting)

Comparison

and

Contrast

Questions

What are the techniques used to produce a desired speed?The most commonly used technique is Pulse Width Modulation in which a DC signal is switched on and off very rapidly, producing a sequence of electrical pulses to the inductor windings.

Questions

Difference between DC motors and Induction MotorsThe induction motor has no brushes and is easy to control, many older DC motors are being replaced with induction motors and accompanying inverters in industrial applications.

Questions

What is the difference between Synchronous and Induction Motors?Induction Motors are asynchronous and Synchronous motors are… well.. Synchronous.

Questions

How an Induction Motor is started? Why the Starter is used?

When the motor is started, the slip is equal to 1 as the rotor speed is zero, so the induced emf in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains. Similarly, when an induction motor starts, a very high current is drawn by the stator, on the order of 5 to 9 times the full load current. This high current can damage the motor windings and because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, the starting current should be limited. A soft start starter is a device which limits the starting current by providing reduced voltage to the motor. Once the rotor speed increases, the full rated voltage is given to it.

Questions

Induction Motor = Rotating Transformer Why?An induction motor is sometimes called a rotating transformer because the stator in essence the primary side of the transformer and the rotor is the secondary side.

References:• http://www.allaboutcircuits.com/textbook/alternating-current

/chpt-13/single-phase-induction-motors/

• http://www.slideshare.net/vashielectricals/induction-motorits2maintypes

• Storey: Electrical & Electronic Systems, Pearson Education Limited, 2004

• http://www.electrical4u.com/construction-of-three-phase-induction-motor/