SPECIAL ELECTRICAL MACHINESINTRODUCTION

ELECTRICAL MACHINES- IN GENERAL

DC MACHINES- MOTORS

AC MACHINES- GENERATOR

INDUCTION MOTORS- MOTOR

LIMITATIONS

DC Motor Armature and field voltage control

IM Motor Frequency and Stator voltage control

Alternator Designed for low synchronous speed

STATIC DEVICES- AFTER 1960’S

Innovation of Static Devices such as SCRs etc. DC motor- Variable DC supply IM Motor- Variable frequency drive

Later years development of Power transistors, MOFET, IGBT etc. increases the ease control

DEVELOPMENT OF TECHNOLOGY

Computer peripheral equipments Robotics Computer Numeric Control (CNC)

Machines Electrical vehicles

OVERALL COST

high material price, such as permanent magnet, copper, and iron

To reduce the cost Need to improve torque density

NON CONVENTIONAL MACHINES

Stepper Motor- position Control Reluctance Motor- medium traction Brushless DC motor- High Torque

Applications Linear IM- High Speed transportation

ADVANTAGES

Specific applications High torque density Low cost Better performance Easy to implement digital control

SYLLABUS-COVERAGE

Reluctance Motor- 2nd Module

Permanent Magnet Synchronous Motor- 4th Module

RELUCTANCE MOTORMODULE 2

ELECTRICAL MACHINES- CLASSIFICATION broadly classified into two categories

on the basis of how they produce torque electromagnetically or by variable reluctance.

TORQUE- ELECTROMAGNETICALLY

Motion is produced by the interaction of two magnetic fields, one generated by the stator and the other by the rotor.

Two magnetic fields, mutually coupled, produce an electromagnetic torque tending to bring the fields into alignment.

The same phenomenon causes opposite poles of bar magnets to attract and like poles to repel.

TORQUE- RELUCTANCE

In the second category, motion is produced as a result of the variable reluctance in the air gap between the rotor and the stator.

When a stator winding is energized, producing a single magnetic field, reluctance torque is produced by the tendency of the rotor to move to its minimum reluctance position.

This phenomenon is analogous to the force that attracts iron or steel to permanent magnets.

In those cases, reluctance is minimized when the magnet and metal come into physical contact.

RELUCTANCE

Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits.

It is analogous to resistance in an electrical circuits, but rather than dissipating electric energy it stores magnetic energy.

In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance

RELUCTANCE MOTOR-DEFINITION

A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance.

ADVANTAGES

Reluctance motors can deliver very high power density at low cost, making them ideal for many applications.

Washing machines. Control rod drive mechanisms of nuclear

reactors.

DISADVANTAGES

Disadvantages are high Torque Ripple (the difference between maximum and minimum torque during one revolution) when operated at low speed, and

Noise caused by torque ripple

CLASSIFICATION

Synchronous reluctance motor

Variable reluctance motor or Switched Reluctance Motor

SYNCHRONOUS RELUCTANCE MOTOR

Synchronous reluctance motors have an equal number of stator and rotor poles.

As the rotor is operating at synchronous speed and there are no current-conducting parts in the rotor, rotor losses are minimal compared to those of an induction motor

Once started at synchronous speed, the motor can operate with sinusoidal voltage. Speed control requires a variable frequency drive.

VARIABLE RELUCTANCE MOTOR

The switched reluctance motor (SRM) is a form of stepper motor that uses fewer poles. The SRM has the lowest construction cost of any industrial electric motor because of its simple structure.

Common uses for an SRM include applications where the rotor must be held stationary for long periods, and in potentially explosive environments such as mining because it does not have a mechanical commutator.

SUMMARY

Conventional Electrical Machines are mainly used bulk energy conversions.

Special Electrical Machines- invented for specific control applications.

Reluctance motors Synchronous reluctance Motor Switched reluctance Motor

SWITCHED RELUCTANCE MOTOR

The Switched reluctance motor is an electric motor in which torque is produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is maximized.

CONSTRUCTION

CONTINUED

It has wound field coils of a DC motor for its stator windings and has no coils or magnets on its rotor.

It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient machine.

STATOR

Built by stacking suitably punched silicon laminations to appropriate length.

Has salient poles. Poles carry concentric windings. The coils on the opposite poles are

connected in series to form phases.

ROTOR

The rotor contains no windings or permenant magnet.

Build up of steel laminations and laminations are stacked to the shaft.

DIFFERENT CONFIGURATIONS

WORKING PRINCIPLE

The rotor is aligned whenever the diametrically opposite stator poles are excited.

In a magnetic circuit, the rotating part prefers to come to the minimum reluctance position at the instance of excitation. While two rotor poles are aligned to the two stator poles, another set of rotor poles is out of alignment with respect to a different set of stator poles.

Then, this set of stator poles is excited to bring the rotor poles into alignment.

CONTD…

CONTD…

the movement of the rotor, hence the production of torque and power, involves a switching of currents into stator windings when there is a variation of reluctance, this variable speed motor is referred to as a switched reluctance motor (SRM).

PRINCIPLE OF OPERATION

DESIGN ASPECTS OF STATOR AND ROTOR POLE ARC

s = Stator Pole Arc s = Stator Slot Arc r = Rotor Pole Arc r = Rotor Slot Arc

s + s = 360 / Ns

r + r = 360/ Nr

Ns, Nr - Stator projections and Rotor projections respectively.

Step Angle = (1/ Nr – 1/ Ns ) * 360

AIR GAP INDUCTANCE

Tc= No of turns R= radius of the rotor at air gap g= gap length; = overlap angle

AIR GAP INDUCTANCE

Lphase= 2 Lcoil

SPEED EQUATION

Speed (rpm)= f * step angle in deg. * no of stator phases * 60

360

f- switching frequency

LAWRENSON ANALYSIS

Method to select the tooth and slot dimensions of the stator and the rotor so as to obtain feasible and optimised values for Lmax and Lmin.

Torque = dL/dt * I2

CONTD…

To allow a quick build up of the current from a voltage source, it is desirable that the winding be switched when the inductance is low and fairly constant.

This is possible only when the stator pole arc is less than the rotor slot width.

s Should be less than 2/Ns

s < r or s > r ??

s < αs ; Larger stator slot width allow more ampere-conductors.

so s < r

s > Step angle, ε

R POHL. THEORY OF PULSATING MACHINES

rotor tooth arc be chosen as appox. 40% of the rotor slot pitch for maximizing the difference Lmax - Lmin.

FOR 8:6 SRM

Rotor slot pitch= 360/Nr= 60

so r = 24; r = 36

Now s < 24 and > step angle 15 s + s = 360 / Ns ie 45; So s > 21 assume so s = 21 so s = 21

DWELL ANGLE

So dwell angle,

TUTORIAL

A four phase eight pole switched reluctance motor has six rotor teeth. Find the step angle and commutation frequency.

A three-phase SRM has six stator poles and four rotor teeth. Draw the feasible zone for stator and rotor pole arcs. Design the pole arc and rotor tooth arc. Sketch the L-θ profile.

SRM DRIVE SYSTEM

COMPONENTS

Converter Topology Position Sensors Control Circuitry

POWER CONVERTER FOR SRM

ASYMMETRIC BRIDGE CONVERTER

When T1 and T2 are ON Va1a2= V

When T1 and T2 are OFF Va1a2= -V

Ie D1 and D2 become forward biased and send power back to the dc bus.

When T1 or T2 is OFF Va1a2= 0 V, current free wheels during this period.

N+1 SWITCHING DEVICES AND N+1 DIODES

Higher torque ripple Higher switching stress for T

BIFILAR TYPE CONVERTER TOPOLOGY

Poor copper utilization Voltage spikes due to imperfect

coupling

C-DUMP CIRCUIT

POWER CONVERTER

A three-phase, 6/4-pole reluctance machine, in which i is the current of a single phase

POSITION SENSORS

In the SRM drives, rotor position is essential for the stator phase commutation and advanced angle control. The rotor position is usually acquired by the position sensors.

The commonly used position sensors are phototransistors and photodiodes, Hall elements, magnetic sensors, pulse encoders and variable differential transformers.