Switched Reluctance Motors and brushless dc motor operation

May 14 • Notes • 6495 Views • 5 Comments on Switched Reluctance Motors and brushless dc motor operation

Switched Reluctance Motors

The switched reluctance motors are synchronous motor and almost similar to the brushless permanent magnet motors only both differ in the rotor construction such that the rotors are made up of the laminated soft magnetic material shaped such as to form the salient.

Principle of operation

In the magnetic field when a magnetic material is kept such that it will align itself with the direction of the magnetic field such that the reluctance of the magnetic circuit is minimized.When it is kept it in the reverse direction as it was kept before then it will direct itself towards the magnetic poles thereby creating the field.A torque is created on the rotor known as the reluctance torque.

The magnetic circuit of the motor will have high magnetic reluctance when the spaces between the  rotor poles are opposite to the stator poles and similarly when the rotor poles and the stator poles are in the same direction then magnetic circuit of the motor will have low magnetic reluctance.To minimize the reluctance path a pair of the stator pole if energized with the nearest stator poles it can be analyzed.

The reluctance motor requires the position feedback so that the motor phase commutation can be controlled.By the use of the position sensors the feedback control can be provided.Even the sensor less position control is possible if more complex electronic circuits and the software are used.Motor torque can be maximized by synchronizing the controller switching phase with rotor position so that torque angle is held at its maximum of 90⁰.

The motor design are double salient i.e. both the stator and the rotor have the salient poles.The rotor has comparatively less number of poles than the stator which enables self starting and the bidirectional control.

No back emf is generated as the rotor is not made up of the permanent magnet,it is made up of iron such that it helps the motor to reach high speed as similar to the permanent magnet motors

DISADVANTAGE

As it consists of the double salient poles the motor design suffers from various problems such as resonance,torque ripple and noise.Various methods such as multiple pole and the pole shaping are needed to remove these variations.

Characteristics

No I²R loss in the rotor
Cost is low.
It has low weight and size.

It can be easily manufactured and highly inexpensive.

It has high efficiency,

Rugged construction

High speed  of operation.

It suffers from excessive noise.

APPLICATIONS

It can be used as the 3 or 4 phase motors in the fans or scooters etc.

In the house hold appliances

High speed pumps and the compressors.

For the traction drives,power steering system,pumps etc

.Brush less permanent magnet dc motors

By using the rotor winding of the permanent magnet the mechanical commutation can be eliminated thereby reducing the cost and the reliability.The efficiency is also improved and the frictional losses are reduced.

• Synchronous Operation

The brush less permanent magnet dc motors are not just dc motors.The stator field windings are pulsed dc fed such that a rotating magnetic field is created and they operate at the synchronous speed.This type of the motors uses the electronic commutation process which is provided to the rotating field.

• Rotating Field and Speed Control
  The pair of the poles named A is given the DC pulse which as the result magnetises the pole A1 as the south pole and pole A2 as the north pole.As the magnet passes through the pole pair which is magnetized first i.e A1 and A2 there by the current to the pole pair A is switched off and fed to the next pole pair i.e B.The magnet then rotates clockwise to align itself with the pole pair B.By pulsating the pair of the poles-the magnet continues to rotate itself in the clockwise direction in order to align itself with the energized pole pair

The working of the permanent magnet brush less dc operation

It is to be noted that only two out of the three pole pairs are energized at a time.We can also say that only only two of the six inverters switches conduct at a time.

Mechanical construction

In the rotors no current or the supply is fed which are constructed from the permanent magnet.As there is no current in the rotor machines have no I² R losses

The rotor have low rotor inertia as it doesn’t uses the mechanical commutation allowing the higher speed to be achieved with the elimination of the high current mechanical switch.  The stator windings are easy to manufacture and install.The heat dissipation is easier to control and the high current and the motor power can be achieved as all the heat generating circuits are in the stator.

characteristics.

. It is compact and has light weight

It has high torque and power density.

Due to the losses it has more torque per amp,

Low cost due to the elimination of the stator fields.

It has no conduction losses as no field winding is needed to produce the flux

Speed upto 100,000 RPM.

It has low maintenance and longer life  as it doesn’t have commutator mechanically.

Disadvantage

The current transitions gives rise to the torque transitions resulting in the cogging and some noises which hinders in the usage of some applications

Applications

It is suitable for the traction applications from low power to the high power uses

Preferred in powering the electric bikes as they does not produce friction with the commutator brushes.

RELATED QUESTIONS AND ANSWERS

WHAT IS THE BASIC IMPORTANCE OF THE SWITCHED RELUCTANCE MOTORS??

The switched reluctance motors are synchronous motor and almost similar to the brushless permanent magnet motors only both differ in the rotor construction such that the rotors are made up of the laminated soft magnetic material shaped such as to form the salient.In the magnetic field when a magnetic material is kept such that it will align itself with the direction of the magnetic field such that the reluctance of the magnetic circuit is minimized.When it is kept it in the reverse direction as it was kept before then it will direct itself towards the magnetic poles thereby creating the field.A torque is created on the rotor known as the reluctance torque.

GIVE SOME CHARACTERISTICS OF THE SWITCHED RELUCTANCE MOTORS.

No I²R loss in the rotor
Cost is low.
It has low weight and size.

It is highly inexpensive.

It has high efficiency.

Rugged construction

It has high value of the start up torque and high speed of the operation is possible.

reluctance motors suffer from extreme noise.

HOW THE SPEED OF THE SWITCHES RELUCTANCE MOTOR CAN BE MATCHED UP WITH THE PERMANENT MAGNET BRUSH LESS DC MOTOR??

No back emf is generated as the rotor is not made up of the permanent magnet,it is made up of iron such that it helps the motor to reach high speed as similar to the permanent magnet motors.

EXPLAIN THE WORKING OF THE BRUSH LESS OPERATION OF THE PERMANENT MAGNET BRUSH LESS DC MOTOR

The pair of the poles named A is given the DC pulse which as the result magnetises the pole A1 as the south pole and pole A2 as the north pole.As the magnet passes through the pole pair which is magnetized first i.e A1 and A2 there by the current to the pole pair A is switched off and fed to the next pole pair i.e B.The magnet then rotates clockwise to align itself with the pole pair B.By pulsating the pair of the poles-the magnet continues to rotate itself in the clockwise direction in order to align itself with the energized pole pair

The working of the permanent magnet brush less dc operation

It is to be noted that only two out of the three pole pairs are energized at a time.We can also say that only only two of the six inverters switches conduct at a time.

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