SPECIAL ELECTRICAL MACHINES

1

PERMANENT MAGNET BRUSHLESS DC MOTORS

PERMANENT MAGNET BRUSHLESS DC MOTORS

• It is a brushless dc motor.

• It has a PM rotor type.

• Why we are choosing PMBLDC Motor…..

• Comparison between conventional dc motor & PMBLDC Motor.

2

FEATURES CONVENTIONAL DC MOTOR

PMBLDC MOTOR

Maintenance High low

Commutation method Mechanical contact between brushes & commutator.

Electronic switching using power semiconductor devices ie,transistor,MOSFET

Detecting method Automatically detected by brushes

Rotor position can be detected by using sensorie, Hall sensor, optical sensor

controllable Speed controllable is difficult

Speed can be easily controllable, so that it is possible to have very high speed

Conventional dc motor

PMBLDC MOTOR

Construction of PMBLDC motor

• It consist of two parts mainly stator & rotor.

• Stator is made up of silicon steel stampings with slots.

• The slots are accomodated armature windings.

• This winding is wound with specified no.ofpoles.(even number).

• This winding connected a dc supply through a power electronic switching circuits.

• Rotor accomodates PM.

• The rotor shaft carries a rotor position sensor.

• Sensor provides information about the position of the shaft.

• This shaft position signal send to electronic commutator.

advantages

• There is no mechanical commutator, so that size become very small.

• Speed can be easily controlled.

• Regenerative braking is possible.

applications

• Automotive application.

• Textile and industries.

• Computer and robotics.

• Small appliances such as fans, mixers etc

Commutation in DC motors

(1) Mechanical commutator.

(2) Electronic commutator.

• Construction of Mechanical commutator

• Commutator is made up of specially designed commutator segments,made of copper.

• These segments are insulated from each other by a thin layer of mica.

• It forms a cylindrical shape.

Mechanical commutator

• It consist of 2 pole machine with 12 commutator segments.

• Carbon brush A contacts with CS 1 and brush B contacts with CS 7.

• When a dc supply is connected across A & B, a dc current passes through A-CS1-tapping1-tapping 7-CS 7 and through B.

• The current has 2 parallel path in the armature winding.

• Parallel path 1= 1-2-3-4-5-6-7

• Parallel path 2=1-12-11-10-9-8-7

• The current crossing through the armature conductor setup an mmf along the axis A and B

• The commutator rotates along the clockwise direction, now the brush A makes contact with CS2 and brush B with CS 8.

• Now there are two parallel path 1 is 2-8 & parallel path 2 is 2-1-12-11-10-9-8.

• Function of commutator and bush arrangement is to setup an armature mmf whose axis is always in quadrature with the main field mmf irrespective of the speed of rotation of the motor.

Electronic commutator

Switching circuit of electronic commutator

Step 1: S1 & S1’ are closed.other switches are open

• current has 2 parallel path in armature winding.

• Parallel path 1 =1-2-3-4-5-6-7

• Parallel path 2 =1-12-11-10-9-8-7

• These current setup in the armature mmf.

Step 2: S1 & S1’ are opened,S2 & S2’ are closed

• Now the current passes through tapping 2-8.

• Thus operating switches in the sequential manner, we are getting a revolving magnetic field.

• For normal electronic commutator , 6 switching devices are employed.

• Here the windings may be connected either star or delta connection.

• Therefore the winding should have 3 tappings.

• Figure draw ..

Comparison between mechanical and electronic commutator

Mechanical commutator Electronic commutator

Commutator is made up of commutatorsegments and mica insulation. brushes are made up of carbon.

Power electronics switching devices are used in the commutator.

Shaft position sensing is inherent in the arrangements.

It requires a separate rotor position sensor.

Commutator arrangement is located in the rotor

Commutator arrangement is located in the stator.

Sliding contact between commutator and brushes.

No sliding contacts.

Sparking takes place. There is no sparking.

It requires a regular maintenance. It requires less maintenance.

Number of commutator segments are very high.

Number of switching devices is limited to 6.

Difficult to control the voltage available across tapping

Voltage available across armature tappings can be controlled by PWM techniques.

Highly reliable Reliability depends on the switching devices.

OPERATION OF PMBLDC MOTOR• When dc supply is given to the motor,the

armature winding draws a current.

• The current distribution with in the stator armature winding depends upon the rotor position.

• The mmf perpendicular to the permanent magnet field is setup.

• Then the armature conductor experience a force.

• The force develops a torque in the rotor.

• If this torque is more than the opposing frictional and load torque, the motor starts.

• It is self starting motor.

Hall sensor

Hall effect

Optical sensor

grating

light emitter

light sensor

decode circuitry