MODULE 1

PMDC,PMBLDC & PMSM

MOTORS

AJITH VIJAYAN,
EEE DEPT.
MITS
 SYLLABUS
• Permanent Magnet DC Motors – construction – principle of
operation.
• PM Brushless DC motor- Brushless DC motor-construction -
permanent magnets – different types- demagnetization
characteristics – arrangement of permanent magnets –
magnetization of permanent magnets – axial and parallel
magnetizations- principle of operation – Control of BLDC motor -
applications.
• Permanent Magnet Synchronous Motors-construction - principle
of operation –Control of PMSM - Self control - Sensor less
Control– applications - Comparison with BLDC motors.
PERMANENT MAGNET DC MOTORS
(PMDC)
CONVENTIONAL DC MOTOR vs PMDC
 INTRODUCTION
• In a DC motor, an armature rotates inside a magnetic field. (ie a stationary field
system which produces main magnetic flux)
• Working principle of DC motor - whenever a current carrying conductor is
placed inside a magnetic field, there will be mechanical force experienced by
that conductor.

• The magnetic field is established either by using an electromagnet or a

permanent magnet
• For low power applications (in control systems) electromagnets leads to lower
efficiency due to field copper loss.

• A Permanent Magnet DC motor (or PMDC motor) is a type of DC motor that

uses a permanent magnet to create the magnetic field required for the
operation of a DC motor.

• These types of motors are simple in construction, less space requirement, high efficiency
and better cooling.

• They are commonly used as a starter motor in automobiles, windshield wipers, washers,
for blowers used in heaters and air conditioners, to raise and lower windows – and they
are extensively used in toys.
 CONSTRUCTION OF PMDC MOTOR

• Permanent magnet DC motor is similar to an

ordinary d.c. shunt motor except that its field is
provided by permanent magnets instead of
salient-pole wound-field structure.

• A PMDC motor mainly consists of two parts.

➢ stator

➢ Armature(rotor)
 STATOR
• The permanent magnets are mounted in such a way that the N-pole and S-pole of each
magnet are alternatively faced towards armature as shown in the figure.

• If N-pole of one magnet is faced towards armature then S-pole of very next magnet is
faced towards armature.
• In addition to holding the magnet on its inner periphery, the steel cylindrical stator
also serves as low reluctance return path for the magnetic flux.
STATOR FRAMES
Types of permanent magnets
• There are different types of permanent magnets used for such motors. The
materials used have residual flux density and high coercivity.
• Alnico magnets – They are used in motors having ratings in the range of 1 kW to
150 kW. Can be operated at high temp applications. Alnico 5, Alnico 7, Alnico
9(highest quality)

• Ceramic (ferrite) magnets – They are much economical in fractional kilowatt

motors. It is the cheapest. They have lower remanence (max flux that can be
maintained by the magnet) and higher coercive force. Operation upto 100⁰C
• Rare-earth magnets – Made of Samarium Cobalt and Neodymium iron Boron
which have the highest energy(BH) product. Such magnetic materials are costly
but are best economic choice for small as well as large motors. Operation upto
200⁰C
 ROTOR
**The rotor or armature of permanent magnet DC motor consists of core, windings, commutator & brushes.

**Armature core is made of number of varnish insulated, slotted circular lamination of steel sheets. By fixing these
circular steel sheets one by one, a cylindrical shaped slotted armature core is formed.

**The varnish insulated laminated steel sheets are used to reduce eddy current loss in armature of PMDC.

**The armature conductors are placed in armature slots and are connected in a suitable manner which gives rise to
armature winding.

**The end terminals of the winding are connected to

the commutator segments placed on the motor shaft.

**Like other DC motor, carbon or graphite brushes

are placed with spring pressure on the commutator

segments to supply current to the armature.

Three types of rotor(armature) structure

• Slotted armature

• Slot less armature

• Moving coil armature.

 SLOTTED ARMATURE
• Made up of silicon sheet steel or carbon steel sheet which
are punched together and mounted on the shaft.

• Slots on its outer periphery.

• Armature conductor is placed in the slots and connected
to form armature windings.

• Large number of slots for reducing the torque ripples and

electrical noise

• Skewing for reducing the cogging.

• Thin slot machines which can house more conductors are called
copper machines (iron machines vice versa)
 SLOT LESS ARMATURE
• There is no slots.

• Conductors are fixed on the outer periphery of the core.

• Effective air gap length is larger(S larger, more mmf required)

• Alnico or rare permanent magnets are used.

• Advantages is reduction in torque ripple ( no detent torque)

• Noise is less- smooth operation

MOVING COIL ARMATURE

• The iron core is replaced by non-magnetic core

• Made up of glass fibre

• Advantages of low inertia and no iron loss in armature

• Commutator and brushes are very small and made up of precious metals like gold, silver, platinum and others

• Small size commutator and brushes helps in stable commutation.

 COMMUTATOR AND BRUSHES
• The current passing through the brushes should be distributed in the armature
winding so that unidirectional torque is produced . This is achieved by commutator and
brushes.

• The commutator segments are mechanically fixed to the shaft and electrically isolated
from each other and shaft.

• The commutator segments are connected to the armature winding.

• The brushes are fixed and it slides over the commutator segments.
 WORKING OF PMDC MOTORS
• The working principle of PMDC motor is similar to the general working
principle of DC motor.

• When a current carrying conductor is placed inside a magnetic field, a

mechanical force will be experienced by the conductor and the direction of
this force is governed by Fleming’s left hand rule.

• As in a permanent magnet DC motor, the armature is placed inside the

magnetic field of permanent magnet; the armature rotates in the direction
of the generated force.
• Here each conductor of the armature experiences the mechanical force

F = B.I.L Newton

where, “B” is the magnetic field strength in Tesla (weber / m² ),

“I” is the current in Ampere flowing through that conductor

“L” is length of the conductor in meter comes under the magnetic field.

• Each conductor of the armature experiences a force and the compilation of those forces
produces a torque, which tends to rotate the armature.

• Once the armature starts rotating, a back EMF is produced in the armature winding, and the
direction of back EMF is opposite to that of applied voltage.

• The electrical energy for overcoming the opposition is converted to mechanical energy
developed in the armature.
 TORQUE EQUATION
• In conventional DC motor, the generated or back EMF is given by the equation shown
below.

𝐸𝐵 = 𝑘∅𝑁 -------------------------(1)

• The electromagnetic torque is given

𝑇𝑒 = 𝑘∅𝐼𝑎--------------------------(2)

In Permanent Magnet DC motor, the value of flux ϕ is constant. Therefore, the above
equation (1) and (2) becomes

𝐸𝐵 = 𝑘1𝑁 -------------------------(3)

𝑇𝑒 = 𝑘1𝐼𝑎--------------------------(4)
• The circuit diagram of the PMDC is shown below.

𝑉 = 𝐸b + 𝐼𝑎𝑅𝑎-----------------(5)

• Substituting eq (3) in (5)

𝑉= 𝑘1𝑁 + 𝐼𝑎𝑅𝑎 -----------------(6)

𝑉−𝐼𝑎𝑅𝑎
𝑁= -----------------------(7)
𝑘1

• Where k1 = k ϕ and is known as speed-voltage constant or torque constant.

• Its value depends upon the number of field poles and armature conductors.

• The speed control of the PMDC motor cannot be controlled by using flux control method as the flux remains
constant in this type of motor.
• Both speed and torque can be controlled by armature voltage control, armature rheostat control, and chopper
control methods.

• These motors are used where the motor speed below the base speed is required as they cannot be operated
above the base speed.
 PERFORMANCE CHARACTERISTICS
• Its speed-torque curve is a straight line which
makes this motor ideal for servo applications.

• Input current increases linearly with load

torque.
• The efficiency of such motors is higher as
compared to wound-field dc motors because,
in their case, there is no field Cu loss.
 ADVANTAGES OF PMDC MOTOR
• No need of field excitation arrangement.
• No input power is consumed for excitation which improve efficiency of
DC motor.

• No field coil hence space for field coil is saved which reduces the overall
size of the motor.

• Cheaper and economical for fractional kW rated applications

• For small rating, manufacturing cost is less.

 DISADVANTAGES OF PMDC MOTOR
• The armature reaction of DC motor cannot be compensated hence the magnetic strength of
the field may get weak due to the demagnetizing effect of the armature reaction.

• There is a chance of getting the poles permanently demagnetized (partial) due to excessive
armature current during the starting, reversal, and overloading conditions of the motor.

• The field in the air gap is fixed and limited – it cannot be controlled externally. This makes it difficult
for this type of motor to achieve efficient speed control of DC motor in this type of motor is
difficult.

• Torque per ampere turns are less – flux produced by PM is less compared to shunt motor
for the same rating.

• The magnetic field of PMDC motor is present even when the motor is not being used.
 APPLICATIONS OF PMDC MOTOR
The PMDC motors are used in various applications ranging from fractions to several HP.
They are developed up to about 200 kW for use in various industries. The following applications
are given below.

• PMDC motors are mainly used in automobiles to operate windshield wipers and washers, to
raise the lower windows, to drive blowers for heaters and air conditioners etc.

• They are also used in computer drives.

• These types of motors are also used in toy industries.

• PMDC motors are used in electric toothbrushes, portable vacuum cleaners, food mixers.

• Used in a portable electric tool such as drilling machines, hedge trimmers etc.
BRUSHLESS DC MOTOR
(BLDC)
 INTRODUCTION
🞇 Conventional DC motors requires regular maintenance since the commutator
and brushes are subject to wear and tear.

🞇 The functions of commutator and brushes are implemented by Electronic

commutator by means of solid state switches in a BLDC motor.
🞇 The main advantage of BLDC motor is that the brush maintenance is no
longer required and many problems associated with brushes are removed.
🞇 The efficiency of a BLDC motor is higher than that of a commutator motor of
equal size and absence of brush friction.

🞇 BLDC have better cooling and smaller in size.

 BLDC MOTOR
🞇 BLDC motor is a type of synchronous motor with permanent magnet rotor and is
supplied current from a DC source through an inverter which is automatically synchronized.

🞇 As the name indicates, these motors have no brushes.

🞇 BLDC motor is a combination of auto-synchronized inverter and a synchronous motor
w h i c h will exactly be equivalent in behavior to that of a DC motor but without the
commutator, brushes or slip rings.

🞇 Come in single phase, 2 ph and 3ph configurations. Out of these, 3 ph motors are most popular
and widely used.
🞇 In addition, the ratio of torque delivered to the size of the motor is higher making it useful
in applications where space and weight are critical factors
How does BLDC work ?
https://youtube.com/watch?v=bCEiOnuODac
Can you relate ?
 CONSTRUCTIONAL FEATURES
1. STATOR
• The stator of a BLDC motor is similar to that of a
conventional induction motor.

• The stator core is made up of silicon steel stampings

which are stacked together and fixed on the stator
frame.

• The stator core has slots in the inner periphery to house

the armature conductors.
The armature windings are wound for a specific no. of poles. (no. of poles should be an even no.)
The armature is connected to the DC source through suitable semiconductor switching circuit.
Inverter switching pattern and frequency is controlled by the rotor position and speed.
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BLDC WITH EXTERNAL STATOR AND INTERNAL
STATOR
INRUNNER & OUTRUNNER MOTORS
• Inrunner runs faster (For
more voltage, outrunner
requires more magnets, which
slows it)
• Outrunner has more
torque (a larger air gap
surface area and a longer torque
arm both lead to higher torque)
• https://www.magneticinnovations.com/faq/outrunner-
motor/#:~:text=Consequently%20a%20larger%20air%20gap,equipped%20
with%20transmissions%20or%20gearboxes.
36
2. ROTOR
• Rotor is made up of forged steel and accommodates
permanent magnet.

• No. of poles on the rotor is the same as that of the stator.

• The rotor shaft carries a rotor position sensor which provides
information about the position of the shaft at any instant to the
controller which sends suitable signals to the electronic
commutator.

• The permanent magnet can be fixed on the rotor in different

ways
• Surface mounted permanent magnet rotor
• Embedded permanent magnet rotor
• Interior permanent magnet rotor.
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STATOR
ROTOR
COIL WINDING FOR ALTERNATE POLARITY
WINDING ARRANGEMENT

• Lucas, Retzbach and Küfuss (delta)

• Connect together: Start A - End C, Start B - End A , Start C - End B.
CONTROLLER
SENSOR REQUIRED FOR SENSING ROTOR
POSITION
HALL EFFECTSENSOR- FOR ROTOR POSITION
SENSING
 Surface Mounted PM DC motor

• Here the magnets are mounted on the surface of the rotor as shown in fig.

Drawbacks of surface mounted PM DC machines

1. Since the magnets are mounted on the surface of the rotor, the machine cannot be operated at high
speeds. (limited due to centrifugal force)

2. The effective airgap is high so that the effect of armature reaction ampere turns is not
significant. Hence control in the constant torque region only is possible. Control in constant power,
flux weakening region is not effective.

46
• EMBEDDED TYPE ROTOR/INSERT MAGNET TYPE

• Magnets are placed under the surface of rotor core

• More robust compared to projecting magnet type

• Can be used for high power application

Interior PM DC motor
• Here the magnets are placed in groves or slots as shown in fig.
• Even though the rotor appears to be of smooth cylindrical shape, magnetically
it is a projecting pole rotor..

• The interior type rotor has more magnetic saliency than that of a
surface mounted PM rotor.

• Advantages of interior PM rotor

1. The rotor construction is more robust permitting higher speeds of
operation.

2. The effective airgap is low, the armature reaction effect is dominant and
hence control in constant torque as well as constant power (flux
weakening) is possible by suitably controlling the demagnetizing
armature ampere turns.

3. Higher torque to ampere ratio.

48
49
ROTOR MATERIAL

• Magnetic material for rotor is chosen based on the required magnetic flux density of the rotor.

• Ferrite magnets are traditionally used to make Permanent Magnets since it is less expensive but it has
low flux density for a given volume.

• As technology advances, rare earth magnets are gaining popularity.

• Alloy material has high magnetic density and enables use of smaller rotor for the same torque.

• Neodymium(Nd), Samarium Cobalt (SmCo) & alloy of Neodymium, iron, boron (NdFeB) are some
examples of rare earth alloy magnets.

• Continuous research going on to improve the flux density to compress the rotor further.

50
 ELECTRONIC COMMUTATION
• The most important advantage of BLDC motor is the electronic commutator in place of
mechanical commutator.
• In electronic commutator functions of commutator and brushes are performed by power
semiconductor devices. DC supply is connected to the armature through these devices.

• So it is essential to have stationary armature and rotating field system for implementing
electronic commutator.

• Removal of commutator and brushes, reduces the length of the motor allowing high speed
operation.

• Inertia of the motor is reduced.

• The heat produced can be easily conducted through the frame.

 MECHANICAL COMMUTATOR AND BRUSHES ELECTRONIC COMMUTATOR
1 Commutator is made up of copper segments and mica Power electronic switching devices are used.
insulation. Brushes are of Carbon or Graphite

2 Commutator arrangement is located on the rotor Commutating switches are connected to the stationary
armature winding.
3 Shaft position sensing is inherent in the arrangement. Separate rotor position sensor is required.
Position feedback is not required.

4 Large no. of Commutator segments and tappings are No. of switching devices is limited to 6.
needed.
5 There exists sliding contact between Commutator and No sliding contact and hence no sparking. Stored energy in the
brushes. Sparking may occur. Requires regular magnetic field can be fed back to the supply. Requires less
maintenance. maintenance.

6 Difficult to control the voltage available across the The voltage available across armature tappings can be
tappings. controlled by employing PWM techniques.
7 Highly reliable Reliability is improved by specially designing the devices and
protective circuits.
8 Interpole windings are employed to have sparkless By suitable operating the switching devices, better
commutation performance can be achieved.
 PRINCIPLE OF OPERATION
• When current flows through one of the stator windings, that generates a magnetic pole
which attracts the nearest permanent magnet of the opposite pole.

• The rotor will move if the current shifts to an adjacent windings.

• Sequentially changing each winding will cause the rotor to follow a rotating field.

• If the switching can be synchronized with rotor position, then the fixed angle between
stator field and rotor field can be maintained.

• Switching the stator phases synchronized with emf wave make the stator & rotor emf
rotate in synchronism.

• Thus the inverter acts like electronic commutator that receives switching logical pulses
from the rotor position sensor. This is why BLDC drive is commonly known as an
Electronically Commutated Motor (ECM).
• Which stator coil or when to energies to get continuous rotation is decided by commutation scheme. –

electronic commutation

• A sensor determines the position of the rotor and based on this information, the controller decides which

coil to energies.

• Position feedback can be achieved by

• Hall effect sensors, encoders or resolvers
• Sensor less commutation techniques - how?
• The most popular position sensor device is hall sensor

• Most BLDC motor have 3 Hall Sensors embedded into the stator on the non-driving end of the
motor.

• Based on the combination of the 3 Hall Sensor signals, the exact sequence of commutation can be
determined.(Measures the exact magnetic field. )
TRANSVERSE SECTIONAL VIEW OF BLDC MOTOR
 PRINCIPLE OF OPERATION
• When supply is switched ON, current flows through the armature winding whose distribution depends
on rotor position and switching ON of devices at various instants.

• Due to interaction between current and magnetic field, rotor experiences a torque and if this torque is
greater than load torque, the rotor starts rotating.

• BLDC motor is a self starting motor.

• As the rotor moves, a relative velocity exits between the rotor and the stator. This results in the emf
induction in armature winding whose direction is opposite to the applied voltage.

• Due to this back emf, a momentary change in torque and speed will occur. The armature draws more
current and increases the torque and hence the speed returns to the same value. These changes are very
fast and the performance is not affected.

37
The torque depends on:
--Current amplitude and number of turns on stator windings
--strength and size of PM
--Air gap between rotor and windings
--length of rotating arm
PRINCIPLE OF OPERATION
 PRINCIPLE OF OPERATION
• Consider a BLDC motor supplied by a VSI. The inverter
converts DC voltage to variable frequency voltage.

• 4 quadrant operation is possible.

• The motor is provided with a position sensor which

provides necessary signals for switching the appropriate
power switches of the inverter.

• The switching is done in such a way that all the 3 phases

conduct at all time. (180o conduction).

• switching interval is 60o.

• Due to the spatial arrangement of the phases of the

armature winding, the mmf produced by the stator
rotates in space. 40
• Assume the initial position of the rotor as shown in fig.

• Case 1: When S1,S5 and S6 are turned ON ➔ Current flows through phase R and divides into equal amounts
and completes its path through Y phase and B phase (S5 and S6).

• This current produces mmfs which can be represented by stator north pole & south poles as shown in fig a.

• Due to these stator poles, the rotor experiences torque and move in clockwise direction

• The rotor moves by 60o and occupies the position shown in fig b.
60
• Case 2: When S1,S2 and S6 are turned ON ➔When the rotor move by 60˚, and occupies
position b(in previous fig), switches S2 is turned ON and S5 is turned OFF keeping S1 and S6 ON.

• When S1,S2 and S6 are conducting with R and Y phases connected to positive and B phase
to negative of the supply, the stator poles are shifted by 60o as shown in fig a.

• This will cause the movement of rotor by 60o in clockwise and its position as shown in fig b.
61
• Case 3: When S2,S4 and S6 are turned ON ➔When the rotor move by 60o, and occupies position b(in
previous fig.), switches S4 is turned ON and S1 is turned OFF keeping S2 and S6 ON.

• When S2,S4 and S6 are conducting with Y phase is connected to positive and R and B phases to
negative of the supply, the stator poles are shifted by 60o as shown in fig a.

• This will produce torque on the rotor and hence it rotates by 60o when the next switching operation is
done as shown in fig b.
62
• For forward (Clockwise) rotation of the BLDC motor, the switches should be operated in the sequence given
below.

1,2,6➔2,4,6➔2,3,4➔3,4,5➔1,3,5➔1,5,6➔1,2,6➔…….

• For reversing the direction, the sequence is

1,2,6➔1,5,6➔1,3,5➔3,4,5➔2,3,4➔2,4,6➔1,2,6➔……..

• For stopping, keep the switches in the same condition (ON/OFF) corresponding to the last operation.
63
64
 CLASSIFICATION OF BLDC MOTOR
• 1. Based on rotor construction
• Surface mounted PM motors
• Embedded PM motors
• Interior type PM motors

• 2. Based on Length of the pole arc

• 180 degree pole arc BLDC
• 120 degree pole arc BLDC

• 3. Based on number of phases and pulses

• One-phase-one-pulse
• One-phase-two-pulse
• Two-phase-two-pulse
• Three-phase-three pulse
• Three-phase-six-pulse
• Multiphase-Multi pulse
 CLASSIFICATION OF PM MOTORS
Based on the shape of the back emf induced in the stator windings, the PM motors are
classified as:

1. Permanent magnet Synchronous Machine with Sinusoidal back emf

2. Brushless Permanent Magnet DC machine with Trapezoidal back emf

 TRAPEZOIDAL SURFACE MAGNET MACHINE
• A non-salient surface mounted machine similar to a sinusoidal SPM machine except its three phase
stator winding (normally star connected ) has concentrated full pitch distribution

• As the machine rotates, most of the time flux linkage in a phase windings varies linearly, except when
the magnet gap passes through the phase axis.

• If the machine is rotated by a prime mover, the stator phase voltage will have symmetrical trapezoidal
wave shape.
• Full pitched and concentrated
windings for trapezoidal back EMF

• High power density

• Hall effect probes to detect the correct

current switching positions
 SINUSOIDAL SURFACE MAGNET MACHINE -PMSM

• The stator has three phase sinusoidal winding

• If the machine is rotated by a prime mover, the stator winding

generate balanced 3 phase sinusoidal voltages.

• Distributed and fractional – slot windings for sinusoidal back

EMF
and smooth operation

• Better control and Extended field weakening

• Shaft encoder to control current (high cost)

• Suitable for servo drives and drives requiring excellent field

weakening capabilities
 PM BLDC MOTOR
• PM BLDC Square wave motors are classified on the basis of the flux density distribution in the airgap of
the motor.

1. PM BLDC Square wave motor with 180o pole arc

2. PM BLDC Square wave motor with 120o pole arc

53
 BLDC SQUARE WAVE MOTOR WITH 180 POLE ARC

• Consider a three phase BLDC motor.

• Two rotor poles having an arc length of 180 each.
The number of slots per pole per phase is two.

• Two adjacent coils in the armature forms the phase

• The coils are assumed to be full pitched and has N
turns each.

• The slot pitch is 30 and single layer winding is

used.
• The commutation circuit requires six
switching devices and two of them should
be ON at any instant.

• The inverter circuit for star connected BLDC motor

is shown.The converter circuit is implemented using
MOSFET..

For achieving the phase currents in table, the switches

are turned ON in the sequence
S2,S6: S2,S4: S3,S4:S3,S5:
S1,S5: S1,S6;S2,S4………………
while all other switches
are in OFF position.
 BLDC MOTOR WITH 120˚ MAGNET ARC
• In this configuration, current flows
through all the phases at all instants.

• The motor should be delta connected.

• The phase currents are 180˚ square wave

form.
• For achieving this, the transistor should be
switched ON in the sequence T1, T6; T1,T2;
T2,T3;T3,T4;T4,T5;T5,T6;T1,T6; with all
others in OFF state.
The phase currents during switching intervals

The line currents during switching intervals

 TYPES OF BLDC MOTOR
• Depending upon the no. of phases and no. of pulses applied during each cycle, BLDC motors can be
classified into,

1. One- phase-one- pulse BLDC motor

2. One-phase and two pulse BLDC motor

3. Two-phase and two pulse BLDC motor

4. Three-phase and three pulse BLDC motor

5. Three- phase and six pulse BLDC motor

6. Multiphase BLDC motor.

77
 1. One-phase and one pulse BLDC motor
• It has only one armature winding and it uses only one semiconductor
switch.

• When the rotor position sensor is influenced by N pole, the switch is turned
ON and it is turned OFF when the sensor is influenced by S pole. The rotor
gets torque whenever the rotor position is under the influence of N pole.
Thus torque is developed only for half cycle.

• The current and torque waveforms are shown in figure.

Advantages
• One transistor and one position sensor is sufficient.
• Inertia should be such that the rotor rotates continuously.
Disadvantage
• Here the utilization of transistor and winding is 50%.
78
 2. One-phase and two pulse BLDC motor
• The motor has one armature winding and two static switches which are used to supply the
armature.

• There is only one position sensor. When the rotor position sensor is under the influence of N
pole, switch S1 is in ON and S2 is in OFF state and when it is under the influence of S-pole,
S2 is ON state and S1 is in OFF state.

• In the 1st case, the winding carries current from A➔ B and when S2 is ON, the winding
carries current from B➔A.

• The polarity of the flux setup by the winding gets altered depending upon the position of the
rotor. This provides unidirectional torque as shown in fig.
Advantages
• Winding utilization is better
• Torque developed is more uniform
• Requires only one rotor position sensor.
Disadvantages
• Transistor utilization is less
• The current needs a 3-wire dc supply.
79
 3. Two-phase and two pulse BLDC motor
• The stator contains 2 phase windings which are displaced by
180o electrical. It makes use of 2 semiconductor switches.

• Performance is similar to one-phase-two-pulse motor. The

torque waveform is unidirectional as shown in figure. However
it requires 2 independent phase windings.

Advantage
• Better torque waveform
Disadvantage
• Utilization of transistor and windings are only 50% which is
less.
• Cabling with rotor position sensor should be made proper.
80
 4. Three-phase and three pulse BLDC motor

• The stator contains 3 phase windings which are displaced by

120o electrical apart.

• Each phase winding is controlled by a semiconductor switch

which is operated depending upon the position of the rotor.

• Three position sensors are required for this purpose.

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 5. Three-phase and six pulse BLDC motor

• Most commonly used.

• It has 3 phase windings and 6 switching devices as shown in figure.
• Winding may be either star or delta connected.
• This circuit produces unidirectional torque in all the three phases
as shown in figure.

Advantages
• Utilization factor of winding is high.
• Torque pulse and current ripple frequency components are less.
• Most popular one

Disadvantages
• Device utilization factor is less.
• Possibility of shoot through faults. Usually 120o or 150o conduction is
adopted.
82
 6. Multiphase BLDC motor
• A multiphase bipolar BLDC motor having an embedded

permanent magnet as a rotor, an armature as stator

and the multiphase windings are sub wound on the

stator in parallel and independently phase-connected.

• Schematic diagram of multiphase BLDC motor is shown

in figure.

• A commutation optical encoder defining light shielding

and light detecting portions is fixedly mounted on the

shaft of the rotor to be rotated and a photo sensor is

coupled operatively and connected with a driving circuit

to improve the performance of the motor

83
ESSENTIAL ELEMENTS OF A TYPICAL BLDC MOTOR

84
 CONTROL OF BLDC MOTOR

• Main blocks in the closed loop control

are
1. Power circuit
2. Control circuit
3. Rotor position sensor

Control circuit➔ Electronic commutation based on rotor position sensor output.

wref and wactual are compared
➔ error signal is used to generate iref and compared with actual current
➔ error signal is fed to monostable multivibrator
➔ o/p of this is ANDed with high freq pulses to drive the lower leg swiching devices
➔ derived torque meets the load torque. 85
• The shaft position is sensed by Hall effect sensors. The Hall effect sensors give necessary information for switching the devices.

• A position decoder is used to generate signals for 120o conduction for armature phases. The decoding is done by the commutation
logic block.

• An inverter is implemented using power transistors T1 to T6. It controls the current and commutation.

• For 120o conduction, only 2 transistors carry current at any instant. i.e. one of the upper transistors T1,T3 and T5 and one of the
lower transistors T2,T4 and T6 conduct at any moment.

• PWM technique is used to control the current through the winding to match the rectangular reference current waveform.

• For 0-60o conduction➔ T1&T6 ON➔ During this interval, T1 is kept ON and T6 is chopped ON/OFF using a current controller with
fixed switching frequency.

• For 60o-120o conduction➔ T1&T2 ON

• For each interval of 60o, one of the upper transistors (T1,T3,T5) is turned ON for the entire interval and one of the lower transistors
(T2,T4,T6) is always chopped for 120o interval.

• The duty cycle depends on the desired (reference) speed.

• The application of PWM techniques to one transistor reduces current ripples.

• The AND gates combines commutation signals and chopping signals for providing the input to lower transistors.
 MERITS
• There is no field winding. Therefore there is no field copper loss.
• The length of the motor is less as there is no mechanical commutator.
• Size of the motor becomes less.
• Better ventilation as armature is accommodated in the stator.
• It is possible to have very high speeds.
• Motor can be operated in hazardous atmosphere.
• Efficiency is higher.
• It is self starting motor. Speed can be controlled.
• Regenerative braking is possible.
• Elimination of radio frequency & electromagnetic interference as compared to conventional dc motors.
• Long life, less maintenance.
• Operation from a low voltage dc supply is possible.
• Motor can be designed for higher voltage, subjected to the constraints posed by the power semiconductor
device.
87
• Field cannot be controlled.
DEMERITS
• Power rating is restricted because of the maximum available size of permanent magnets.

• A rotor position sensor/ indirect rotor position sensing device is required.

• A power electronic switch circuitry is required.

• Increased complexity due to the electronic controller.

• With some types of permanent magnets, there are severe temperature limits since the magnetic
properties deteriorate with rise in temperature.

• Lack of simple method of field weakening for increasing speed.

• High cost of magnets, particularly for large–size machines.

88
COMPARISON BETWEEN BLDC MOTOR AND IM
• In the same frame, for same cooling, BLDC motor has better efficiency and pf and
therefore greater output power. The difference may be in the order of 20-50% higher.

• Power electronic converter required is similar in topology to the PWM inverters used in
IM drives.

• The device ratings may be lower for constant torque requirements. Such type of control
achieved with IM offers inferior performance (efficiency, stability, response and controlled
speed range)

• The operation in field weakening mode can be easily achieved in induction motor, which
provides a constant power capability at high speed whereas it is difficult in BLDC motor.

89
90
 COMPARISON BETWEEN BLDC MOTOR AND
CONVENTIONAL DC MOTOR
FEATURES CONVENTIONAL DC MOTOR BLDC MOTOR
Mechanical structure Field magnets on stator Field magnets on rotor
Maintenance Maintenance is high Low maintenance
Commutation method Mechanical contact between Electronic switching using power
brushes and commutator semiconductor devices.
Detecting method Automatically detected by brushes Rotor position sensor (Hall sensor /
optical sensor) is required

Reversing method By reversal of terminal voltage Rearranging logic sequences

Winding connection Ring type armature winding Star or delta connected armature
winding

Life Shorter Longer 73

 COMPARISON BETWEEN BLDC MOTOR AND
CONVENTIONAL DC MOTOR
FEATURES CONVENTIONAL DC MOTOR BLDC MOTOR

Efficiency Moderate High- since no voltage drop across brushes

Rotor inertia Higher- which limits the dynamic Low- bcz it has PM on rotor and it
characteristics improves the dynamic response.
Output power/ Frame Moderate/ Low – the heat produced by the High / reduced size due to superior
size
armature is dissipated in the airgap thus thermal characteristics bcz BLDC motor
increasing the temperature in the airgap and has the windings on the stator which is
limiting specifications on o/p power and connected to the case and heat dissipation
frame size. is better.
Torque - speed Moderately flat- at higher speeds, brush Flat- enables operation at all speeds with
characteristics
friction increases thus reduces useful torque. rated load
92
 COMPARISON BETWEEN BLDC MOTOR AND
CONVENTIONAL DC MOTOR
FEATURES CONVENTIONAL DC MOTOR BLDC MOTOR
Speed range Lower- due to mechanical limitations by the Higher
brushes

Noise Arcs in brushes may generate noise causing Low

Electromagnetic interference in the equipment
nearby.

Cost low High – since the PMs are costly

Control Simple & Inexpensive Complex & expensive
Control No controller is required for fixed speed & a A controller is always required to keep the
requirements controller is required for variable speed motor running and the same controller
operation. can be used for variable speed control.
93
94
Permanent Magnet Synchronous Motors
(PMSM)
• In PMSM, the electromagnetic field system(like in synchronous
motor), slip rings and brushes are replaced by permanent magnets.
• Results in higher efficiency, cost is reduced.
Construction
• Consists of stator and rotor.
• Stator core is made up of soft steel laminations, fixed to the frame.
• Slots are provided in the inner periphery and armature winding (mostly
double layer) is housed in these slots.
• Short chording is done to reduce harmonics
• The rotor carries permanent magnet. The poles are inherently salient pole.
Smooth cylindrical shape rotors are also magnetically salient.
*(Salience or saliency is the state or quality by which something stands out
relative to its neighbors. Magnetic saliency describes the relationship
between the rotor’s main flux (d axis) inductance and the main torque-
producing (q axis) inductance. The magnetic saliency varies depending on
the position of the rotor to the stator field, where maximum saliency occurs
at 90 electrical deg from the main flux axis (d axis)
Constructional details
• Different types of rotor based on ways of fixing magnets
• Peripheral rotor
• Interior rotor
• Claw pole
• Traverse rotor
• Based on direction of magnetic flux
• Radial field – field along radial direction
• Axial Field – field along direction parallel to shaft
• Axial field machines characterised by higher power density. Radial
field are most common.
Different configurations of PMSM rotor
PRINCIPLE OF OPERATION
• PMSM IS NOT SELF STARTING
EMF EQUATION OF PMSM
• The above emf equation is for an ideal winding. In practice, armature
windings are short chorded (short pitched) and distributed.
Moreover, armature slots are skewed. Therefore, the emf induced in
the armature reduces by skew factor, chording factor and distribution
factor.
• The winding factor for fundamental is given by Kw1 = KS1 Kc1 Kd1
• The emf equation for a practical PMSM, Eph = 4.44 f Φ Kw1.Tph
APPLICATIONS
CONTROL OF PMSM
• PMSM can be used in variable speed drives for low- and medium-power
applications.
• The speed of the motor depends on the frequency of the inverter. For a
given frequency, PMSM is a constant speed machine. It will run at
synchronous speed or will not run.
• PMSM can be driven in open-loop or closed-loop mode.
• In open-loop control, the frequency of the inverter is independently
controlled.
• In closed-loop control, the control signals are generated by an absolute
position encoder mounted on the shaft. There are various types of closed-
loop control schemes.
Commonly used control schemes
• Vector control
• Self Control of PMSM
• Sensorless Control of PMSM
• Microprocessor-Based Control of PMSM
• DSP-Based Control of PMSM
Principle of self control
• As the rotor speed changes, the armature supply frequency also changes
proportionally so that the armature field always moves (rotates) at the
same speed as the rotor.
• The armature and rotor field move in synchronism for all operating points.
Here accurate tracking of speed by frequency is realized with the help of
rotor position sensor.
• Self-controlled PMSM is fed by an invertor whose frequency is controlled
by using the information about the rotor speed.
• The semiconductor switches of the invertor are turned ON and OFF
according to the position of the rotor.
• The control is achieved by adjusting the angle between the stator mmf and
the rotor magnetic field (i.e. the torque angle).
• When the rotor makes certain predetermined angle with the axis of the
armature phases, the firing pulses to the converter feeding the motor also
changes.
• The switches are fired at a frequency proportional to the motor speed.
Thus the frequency of the voltage induced in the armature is proportional
to the speed.
• Self-control ensures that for all operating points the armature and rotor
fields move exactly at the same speed. otherwise, the rotor will
experience rapidly alternating positive and negative torque. This results in
less than optimal torque production, and excessive mechanical vibration,
noise, and mechanical stresses on the machine parts
• The torque angle is adjusted electronically hence there is an additional
controllable parameter possessing greater control of the motor behavior by
changing the firing of the semi-conductor switches of an inverter
• The inverter is implemented using power transistors T1 to T6 when
armature windings carry current, mmfs are established in the air gap.

• The switching of transistors in this order produces armature field initially at

120° away from rotor position and decreases by 60° when next switching is
done. Two transistors and hence two phases of armature conduct at all
instants.
Inner current loop and outer speed loop
• The scheme employes an inner current loop and outer speed loop
• The control scheme for implementing self control is shown as a block
diagram in Fig.
• The phase controlled thyristor rectifier on the supply side of the DC
link has the current regulating loop and operate as a control current
source.
• The regulated DC current is delivered to the DC link inductor and to
the thyristor of load commutator inverter which supplies line current
to the synchronous motor.
• When the current is zero, the motor side converter is switched on to a
new conduction period and supply side converter is turned on.
• The time required for the motor current to fall to zero can be
significantly reduced by placing a shunt thyristor in parallel with the
dc link inductor.
• If current zero is required the line side converter can be made as
inversion and the auxillary thyristor is gated.
• Auxillary thyristor is turned off when line side converter is turned off.
 Sensorless Control of PMSM
• PMSM is a salient pole machine whose synchronous inductances at
direct axis and quadrature axis are different.
• By using information on the phase inductances of PMSM the rotor
position signal for control of PMSM, can be obtained.
• The phase inductances for a given position
of rotor ϴe (electrical radian) are given by

REF :
https://in.mathworks.com/help/sps/ref/synchronousmachinemodel10.html
• where Lds = direct axis synchronous inductance, Lqs = quadrature axis
synchronous inductance
• By knowing the phase inductances, ϴe can be found out from the
variations of phase inductances for different rotor positions as shown
in Fig.
5. A three-phase–four-pole BLDC motor has 36 stator slots. Armature
winding is made of three coils per phase per pole. Each coil has 20
turns. The coil is short chorded by 2 slots. Flux per pole is 1.8 mWb.
Find the no load back emf at 3000 rpm.
• A three-phase, four-pole star connected synchronous motor has 72
slots with 20 conductors per slot. The flux/pole is 0.05 Wb and the
speed is 1500 rpm. Assuming the full-pitched coil, find the line and
phase voltage.
THANK YOU