The document provides information about permanent magnet DC motors and brushless DC motors. It discusses the construction, working principle, and applications of permanent magnet DC motors. It then describes the construction of brushless DC motors including the stator, rotor, position sensors. It explains the working of trapezoidal and sinusoidal brushless DC motors. Trapezoidal BLDC motors have trapezoidal back-EMF and current waveforms for smooth torque production, while sinusoidal BLDC motors have sinusoidal waveforms.

1. EE402 Special Electrical
Machines
Module 5

2. Syllabus
 Permanent Magnet DC Motors – construction – principle of
working. Brushless dc motor – construction – trapezoidal type-
sinusoidal type – comparison – applications.

3. Permanent Magnet DC Motors
 When permanent magnet is used to create magnetic field in a DC motor, the
motor is referred as permanent magnet DC motor or PMDC motor.
 We can replace electromagnet with permanent magnet, which result in higher
efficiency, less space requirement and better cooling.
 PMDC are best solutions to motion control and power transmission
applications where compact size, wide operating speed range, ability to adapt
to a range of power sources or the safety considerations of low voltage are
important.
 The magnets are radially magnetized and are mounted on the inner periphery
of the cylindrical steel stator.
 It has a commutator segments and brushes, similar to DC motor, .

4. Construction

5.  The field poles of this motor are essentially made of permanent magnet.
A PMDC motor mainly consists of two parts. • Stator • Rotor
Stator:-
 Stator is a steel cylinder and the magnets are mounted in the inner periphery
of this cylinder.
 The steel cylindrical stator also serves as low reluctance return path for the
magnetic flux. The permanent magnets are mounted in such a way that the N-
pole and S-pole of each magnet are alternatively faced towards armature.
 Although field coil is not required in permanent magnet dc motor, sometimes
they are used along with permanent magnet. This is because if permanent
magnets lose their strength, these lost magnetic strengths can be compensated
by field excitation through these field coils.

6. Rotor
 The rotor of PMDC motor is similar to the armature of DC Motors. It is made of number of
silicon steel sheets to reduce eddy current loss.
 Consists of core, windings, commutator and brushes. There are three types of armature
structures available:
1. Slotted Armature
2. Slotless Armature
3. Moving Coil
Slotted armature
 Slotted armature is made up of silicon sheet steel or carbon sheet steel which are punched
together or mounted on the shaft.
 The armature has slots on its periphery. Armature conductors are placed on this, slots and
properly connected to form armature windings. A core having many slots is usually desirable,
because the greater the number of slots, the less the cogging torque and electromagnetic noise.
 Cores having even numbers of slots are usually used due to ease of production. But core with
odd numbers of slots are preferred due to low cogging torque (torque ripples).

7. Slotless armature
 In this type of construction, the conductors are fixed on the outer periphery of the
core, slots are absent.
 Advantage of this construction is the reduction in torque ripples.
 In this case the torque is exerted on the conductors uniformly distributed on the
rotor surface.
 Flux decreases due to larger airgap. Thus, larger volume of PMs must be used to
get sufficient magnetic flux.
Moving coil Armature
 In this type of construction, the iron core is replaced by non-magnetic core which
is usually made of glass fibre.
 This has the advantage of low inertia and no iron los in the armature.
 The commutator and brushes are very small and made up of metals like gold,
silver platinum etc. Small sized commutator and brushes helps in stable
commutation.

8. Types of Permanent Magnets used in Electrical Machines
Alnico
 Commonly used in high temperature applications because of higher thermal stability.
Alnicos has a low coercive magnetizing intensity and high residual flux density.
Demagnetization is high. It is used where low current and high voltage is required.
Ceramic or Ferrite
 It is the cheapest possible permanent magnet that can be use in PMDC motor. It is suitable
for moderate temperature. They have linear demagnetization characteristics in the second
quadrant of BH curve. It’s performance is good upto 100◦C.
Somarium Cobalt (Sn2Co17)
 They are suitable for operation up to 200◦C. They have large remanence and high coercive
force. It has high energy density and linear demagnetization characteristics. But it is
expensive due to an inadequate supply of samarium.
Neodymium Iron Boron
 They are comparatively the latest member in the family of permanent magnets.
 It has the highest energy density and very good coercivity. The Neodymium iron boron is
cheaper as compared to Samarium cobalt. They can withstand higher temperature. They
have the highest (BH)max product.

9. Principle of Operation
 The working of PMDC motor is based on the Principle that when a current
carrying conductor is placed in a magnetic field, it experiences a force
whose direction is given by Flemings left hand rule and magnitude is
given by F = BIl
B- flux density
I-current through the conductor
L – length of the conductor.
 Consider a part of PMDC motor shown in Fig. 2
Fig. 2 Part of PMDC Motor

10. Let the armature carry current in the direction shown in Fig. 2, when the supply is
switched ON.
 Due to the interaction between the current and magnetic field , conductor
experiences a force in the direction shown in Fig. 2.
As the conductors are placed on the rotor, rotor starts rotating in CCW direction.
 When the armature starts rotating, a back emf is produced in the armature winding.
The direction of back emf is opposite to the applied voltage.
The applied voltage has to force the current through the armature conductors
against this back emf.
The electrical energy for overcoming this opposition is converted into mechanical
energy developed in the armature.

11. Performance curves of PMDC Motors
The torque – speed characteristic is almost straight line which indicates the
suitability of PMDC motors for servo applications.
 Linear relationship exists between input current and torque.
 The PMDC motors have better efficiency and is much higher than conventional
DC motors.

12. Brush less DC Motor (BLDC)
Main problem associated with PMDC motor is the commutation with the help of
brushes. BLDC motor eliminates this problem. There is no commutator and brushes.
 Since the commutator and brushes are absent, maintenance requirement is very
less, and many problems associated with brushes are removed.
 The armature is stationery and PM field system is mounted on the rotating shaft.
The commutation is achieved by using semi-conductor switches instead of
mechanical commutator.
 Compared with PMDC motors, BLDC motors have higher efficiency, smaller size
and better cooling.

13. Construction of PMBLDC Motor
Figure 4: Construction Details of BLDC Motor
 The field poles of this motor are essentially made of permanent
magnet. Which is the rotor.
 A BLDC motor mainly consists of two parts:
• Stator
• Rotor

14. Stator
 Stator of a BLDC motor is made up of stacked steel laminations to carry the
windings.
 These windings are placed in slots which are axially cut along the inner
periphery of the stator. These windings can be arranged in either star or delta.
However, most BLDC motors have three phase star connected stator.
 Each winding is constructed with numerous interconnected coils, where one or
more coils are placed in each slot. In order to form an even number of poles,
each of these windings is distributed over the stator periphery.

15. Figure 5: Construction Details of Rotor
Rotor
 BLDC motor incorporate a permanent magnet in the rotor. Its function is to produce
the required magnetic field. In order to achieve maximum torque in the motor, the
flux density of the magnet should be high.
 Permanent magnets offer very high flux density. Ferrite magnets are traditionally
used to make permanent magnets.
 Permanent magnet rotates and the armature remains static.
 The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south
and north poles depending on the application requirement. More poles give smaller
steps and less torque ripple.

16. Position Sensor
 Brushless DC motor requires an electronic commutator to rotate the rotor. In
order to rotate the motor, the windings of the stator must be energized in a
sequence.
 The position of the rotor (i.e. the North and South poles of the rotor) must be
known to precisely energize a particular set of stator windings.
 The hall effect sensor provides information about the position of the rotor at
any instant to the controller which sends suitable signals to the electronic
commutator.
 Thus, for the estimation of the rotor position, the motor is equipped with three
hall sensors. These hall sensors are placed every 120◦.
 Each sensor generates Low and High signals whenever the rotor poles passes
near to it. The exact commutation sequence to the stator windings, can be
determined based on the combination of these three sensor responses.

17. Classification

18. Principle Of Operation Of PMBLDC Motor
Figure 6: Drive Circuit of PMBLDC
2
3 5
6

19.  In the case of a BLDC motor, the armature is stationary, and the permanent
magnet is rotating.
 Stator windings of a BLDC motor are connected to a voltage fed inverter
circuit as shown in Figure 6. The inverter converts DC voltage into variable
frequency voltage.
 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 three phases conduct at all
time (180◦ conduction mode). The switching interval is 60◦.

20. Figure 7: Rotor Movement with S1, S5 and S6 ON
 When switches S1, S5 and S6 are turned ON, the current flows through
phase R and divides into equal amounts and completes its path through phase
Y, phase B, through S6 and S5.
 This current produces mmfs which can be represented by stator north pole
and south pole as shown in Figure 7(a), due to these stator poles, the rotor
poles experience and move in clockwise direction.
 When the rotor moves by 60◦ and occupies the position shown in Figure
7.(b) switches S2 is turned ON and S5 is turned OFF keeping S1 and S6 ON.

21.  When switches S1, S2 and S6 are conducting with R and Y connected to
positive and B to negative of power supply, the stator poles are shifted by
60◦ as shown in Figure 8(a). This will cause the movement of rotor by 60◦
in clockwise direction and its position is shown in Figure 8(b).
Figure 8: Rotor Movement with S1, S2 and S6 ON

22. Figure 9: Rotor Movement with S1, S2 and S6 ON
 When the rotor attains the position shown in Figure 9(b) , S1 is switched
OFF and S4 is switched ON. This operation results the position shown in
Figure 9(a). This will produce torque on the rotor and hence it rotates by
60◦ when the next switching operation is done. This is shown in Figure 9
(b)

23. PMBLDC Square Wave Motor With 180◦ Pole Arc (120◦
Commutation Mode )
Figure 10: Three Phase BLDC Motor
Consider a three phase BLDC motor with two rotor poles and having an arc
length of 180◦ . The number of slot per pole per phase is two.
Two adjacent coils in the armature forms a 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 switching interval
is 60◦ .

24. Power converter circuit of BLDC motor
Figure 11: Power Converter Circuit
Figure 12: Current Waveforms
3 5
6 2

25. When ωt = 0 ◦ the phase Y is conducting positive current and
phase B is conducting negative current. ie the switches S3 and
S2 are turned ON.
The polarity of mmf distribution is same as that of the flux
density distribution of rotor, so positive torque is produced.
At ωt = 60◦ the switch S2 is switched OFF and S4 is turned ON.
Now the torque remains unaffected. So the rotor continuous to
rotate.
For achieving the phase current pattern in the Table, the
semiconductor switches are turned ON in the sequence S2, S3 ;
S2, S4; S3, S4 ; S4, S5 ; S5, S6 ; S6, S1 . . . , while all other
switches are in OFF position. The current waveforms are shown
in Figure 12.

26. PMBLDC Square Wave Motor With 120◦ Pole Arc (180◦
Commutation Mode )
Figure 13: Three Phase BLDC Motor
Figure 14: Current Waveforms 

27.  Consider a three phase BLDC motor with two rotor poles. The number of slot per pole per
phase is two. Two adjacent coils in the armature forms a phase. In this configuration, current
flows through all the phases at all the instants. The motor should be delta connected. The phase
currents are of 180◦ square wave form.
 The 120◦ magnetic arc motor uses 180◦ mode of inverter operation. The commutation circuit
requires six switches. Three switches should be ON at any instant. Inverter circuit for star
connected BLDC is shown in figure. In this setup, the converter is implemented using power
transistors. 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 switching interval is 60◦
 . When ωt = 0 ◦ the phase R and B are conducting positive current and phase Y is conducting
negative current. For that, the switches T1 T5 and T6 are turned ON. The polarity of mmf
distribution is same as that of the flux density distribution of rotor. So positive torque is
produced.
 At ωt = 60◦ the switches T6 is switched OFF and T1 is turned ON. Now the torque remains
unaffected. So the rotor continuous to rotate.
 For achieving this, the transistors should be switched ON in the sequence T1,T5 ,T6, ; T1, T6,
,T2 ; and so on.

28. Trapezoidal type BLDC Motor
A sample circuit set up is shown in Figure, based on the signals from the position sensors,
control circuitry consisting torque to current computation block,
current control circuit block etc. will be in operation. Switching signals are produced
accordingly to energize the stator windings of BLDC.
Fig 15.Circuit set up of BLDC

29. Trapezoidal type BLDC Motor contd…
 The BLDC motor is designed with concentrated coils, and the magnetic
structure is shaped such that the flux density of the field because of the
permanent magnets and the induced excitation voltages have trapezoidal
waveforms.
 Fig 16. Waveforms -

30. Trapezoidal type BLDC Motor
 Waveforms show that induced emf efa,(t) in phase a, where the rotor is rotating in a
counterclockwise direction at a speed of ωs, radians per second.
 This emf waveform has a flat portion, which occurs for at least 120” (electrical) during each half-
cycle. The amplitude ef is proportional to the rotor speed.
 Similar voltage waveforms are induced in other phases also displaced by 120 degree and 240
degree.
 To produce as ripple-free torque as possible in such a motor, the phase currents supplied
should have rectangular waveforms as shown in figure.
 The instantaneous electromagnetic torque is independent of time and depends only on
the current amplitude I.
 A current-regulated VSI as shown in Fig.15 is used in which the reference currents are rectangular
reference currents.
 One complete cycle is divided into six intervals of 60 electrical degrees each. In each
interval, the current through two of the phases is constant and proportional to the torque
command. To obtain these current references, the rotor position is usually measured by
Hall effect sensors that indicate the six current commutation instants per electrical cycle
of waveforms.

31. Sinusoidal type BLDC Motor (Permanent Magnet
Synchronous Motor, PMSM)
 The circuit setup is same as in Figure 15. Air gap flux density distribution and the
induced excitation voltages in the stator phase windings are nearly sinusoidal.
 The torque angle, δ is maintained at 90 degree. For controlling the motor, the rotor
field position is measured by means of a position sensor with respect to a stationary
axis.
 A current-regulated VSI as shown in Fig. 15 is used where the rectangular
reference currents are replaced by sinusoidal reference currents.
 It has distributed and fractional slot winding for sinusoidal back emf and smooth
operation. The armature windings are generally double layer and lap wound.

32. Figure 16: Sinusoidal Back emf
 The rotor poles are shaped so that the back
emfs induced in the stator are sinusoidal.
 In addition to the back emf, the phase
current also has sinusoidal variations.
 Sinusoidal back emfs produced by phase a, b
and c are shown in Fig. 16 which are
displaced by 120 degree phase difference.
Sinusoidal type BLDC Motor Contd…

33. Comparison between Trapezoidal and Sinusoidal
Motors

34. Electronic Commutator
 The most important advantage of BLDC motor is the use of electronic
commutator in the place of mechanical commutator. The electronic
commutator is used in PMBLDC, to transfer current to the armature.
 In PMBLDC motor, the function of commutator and brushes are performed
by, power semiconductor switches. The phase windings of PMBLDC motors
are energized using these power semiconductor switches. Thus it is also
called electronically commutated motor.
 It is necessary to have stationary armature and rotating field system for
implementing electronic commutator. As the field winding is rotating it is
necessary to supply DC voltage to the tapping points of the armature winding.
 To accomplish this, each tapping on the armature winding is connected to the
junction of two semiconducting switches in such a way that one conducts in
one direction and the other in OFF position.

35. Comparison between Electronics and Mechanical Commutator

36. Comparison - BLDC with SRM

37. Comparison –BLDC with Conventional DC Motor

38. Torque Speed Characteristics of PMBLDC Motor
 There are two torque parameters used to define a BLDC
motor, peak torque and rated torque.
 During continuous operations, the motor can be loaded
up to rated torque. This requirement comes for brief
period, especially when the motor starts from stand still
and during acceleration. During this period, extra torque
is required to overcome the inertia of load and the rotor
itself.
 The motor can deliver a higher torque up to maximum
peak torque, as long as it follows the speed torque curve.
 Figure shows the torque speed characteristics of a BLDC
motor. As the speed increases to a maximum value of
torque of the motor, the torque of the motor decreases.
 Continuous torque zone is maintained up to the rated .
 The stall torque represents the point at which the torque
is maximum, but the shaft is not rotating.
 The no load speed, ωn, is the maximum output speed of
the motor.
 If the phase resistance is small, as it should be in an
efficient design, then the characteristic is similar to that of
a shunt DC motor.

39. Advantages and Disadvantages of BLDC
Motors
Advantages
 1. Maintenance is less due to the absence of brushes and commutator
 2. Sparking associated with brushes are eliminated
 3. Higher reliability
 4. Low weight
 5. Long life
 6. Reduction in noise
 7. Low radio frequency interference
 8. Low inertia and friction
 9. Faster acceleration and can run in higher speed
 10. Higher efficienc

40. Disadvantages
 1. High cost
 2. Low starting torque
 3. No flexibility in control due to the absence in field winding

41. Applications
 1. Electric vehicles (around 15% longer driving range is possible compared to induction
motors)
 2. Spindle drives (Hard disk drives) of computers
 3. Variable speed fans.
 4. Robotics.
 5. Record players
 6. Used as drives of cooling fans for electronic circuits and heat sinks.
 7. Used in portable electric tools such as drills saber saws etc.
 8. High speed aerospace drives and gyroscopes in the field of aerospace. 9. In the field
of biomedical engineering (Cryogenic coolers, artificial heart pumps)
 10. Used in food mixers, ice crushers and portable vacuum cleaners