Unit - 1
Salient Features Of Various Various Permanent Magnet Materials Materials
B-H-
Loop
Demagnetization Demagnetization Characteristics Characteristics
Comparison Of BLDC :vs.: Conventi Conventional onal Machines
Operating Principle Principle Of BLDC Motor Motor
Principle Of Hall Sensor
Unipolar
BLDC
Bi-polar
BLDC
What is permanent magnet?
1. The first term term is the flux-density flux-density that would exist if the magnet were Removed and magnetising force remained at the value ‘H’ 2. The second term can be regarded as the contribution of of magnet to the flux-density with in its volume
Note: clearly the demagnetizing curve is a straight line. And its relative slope and recoil permeability is unity
Magnetic Materials 1. 2. 3.
Neodymium Neodymium - iron – Born Cera Cerami mics cs Alni Alnico co
FIG. 1 1 FLU X DENSITY VERSUS MAGNETIC FIELD OF PERMANENT MAGNET MATERIALS
Magnet Properties
Material
Coercivity (T)
Remanence (T)
(BB0/μ0)max (kJ/m3)
BaFe12O19
0.36
0.36
25
Alnico IV
0.07
0.6
10.3
Alnico V
0.07
1.35
55
Alcomax I
0.05
1.2
27.8
MnBi
0.37
0.48
44
Ce(CuCo)5
0.45
0.7
92
SmCo5
1.0
0.83
160
Sm2Co17
0.6
1.15
215
Nd2Fe14B
1.2
1.2
260
B-H Loop
Demagnetizing characteristics
Absence of ampere-turn, the operating point is the intersection of load line and demagnetizing characteristics
Slope= permeance of external circuit x μo
B and H varies with external circuit permeance
J – magnetisation of the magnet
Linearization of non-linear behaviour
Demagnetisation Curve Of PM (a) Illustrates Working Point Below The Knee Of BH Curve (b) High Temperature Causing Demagnetisation
Losses in Permanent Magnets High-Temperature
Effects
Reversible Losses
Irreversible Losses Recoverable By Re-Magnetisation
(a) Long term stability of magnets at 24oc (b) i ll us tration of i rreversib le l oss b y reversib le changes in the B-Hloop
Applications Permanent magnets provide a motor with lifelong excitation. High initial cost is the disadvantages because it will lead the cost of the motor. The factor considered while selecting the magnets for motor applications are:
Power Density
Operating Temperature Range
Severity of Operational Duty
Structure of Controller for PMBLDC motor
How it Works
Halls Sensors sense the position of the c oils The Dec oder C irc uit turns appropriate switc hes on and off The voltage through the spec ific c oils turns the motor
Images courtesy of Servo Ma gnetics (http:// www .servomag.c om/ flash/2-pole/2po le-bldc- motor.html)
Constructions of PMBLDC Motor
Stator
Fig. 2.2 back EMF of a PMBLDC motor (a) trapezoidal (b) sinusoidal back EMF
Rotor
Fig. 2.3 rotor of PMBLDC motor (a) circular core with magnets on the periphery (b) circular core with rectangular magnets embedded in the rotor (C) circular core with rectangular magnets inserted into the rotor core
View of PMBLDC motor Assembling
Components of PMBLDC motor
Classification Of PMBLDC Motor PMBLDC
sine wave motor
PMBLDC
square wave motor
It is based on the flux density distributed in the air-gab, in addition to that further classified based on number of phase and number of pulse given to the device during each cycle is
One phase winding and one pulse PMBLDC motor
One phase winding and two pulse PMBLDC motor
Two phase winding and two pulse PMBLDC motor
Three phase winding and three pulse PMBLDC motor
Three phase winding and six pulse PMBLDC motor
Mechanical Commutation
Fig. 2.6 Mechanical Commutation Components (a) Commutator Segment Assembly b) Its Connection With Armature Windings
Fig. 2.7 Armature Windings Of DC Motor
Electronic Commutation
Fig. 2.8 Phototransistor controlling the IGBT with antiparallel diodes
Fig. 2.9 Space representation of the electronic commutator
Advantages of Electronic Commutation
Solid state high power switching devices are used Commutator arrangement is located in the stator
No sliding contacts
No chances of sparking
less maintenance
applied voltage may be controlled by PWM technique
reliable in operation
Advantages
Increased Reliability & Efficiency Longer Life Elimination of Sparks from Commutator Reduced Friction Faster Rate of Voltage & Current Precision Voltage & Current Applied to Field Coils
Applications
Low Power
Consumer Electronics
Medical Field
(Hard Drives, CD/DVD Drives, PC cooling fans, toys, RC air planes, air conditioners) (drills, operation equipments )
High Power
Industrial Machinery(Servo drives) Vehicles (Electronic power steering) Airplanes(Antares 20E, an electric self-launching sailplane, flies with a 42kW DC/DC brushless motor and Li-Ion batteries and can climb up to 3000m with fully charged cells )
Disadvantages
Requires Complex Drive Circuitry
Requires additional Sensors
Higher Cost
Some designs require manual labor (Hand wound Stator Coils)
Position Sensors Rotor position Sensors of BLDC motor
It converts the information of rotor position into a suitable electrical signal. This signal is used to switch ON and OFF of vari variou ouss sem semicon icondu duct ctor or devi device cess of elec electr tron onic ic swit switch chin ingg circ circui uitr tryy of BLPM motor Types of RPS:
Optical position sensor
Hall effect position sensor
Cont…
Cont…
Principle of position detection using Hall sensors
Comparison of BLDC motor Synchronous motor SL. No
Feature
PMBLDC motor
Synchronous Machine
1
Mechanical structure
Field magnet on the rotor
May be in the rotor or stator
2
Maintenance
Low
High
3
Winding connection
The highest grade: Δ or Y 3-Ø Connection The normal grade: Y 3-Ø Connected winding with grounded neutral point or 4-Ø connection The simplest grade: 2-Ø Connection
Δ or Y
4
Commutation methods
Electronic switching using power semiconductor switches Ex: MOSFET, TRANSISTOR
Commutation is not required but 2 Nos. of slip ring is sufficient
5
Detecting method (zero cross point/rotor position)
Rotor position could detected by sensors Ex: Hall sensors, optical encoder
Need not be required
6
Reversing method
Rearranging logic sequence
By reversing any two of the stator terminals
3-Ø Connection
Comparison of BLDC and Brushed DC motor
Comparison of BLDC motor Induction motor SL. No
Feature
PMBLDC motor
Induction Motor
1
Mechanical structure
Field magnet on the rotor
Self-excitation
2
Maintenance
Low
low
3
Winding connection
The highest grade: Δ or Y 3-Ø Connection The normal grade: Y 3-Ø Connected winding with grounded neutral point or 4-Ø connection The simplest grade: 2-Ø Connection
Δ or Y
4
Commutation methods
Electronic switching using power semiconductor switches Ex: MOSFET, TRANSISTOR
Commutation is not required
5
Detecting method (zero cross point/rotor position)
Rotor position could detected by sensors Ex: Hall sensors, optical encoder
Need not be required
6
Reversing method
Rearranging logic sequence
By changing any of two stator terminals
3-Ø Connection
Comparison of BLDC motor Induction motor cont…
Principle of Operation
Torque Production The key to successive torque and speed control of a BLDC motor is based on simple torque and Back EMF equations. The Back EMF magnitude can be written as:
Different Mode of Operation The simplest unipolar drive consists of a single switch
Uni - Polar Mode
in series with each winding and a zener diode or dump resistor in the freewheeling path as shown in Fig. 2.11 This drive is inefficient because the stored energy in the phases is dissipated. Better performance can be obtained by using topologies such as the Cdump converter shown in Fig. 2.12, which offers full regenerative
Fig. 2.11 Three-phase Inverter For BLDC Motor Drives
control.
However,
it
has
the
disadvantage of requiring a complicated control for the dump capacitor voltage, the failure of which could be catastrophic.
Fig. 2.12 C dump converter For unipolar BLDC Motor Drives
Fig. 2.13 Back-emf, Current, Torque Waveforms Of Unipolar Excitation Of BLDC Motor Drives
Advantages of Uni-Polar mode
There is only one device in series with each phase, minimizing conduction losses.
The risk of shoot-through faults is eliminated.
Switching of devices connected to the supply rails, which generally requires some isolation circuitry, can be avoided.
Research/Project Area Identified In PMBL DC Motor Operation
Bi-Polar Mode
Fig. 2.15 Bi-polar BLDC Motor Drive
Fig. 2.16 Back-emf, Current, Torque Waveforms Of Bi-polar Excitation Of BLDC Motor Drives
Bi-Polar Start Uni-Polar Run Modes
Fig. 2.17 Inverter Circuit For Bipolar-starting And Unipolar–running Drive
Fig. 2.18 Theoretical Inverter Circuit For Bipolar-starting And Unipolar–running Drive, Negative Back –EMF Would Driving Current
.
Speed Control of PMBLDC Motor This control is split into two distinct operations:
Stator and rotor flux synchronization and
Current
control
Both operations are perceived through the three phase inverter illustrated in the Fig.2.20.
FIG. 2.20 Three-phase inverter for BLDC motor drives
The flux synchronization is derived from the position information using sensors, or sensorless techniques. From the position information, the controller decides the suitable pair of transistors (Q1 to Q6) which must be driven. The control of the current to a fixed 60 degrees reference can be perceived in either of the two modes: The
Pulse Width Modulation (PWM) Mode
The
Hysteresis Mode
Closed Loop Speed Control
(with Position sensors)
Fig. Block Diagram Of Closed Loop Position Control With Speed And Current Loops Of PMBLDC Motor Drive.
Sensor less Control
Technique used to estimate the rotor position is Back-EMF
based methods
Flux calculation based methods.
Observer Other
based methods.
methods
Techniques used in Back-EMF based methods
Terminal Voltage Sensing
Back EMF Integration Methods
Sensing of Third Harmonic Of The Back-emf
Freewheeling Diode Conduction
Flux Calculation
Terminal Voltage Sensing
For typical operation of a BLDC motor, the phase current and back-EMF should be aligned to generate constant torque. The current commutation point shown in Fig. 2.21 can be estimated by the zero crossing point (ZCP) of back-EMFs and a 30 phase shift to detect the ZCPs, the phase back-EMF should be monitored during the silent phase. °
Dis Advantages / Drawbacks
It is not possible to use the noisy terminal voltage to obtain a switching pattern at low speeds since back-EMF is zero at standstill and proportional to rotor speed.
Also, the estimated commutation points have position error during the transient period when the speed is accelerated or decelerated rapidly, especially for a system that has low inertia.
With these methods, rotor position can be detected typically from 20% of the rated speed.
Nevertheless, the terminal voltage sensing method is widely used for low cost industrial applications such as fans, pumps and compressor drives where frequent speed variation is not required
Back EMF Integration Methods
In this method, the commutation instant is determined by integration of the silent phase’s back-EMF. The basic idea of this method is that the integrated area of the back-EMFs shown in Fig. 2.23 is approximately the same at all speeds. The integration starts when the silent phase’s back-EMF crosses zero. When the integrated value reaches a pre-defined threshold value, which corresponds to a commutation point, the phase current is commutated.
Dis Advantages / Drawbacks
If flux weakening operation is required, current advance can be achieved by changing the threshold voltage.
The integration approach is less sensitive to switching noise and automatically adjusts for speed changes, but low
speed
operation
is
poor
due
to
the
error
accumulation and offset voltage problems from the integration.
Sensing of Third Harmonic Of The Back-emf
This method utilises the third harmonic of the back-EMF to determine the commutation instants of the BLDC motors. Since the third harmonic of the back-EMF has three times greater frequency, this method is not as sensitive to time delay of a LPF, which is a problem of the terminal voltage sensing method. The back-EMF terms can be represented using the Fourier expression
As illustrated in Fig.2.24, the zero crossings of the integrated third harmonic flux linkage are commutation points. To sense the third harmonic of the back-EMF, an external hardware circuit is required. The third harmonic based method has a wider speed range and smaller phase delay than the terminal voltage sensing method. However, at low speed, Fig. 2.24 third harmonic of the back –EMF and commutation points
the integration process can cause a serious position error, as noise and offset error from sensing can be accumulated for a relatively long period of time.
Freewheeling Diode Conduction
This method utilises current flowing through a freewheeling diode in silent phase. Right after the ZCP of the back-EMF in the silent phase, a tiny current is flowing through the freewheeling diode during the active phase switches are turned off under alternate chopper control. This silent phase current starts to flow approximately at the time where the back-EMF of the open phase crosses zero.
Dis Advantages / Drawbacks
This
method
also
has
a
position
error
of
commutation points in the transient state as other back-EMF based methods.
The most serious drawback of this method is the use of six isolated power supplies for the comparator circuitry to detect current flowing in each freewheeling diode.
The drawback prohibits this method from practical applications
Flux Calculation
In this method, the flux linkage is estimated from measured voltages and currents and then the position is predicted by polynomial curve fitting. The fundamental idea is to take the voltage equation of the machine
Figure 2.25 shows the block diagram of the position estimation algorithm. The current loop for current estimation 1 is used for prediction, estimation and the correction of the position. The other current loop for current estimation 2 is used for correction of the calculated flux Fig. 2.25 Schematic Diagram Of The Flux Linkage Calculation Based Method
linkage values
Observer-based methods In this category, various types of observers are used to estimate rotor position. The fundamental idea is that a mathematical model of the machine is utilised and it takes measured inputs of the actual system and produces estimated outputs. Then, the error
between the
estimated
outputs and
measured quantities is fed back into the system model to correct the estimated values. The biggest advantage of using observers is that all of the states in the system model can be estimated including states that are hard to obtain by measurements.
Speed sensor-less control system of PMSM is a three-loop control system, the most outer ring is the location loop, and second ring is for the speed loop, the most inner loop as the current loop.
Other Methods Method -1
In most cases, from fundamental motor voltage equations, the inductance is calculated based on other measured quantities.
To detect rotor position at standstill, a
diagnostic current is injected. Based on the current, the rotor pole sign is estimated. This method detects the difference of the saturation level based on the sign of rotor magnet flux. Two voltage pulses with opposite signs are applied for a predetermined time for the purpose.
Method -2
A similar approach is done in to detect the rotor position at standstill. If there is a sufficient magnetic saturation level difference when a different PM Pole is located, the rotor pole can be identified at standstill by applying positive and negative voltage pulses and measuring magnitude of the resultant current
Method -3
A high-frequency signal is injected to detect inductance, which is a function of rotor position for salient PM motors. Once there is a significant magnetic saliency in the motor, the initial rotor position at standstill can be detected based on the incremental inductance calculation
Method -4
To create artificial saliency, a thin non-magnetic material is attached to the round surface of the PMs. In this case, the stator inductance changes with rotor position as the eddy currents in the aluminium act to decrease the inductance. By applying rectangular
voltage
pulses and
measuring
the
maximum current amplitudes, the position can be estimated. However, the additionally attached nonmagnetic material can increase the air-gap of the motor, and hence can decrease the efficiency.
Method -5
The DC bus current shape is used to estimate commutation error. When the motor phase current is not synchronised with the back-EMF, the DC bus current is distorted. Based on the DC bus current observation, when the phase current leads and lags the back-EMF, DC link input voltage is updated to align the current and back-EMF, and continuously maintains the synchronism of the motor.
Speed Torque Characteristics Figure 2.26 shows an example of torque/speed characteristics. There are two torque parameters used to define a BLDC motor, peak torque (TP) and
rated
torque (TR).
During continuous
operations, the motor can be loaded up to the rated torque. As discussed earlier, in a BLDC motor, the torque remains constant for a speed range up to the rated speed. The motor can be run up to the maximum speed, which can be up to 150% of the rated speed, but the torque starts dropping. Fig. 2.26 Speed – Torque Characteristics of PM BLDC motor
Ex1. A BLPM d c motor has no load speed of 600 rpm. When connected to 120V dc supply. The armature resistance is 2.5Ω. Rotational & iron losses may be neglected. Determine the speed when the supply voltage is 60V and torque is 0,5N
Given data Na=600 rpm, V=120V,Ra=2.5Ω,T=0.5N Solution Na=600 rpm, V=120V,Ra=2.5Ω,T=0.5N K e
V
K e
I
mo
T
V
But , E b
m
0 . 19 volt / rad / sec
60 K t and T K t I
K t Back emf E b
2
120 6000
0 .5
0 . 19 I a R a
60
2 . 63 A
( 2 . 63
2 .5 )
53 . 43 volts
K e m
E b K e
53 . 13 0 . 19
281 . 18 rad / sec
2685 rpm
