BRUSHLESS DC MOTOR
BLDC Motor
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Introducti Intro duction on - BLDC motor BLDC motors are a type of Synchronous Motor .
This means the magnetic field generated by the stator and the mag magnet netic fiel field d gene genera rate ted d by the the roto rotorr rot rotate ate at the the same same frequency. BLDC motors do not experience the “slip” that is normally seen in induction motors.
BLDC BLDC moto motors rs come come in sing single le-p -pha hase se,, 2-pha 2-phase se and and 3-ph 3-phas ase e config configura uratio tions. ns. Out of these, these, 3-phase 3-phase mot motors ors are the most popular and widely used.
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Introduction - BLDC motor
Conventional DC motors - highly efficient - Only drawback commutator and brushes - subjected to wear and tear require maintenance.
Brushless DC motors - When the functions of commutator and brushes implemented by means of solid state switches maintenance free motors realized.
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Difference between Mechanical and Electronic commutator
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Mechanical commutator
When a commutator segment comes in contact with the brush, current flows through the section of the winding connected to the DC supply through commutator and brushes.
Commutator serves the purpose of switching current from one section of the armature winding to the other at correct instant.
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Electronic commutator
In BLDC Motor, PM field rotating, stationary armature supplied by an inverter controlled by shaft position sensing signals.
Static inverter – Solid state switches does the function of Commutator - Electronic commutator.
This facilitates operation of a BLDC motor as a versatile variable speed DC motor having no mechanical commutator and brushes.
Maintenance free motor - BLDC motor.
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Difference between BLDC & PMSM
Construction identical to that of a synchronous motor with a permanent magnet rotor.
But the difference between a PMSM and a BLDC motor – manner in which phase windings excited.
In PMSM - the phase windings excited by 3 phase supply of line frequency - permanent magnet will follow stator rotating field.
In BLDC motor - stator windings – switched with DC current or voltage - through a 3 phase inverter - Electronic commutation.
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Advantages of BLDC motor (over Brushed DC motors and Induction motors) High dynamic response High efficiency and reliability Long operating life – absence of brushes & no sparking Noiseless operation Higher speed ranges Better speed versus torque characteristics High ratio of torque delivered to the size Better power factor
Disadvantages of BLDC motor Need for rotor position sensing Increased complexity in electronic controller Increased cost
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Applications of BLDC motor Finds application in every segment of the market such as: Automotive Aerospace Consumer Equipment & Instrumentation Home Appliances Industrial automation Medical equipments
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Applications of BLDC motor (contd.) We can classify the BLDC motor control into 3 major types:
(i) Applications With Constant Loads
These are the types of applications where a variable speed is more important than keeping the accuracy of the speed at a set speed. In these types of applications, the load is directly coupled to the motor shaft. For example, fans, pumps and blowers come under this. These applications demand lowcost controllers, mostly operating in open-loop.
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Applications of BLDC motor (contd.)
(ii) Applications With Varying Loads These are the types of applications where the load on the motor varies over a speed range. These applications may demand a high-speed control accuracy and good dynamic responses. In home appliances - washers, dryers and compressors, in automotive - fuel pump control, electronic steering control, engine control and electric vehicle control, in aerospace - centrifuges, pumps, robotic arm controls, gyroscope controls and so on. These applications may use speed feedback devices and may run in semi-closed loop or in total closed loop. These applications use advanced control algorithms and this increases the price of the complete system.
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Applications of BLDC motor (contd.)
(iii) Positioning Applications Most of the industrial and automation types of applications come under this category. In these applications, the dynamic response of speed and torque are important. Also, they may need frequent reversal of rotation. The load on the motor may vary, causing the controller to be complex. These systems mostly operate in closed loop. There could be three control loops functioning simultaneously: Torque Control, Speed Control and Position Control Loop. Optical encoder or synchronous resolvers are used for measuring the actual speed of the motor.
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Construction of BLDC motor Stator
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The stator consists of stacked steel laminations with windings placed in the slots that are axially cut along the inner periphery similar to that of an induction motor; however, the windings are distributed in a different manner - trapezoidal stator windings Windings connected in star Sequentially switched with DC current or voltage through a 3 phase inverter Inverter switching pattern and frequency controlled by the rotor position and speed Back EMF trapezoidal in nature
Construction of BLDC motor (contd.) Rotor The rotor consists of a shaft and a hub with PM to form two to eight pole pairs. Based on the required magnetic field density in the rotor, the proper magnetic material is chosen to make the rotor. Ferrite magnets & rare earth alloy magnets are commonly used. The ferrite magnets are less expensive but they have the disadvantage of low flux density for a given volume. In contrast, the alloy material has high magnetic density per volume and enables the rotor to compress further for the same torque. Also, these alloy magnets improve the size-to-weight ratio and give higher torque. Neodymium (Nd), Samarium Cobalt (SmCo) and the alloy of Neodymium, Ferrite and Boron (NdFeB) are some examples of rare earth alloy magnets.
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Construction of BLDC motor (contd.) Rotor (contd.)
Two types of rotor constructions are possible. (i) Surface mounted PM rotor - if the permanent magnets are mounted on the surface of the rotor (ii) Interior PM magnet rotor - if the magnets are embedded or placed in the groves or slots on the rotor.
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Construction of BLDC motor (contd.) Rotor position sensing – Using
Hall effect sensors
Most BLDC motors have three Hall sensors embedded into the stator on the non-driving end of the motor.
Whenever the rotor magnetic poles pass near the Hall sensors, they give a High or Low signal, indicating the N or S pole is passing near the sensors.
Based on the physical position of the Hall sensors, there are two versions of output. The Hall sensors may be at 60° or 120° phase shift to each other. Based on this phase shift, the motor manufacturer defines the commutation sequence, which should be followed when controlling the motor.
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Principle of operation
The stator windings are fed from a three phase inverter. When the stator windings are energized, a rotating magnetic field is created, which will attract the rotor poles. So, for continuous rotation of rotor, the windings must be energized in a particular sequence based on the rotor position.
Rotor position is sensed using Hall sensors. Based on the combination of these three Hall sensor signals, the exact sequence of commutation can be determined.
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Principle of operation (contd.) Figure shows an example of Hall sensor signals with respect to back EMF and the phase current. Every 60 electrical degrees of rotation, one of the Hall sensors changes the state. In synchronous with every 60 electrical degrees, the phase current switching should be updated.
Basic Block diagram – Controller + BLDC 18
Principle of operation (contd.) (00-600),
For example, during I A is positive, IC is negative and phase B is unenergized. Also, the hall sensor output in this interval is 001 (HA HB HC). Hence the switches Q1 & Q4 will turn ON. For (60 0 – 1200), I A is positive, IB is negative and phase C is unenergized. The hall sensor output in this interval is 000. Hence the switches Q1 & Q2 will turn ON. Thus, by sensing the Hall sensor output, the switching sequence (or commutation) can be determined.
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Table 1 Rotor position θ
HA
HB
HC
IA
IB
IC
Switches
00 - 600
0
0
1
+
0
-
Q1 & Q4
0
0
0
+
-
0
Q1 & Q2
0
0
60 - 120
1200 - 1800
1
0
0
0
-
+
Q5 & Q2
0
180 - 240
1
1
0
-
0
+
Q5 & Q0
2400 - 3000
1
1
1
-
+
0
Q3 & Q0
0
1
1
0
+
-
Q3 & Q4
0
0
0
300 - 360
Table 1: Shows the switching sequence that should be followed with respect to the Hall sensors for clockwise (CW) rotation.
Principle of operation (contd.) Table 1 Rotor position θ
HA
HB
HC
IA
IB
IC
Switche s
00 - 600
0
0
1
+
0
-
Q1 & Q4
600 - 1200
0
0
0
+
-
0
Q1 & Q2
0
120 - 180
1
0
0
0
-
+
Q5 & Q2
1800 - 2400
0
1
1
0
-
0
+
Q5 & Q0
0
240 - 300
1
1
1
-
+
0
Q3 & Q0
3000 - 3600
0
1
1
0
+
-
Q3 & Q4
0
Figure on left shows the winding energizing sequence with respect to the Hall sensors for clockwise (CW) rotation.
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Principle of operation (contd.) Table 2
Table 2 shows the switching sequence that should be followed with respect to the Hall sensors for counter clockwise (CCW) rotation.
Rotor position θ
HA
HB
HC
IA
IB
IC
Switche s
00 - 600
0
1
1
0
-
+
Q5 & Q2
600 - 1200
1
1
1
+
-
0
Q1 & Q2
0
120 - 180
1
1
0
+
0
-
Q1 & Q4
1800 - 2400
0
1
0
0
0
+
-
Q3 & Q4
0
240 - 300
0
0
0
-
+
0
Q3 & Q0
3000 - 3600
0
0
1
-
0
+
Q5 & Q0
0
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Basic configuration of BLDC Motor drive
Figure shows a block diagram of the controller used to control a BLDC motor.
Q0 to Q5 are the power switches controlled by the PIC microcontroller. Based on the motor voltage and current ratings, these switches can be MOSFETs, or IGBTs, or simple bipolar transistors. If the signals marked by PWMx are switched ON or OFF according to the sequence, the motor will run at the rated speed. To vary the speed, these signals should be Pulse Width Modulated (PWM) at a much higher frequency than the motor frequency. 22
Controller circuit – BLDC Drive
Rotor position sensed by a position sensor.
Decoded by combinational logic to provide firing angles for the three phases.
Commutation logic or rotor position decoder has six outputs which control upper and lower phase leg transistors.
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Drive and Power circuit
Base drive circuit
Position signal pulses from logic circuit between +5V and 0V
Do not have enough power content in them to turn the transistor ON and OFF
Need of base driver circuit
Power content in the pulse needed to be enhanced
Isolation of controller circuit from the power circuitoptoisolator
Power circuit
Three phase bridge inverter
Feedback diodes across each switch
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Torque-Speed Characteristics
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Torque Vs Speed characteristics - BLDC
Peak Torque (TP) or Stall torque Maximum torque that motor can deliver for a short duration of time.
Rated Torque (TR) Torque available on the shaft for the given speed range.
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Torque Vs Speed characteristics – BLDC (contd.) Characteristics can be divided into: (i) C o n t i n u o u s To r q u e Zo n e
Torque remains constant (TR) for a speed up to rated speed. Motor can be run up 150% of the rated speed (maximum) but, torque starts drooping.
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Torque Vs Speed characteristics – BLDC (contd.) (ii) In t e r m i t t en t To r q u e Z o n e
Applications that need frequent starts & stops and frequent reversals of rotation with load on the motor demand, more torque than the rated torque. eg: when the motor starts from standstill and during acceleration. ie. extra torque required to overcome the inertia of the load and rotor itself.
Motor can deliver higher torque, maximum up to peak torque (TP).
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