BRUSHLESS MOTOR

1

ABES Institute of Technology

Submitted to:Yogesh Chandra GuptaDept. of Electrical Engineering

Submitted by:Amit Kumar B Tech. EEE- 3rd Year0729021006

Brushless DC Motors Standard DC motor – Magnetic field is

stationary in stator, rotor poles switch polarity due to commutation to provide constant rotation.

Brushless DC motor – Magnetic field of rotor is fixed. Magnetic field in stator poles is electronically commutated, provides rotating magnetic field.

Motor contains internal position encoder to provide position feedback to the control system.

2

3

ApplicationsCPU cooling fansCD/DVD PlayersElectric automobiles

Pros (compared to brushed DC)Higher efficiencyLonger lifespan, low maintenanceClean, fast, no sparking/issues with brushed contacts

ConsHigher costMore complex circuitry and requires a controller

Brushless DC motor applications

4

Advantages:High efficiency (no losses across brushes)Absence of arcing at brushes – reduces electrical

noise.High-speed operation possibleHigh reliability, low maintenance

5

Characteristics of Brushless DC motors

Pros • Very Power efficient (95%) • Good Power to Weight ratio • No mechanical wearing of Brushes • Very good control of speed and position • Low electrical noise • High starting torque

Cons • Rely on electronic commutation (microcontroller or ASSP) • More complex driver stage than Brushed DC

6

The basic concept • Rotor consists of permanent magnets • Stator consists of a number of windings • Current through windings generates a magnetic field that

attract rejects the rotor magnets • Position sensors used to determine when to “commutate”

7

Split a rotary servo motor radially along its axis of rotation:

Flatten it out:

Result: a flat linear motor that produces direct linear force instead of torque

How Linear Brushless DC Motors work

8

9

Essential difference - commutation is performed electronically with controller rather than mechanically with brushes

10

Brushless DC Motor Commutation

Commutation is performed electronically using a controller (e.g. HCS12 or logic circuit)Similarity with stepper motor, but with less

number of polesNeeds rotor positional closed loop feedback:

hall effect sensors, back EMF, photo transistors

Basic of brushless Rotation To begin to understand how brushless motor operate, refer to the

following figure. Power is applied to winding “R” and current flow sets up a ‘north’ pole which the permanent magnet will react to, and begin movement. This movement will cease when the ’south’ pole of the magnet aligns it. However, if, at the appropriate time, current is shut off in winding

“R”, and turned on in winding “S”, then the rotor continues to move. Again at the appropriate time, shut off “S”

and turned on “T”.

11

By continuation of this timing sequence, complete rotation occurs. What is occurring, is that the field set up by the stator is

being switched, and the rotor tries to catch up to it. In this example, the explanation was simplified by

exciting only one winding at a time. In reality, the stator

consists of a three phase Y–connected winding, and two or three windings are actually energized.. This

makes efficient use of windings and development of higher motor torques.

12

Brushless DC motors are used in a growing number of motor applications as they have the following advantages:

They have no brushes so they require little or no maintenance. They generate less acoustic and electrical noise than universal

brushed DC motors. They can be used in hazardous operation environments (with

flammable products). They also have a good weight/size to power ratio. Such motors have little rotor inertia. The coils are attached to

the stator. The commutation is controlled by electronics. Commutation times are provided either by position sensors or

by coils back Electromotive Force measurements. In sensor mode, Brushless DC motors usually consist of three

main parts: a Stator, a Rotor and Hall Sensors.

13

Theory of Operation

BRUSHLESS SERVOMOTOR

14

Brushless motor rotation is due to two magnetic fields interaction, which result in movement.

In the case of AC motors, the stator winding sets up one magnetic field while inducing the second interacting field onto the squirrel cage rotor.

With DC motors, the permanent magnet stator sets up the first magnetic field, and the rotor windings produce the second field.

These two magnetic fields interacting, results in rotation. In the DC motor, the two fields try to align.

However the commutator continually switches power from winding to winding. Thus, maintaining the two magnetic fields at a 90 degree relationship. If they did indeed align, motor rotation would not occur.

Compared to DC motors, brushless technology has been termed an “inside out” design. That is, the

permanent magnets are on the rotor, and the stator consists of windings. The design still consists of two

magnetic fields interacting.15

Analysis Method

Analysis is similar to that of rotary machines.Linear dimension and displacements

replace angular onesForces replace torques

16

Brushless motor

17

Stator A basic three phases BLDC motor Stator has three coils. In many motors the number of coils is replicated to

have a smaller torque ripple. Figure shows the electrical schematic of the stator. It consists of three coils each including three elements in series, an inductance, a resistance and one back electromotive force.

18

Rotor The rotor in a BLDC motor consists of an even number of

permanent magnets. The number of magnetic poles in the rotor also affects the

step size and torque ripple of the motor. More poles give smaller steps and less torque ripple. The permanent magnets go from 1 to 5 pairs of poles. In certain cases it can go up to 8 pairs of poles. Figure shows three phase, three coil BLDC motor stator and

rotor The coils are stationary while the magnet is rotating. The rotor of the BLDC motor is lighter than the rotor of a

conventional universal DC motor where the coils are placed on the rotor.

19

20

Delta               Wye

BLDC (3-Pole) Motor Connections

Has 3 leads instead of 2 like brushed DC Delta (greater speed) and Wye (greater torque)

stator windings 

• ”Sensor less control” is when position sensors are not used • Back Electromotive Force (B-EMF) is the key to sensor less

control • B-EMF is the voltage generated in the stator windings when

motor rotates - the motor is a generator while being a motor • B-EMF voltage is depending on the motor speed • B-EMF voltage not available when starting

21

Single-phase BLDC • Single-phase BLDC motor • Used for fans • Advantages • Low cost mechanical solution • Disadvantages • Electronical control requires hall-sensor • Noisy due to high torque ripple • Costly driver stage • Requires dead-time between commutations

22

Two-phase BLDC • Two-phase BLDC motor • Used in fans

Advantages • Low mechanical cost • Simple to control (and no dead band required) • Inexpensive driver stage

Disadvantages • Difficult to control without sensors

23

Three-phase BLDC Three-phase BLDC motor Used in various applications like toys, power tools, white goods. Advantages Easy to control High power to weight ratio Good position control Low cost solution Implementation needs few resources, CPU cycles available for other tasks HW fault protection (Tiny26 family) Disadvantages • Noisy

24

The control consists of logic circuitry and a power stage to drive the motor.

The control’s logic circuitry is designed to switch current at the optimum timing point. It receives information about the shaft/magnet location and outputs a signal, to turn

on a specific power device, to apply power from the power supply to specific windings of the brushless motor.

25

EVALUTION OF THE BRUSHLESS MOTOR

• Brushed motors are thermally limited due to the heat dissipation of the motor• The heat generated in the windings must dissipate through the air gap to the magnets, through the case to the outside air or through the shaft• In order to make it more efficient, the windings were placed on the outside (stator), and the magnets were moved to the inside (rotor)

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

BRUSHLESS MOTOR INTERFACES

42

43

44

45