Chpt 25 Brushless DC Motors

Question 1

what does a simplified brushless DC motor consist of

Answer 1

three pairs of coils wound on the pole pieces of the fixed housing of the stator, and a rotor that incorporates a permanent magnet

Question 2

what is the main difference from a brushed DC and a brushless one

Answer 2

the coils are placed on the stator in a BLDC motor which means that it is no longer a need to supply current across a moving boundary which was the function of the brushes and commutator

Question 3

in a BLDC motor what is in place of the commutator

Answer 3

separate drive electronics

Question 4

what are ‘phases’ in BLDC motors

Answer 4

the number of individually controllable coils

even if there are physically separate coils, but are wired and controlled in pairs

Question 5

what are the two configurations of wiring

Answer 5

wye and delta

Question 6

how do you know the position of the rotor relative to the coils

Answer 6

incorporate a coarse position sensor into the motor

the motors position sensor is formed by the commutation magnets and the hall effect sensors mounted at one end of the motor.

Question 7

how many stages are there in an electrical cycle and how many times will the windings be excited in this time

Answer 7

there are six stages in an electric cycle

during a complete cycle each of the windings will be excited four times

twice in one current direction and twice in the other

Question 8

what is one mechanical cycle

Answer 8

one complete revolution of the rotor

Question 9

what affect will adding more electromagnetic poles do

Answer 9

this will result in smoother rotation with less torque ripple and requires that the drive electronics cycle through the six stages of the electrical cycle multiple times

Question 10

how is the electrical cycle measured

Answer 10

in 360 degrees

Question 11

how are the mounting locations of the hall effect sensors and the number of poles on the commutation magnet arranged

Answer 11

that each of the three sensors is active for 180 electrical degrees and the three sensors are offset from one another by 60 electrical degrees

Question 12

what does sensor-less communication require to work

Answer 12

that the magnet of the rotor be moving fast enough to induce a detectable voltage in an unenergised coil of the stator

Question 13

how do you need to start the motor for sensor-less communication case

Answer 13

the simplest way is to start the motor slowly. if communication begins slowly enough that you are sure that the rotor will have to align itself with the moving magnetic field, you can accelerate the commutation rate and “pull” the rotor up to a speed that will produce enough back EMF. at that point you can transition to commutation based on signals derived from the back EMF

this approach has a potential anomaly at startup which may lead to the motor initially going in the wrong direction

Question 14

what is another approach to start a motor if the starting direction is important

Answer 14

inject a small amount of current into each of the coils which is used to measure the inductance of the coils, from this measurement it is possible to identify the first coil to energise so that the rotation can be guaranteed to be in the desired direction

Question 15

at the same drive voltage which wiring configuration has a higher no load speed and a higher stall torque

Answer 15

delta configuration
because its torque constant is approximately 1/SQRT(3) to that of the wye configuration, it will require correspondingly higher current to produce the same torque output

Question 16

what is block communication

Answer 16

the coils of the motor were switched on and off as the output of the hall sensors changed

Question 17

what are the draw backs of block communication

Answer 17

the abrupt nature of the change in drive and the relatively coarse position information available result in variation in available torque as the motor moves through a single rotation
this is known as torque ripple

Question 18

what is a way of overcoming the torque ripple

Answer 18

if the motor is equipped with a higher resolution position sensor, it is possible to use an alternative drive technique to virtually eliminate the torque ripple

to do this drive voltages to the coils are modulated rather than abruptly being switched on and off

Question 19

what is sinusoidal communication

Answer 19

as the rotor rotates the drive voltages are smoothly varied between the coils, producing a constant torque output independent of position. because the required variation in drive current is sinusoidal the technique is referred to as sinusoidal communication

Question 20

what are the pros and cons of sinusoidal communiaction

Answer 20

because the current in each coil is a function of the combination of the applied voltage and back EMF, achieving good sinusoidal communication is a nontrivial task. it requires feedback from the coils and sophisticated digital signal processing to perform the calculations and generate the required waveforms

the result of this is excellent torque control all the way to zero speed

Question 21

TRUE/FALSE

BLDC are acoustically quieter

Answer 21

TRUE

without brushes, bearings are the only moving surface in contact and they produce much less noise than the brushes

Question 22

TRUE/FALSE

Brushed DC motors are quieter electrically

Answer 22

FASLE

without brushes, there is no arcing at the brush/commutator interface

Question 23

TRUE/FALSE

BLDC are safer in explosive environments

Answer 23

TRUE

without brushes there is no source of sparks

Question 24

TRUE/FALSE

Brushed DC motors are more relaible

Answer 24

FALSE

the brushes are likely to fail and therefore as BLDC have no brushes

Question 25

TRUE/FALSE

BLDC are more efficient than brush type motors

Answer 25

TRUE
the brush commutator interface adds a relatively high electrical resistance element in the current path as well as mechanical friction, both of which reduce the DC motors efficiency

Question 26

TRUE/FALSE

BLDC motors have higher max speeds

Answer 26

TRUE
the max speed of a brushed DC motor is limited by the dynamics of the brush/commutator interface
at a very high speeds the brushes tend to bounce off the commutator segments

brushed are limited to ~ 10 000 rpm
BLDC are limited to ~ 100 000 rpm

Question 27

TRUE/FALSE

Brushed DC are easier to cool

Answer 27

FALSE
in a brushed DC motor the heat is generated in the coils on the rotor, with very limited means of exhausting that heat through the bearings and brushes or motor housing
since the coils of the brushless motor are stationary and mounted on the housing of the motor it is easier to expell the heat