Chpt 25 Brushless DC Motors Flashcards
what does a simplified brushless DC motor consist of
three pairs of coils wound on the pole pieces of the fixed housing of the stator, and a rotor that incorporates a permanent magnet
what is the main difference from a brushed DC and a brushless one
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
in a BLDC motor what is in place of the commutator
separate drive electronics
what are ‘phases’ in BLDC motors
the number of individually controllable coils
even if there are physically separate coils, but are wired and controlled in pairs
what are the two configurations of wiring
wye and delta
how do you know the position of the rotor relative to the coils
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.
how many stages are there in an electrical cycle and how many times will the windings be excited in this time
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
what is one mechanical cycle
one complete revolution of the rotor
what affect will adding more electromagnetic poles do
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
how is the electrical cycle measured
in 360 degrees
how are the mounting locations of the hall effect sensors and the number of poles on the commutation magnet arranged
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
what does sensor-less communication require to work
that the magnet of the rotor be moving fast enough to induce a detectable voltage in an unenergised coil of the stator
how do you need to start the motor for sensor-less communication case
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
what is another approach to start a motor if the starting direction is important
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
at the same drive voltage which wiring configuration has a higher no load speed and a higher stall torque
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
what is block communication
the coils of the motor were switched on and off as the output of the hall sensors changed
what are the draw backs of block communication
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
what is a way of overcoming the torque ripple
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
what is sinusoidal communication
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
what are the pros and cons of sinusoidal communiaction
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
TRUE/FALSE
BLDC are acoustically quieter
TRUE
without brushes, bearings are the only moving surface in contact and they produce much less noise than the brushes
TRUE/FALSE
Brushed DC motors are quieter electrically
FASLE
without brushes, there is no arcing at the brush/commutator interface
TRUE/FALSE
BLDC are safer in explosive environments
TRUE
without brushes there is no source of sparks
TRUE/FALSE
Brushed DC motors are more relaible
FALSE
the brushes are likely to fail and therefore as BLDC have no brushes
TRUE/FALSE
BLDC are more efficient than brush type motors
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
TRUE/FALSE
BLDC motors have higher max speeds
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
TRUE/FALSE
Brushed DC are easier to cool
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
