MAXON MOTORSEC AND BLDC

CONCEPTSSHREYANSH VATS (The Technocrats)

Sub-system : Drive by wire

CONVENTIONAL DC MOTOR

So…. Why do we need Brushless motors? The very first and foremost point is that the brush assembly, being

made from carbon gets rubbed and wears off. So, they need to be updated often.

Next, due to the friction between the rotating ROTOR and the stationary BRUSHES, friction losses occur and some amount of input power gets wasted in this.

Often possibility of sparking occurs between the brush contacts with the rotating coil which damages the critical parts of the motor.

WHAT’s BLDC??? We all know that motors have brushes to commutate the current from supply

to the coil… So how could this commutation be possible without the brushes? In BLDC motors, the entire ideology of using a brush is blown off. Here, the

stator is the one that is the current carrying coil (stator is electromagnet) and the rotor is made up of permanent magnet (High grade magnets like NiFeB or an alloy of Nd).

Brushless DC electric motor (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) aresynchronous motors that are powered by a DC electric source via an integrated inverter/switching power supply, which produces an AC electric signal to drive the motor (AC, alternating current, does not imply a sinusoidal waveform but rather a bi-directional current with no restriction on waveform); additional sensors and electronics control the inverter output amplitude and waveform (and therefore percent of DC bus usage/efficiency) and frequency (i.e. rotor speed).(Courtesy : Wikipedia)

Using BLDC….

First of all, there is no brush assembly present in the motor, so no loss due to friction at the brush contacts. Also no brush means no maintenance with brush point of view, thus increasing the efficiency of the motor while decreasing the mechanical wear.

 An electronic controller replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using a solid-state circuit rather than the brush/commutator system.

Brushless vs Brushed…  more torque per weight more torque per watt (increased efficiency)  increased reliability  reduced noise longer lifetime (no brush and commutator erosion) elimination of ionizing sparks from the commutator overall reduction of electromagnetic interference (EMI) With no windings on the rotor, they are not subjected to centrifugal forces, and

because the windings are supported by the housing, they can be cooled by conduction, requiring no airflow inside the motor for cooling. This in turn means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter.

Brushless motors are more efficient at converting electricity into mechanical power than brushed motors. This improvement is largely due to the brushless motor's velocity being determined by the frequency at which the electricity is switched, not the voltage. Additional gains are due to the absence of brushes, alleviating loss due to friction.

Source : Wikipedia

BLDC – The dark side….

The maximum power that can be applied to a brushless motor is limited almost exclusively by heat; too much of which weakens the magnets, and may damage the winding's insulation.

A brushless motor's main disadvantage is higher cost, which arises from two issues.

Firstly, brushless motors require complex electronic speed controllers (ESCs) to run. In contrast, brushed DC motors can be regulated by a comparatively simple controller, such as a rheostat (variable resistor). However, this reduces efficiency because power is wasted in the rheostat.

Secondly, some practical uses have not been well developed in the commercial sector. For example, in the radio control (RC) hobby arena, brushless motors are often hand-wound while brushed motors are usually machine-wound.

Controller Implementations…

Generally Hall effect sensors(or rotary sensors) are used. http://en.wikipedia.org/wiki/Hall_effect_sensor Advanced controllers employ a microcontroller to manage

acceleration, control speed and fine-tune efficiency.

Source : Wikipedia

Electronic Commutation…

There are different systems. maxon uses the following three: •Block commutation with or without Hall sensors •Sinusoidal commutation. As you can see the different maxon

controller families perform different commutation types. Common to all these systems is that they should apply the current in a way, that the generated torque is as high as possible. As we have learned this is achieved by a perpendicular orientation of the magnetic fields of permanent magnet and winding.

The SENSORED Block Communication…

The actual position of the control magnetin the diagram generates the following signals:

• The blue Hall sensor sees thenorth pole. Thus the signal output level ishigh and will remain high for the next 120°. • The green Hall sensor is close tothe south pole. The output level is low forthe next 60°.Then the north pole approachesand the output signal will switch to a high state. • The red Hall sensor has just switchedfrom high to low where the signal level willstay for the next half a turn. The combination

of the three Hall sensor signals is unique for each 60° of rotation. Looking at these signalsallows to know the rotor position within 60°.That exactly what we need for commutation.Remember there were 6 different ways of current flow through the motor at acommutation angle of 60°.

First we have to look at the Hall sensor feedback signals. In the back of the motor there are three Hall sensor mounted on the PCB at an angle of 120°. The Hall sensor detect the magnetic poles of the control magnet which is mounted on the shaft. The control magnet exhibits the same two magnetic poles in the same orientation as the power magnet. (Basically the Hall sensors could monitor the power magnet directly but the control magnet offers two advantages: The magnetic transitions between north and south pole are more precisely defined. And an angular misalignment and tolerances between the relative position of winding and Hall sensors can be adjusted.) The digital Hall sensors used probes. They generates a high output signal (5V) if the north pole of the control magnet is close to them. A south pole produces a low level (Gnd).

The UNSENSORED (Sensorless) Block Communication…

There is another way of getting the necessary position information from the motor. Let's consider a motor with one pole pair with a winding in star configuration.

There is always one phase of the winding which is not powered. But this phase will see the rotating permanent magnet which induces a sinusoidal voltage in this phase – the back EMF. One can show that exactly in the middle of the 60° of block commutation (when the phase is not powered) the induced voltage crosses zero. This voltage crossing can be detected if the star point of the winding is accessible as well.

Then one has to wait until 30° of rotation have passed and do the next switching of block commutation. (The tricky thing is to have speed information as well in order to know when the 30° have passed. But this can be done more or less precisely, e.g. from the time distance of the previous zero-crossings).

During the next commutation interval one looks at the next phase that is not powered and so on. There is one problem. When speed is low the amplitude of the EMF becomes smaller and smaller.

The slope of the EMF voltage becomes flatter and flatter and it is difficult to determine exactly the zero crossing. Even worse, at zero speed (e.g. during start-up) there is no back EMF at all!

This means that sensorless commutation does not work well at low speeds (typically below approx. 1000 rpm for motors with 1 pole pair) and it needs a special starting procedure which is done similar to a stepper motor. I.e. the windings are powered according to the block commutation sequence without taking note of the EMF. The commutation frequency is enhanced and if anything goes well the rotor will speed up. Once a certain minimum speed is reached the back EMF is taken into consideration and the real sensorless block commutation is established.

In order to get a reliable starting up the parameters of the start-up procedure must be selected carefully depending on motor characteristics and load (friction, mass inertia, …).

T H A N K S