HARSFEN0602
In the stepper commutation policy, the windings field is set to point at the desired rotor
position. The commutating device doesn't have to know where the rotor is – it just assumes
that the rotor will come to rest at the field position.
The stepper commutation has the advantages of simplicity and reliability. The main
drawback is that normally 90<<θ , thus to generate a given torque large currents are
required.
At the steady state, the motor torque is zero, and indifferent to the motor current. The
sensitivity of the motor torque to deviation of the rotor angle is maximal.
The large sensitivity of the torque to the rotor angle generates a fast, but oscillatory position
feedback.
9.1.3 The BLDC commutation policy
In the stepper commutation policy, the windings field is set to point 90 away from the rotor
position. The commutating device has to know where the rotor is in order to keep the field
direction 90 away.
The BLDC commutation has the advantage of maximum torque per given motor current,
and of smooth, controllable torque. The BLDC policy involves much real time calculation
and it requires a rotor position sensor. The commutation dependence in the sensor decreases
motor reliability.
The torque is not sensitive to the rotor angle.
The BLDC commutation is ideal for servo applications.
9.2 Mechanical and electrical motion Figures are missing
Most of the brushless motors has two or three phases (windings). The figure below shows a
two phased linear motor and a three phased linear motor.
Figure 3: Two phased linear motor
Figure 4: Three phased linear motor
The Harmonica suits only three phased motors.
When the rotor travels, the coils of the 3-phased motor are powered in the sequence A-B-C-
A-B-C... and so on. Although the moving part (rotor) travels continuously, it sees the
windings in a repetitive, cyclical pattern. When the rotor passes from a location over the A
coil to the next A coil, it covers an electrical cycle.
"Rolling" the linear motor of Figure 4 so that its right hand C phase rolls towards the left
hand A phase makes a rotary three-phase motor.
Figure 5: Three phase rotary motor
The motor of Figure 5 has two coil sets (and also two magnetic pole pairs in the rotor). The
rotor follows two electrical cycles in each mechanical revolution.
For rotary motors, normally the number of coil sets equals the number of magnet pole pairs,
so the term "number of pole pairs" is commonly used as substitute to "number of electrical
cycles in a shaft revolution".
You can read the electrical and the mechanical angle of the motor using the following
commands:
Command Description