ElmoMC SimplIQ Software Manual - Manual (Page 112)

SimplIQ Software Manual Commutation
MAN-SIMSW (Ver. 1.4)
8-2
ƒ CA[25], motor direction is set to 0 or 1 so that the motor will rotate in the desired
direction for positive torque commands.
The values of CA[16] and CA[25] must be coordinated; otherwise, the feedback direction will
be incorrect and the encoder will count negative displacement for positive torques. In
addition, the motor will immediately “run away” as soon as speed or position control is
attempted.
If you intend to use the SimplIQ drive with DC motors, the rest of this chapter is irrelevant.
8.1.2 Stepper Commutation
With stepper commutation, the windings field is set to point at the desired rotor position.
The commutating device need not be informed where the rotor is; it simply assumes that the
rotor will come to rest at the field position.
Stepper commutation is simple and reliable. Its main drawback is that normally, |θ| << 90º;
therefore, large currents are required to generate a given torque. In its steady state, the
motor torque is zero and not affected by motor current. The sensitivity of motor torque to
deviation of the rotor angle is maximal. The great sensitivity of the torque to rotor angle
generates a fast, but oscillatory position feedback.
8.1.3 BLDC Commutation
With stepper commutation, the windings field is set to point 90º away from the rotor
position and the commutating device must know where the rotor is in order to maintain this
field direction.
The advantage of BLDC commutation is its maximum torque per given motor current, and
its smooth, controllable torque. It is ideal for servo applications. BLDC involves a
considerable amount of real-time calculation and requires a rotor position sensor, which
decreases motor reliability. The torque is not sensitive to rotor angle.
8.2 Mechanical and Electrical Motion
Most brushless motors have two or three phases (coils, or windings). The SimplIQ drive is
applicable for three-phased motors only, called A, B and C.
When the rotor travels, the coils of the three-phased motor are powered in the sequence A-
B-C-A-B-C . . . and so on. The powered phases generate a magnetic field, which attracts the
permanent magnet of the moving part (rotor). When the phases are powered sequentially,
the magnetic field moves and the rotor follows continuously. When the rotor passes from a
location over the A coil to the next A coil, it covers an electrical cycle.
Several coil sets — with corresponding fixed-magnet pole pairs on the rotor — may be
located around a motor circumference. For example, a motor with three pole pairs complete
three electrical cycles in one mechanical shaft revolution.