ElmoMC SimplIQ Software Manual - Manual (Page 208)

SimplIQ Software Manual Limits, Protections, Faults and Diagnosis
MAN-SIMSW (Ver. 1.4)
13-13
13.11 Commutation is Lost
The drive uses the feedback (encoder) counts to calculate the electric angle of the rotor.
This calculation is used to set the currents at the stator so that the magnetic field of the
stator points 90º away from the rotor. The angle between the magnetic field in the stator
and rotor is called the “commutation angle”. The motor torque is:
) sin( I Ke
2
3
T
r s
θ − θ ⋅ ⋅ =
where:
T is the torque.
Ke is the motor constant.
I is the motor current.
s
θ is the stator field angle.
r
θ is the rotor angle.
The difference
r s
θ − θ = θ is called the commutation angle. Obviously, θ must be
maintained near 90º for the motor to function properly. If the commutation angle is set
incorrectly, the motor will lose torque. For a given torque command (given either directly
in UM=1 or by external control loops in other modes), the motor current remains the
same. The torque falls because the proportion between the current and the torque is
cos(Δθ), where Δθ is the commutation angle error.
Note that in extreme cases, where abs(Δθ) > 90º, the motor torque becomes reversed with
respect to what is expected by the current defined in the command.
13.11.1 Reasons for and Effects of Incorrect Commutation
Commutation errors may be disastrous to drive operation. The most common incorrect
commutation behaviors are:
ƒ
The commutation error is static (that is, Δθ does not change in time)
Static commutation errors occur due to bad setup data or due to an exceptional load
in automatic alignment (at MO=1, when the only available sensor is an incremental
encoder). A static commutation error leads to motor torque reduction, reduced
efficiency, degraded dynamic response and possible speed or position loop
instability.
ƒ The commutation is static (that is, θ
s
does not change as a function of the motor
position)
Static commutation occurs in encoder systems when an encoder wire is broken. In
this situation, the direction of the stator field is constant and, at the torque command,
the rotor seeks equilibrium aligned to the magnetic field. If the motor is driven by an
external speed or position controller, it will receive a full-torque command and
dissipate the corresponding heat, without generating any motion.