Elmo HARSFEN0602 191, Commutation is lost General, Reasons and effect of incorrect commutation

Commutation errors may be disastrous to drive operation. The most common incorrect commutation behaviors are:

HARSFEN0602ElmoHARmonicaSoftwareManual PRELIMINARYDRAFT

taken since almost every motion system applies high torques for short acceleration periods while the speed is slow.

CL[2] defines the tested torque level as a percentage of the continuous current limit CL[1]. CL[3] states the absolute threshold main sensor speed under which the motor is considered not moving.

Do not set CL[3] to very small number. When a motor is stuck, a vibration may develop that will induce speed-reading. When encoder wire is damaged, the motor will run-away with the encoder readout vibrating +/- bit. This also creates speed-reading.

Example

If CL[2]=50 and CL[3]=500, the Amplifier will abort (reset to MO=0) if the a torque level is kept at least 50% of the continuous current, while the shaft speed do not exceed 500 counts/sec for continuous 3 seconds.

191

14.11Commutation is lost

14.11.1General

The Amplifier uses the feedback (encoder\resolver\tacho) counts to calculate the electric angle of the rotor. This is used to set the currents at the stator so that the magnetic field of the stator will point 90 degrees away from the rotor (90+/-30 degrees at the Clarinet).

The angle between the magnetic field in the stator and the rotor is called the “commutation angle” The motor torque is

T = 32 Ke ⋅ I ⋅ sin(θs − θr )

Where T is the torque, Ke is the motor constant, I is the motor current, θs			is the stator field angle, and θr is
the rotor angle.
The difference θ = θs − θr	is called the commutation angle. Obviously,		must be kept near 90 degrees for
the motor to function properly.		θ		∆θ
If the commutation angle is incorrectly set, the motor loses 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 (heating…)
The torque falls, since the proportion between the current and the torque is cos(∆θ) where				is the
commutation angle error.

Note than in extreme cases, where abs(∆θ) > 90o, the motor torque becomes reversed with respect to what is expected by the commanded current.

14.11.2Reasons and effect of incorrect commutation

The commutation error is static (i.e. Does not change in time):

Static commutation error occur because∆ofθbad setup data, or by exceptional load in automatic alignment (at MO=1, when the only sensor available 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.

The commutation is static (i.e. θs does not change as a function of the motor position): Static commutation occurs in encoder systems when an encoder wire is broken.