Etel DSC2P - Page 105

ETEL Doc. - Operation & Software Manual # DSC2P 903 / Ver. F / 3/6/05 Chapter C: System functions
Operation & Software Manual
Direct Drives & Systems 105
Remark: Refer to §12.8.1 for more information about the autosetting with constant current.
Phasing by constant current:
Supposing the user does a phasing by applying a constant current to one of the phase and a 0 current to the
others. The Laplace force F1 generated by the current and the magnetic field will move the motor. When the
powered coil reaches a magnet pole, this force becomes null and the motor stops.
There are two possible configurations: either the coil has reached a stable balance point, or a unstable one. In
both cases the motor stops, but if the balance point is unstable, an external force, even weak, is enough to
eject it from this position to the next stable balance point.
Stable and unstable balance points alternate every 16mm (for most ETEL's motors) if the magnetic period is
equal to 32mm or every 32mm if the magnetic period is equal to 64mm.
In fact, during a phasing, distinct constant current are sent into each phase of the motor instead of a single
current in a single phase. In this case the principle remains exactly the same but the stable and unstable
balance points are shifted with respect to the magnet pole according to the ratio between the injected currents.
If a motor has reached an unstable balance point, it means that was already in this position at the beginning of
the phasing. This case must be avoided because the motor will not move when constant current are sent and
it will be in ’inverted phase shift adjustment’ (force F generated by the motor in opposition to force reference Fc).
To avoid that problem, we move linearly in function of the time, the pointers position in the look-up table. Which
amounts to move the position of the stable and unstable balance points against the magnets. To simplify the
drawing, the look-up table of one phase is represented below:
32mm
I
SN
S
B
B
B
Motor motion
Magnets
Coil
Poles
B Magnets magnetic field
I Current in the motor winding
B
1
B
3
NN
I
B
2
I
B
1
F
l
B
2
B
3
NSN
B
1
B
3
N
S
N
B
2
I
S
F
l
F
l
Magnets
Coil
Stable balance point Intermediate position Unstable balance point