7.6 Asynchronous motor field weakening
Up to rated speed the asynchronous motor runs with a full magnetic field and so is
able to develop a high torque. Above the rated speed, the magnetic field is reduced
because the maximum output voltage of the Servo Drive has been reached. The
motor is operated in the field-weakening range at reduced torque.
To use field weakening for asynchronous motors, the motor parameters must be
known very accurately. This applies in particular to the dependency of the main
inductance on the magnetizing current. The field-weakening mode requires that a
motor identification and an optimization in the basic setting range be carried out. In
the process, default values for the control circuits and the "magnetic operating point"
are set based on the rated motor data and the magnetizing current presetting in
P 340[0] - CON_FM_Imag. For field weakening when using an asynchronous motor,
a characteristic curve is always internally calculated (independently of P 435[0] -
CON_FM_FWMode) and a voltage controller is superimposed.
Configuring the characteristic curve with P 341[0] - ImagSLim:
l
Variant 1:
P 341[0] ≠ 0
signifies selection of the 1/n characteristic (default)
l
Variant 2:
P 341[0] = 0
signifies selection of the modified 1/n characteristic
isd = f(n).
MOOG
ID No.: CB40859-001 Date: 11/2020
MSD Servo Drive - Device Help
143
7 Control
n
act
P 0415
CON_SCALC_ActSpeed
IhAct
PD-T -Glied
1
K*TR
P 0415
CON_SCALC_ActSpeed
CON_FM_ImagTab
P 0347
CON_FM_VRef
Available U
zk
vmot
soll
vmot_filt
P0345
P0346
vreg_max-isdref_tab
vreg_min-isdref_tab
vreg_isdref
isdref
isdref_tab
Lh-Kennlinie
1
Ti
d-current control
1
P 0344
CON_FM_VConTF
Tf
Vmot
udref
uqref
Voltage
Calculation
Kp
Tn
CON_FM_Imag
CON_FM_SpeedTab
MOT_FM_SNom
x
y
x
2
x
y
x
y
P 0343
P 0340
x
y
P 0342
P 0348
1
CON_FM_ImagSLim
P 0341
= 100 default
y
2
CON_FM_ImagSLim
P 0341
= 0 default
CON_FM_ImagSLim
P 0341
≠ 0
1
2
3
Image 7.33: Structure of field weakening for asynchronous motors
7.6.1 Variant 1: Default
Combination of "pre-control via 1/n characteristic" + voltage controller. The motor
identification sets the voltage controller so that the voltage supply in a weakened
field is adequate. If the Servo Drive is at the voltage limit, it reduces the d-current and
thus the rotor flux. The 1/n characteristic curve is calculated with operating speed
SNom * ImagSLim.
As the Servo Drive has only limited dynamism, and starts to oscillate if larger gain
factors are set, it is possible to use variant 2.