ASM field-weakening

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ASM 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 rated speed the magnetic field is reduced because the maximum output voltage of the inverter has been reached and the motor is run in the so-called field-weakening range with reduced torque.
For field-weakening of asynchronous motors, the motor parameters must be known very precisely. This applies in particular to the dependency of the main inductance on the magnetizing current. It is essential to carry out a motor identification and an optimization in the basic setting range for field-weakening mode. 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 0340 CON_FM_Imag. Two variants are available for operation in field-weakening mode.

There are two variants for field-weakening of an asynchronous motor. The choice of variant 1 or 2 is made via parameter P 0435 CON_FM_FWMode.

Bild: Field weakening

Variant 1: (Table)

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 drive controller is at the voltage limit, it reduces the d-current and thus the rotor flux. Since the controller has only limited dynamism, and starts to oscillate if larger gain factors are set, it is possible to use variant 2.

Variant 2: (Calc)

Combination of "pre-control with modified 1/n characteristic (isd=f(n)) + voltage controller.

This characteristic describes the magnetizing current as a percentage of the nominal value of
P 0340 CON_FM_Imag dependent on the speed.

The choice between the modified 1/n characteristic and the static characteristic is based on parameter P 0341 CON_FM_ImagSLim.

P 0341 ≠ 0 signifies selection of the 1/n characteristic (default )

P 0341 = 0 signifies selection of the modified 1/n characteristic isd = f(n).

After a motor identification the voltage controller is always active, as the controller parameters are preset. With P 0345 CON_FNVConKp = 0 the voltage controller is deactivated.

Parameterizing variant 2:

Setting the d-current dependent on the speed. The speed is specified relative to the rated speed in P 0458 MOT_SNom, the d-current relative to the magnetizing current in parameter P 0340. Up to the field-weakening speed, a constant magnetizing current is injected
P 0340.

Voltage controller:

Vorgehensweise: Selection of modified characteristic

P 0341 = 0 (selection of modified characteristic) + voltage controller
Approach desired speeds slowly
Adjust scope: Isdref
SQRT2*Imag = %-speed value
The maximum amount of the "field-forming" d-current is defined by parameter
P 0340 CON_FM_Imag (specification of effective value).
Enter values in table; P 0342 CON_FM_SpeedTab

Example:

Index	P 0348 rated speed P 0340 I_mageff	P 0342 (0-7)Field-weakening speed in [%]	P 0343 (0-7)Magnetizing current in field-weakening mode in [%]
(0)	I_rated= 1800 rpm I_{mag eff} = 100%	100	100
(1)		110	100
(2)		120	100
(3)		130	100
(4)		140	90
(5)		150	70
(6)		160	55
(7)		170	0

Tabelle: Example of modified characteristic

Parameters

P. no.	Parameter name	Function
P 0340	CON_FM_Imag	Effective value of the rated current for magnetization
P 0341	CON_FM_ImagSLim	Field-weakening activation point (as % of P 0348 MOT_SNom). This effects the switch to the 1/n characteristic (P 0341 ≠ 0). For P 0341 = 0 the field-weakening works via the modified characteristic isd = f(n). For a synchronous machine this value must be set to 0.
P 0342	CON_FM_SpeedTab	Speed values scaled as % of P 0458 n_rated to populate the modified table.
P 0343	CON_FM_ImagTab	d-current scaled as % of P 0340 Imag eff. to populate the modified table.

Tabelle: Parameters for field-weakening