Synchronous machine field-weakening

NAVIGATION: Closed-loop control > SM field-weakening > Synchronous machine field-weakening

Synchronous machine field-weakening

Synchronous motors can also be operated above their rated speed at rated voltage, by reducing their voltage consumption based on on injection of a current component.

Features

The method is relatively robust against parameter fluctuations.
The voltage controller can only follow rapid speed and torque changes to a limited degree.
A non-optimized voltage controller may cause oscillation; the controller must be optimized.

Conditions

To effectively reduce the voltage consumption, the ratio of stator inductance P 0471 MOT_Lsig multiplied by the rated current P 0457 MOT_CNom to rotor flux P 0462 MOT_FluxNom must be sufficiently large. In contrast to field-weakening of asynchronous motors, synchronous motors can also be operated in the "field-weakening range" with full rated torque at the nominal value of the q-current. Power beyond the rated power output can therefore be drawn from the machine in field-weakening mode, even at rated current. This must be taken into consideration when configuring the motor.

Voltage demand

ATTENTION:

If the speed achieved by field-weakening is so high that the induced voltage exceeds the overvoltage threshold of the device (for 400 V devices approximately 800 V, for 230 V devices approximately 400 V), this will result in destruction of the servocontroller if no additional external safety measures are applied.

Field-weakening for the synchronous motor:

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

	Characteristic isd = f(n)
	Calculated map
	Voltage controller

Bild: Field-weakening variants 1 and 2

Vorgehensweise: Variant 1: Characteristic isd = f(n) (Table)

Deactivate table: P 0341 CON_FM_ImagSlim = 0
P 0435 CON_FM_FWMode = (1) Select Table
Approach desired speeds slowly
Adjust scope: Isdref/SQU2*Imag = % = field-weakening speed. The maximum amount of the "field-weakening"
d-current is defined by parameter P 0340 CON_FM_Imag (specification of effective value).
Enter values in table P 0342 CON_FM_SpeedTab

Vorgehensweise: Variant 2: "Calculated map" (calc)

In the case of very rapid speed or load changes in the field-weakening range, the setting
P 0435 CON_FM_FwMode = 2 is selected. A characteristic for a higher control dynamism is calculated internally.

Features

Very fast adaptations, with high dynamism, are possible (open-loop control method).
Motor parameters must be known quite precisely.
If continuous oscillation occurs (voltage limit) the preset negative d-current value is then not sufficient. Scaling parameter
P 0436 CON_FW_SpeedScale > 100 % is used to evaluate the map at higher speeds.

The voltage controller overlaid over the map (setting as described in variant 1).

The set combination of voltage controller and map entails more commissioning commitment, but it enables the best stationary behaviour (highest torque relative to current) and the best dynamic response to be achieved.

Example

The speeds in P 0342 CON_FM_SpeedTab must continuously increase from index 0 -7.

Example:

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

Tabelle: Speeds in P 0342 CON_FM_SpeedTab

Parameters

P. no.	Parameter name	Function
P 0435	CON_FM_FWMode	Selection mode for field-weakening of synchronous motors
(0)	None	Field-weakening is off, regardless of other settings.
(1)	Table	Field-weakening is effected by a characteristic which specifies the d-current dependent on the speed isd = f(n) P 0342 CON_FM_SpeedTab parameter and P 0343 Con_TAB_POSDelta.
(2)	Calc	Field-weakening is effected by way of a characteristic which is set internally via the motor parameters. The d-current reference is then calculated dependent on the speed AND the required q-current: isd = f(n, isq_ref). The inaccuracies with regard to the motor parameters, the available voltage etc. can be compensated by way of the Scale parameter P 0436 CON_FW_SpeedScale.

Tabelle: Selection mode for field-weakening

ATTENTION:

When configuring projects, it must be ensured that the speed NEVER exceeds the value of the product of
P 0458 MOT_SNom x P 0328_CON_SCON_SMax. It should be ensured as a matter of principle that the induced voltage does not exceed the voltage limits. The maximum system speed must not be exceeded.