Functional Software description
DCS 500 Software Description 61
Selection of
EMF-control
The EMF-control function can be activated by means of parameter
FIELD MODE (1001)
0 No EMF-control (constant field) without field reversal
1 EMF-control without field reversal
2 No EMF-control (constant field) with field reversal
3 EMF-control with field reversal
4 OPTITORQUE without EMF-control
5 OPTITORQUE with EMF-control
Field reversal is normally used on two-quadrant drives. The field
exciter must also be such that the field current can be controlled,
e.g. SDCS-FEX-2, DCF50x-0050 or DCF500/DCF700.
Resistive and in-
ductive voltage
drop
To enable an accurate control, the EMF-voltage must be calculated.
Without load the measured DC-voltage equals about the EMF but
when the motor is loaded, the EMF starts to decrease because of
losses in the motor. The EMF is calculated for the controller using
the formula:
EMF = U
dc
- ( R
A
* i + L
A
* di/dt )
where
R
A
= armature resistance in ohms
L
A
= armature inductance in mH
i = armature current
U
dc
= measured armature voltage
For more information see paragraph Speed Measurements.
Field weakening
area
Above a certain speed the motor FLUX must be reduced in order to
avoid armature overvoltage. This area is called the "field weaken-
ing area" and the speed where the field reduction starts is called the
"field weakening point". Above the field weakening point the motor
FLUX is reduced by ratio 1/n. Two parameters are needed to carry
out the function:
Speed
2nn
NN
Flux
100%
50%
Figure 45 The flux as a function of speed
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