Parameters
04/10 MN04020001Z-EN
114
On the constant three-phase AC supply, the three-phase
asynchronous motor has a constant rotor speed (n
1
, P7.3, rating
plate specifications) according to the number of pole pairs and
mains frequency. The slip here represents the difference between
the rotating field of the stator and that of the rotor. In static
operation, the slip is constant.
Load changes (a) at the motor shaft cause a larger slip (Dn) and
thus a reduced rotor speed (b). In controlled operation
(V/F-characteristic), the frequency inverter cannot compensate this
load-related speed difference. The speed behavior of the motor is
even, as in a constant AC supply.
In “Speed control” mode (P11.8 = 1), the frequency inverter can
compensate these load-related deviations. From the measured
voltage and current values of the stator winding (u
1
, i
1
) the
internal motor model calculates the required manipulated
variables for the flux variable i
µ
and the torque variable i
w
. In the
equivalent circuit diagram of the three-phase motor, the load-
related slip is shown as the resistance R’
2
/s. In idle operation
without a load, this resistance approaches infinity, and
approaches zero as the load increases.
An exact calculation requires the precise rating specifications of
the motor (parameter group 7). The speed control (P11.8 = 1) can
then compensate the load-related slip deviations. The simple
illustration shows that, as the load torque increases (
a), the
resulting speed reduction is compensated by an increase in the
output frequency (
b) (see figure).
Figure 98: Speed behavior without slip compensation
Figure 99: Equivalent circuit diagram for an asynchronous motor
a Stator winding
b Air gap
c Transformed rotor winding
Figure 100: Speed behavior with slip compensation
R
1
acb
X'
2
X
1
i
1
i
w
u
1
X
h
i
m
R'
2
s
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