6-4
Application Manual CDA3000
6 Control modes
The synchronous speed (n
s
) of a motor is determined by the number of
pole pairs (P) and the feed frequency (f
1
) of the stator.
Based on the induction from the stator rotating field, asynchronous
motors develop a torque which drives the rotor and which attempts to
reduce the speed relative to the stator rotating field and thus to counteract
the cause of induction. Without the induction of a voltage in the rotor,
however, no current (i
2
) capable of forming a torque will flow. Conse-
quently, a relative difference is established between the stator speed (n
1
)
and the rotor speed (n), which is defined as the slip (s).
The asynchronous operating speed (n
b
) is thus composed of the
synchronous speed (n
s
)andtheslip(s).
Low-loss speed control is only possible by means of a change of
frequency. In order to retain a constant motor torque in the event of a
speed adjustment, the magnetic flux Φ
1
in the stator winding must remain
constant. The voltage U
1
must therefore be adjusted proportional to the
stator frequency f
1
.
and
A frequency/speed adjustment by means of the frequency inverter thus
results in a parallel shift of the characteristic in the basic setting range
along the speed axis (see Figure 6.1 diagram on right).
If the stator frequency is increased further when the rated frequency f
N
and rated voltage U
N
are reached, even though the maximum output volt-
age of the frequency inverter has been reached (U=Const.), the result is a
field weakening.
As the speed rises, this results in a drop in torque with
n
s
f
1
60⋅
p
--------------=
s
∆n
n
1
------
n
1
n–
n
1
--------------==
n
b
f
1
60⋅
p
-------------- 1s–()⋅=
M Φ
1
i
2
⋅∼Φ
1
U
1
f
1
-----∼
M
1
n
2
-----∼
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