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Closed-loop operation above approx. 1 Hz (p1755) and the ability to start or reverse at 0 Hz
directly in closed-loop operation (p1750) produce the following benefits:
● No switching action within the closed-loop control necessary. This means smooth behavior,
● Stationary speed–torque control up to approx. 1 Hz.
8.5.1.3
U/f control
As an alternative to vector control, a U/f control is available for induction motors. This technique
is only appropriate when there are no highly dynamic requirements, e.g. pumps and fan drives.
V/f control aims to maintain a constant flux Φ in the motor whereby the flux (Φ) is proportional
to the magnetization current (Iµ) and the ratio of voltage (V) to frequency (f).
Φ ~ Iµ ~ V/f
The torque (M) generated by the induction motors is, in turn, proportional to the vector product
(Φ x I) of the flux and current. The V/f characteristic is derived from these basic premises:
To generate as much torque as possible with a given current, the motor must function using the
greatest possible constant flux. To maintain a constant flux (Φ), therefore, the voltage must be
changed in proportion to the frequency f. The voltage change ensures that a constant
magnetization current Iµ flows.
Figure 8-4
Adjustable parameters
Parameter
p1300
SINAMICS SM150 6SL3815-7NP41-0AA1
Operating Instructions Rev.201910281301 EXAMPLE
no frequency drops.
Note
For controlled reversing or controlled starting for 0 Hz, under the following condition the
closed-loop control is switched automatically from closed-loop controlled operation to open-
loop controlled operation:
● Dwell-time > 2 seconds or > p1758 in the range of 0 Hz
Operating areas and characteristic curves for the induction motor with drive supply
Setting
Open-loop/closed-loop operating mode / control type
Operation
8.5 Functions
127
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