Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 27)

frequency to keep the magnetic flux constant and to avoid saturation of the ferromagnetic
circuits. This means that the magnitude of the breakdown torque remains roughly constant.
Motors that are operated for long periods at lower speeds must be externally ventilated due to
the decreasing efficiency of their internal ventilation.
If the frequency rises above the supply frequency then a constant voltage is usually available
from the frequency converter. This results in a weakening of the magnetic field with increasing
frequency and consequently in a reduction of the breakdown torque in proportion to the square
of the frequency. Up to the maximum speed, such drives can typically be operated with constant
power output.
0.00
0.50
1.00
1.50
2.00
2.50
0 50 100 150 200
f/f
n
[%]
T/T
e
Fig. 1.7-8
Typical torque characteristic with variation of the frequency.
Voltage proportional to frequency in the range 0 … f
n
Voltage constant in the range > f
n
When selecting the line-side switching and protective equipment it should be remembered that
frequency converters have large controlled or uncontrolled rectifier circuits on the input-side with
large storage capacitances. This results in high charging current surges and in a high harmonic
content in the current.
LVSAM-WP001A-EN-P - April 2009 1-17