Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 107)

Fig. 3.10-3
U/f characteristic curve
At small frequencies (< approx. 5 Hz) the voltage drop across the internal ohmic resistances of
the motor (independent of frequency) relative to that across the motor inductances (proportional
to the frequency) is growing. This results in insufficient magnetization and as a consequence in
a fall in torque. In order to counter this effect, a voltage boost is provided at low speeds
(
Fig. 3.10-4 ).
Voltage
Frequency
Fig. 3.10-4
Voltage boost at low frequencies to prevent a drop in torque
3.10.3 Change of sense of rotation and braking
As the rotating field in a frequency converter is generated electronically, changing the direction
of rotation can be performed by means of a control command.
If the frequency is reduced when the motor is running, then the rotor turns faster than the
rotating field. The motor runs in the so-called over-synchronous mode and acts as a generator.
Energy is thus fed back from the motor to the frequency converter, where it is in turn stored in
the intermediate circuit. This results in a voltage rise and as a consequence possibly in a
protective shutdown, if the electrical energy is not removed in an appropriate manner, e.g. by
means of:
- Electronically controlled dissipation of the energy via a resistance
- Feedback of the energy into the power supply by means of an inverter
- Connection of the intermediate circuits of several frequency converters and exploiting the
braking energy for the operation of the total of connected motors.
3.10.4 Motor protection
Frequency converters usually have integrated electronic motor protection. No additional
protection is normally required.
For special applications, for example for supplying several motors via one inverter, additional
motor protection for each motor is required. If overload protective devices with bimetal tripping
mechanisms are deployed for the protection of the individual motors, it must be remembered
LVSAM-WP001A-EN-P - April 2009 3-31