Application and Operation
6.16 Dual Frequency Braking
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
153
Using this method, the first vector set controls the torque and flux in the motor, and is nearly
synchronous. The second vector set induces losses in the motor to absorb the braking
power returned by the first vector set. The amplitudes of the two vector sets are coordinated
to best utilize the current and voltage limitations of the converter. The frequency of the loss-
inducing vector set is chosen with the goal of maximizing losses per ampere. This
automatically minimizes the torque pulsations by minimizing the loss-inducing current.
The dominant losses in a motor are conduction losses, proportional to I
2
R. Maximum losses
per ampere require a large value of R. The nominal resistance of the motor windings is fixed
by the design. Fortunately, the effective resistance depends on the frequency. The rotor
windings are deliberately designed to exhibit a strong "deep-bar" effect, so that their
resistance (above a low threshold) increases roughly proportional to frequency.
In principle, the frequency of the loss-inducing vector set should be as high as possible for
maximum effective resistance. Since this high loss-inducing frequency produces negative
slip, it will have negative sequence. The maximum applied frequency is limited by the control
bandwidth of the converter, and also by the available voltage. However, because the loss-
inducing vector set is negative sequence, the rotor frequency will be higher than the stator
frequency due to the rotational speed.
6.16.3 Limitations
The drive output current plus the braking current must not exceed the current capability of
the cells in the drive. Hence the braking torque is limited in the drive and is greatest at slow
speed and smallest at high speed. Figure "Typical Braking Torque with Dual Frequency
Braking" shows the typical braking torque that can be expected with Dual Frequency
Braking.
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Figure 6-26 Typical Braking Torque with Dual Frequency Braking
With high efficiency motors and inverter duty motors, the braking torque that can be
achieved with DFB is lower than the values shown in Figure 5-26. Contact Siemens
Engineering with the following motor-related data to determine the braking torque capability
with a higher efficiency motor:
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