Dual Frequency Braking; Introduction To Dual Frequency Braking; Operation - Siemens 6SR41 series Product User Manual

6.16

Dual Frequency Braking

6.16.1

Introduction to Dual Frequency Braking

There are many applications for VFDs that need occasional negative torque for braking.
Unfortunately, at present the most popular static converters used for VFDs are not capable
of returning energy to the utility. Such applications therefore require additional circuits to
regenerate the braking energy into the AC mains, or to dissipate the braking energy in a
resistor. Both of these solutions add cost to the VFD, and are especially undesirable for large
modular medium-voltage VFDs.
Additional power devices can be avoided by using the existing circuits to inject DC current
into the motor windings. This method dissipates the braking energy in the motor, and adds
little cost to the VFD. However, DC injection braking is not very effective unless the available
current is several times rated, especially for large motors. Another drawback is that
estimation of motor speed is very difficult during DC injection braking.
Dual Frequency Braking (DFB) is another method in which braking energy can be dissipated
in the motor. Dual Frequency Braking provides much higher torque per ampere than DC
injection braking, and permits continuous estimation of motor speed. Like DC injection
braking, this approach is implemented in software and requires no additional hardware that
can reduce the reliability of the drive.
Siemens has a patent on Dual Frequency Braking (US 6,417,644).
6.16.2

Operation

Dual Frequency Braking causes extra losses to be induced in the motor by applying a
second set of three-phase voltage vectors to the motor, in addition to the normal set of
voltage vectors used for speed control. These extra losses are used to absorb the kinetic
energy released during braking.
There are two side effects of Dual Frequency Braking (DFB) against which protection is
applied as follows:
1. Torque pulsations: The motor can be subjected to as much as 1 per-unit torque pulsation
2. Motor heating: The losses generated during DFB cause motor heating and limit the
The second set of voltage vectors creates a counter-rotating flux vector that produces high
slip in the machine and generates these additional losses in the motor. The injection
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
at the pulsation frequency with DFB. However, the customer can select the torque
pulsation frequency via the menu entry for Pulsation Frequency to avoid any mechanical
resonance frequencies.
number of deceleration ramps (from full speed to zero) that can be performed repetitively.
Motor heating due to the additional losses is designed to be no worse than a line start.
The software motor thermal model in NXG monitors motor heating due to these losses,
and can provide an alarm and/or a trip to indicate excessive heating. The number of
repetitive deceleration ramps (from full speed to zero) is limited to 2-per-hour (based on
MG-1, Part 20, which assumes that the motor has cooled down to its rated temperature
before the second ramp down). This recommendation applies when the load inertia and
load torque are those for which the motor is designed. With lower values of load inertia
and/or smaller speed reductions, DFB can be used more frequently.
Application and Operation
6.16 Dual Frequency Braking
151