Siemens LMV5 Series Technical Instructions page 158

Technical Instructions
Document No. LV5‐1000
The DC bus can absorb a small amount of energy in the DC bus capacitors. However, if the motor
generates more than what these capacitors can absorb, the DC bus voltage will rise to critical levels and
one of two actions will be taken by the VFD. Depending on the parameter setting of the VFD, the VFD
will either stop decelerating (stall prevention) or the VFD will alarm and shut down. Either one of the
actions is not a desirable result on a combustion air application.
To avoid DC bus overvoltage issues, a braking resistor can be added to the VFD so that the excess
electrical energy generated by decelerating the blower wheel can be turned to heat. This process
happens seamlessly so that the VFD can decelerate the blower smoothly.
Due to a number of variables, it is difficult to determine if a braking resistor will be needed on a
particular application unless that application has been tested. The only disadvantage of having a braking
resistor and not needing it is cost and possibly the space for the resistor. Burners having the following
characteristics will typically need a braking resistor:
1. A heavy blower wheel ‐ Kinetic energy is stored in a spinning wheel. The heavier the blower wheel,
the greater the stored energy. When this wheel is slowed down, the kinetic energy must go
somewhere, and it is usually "pushed" back to the VFD as electrical energy.
2. Fast ramp times ‐ The faster the ramp times, the faster the blower wheel must be accelerated and
decelerated. Just like a car, more energy is required to accelerate quickly (bigger engine) and more
energy is required to be dissipated when decelerating quickly (bigger brakes). Decelerating a given
blower wheel more quickly will push more electrical energy back to the VFD.
3. Mostly closed air damper ‐ A motor spinning at 3600 RPM draws fewer amps with a closed or nearly
closed air damper as compared to a wide open air damper. Thus, the horsepower used by the
motor and the drag (braking) on the blower wheel will be much less with a closed or nearly closed
air damper. Decelerating a given blower wheel with reduced drag will also push more electrical
energy back to the VFD.
As one might expect, the above points compound one another. Decelerating a heavy blower wheel
with a fast ramp time and a mostly closed air damper will push a large amount of electrical energy back
at the VFD and will likely cause DC bus overvoltage issues if a braking resistor is not installed.
In contrast, a light blower wheel (sheet metal instead of cast iron), a slower ramp time (90 seconds
instead of 30 seconds), and slowing the blower down on a more open air damper are characteristics that
will greatly reduce the amount of electrical energy pushed back to the VFD and should allow the braking
resistor to be omitted in most cases.
On some models of VFDs, braking resistors can be added after the VFD is installed if necessary. This is a
point to consider when installing VFDs for combustion air applications.
Section 5 Page 6 SCC Inc.
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