Siemens LMV5 Series Technical Instructions page 169

LMV Series

The LMV52 / VFD / blower response can be watched to check for proper operation. This can be
done in standby Phase 12 (burner off) by using the following menu path:
Params & Display > RatioControl > Gas/Oil Settings > CurveParams or Curve Settings
When the blower is at a speed other than what is displayed, an arrow (>) is displayed on the AZL
next to the VFD position. When the blower is at the displayed speed, a colon (:) is displayed on the
AZL next to the VFD position. By ramping the VFD up and down, the response of the VFD can be
observed. Transitioning back and forth between a colon (:) and a (>) after ramping the VFD up or
down indicates a speed control issue.

The LMV52's closed loop speed control is based on PI (proportional‐integral) logic. The response of
this loop can be tuned using the following menu path:
Params & Display > VSD Module > Configuration > Speed > Setteling Time
In most cases, this does not need to be tuned. See Section 3 (Parameters) for a full explanation.

The absolute speed as read in real time by the speed wheel can be watched at any point in time.
When the LMV52 is not modulating or when it is at a fixed VFD speed (pre‐purge, post‐purge,
ignition, etc.), the speed should be steady, not deviating by more than +/‐ 10 RPM. This can be
viewed at:
Params & Display > VSD Module > Configuration > Speed > Absolute Speed

The combustion air pressure switch should be set by taking the VFD to 10% below the lowest
anticipated low fire speed (if low fire is 50%, take the VFD to 40%) and setting the switch to open at
that point. This should maximize the safety potential of the combustion air pressure switch and
minimize nuisance trips. This can be done in standby by setting the home position of the VFD to
40% and setting the switch.

In most applications with an air damper, there is little reason to decrease blower speed below about
50% VFD (30 Hz). Power consumption decreases by the cube of the RPM even without the
additional restriction of an air damper. Referencing information from the "Centrifugal Blower
Fundamentals" section on the previous pages, decreasing the speed of a 25 HP (18.62 kW) motor
from 3600 RPM (60 Hz) to 1800 RPM (30 Hz) will cause the power consumption to be reduced from
18.62 kW down to 2.32 kW, an electrical savings of over 800%.

A VFD alone without an air damper or sliding head offers limited accuracy and repeatability for the
airflow at higher burner turndowns. For most boiler burners, modulating the VFD alone without an
air damper is okay for turndowns of 4‐to‐1 or less. Using only a VFD for airflow regulation at higher
turndowns may lead to airflow repeatability issues.
SCC Inc. Page 17
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Technical Instructions
Document No. LV5‐1000
Section 5