Blower Speed Monitoring - Siemens LMV5 Series Technical Instructions

LMV Series
If the analog signal is decreased to the minimum allowable signal and the blower RPM is still high, a
message will be displayed on the AZL that states "Control range limitation VSD Module".
The reason that the standardization is done at 19.5mA instead of at 20mA is to give the LMV52 some
additional "room" to increase the analog signal for a low RPM condition at high fire. Because the
standardization is done at 19.5mA, the analog input on the VFD is spanned so that 20mA = 62 Hz. This is
done so that the blower will still achieve full 60 Hz blower speed at high fire on jobs where the blower is
just large enough.
NOTE: Most VFDs can be scaled to output 400 Hz or more. Consult the blower and / or motor
manufacturer before over‐speeding the motor and blower, since blower wheels and motor rotors can
catastrophically fail if RPM limits are exceeded.
In addition to limits on how much the 4‐20mA signal can be compensated, the LMV52 also has limits on
how far the blower speed can deviate from the standardized speed line. The next section explains how
the blower speed is monitored when the burner is in operation.
Blower Speed Monitoring
Blower speed has a large impact on the airflow delivered to the burner and thus the fuel‐air ratio. Fuel‐
air ratios must be kept in a safe range while a flame is present in the boiler. If the fuel‐air ratio cannot
be kept in a safe range, the burner must be shut down.
To help ensure that the burner is either operated at a safe fuel‐air ratio or is shut down, the blower
speed is constantly monitored while a flame is present in the boiler. The speed is monitored so that
nuisance shutdowns are eliminated, but fast shutdowns will occur if the speed deviation is large. To do
this, the LMV52 evaluates the magnitude of the speed deviation in combination with how long the
speed deviation exists. To accomplish this, three distinct bands and one limit centered about the
standardized speed line are used. These bands are:
1. Neutral Band ‐ if the speed is within this band, it is considered to be OK and no action is taken. The
width of this band is +/‐ 0.5% of the standardized speed.
2. Low Risk Band ‐ if the speed is within this band, the analog signal to the VFD is adjusted to bring the
speed back into the Neutral Band. If the Neutral Band speed cannot be achieved in 8 seconds, a
lockout will occur. The width of this band is +/‐ 2.0 % of the standardized speed.
3. Outside Low Risk Band ‐ if the speed is outside the Low Risk Band but does not exceed the High Risk
Limit, the analog signal to the VFD is adjusted to bring the speed back into the Low Risk Band and
then ultimately into the Neutral Band. The width of this band varies with the setting of the High Risk
Limit (parameter TolQuickShutdown). If the Low Risk Band speed cannot be achieved in 3 seconds,
a lockout will occur.
4. High Risk Limit (parameter TolQuickShutdown) ‐ if the speed exceeds the High Risk Limit threshold
for more than 1 second, a lockout will occur. This limit is determined by parameter
TolQuickShutdown, which can be set by the burner OEM. The default setting is +/‐ 10% of the
standardized speed.
SCC Inc. Page 13
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Technical Instructions
Document No. LV5‐1000
Section 5