Additional Tips For Burners With Pwm Blowers - Siemens LME7 Series Technical Instructions

LME Series

Additional Tips for Burners with PWM Blowers

Most of the time, speed faults that are seen on the LME7 are caused by the PWM blower not being
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able to decelerate quickly enough when the blower is being ramped down. If fast ramp times are
not critical for the application, ramp times (parameters 522 and 523) can be increased and this
should correct the issue.
The LME7 in combination with the PWM blower can be tested for proper operation while the LME7
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is in standby (OFF). After the PWM blower parameters are set, the standby speed of the PWM
blower can be adjusted with parameter 503.00. The actual speed as a % of the maximum blower
speed (parameter 519) can be read back on parameter 936. If different blower speeds are set and
plotted vs. the actual speed (936), the linearity of the PWM blower's speed response can be
assessed.
The ACS410 PC software has a trending package that is very useful when diagnosing PWM blower
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speed issues. In particular, the target speed (parameter 920) and the actual speed (parameter 936)
can be plotted against one another in real time, and can be closely reviewed to see where the
largest deviations occur.
The combustion air pressure switch should be set by taking the PWM blower to 10% below the
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lowest anticipated low fire speed (if low fire is 50%, take the PWM blower 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 air pressure trips. This can be done in standby by setting the
standby speed of the blower to 10% lower than low fire and adjusting the switch to trip at this point.
Most PWM blowers have their own internal speed control. However, when using a PWM blower
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that does not have its own internal speed control, parameter 559 must be set to 1 so that the LME7
can control the speed of the blower. When parameter 559 is set to 1, the LME7 controls the blower
speed using the PID values set by parameters 676, 677, and 678. The default settings of these
parameters are typically adequate.
SCC Inc. Page 9
Technical Instructions
Document No. LME-1000
Section 6