Siemens LMV5 Quick Start Manual page 187

Introduction and Principle of Operation 7-2
The LMV52 VSD controller is typically used to vary the speed of the combustion air blower.
In most cases, the blower RPM is decreased at low fire and increased to maximum or near
maximum when the burner is at high fire. The primary advantages of this capability are less
noise and reduced power consumption especially at low to mid fire.
As previously mentioned the VSD control in the LMV52 is an active closed loop control.
A speed wheel and sensor mounted to the motor / blower shaft provides a pulse feedback to
the LMV52 so that the speed of the blower can be constantly monitored and corrected if
necessary.
Since the speed wheel is asymmetrical, the LMV52 can also determine the direction of
blower rotation.
The VSD control in the LMV52 regards the VSD / motor / speed wheel combination much
the same way as the LMV5 regards an air or fuel actuator.
With an air or fuel actuator, the LMV5 sends the actuator a command over the CANBus to
move to a certain position. The actuator then rotates, and this change is verified and fine
tuned with the feedback potentiometer that is mounted on the actuator's shaft.
Similarly, the LMV52 VSD control sends out a command in the form of an analog signal
(typically 4-20mA) to a VSD. The VSD / blower motor then increases or decreases speed in
accordance with the 4-20mA signal and the change is verified and fine tuned using the
speed wheel feedback that is mounted on the blower shaft.
In ether case if the feedback from the potentiometer or speed wheel is out of range a lockout
will result.
The SQM4 actuators are calibrated (standardized) before they leave the factory. During this
process, the output of the shaft mounted potentiometer is matched to the position of the
actuator shaft. Thus a relationship of ohms per degree is established.
Much like the SQM4 actuator, the VSD / motor combination must be calibrated to the LMV52
before they are put into operation. This procedure is called Standardization and involves
the LMV52 ramping the VSD / motor combination up, automatically recording the peak
RPM, and ramping the VSD back down. This establishes a linear milliamp to pulse
relationship that is correct for the VSD motor combination that is being used with the LMV52
VSD control.
For the standard three fingered speed wheel, three pulses per revolution are produced.
For the special six fingered speed wheel, six pulses per revolution are produced.
The standardization process for a three fingered speed wheel is illustrated in Figure 7-2.1.
The process for the six fingered speed wheel is the same, except that the pulses per minute
would be divided by six instead of three to figure out the RPM.
Typically, the six fingered speed wheel is only used where the blower RPM would be less
than 300 RPM.
Absolute minimum RPM for operation is 170 RPM with the six fingered speed wheel.
LMV5 QSG Rev 4 Sec 7 Pg 3 www.scccombustion.com