AUTOFLAME Mk8 MM, Technical Manual

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2 Flame Safeguard
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MM Application Possibilities
2.8 Mini Mk8 MM Single Servomotor
2.8.1 Single Servomotor with VSD
For applications where only the VSD controls the air going into the burner and no air servomotor is
required, the Mini Mk8 MM can be set for single servomotor with VSD. The MM will make changes to
the fuel servomotor and VSD in synchronisation as the firing rate modulates up and down.
When using a single servomotor with VSD, the MM checks that the VSD feedback is within the fault
tolerance bands set in option 99 as the fuel servomotor drives open to increase the firing rate. If the
VSD feedback is not higher than the tolerance band at that the servomotor angle, then the servomotor
will wait until the VSD ramps up to meet this limit at minimum. This prevents the burner from being too
fuel-rich as the firing rate increases. As the fuel servomotor closes, there is a natural lag in the VSD
feedback as it slows down; the fuel servomotor still waits for the VSD but does not modify the target
VSD speed.
Relative channel
position
Time
Fuel
(servo)
Air
(VSD)
Natural
lag
Air channel
advanced by VSD
tolerance band
High Fire
Low Fire
VSD
Band
Figure 2.8.1.i Single Servomotor with VSD Diagram
Option 4 Air Channel must be set to setting 1 for ‘VSD Channel 4,’ and options 90 to 99 must be set
according to the VSD settings. Option 8 must be set to ‘Channel 1 only.’ Option 89 allows the user to
send the high signal to the VSD only when the T58 is required to come on, to prevent the burner
being forced with air at start-up.
Note: EGA trim will not work with single servomotor with VSD.
For single servomotor with VSD, both the fuel servomotor on channel 1 and the VSD on channel 4 are
wired as normal to the MM. Please refer to the Mini Mk8 MM Installation and Commissioning Guide.