The mod motor takes a resistance, current or voltage signal from the firing rate control and turns its shaft to a matching position, moving the jackshaft and linkages. Auxiliary switches on the motor prove low fire for light-off and high fire for purge. When a mod motor sticks, the burner may light off at the wrong rate or refuse to modulate.
Mod motor's stuck at high fire, so it won't prove low fire for light-off, check the signal before you swap it.
Also heard as
- modulating motor
- modutrol
Related terms
Modulation
Varying a burner's firing rate continuously between low fire and high fire to match the load.
Jackshaft
The shaft and linkage system that lets one motor move a burner's fuel valve and air damper together.
Low-fire start
The rule that a burner must light off at its minimum firing position, proven by a switch, before increasing rate.
Purge
Blowing air through the furnace and flue before ignition or after shutdown to clear out unburned fuel.