Midco 400B-33 - Manual (Page 7)

X MOTOR, BLOWER INTERLOCK &
CENTRIFUGAL ACTUATOR
NOTE: BEFORE SERVICING, mark with a scribe line or
measure opening of primary air controlling shutter, so
that it can be reset to its original position following
servicing.
■ ■ Cleaning of the blower wheel is usually the only
service required. Need for cleaning is indicated if the
character of the flame indicates a deficiency of air. Motor
cooling air vents if present should also be cleaned at this
time.
■ ■ The blower side plate, motor and wheel are removed
as an assembly. Disconnect the motor wires from the
motor terminal strip. Disconnect the motor conduit from
the motor and remove the side plate screws.
■ ■ The blower wheel is equipped with a spring loaded
centrifugal actuator to operate an electrical interlock
switch so as to prevent the burner from firing if the blower
wheel is not running at its operating speed. When the
motor is off, the actuator spring forces the disk against
the switch plunger to push it past its operating point.
When running, the actuator pulls the disk clear of the
plunger.
■ ■ To make a specific test of the interlock circuit:
1. Turn burner power OFF.
2. Turn Manual Gas Cock Knob on Combination
Valve to OFF.
3. Disconnect the motor wire from the motor relay
to keep the motor off.
4. Turn burner power ON and set the operating
control to ON or thermostat to call for heat.
Check for 24V between the Electronic Control
25V and 25V GND terminals.
A. No voltage: Interlock circuit OK.
B. Voltage present: check that switch bracket is
screwed down tight (see Figure 6). If so,
without disconnecting switch wires, remove
switch bracket and manually depress switch
plunger. If voltage is still present, or if the
plunger has to be depressed to where
dimension"A" of Figure 6 is less than 9/32"
when switch clicks over, replace the switch.
5. If the switch tests OK, check dimension "A" of
Figure 6 as follows:
Switch plunger freeÉÉÉÉÉÉ9/32 to 11/32"
Disk freeÉÉÉÉÉÉÉÉÉÉ...7/32 to 1/4"
Disk held all the way inÉÉÉÉ.15/32" min.
6. If plunger dimension is wrong, replace switch. If
disc dimension is wrong, check that the actuator
operates freely with a minimum movement of
1/4" from the free position. If movement is OK,
reposition blower wheel on motor shaft. If not,
replace wheel.
FIGURE 6 Motor, Blower and Interlock Assembly
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