5567611-BIM-B-1218
58
Johnson Controls Ducted Systems
Supply air drive adjustment
At unit start-up, the measured CFM may be higher or lower than
the required CFM. To achieve the required CFM, you may need
to adjust the speed of the drive by changing the datum diameter
(DD) of the variable pitch motor sheave as described below:
Use the following tables and the DD calculated according to the
above equation to adjust the motor variable pitch sheave.
Example
A 4 ton unit was selected to deliver 1,700 CFM with a 1.5 HP
motor, but the unit is delivering 1,400 CFM. The variable pitch
motor sheave is set at 3 turns open.
Use the equation to determine the required DD for the new
motor sheave.
Use Table 24 to locate the DD nearest to 2.28 in. Close the
sheave to 2 turn open.
New BHP
= (Speed increase)
3
• BHP at 1,400 CFM
= (Speed increase)
3
• Original BHP
= New BHP
New motor Amps
= (Speed increase)
3
• Amps at 1,400 CFM
= (Speed increase)
3
• Original Amps
= New Amps
Before you make any blower speed changes, review the
installation for any installation errors, leaks, or
undesirable systems effects that can result in loss of
airflow.
Even small changes in blower speed can result in
substantial changes in static pressure and BHP. BHP
and AMP draw of the blower motor increase by the cube
of the blower speed. Static pressure increases by the
square of the blower speed. Only qualified personnel
can make blower speed changes, strictly adhering to the
fan laws.
1,700 CFM
1,400 CFM
1.88 in. = 2.28 in.
Table 24: Motor sheave datum diameters
1VL34x7/8
(1-1/2 HP motor)
1VL44x7/8
(1-1/2 HP motor)
1VL40x7/8
(1-1/2 HP motor)
1VM50x7/8
(2 HP motor)
Turns open
Datum diameter
Turns open
Datum diameter
Turns open
Datum diameter
Turns open
Datum diameter
0
2.9
0
4.0
0
3.6
0
4.4
1/2
2.8
1/2
3.9
1/2
3.5
1/2
4.3
1
2.7
1
3.8
1
3.4
1
4.2
1-1/2
2.6
1-1/2
3.7
1-1/2
3.3
1-1/2
4.1
2
2.3
2
3.6
2
3.2
2
4.0
2-1/2
2.4
2-1/2
3.5
2-1/2
3.1
2-1/2
3.9
3
2.3
3
3.4
3
3.0
3
3.8
3-1/2
2.2
3-1/2
3.3
3-1/2
2.9
3-1/2
3.7
4
2.1
4
3.2
4
2.8
4
3.6
4-1/2
2.0
4-1/2
3.1
4-1/2
2.7
4-1/2
3.5
5
1.9
5
3.0
5
2.6
5
3.4
You must adjust the belt drive blower systems to the
specific static and CFM requirements for the application.
The belt drive blowers are not set at the factory for any
specific static or CFM. You must adjust the blower speed
and belt tension.
Verify proper sheave alignment. Tighten the blower
pulley and motor sheave set screws after these
adjustments. Re-check the set screws after 10-12 hours
of run time.
