5703827-XIM-A-0319
Johnson Controls Ducted Systems
35
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 have 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 per the above equation to adjust the motor variable pitch sheave.
Example new datum diameter
A 4 ton unit was selected to deliver 1,600 CFM with a 1VL34 motor sheave, but the unit is delivering 1,350 CFM. The variable pitch
motor sheave is set at 4 turns open.
Use the equation to determine the required DD for the new motor sheave,
Use Table 25 to locate the DD nearest to 2.48 in. Close the sheave to 2 turns open.
Example new BHP
= (Speed increase)
3
• Original BHP = New BHP
= (Speed increase)
3
• BHP at 1,350 CFM
Example new motor Amps
= (Speed increase)
3
• Original Amps = New Amps
= (Speed increase)
3
• Amps at 1,350 CFM
CAUTION
Before making 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 will
increase by the cube of the blower speed. Static pressure will increase by the
square of the blower speed. Only qualified personnel should make blower
speed changes, strictly adhering to the fan laws.
Table 25: Motor sheave datum diameters
Turns open
1VP56
1VP65
Datum diameter (in.)
Datum diameter (mm)
Datum diameter (in.)
Datum diameter (mm)
0
5.3
134.6
6.0
152.4
1/2
5.2
132.1
5.9
149.9
1
5.1
129.5
5.8
147.3
1-1/2
5.0
127.0
5.7
144.8
2
4.9
124.5
5.6
142.2
2-1/2
4.8
121.9
5.5
139.7
3
4.7
119.4
5.4
137.2
3-1/2
4.6
116.8
5.3
134.6
4
4.5
114.3
5.2
132.1
4-1/2
4.4
111.8
5.1
129.5
5
4.3
109.2
5.0
127.0
5-1/2
--
--
4.9
124.5
6
--
--
4.8
121.9
1,600 CFM
1,350 CFM
2.1 in. = 2.48 in.
