5370939-TIM-C-0518
Johnson Controls Ducted Systems
43
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, the speed of
the drive may have adjusted 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
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 22 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 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.
1,700 CFM
1,400 CFM
1.88 in. = 2.28 in.
Table 22: 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
Belt drive blower systems MUST be adjusted 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. Adjustments of the blower speed
and belt tension are REQUIRED. Verify proper sheave
alignment; tighten blower pulley and motor sheave set
screws after these adjustments. Re-checking set screws
after 10-12 hrs. run time is recommended.
