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292447-YIM-A-0507
42 Unitary Products Group
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 12.5 ton unit was selected to deliver 4,000 CFM with a 3 HP
motor, but the unit is delivering 3,800 CFM. The variable pitch
motor sheave is set at 2 turns open.
Use the equation to determine the required DD for the new
motor sheave,
Use Table 21 to locate the DD nearest to 4.21 in. Close the
sheave to 1 turn open.
New BHP
= (Speed increase)
3
• BHP at 3,800 CFM
= (Speed increase)
3
• Original BHP
= New BHP
New motor Amps
= (Speed increase)
3
• Amps at 3,800 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.
Table 21: Motor Sheave Datum Diameters
1VM50x7/8
(1-1/2, 2 & 3 HP Motor)
1VP56x1-1/8
(5 HP Motor)
Turns Open Datum Diameter Turns Open Datum Diameter
04.415.3
1/2 4.3 1-1/2 5.2
14.225.1
1-1/2 4.1 2-1/2 5.0
24.034.9
2-1/2 3.9 3-1/2 4.8
33.844.7
3-1/2 3.7 4-1/2 4.6
43.654.5
4-1/2 3.5 5-1/2 4.4
53.464.3