Fan Laws vs. Motor Nameplate
AHU-7 has a belt-drive centrifugal supply fan rated 20,000 cfm at 2.0 in. w.c. total static. The motor is 20 hp, 460 V, nameplate 27 A, service factor 1.15, on a VFD. The motor sheave is adjustable and currently set so the fan turns 1,140 rpm at 60 Hz. With the system complete you measure 18,900 cfm at 1.78 in. w.c. total static, fan 1,140 rpm, motor 24.5 A, drive at 60 Hz (maximum setting). The commissioning agent wants you to "reprogram the drive to 65 Hz for the last 1,100 cfm." The VFD enclosure is mounted on the AHU in a mechanical room running about 100°F.
1. Calculate what the fan actually needs, and determine whether the existing motor can deliver it.
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Required speed from fan laws (flow varies with speed):
- Speed ratio = 20,000 / 18,900 = 1.058
- Required fan speed = 1,140 x 1.058 = 1,206 rpm
Predicted static and power:
- Static = 1.78 x 1.058² = 1.99 in. w.c. - consistent with the 2.0 in. rating
- Power/amps = 24.5 x 1.058³ = 24.5 x 1.185 = 29.0 A
Nameplate is 27 A. 29.0 A is above nameplate, inside the 1.15 service factor (31.1 A) but with essentially no margin, and the motor is sitting in a ~100°F room, which derates it further.
So the answer is: the fan aerodynamically needs about 1,206 rpm, but the installed 20 hp motor cannot carry that duty continuously with any margin. The motor is undersized for the as-built system resistance, which is itself slightly above design if it takes 1.99 in. w.c. to move 20,000 cfm.
2. The CA proposes reprogramming the drive to 65 Hz. Explain why that will not solve the problem and what the real options are.
Read the worked answer
Two separate reasons it will not work:
- The drive frequency is already at its 60 Hz maximum and the fan is turning 1,140 rpm. Going to 65 Hz only reaches 1,235 rpm if the belt ratio allows - which it does - but it does nothing about the current limit. Above 60 Hz the drive is in the constant-horsepower region: voltage cannot rise, so torque falls while the fan's torque demand keeps climbing with the square of speed. The result is a drive that hits current limit and stops accelerating, not 20,000 cfm.
- The motor is the constraint, not the drive. Even the correct 1,206 rpm requires about 29 A against 27 A nameplate.
Real options, all requiring engineer approval and none of which you implement on your own authority:
- Replace with a 25 hp motor and re-set the sheave for 1,206 rpm at 60 Hz.
- Reduce system resistance (filters, coils, obstructions) so the fan reaches 20,000 cfm at a lower speed.
- Accept the reduced airflow by written agreement, with the AHU re-rated accordingly.
Your job is to report the measured operating point, the fan-law projection, and the motor margin - not to chase the number by over-speeding equipment.
3. Before any sheave or drive change is made, what safety and jobsite steps apply?
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- Lockout/tagout the VFD and the motor disconnect, and verify zero energy; VFDs retain DC bus charge - wait the manufacturer's stated discharge time before opening the enclosure.
- Allow the fan to coast to a complete stop and confirm with the tach before going near belts or sheave. Do not rely on the drive display alone.
- Belt guard and sheave work: adjustable sheaves must be set to the calculated diameter and locked; both sheaves aligned; belts re-tensioned and the guard reinstalled before any test run. A thrown belt in an operating AHU is a serious injury risk.
- Verify the fan wheel and bearings are rated for the higher speed, and confirm the motor's ambient rating against the ~100°F mechanical room - a 40°C-rated motor is marginal there.
- Coordinate the electrical work with the electrical contractor and the controls contractor (drive parameters, safeties, and re-commissioning), and get the engineer's written direction before changing any equipment. Record all of it in the field log and the report - sheave changes are a scope change, not a TAB adjustment.