Running power answers whether the motor can sustain the fan at an operating point. It does not answer whether the motor can bring a heavy impeller to speed before the motor, starter or drive reaches a limit.
Obtain inertia in one reference system
Collect motor, coupling and fan inertia, including any gearbox. Confirm units and whether the fan value is referred to its own shaft or the motor shaft. Gear ratio changes the inertia seen by the motor, so adding nameplate GD² or J values without conversion can corrupt the study.
Angular acceleration = (motor torque − fan torque) ÷ total referred inertia
Integrate this relationship across speed to determine run-up time. A single locked-rotor torque value is insufficient because the smallest torque margin may occur later in acceleration.
Define the aerodynamic start condition
| Start variable | Question to settle | Source |
|---|---|---|
| Damper or guide vane | What position is proved before start? | Fan and control supplier |
| Duct pressure | Can another running fan pressurise the system? | System operating study |
| Reverse windmilling | Can process flow rotate the stopped fan backward? | Site observation and fan supplier |
| Minimum voltage | What voltage reaches the motor during starting? | Network study |
| Starts and restarts | What hot restart sequence is required? | Operating philosophy |
Recalculate torque at the motor terminals
Starting current can cause voltage drop in the feeder and upstream network. Available induction-motor torque falls when voltage falls, so use the predicted motor-terminal voltage. Compare that torque curve with the fan curve, not only the catalogue curve at rated voltage.
Coordinate protection with real run-up time
Stall and overload settings need to permit the legitimate acceleration while clearing a failed start before motor thermal capability is exceeded. Record expected speed versus time and use speed supervision where the protection design supports it.
With a VFD, confirm current limit, ramp, low-speed cooling and any skip-frequency bands. A programmed ramp is a command; actual acceleration still depends on available torque.
Inputs for motor selection
Send fan torque-speed data, all referred inertia values, damper condition, starting frequency, supply impedance or predicted voltage, control method, ambient and duty. LEADGO can compare line-fed, VFD and high-voltage motor options against the same acceleration case.