Buildings & data centers
Induction motor and supply fan
Fans follow the cube law. Motors leave a signature in their current.
A supply fan moves air. The fan laws say how much power it takes at any speed. The motor’s current says whether the rotor, the bearings and the load are healthy.
30,000 cfm at 5.0 in. wg, 40 hp, 1,780 rpm
- Input at design flow
- 26.0 kW
- Half flow, damper
- 21.3 kW
- Half flow, drive
- 4.0 kW
- Design airflow
- 30,000 cfm
Anatomy and nameplate
- 1Motor current. Power, load and rotor condition.
- 2Winding temperature. Insulation life.
- 3Motor bearing vibration. Bearing and balance.
- 4Fan bearing vibration. Wheel unbalance and bearings.
- 5Duct static pressure. Setpoint for speed control.
- 6Airflow station. The operating point.
- 7Filter pressure drop. Loading cost.
- 8Drive speed and power. What the drive reports.
| Fan | |
|---|---|
| Type | Airfoil plenum fan, 36.5 in wheel |
| Design airflow | 30,000 cfm |
| Design static pressure | 5.0 in. wg |
| Shutoff pressure | 6.5 in. wg |
| Peak fan efficiency | 72% |
| Input power at design | 26.0 kW |
| Motor and drive | |
| Motor | 40 hp (29.8 kW), 4-pole, NEMA Premium |
| Supply | 460 V, 3-phase, 60 Hz |
| Speed | 1,780 rpm, slip 1.11% |
| Full-load current | 52 A |
| Drive | Variable frequency drive, duct pressure control |
| Sideband frequencies | 58.67 Hz and 61.33 Hz |
The physics
The equations behind every curve on this page.
Fan laws
Q ∝ N Ps ∝ N² kW ∝ N³
Half speed is one eighth of the power.
Air power
hp = Q·Ps / (6356·η)
Air moved against pressure, divided by efficiency.
Slip
s = (n_sync − n) / n_sync
1.11% at 1,780 rpm on a 1,800 rpm field.
Rotor-bar sidebands
f = f_line·(1 ± 2s)
A cracked bar adds peaks 1.3 Hz either side of 60 Hz.
Fan and duct
In a duct system with no static lift, the operating point rides straight down the system curve.
Fan curve and system curve
- 100% speed
- 80% speed
- 60% speed
- Duct system
With no static lift in the ducts, the operating point slides straight down the system curve. Flow follows speed one for one.
Q2 = Q1·(N2/N1) Ps2 = Ps1·(N2/N1)² kW2 = kW1·(N2/N1)³
Power at part flow: damper versus speed
- Fixed speed with damper
- Variable speed
- At half flow
- 17.3 kW less, about $5,518 a year at 5,800 h
A damper adds resistance and keeps the fan working hard. Slowing the fan drops power with the cube of speed.
hp = Q · Ps / (6356 · η_fan)
The motor
The stator current carries the rotor’s condition. A cracked bar leaves two small peaks beside the line frequency.
Motor current spectrum: broken rotor bar
Slip 1.11%, sidebands at 58.67 and 61.33 Hz
- Sound rotor
- Cracked rotor bar
A cracked bar makes the rotor field uneven. That puts small side peaks either side of the 60 Hz line, set by slip.
f_sideband = f_line · (1 ± 2s) s = (n_sync − n) / n_sync
Failure modes
What breaks, what it looks like, what the twin does about it.
| Failure mode | The physics | Signals that show it | How the twin catches it |
|---|---|---|---|
| Broken rotor bar | The rotor field becomes uneven and modulates the current at slip frequency. | Sideband peaks at f(1 ± 2s). Speed ripple. | Reads sideband level against the action line, with slip from load. |
| Bearing wear | Race and ball defects ring at geometric frequencies. | Vibration lines at BPFO or BPFI. Heat. | Band energy at the predicted lines. |
| Wheel unbalance | Dust build-up moves the center of mass. | 1× vibration up. Same phase on both bearings. | Compares the 1× level with the speed-scaled baseline. |
| Filter loading | More resistance lifts the system curve. | Filter pressure drop. Lower flow at the same speed. | Shows the added kW and the best change date. |
| Belt or coupling slip | Torque transfer drops. Speed falls behind the drive. | Fan speed below motor speed. Squeal. | Checks fan speed against motor speed. |
How we catch it early
A physics twin and a PINN, working on one induction motor and supply fan.
The twin is a set of equations for this machine, tuned to its nameplate. Give it the flow, speed and conditions and it returns what every reading should be. It always gives the same answer for the same inputs.
A physics-informed neural network, or PINN, sits on top. It is trained on the plant’s own data and it is penalized whenever it breaks a physics rule. So it can only learn what the equations leave out: wear, fouling, friction. It cannot invent an answer that the machine could never produce.
Step 1
Measure
Flow, pressure, current, temperature, vibration. Whatever the asset already has.
Step 2
Physics twin
The equations of this machine, set to its nameplate. It says what the readings should be right now.
Step 3
Residual
Measured minus expected. Demand, weather and speed are already taken out.
Step 4
PINN
A small neural network that learns the cause of the gap, under the physics rules.
Step 5
Finding
A named cause, a date, and the kWh or hours at stake. An engineer reviews it.
loss = (measured − predicted)² + λ · (physics residual)²
The second term is the rulebook. It is zero when the network obeys the equations.
Why this beats black-box AI
A black-box model learns patterns from history. It needs examples of each failure, and it goes quiet when the pump runs at a point it has not seen. The twin starts from physics, so it works on day one. The PINN adds what physics cannot know about this particular machine.
That is also what makes the savings countable. The baseline is a calculation, not a guess, so the gap between baseline and meter is a number a customer can check.
What the twin checks on this asset
- Power against the cube law at the current speed
- Sideband level against the −45 dB action line
- Bearing band energy
- Filter cost against change date
| Fixed alarm | Black-box AI | Twin + PINN | |
|---|---|---|---|
| Needs failure history to learn | No | Yes, lots | No |
| Says what is wrong | No | Rarely | Yes, by name |
| Works at a flow it has never seen | No | Often fails | Yes, the equations still hold |
| Puts a number on the saving | No | No | Yes, in kWh and dollars |
| Lead time on slow wear | Late | Varies | Weeks to months |
| Every number can be traced | Yes | No | Yes, to an equation |
What the twin puts a number on
Energy and cost, from the equations.
Design input power
26.0 kW
30,000 cfm
Half flow with damper
21.3 kW
Fixed speed
Half flow with drive
4.0 kW
17.3 kW less
What we do for you
We watch it. We catch it. We prove it.
Our reliability engineers run the twin on your induction motor and supply fan, review every finding, and help your crew fix it. After the repair we compare the meter to the baseline and write the result in the ledger.