# AHU Exhaust Fan Motor Arrangements

How a hot exhaust fan's motor stays out of the airstream, comparing a direct\-coupled drive with a belt drive that changes speed through sheave ratios\.

By Omkar Marathe

Topic: Engineering

[Open the interactive Jig](https://ask.jig.com/j/tpkwpZd6nj74)

## Transcript

### 1. Two drives, one boundary

Both exhaust sections carry the same overhung plug fan, and in both the fan shaft passes through an insulated bulkhead into a separate drive compartment\. The motor therefore runs in ambient air while hot exhaust passes only over the wheel\. The left unit is coupled directly; the right unit is belt\-driven\. Drag anywhere to turn the assemblies\.

Rotation is slowed for viewing: the motors actually run at about 1765 rpm\.

### 2. The hot air path

Exhaust air enters the open end and accelerates through the bellmouth inlet cone into the wheel eye\. Nine backward\-curved blades discharge it radially into the casing, which acts as the plenum, and it leaves through the roof outlet\. Only the wheel, shroud, inlet cone and shaft end are wetted by the exhaust; nothing electrical is\.

A plug fan has no scroll housing: the plenum recovers part of the velocity pressure as static pressure\.

### 3. Why the windings stay cool

Standard NEMA motors are rated for a 40 °C ambient\. Insulation life roughly halves for every 10 K of sustained overtemperature, so a motor bathed in 60 °C exhaust would keep only about a quarter of its rated life\. Raise the exhaust temperature: the hot stream intensifies, while both drive compartments stay at room air\.

Class F insulation is rated 155 °C total; many motors pair it with a Class B \(80 K\) rise for margin\.

### 4. Through the bulkhead

Each shaft crosses the insulated double\-wall bulkhead through a sealed penetration, then runs in two pillow\-block bearings on a pedestal outside the airstream\. The wheel is overhung beyond the inboard bearing, the bearing layout AMCA 99 calls Arrangement 1\. The bearings, grease and seal therefore also stay at ambient temperature\.

A short overhang limits shaft deflection and the inboard bearing load\.

### 5. Direct drive: coupled motor

In the left unit a flexible jaw coupling joins the fan shaft to a foot\-mounted motor on an extended base: AMCA Arrangement 8\. Wheel and motor share one speed, n = 120·f / p · \(1 − s\), about 1765 rpm for a four\-pole motor at 60 Hz with 2% slip\. Speed can only be changed electrically, so this motor runs from a variable\-frequency drive; slide its frequency\.

Below rated frequency the fan laws apply: flow ∝ n, pressure ∝ n², shaft power ∝ n³\.

### 6. Belt drive: the sheave ratio

In the right unit the motor sits beneath the fan shaft and drives it through two V\-belts\. Fan speed follows the pitch diameters, n\_fan = n\_motor · d / D, with a 250 mm fan sheave\. Swap the motor sheave: from 150 to 190 mm the fan turns 27% faster, so flow rises 27% and shaft power roughly doubles\.

Check the new shaft power against the motor rating before fitting a larger sheave\.

### 7. Tension, guards and losses

Belts stretch, so the motor rides on a pivoting base with a tensioning screw, and a guard encloses the belts and sheaves \(its cover is cut away here\)\. Belt pull adds a radial load on the fan bearings that the coupled drive avoids\. A V\-belt drive transmits 95–98% of motor power when new and about 93% once it slips; that loss is released as heat in the drive compartment\.

Notched belts recover about 2%; synchronous belts reach about 98%\.

### 8. Choosing an arrangement

Both arrangements keep windings, terminal box and bearings in ambient air, which suits hot, humid or contaminated exhaust\. The coupled drive avoids belt losses and upkeep but needs a VFD and a longer drive compartment\. The belt drive is compact and field\-adjustable by changing a sheave, at the cost of belt maintenance and higher bearing load\.

Arrangement 4 mounts the wheel on the motor shaft, so the motor sits in the airstream: clean, moderate\-temperature air only\.