Blower-performance differences: Where do slight variations in blower output originate?

power variation

Blower performance depends on dozens of electromagnetic and geometric design features. Stack-up of tolerances heightens blower-to-blower performance disparities — even between those supposedly made of identical parts.

Final calibration minimizes such output variation, but slight deviations always persist. This is why mechanical efficiency plots serve as guidelines on capability ranges — and (as covered in other AMETEK FAQs) why feedback and blower controls that compensate for specific motors and installation conditions are so useful. After all, most geometrical blower tolerances aren’t of direct concern to OEMs and design engineers looking to apply blower. Rather, designers are more focused on the overall airflow of a given blower model. We’re selling blowers — selling airflow.

Blower-motor stator construction is one element that affects performance.

1. The motor stator’s lamination-steel composition introduces ± tolerances. Stator resistance (I2R) loss is a major inefficiency source. (Within induction motors, there are also I2R losses in the rotor bars.) Lamination-steel core losses include eddy-current losses from current induced between the laminations and hysteresis losses from the magnetic field’s continual reorientation in the laminations. The magnitudes of these depend in part on the laminations’ ± tolerances.

2. Then there’s variation associated with the lamination geometry — equal to the quantity of laminations in the stator stack multiplied by ± one lamination tolerance.

3. The diameter of copper wire for the winding has a ± tolerance as well. This is largely due to the fact that the wire tension while manufacturing the windings (and adding the wire to the stator) varies slightly ... which in turn makes for slightly inconsistent wire diameter. This in turn can cause very subtle voltage drops.

Several features of the rotor geometry also cause performance variations.

1.The rotor cup’s wall thickness has a ± tolerance which affects the flux-density path of magnet strength — and ultimately, the power of the motor.

2. Within motors that incorporate magnets, subtle differences in magnet composition can introduce ± tolerances in magnet strength. Variations in the magnet geometry — especially its inside diameter — adds another ± tolerance. These variations introduce a subtly varied air gap … which can degrade flux density and introduce axial forces between rotor and stator (which ultimately add load to the motor bearings).

3. Two other rotor-component sources of dissimilarities include magnet strength after magnetization (due to divergent composition) and minutes of variation in the magnet poles’ phase separations (introduced when the magnets are mounted).

Rotating fans for the working side of the blower — as well as the fan for cooling the blower — introduce their own geometric variations.

1. The fan-blades’ widths and lengths both have ± tolerances. Blower cubic feet per minute (CFM) increases with blade width while flow is decelerated.

2. The fan-blades’ curvatures have a ± tolerance. Of course, blowers can have fan blades of various curvature designs — including backwards and forwards and radial designs — but the tolerance mentioned here is between those of identical design.

3. The fan inlet (eye) is a shallow tapered or cone-shaped plate or ring that typically assembles between a blower’s inlet grating and a blower housing cowl. It too a ± tolerance. This can cause minute flow restrictions, jetting, or turbulence.

Stationary fans in blowers — those that collect and redirect working air — have slightly variable geometric features, so introduce different variations.

1. The fan-blades’ widths, lengths, and curvatures have ± tolerances. As mentioned, wider blades are associated with higher CFM and decelerated flow.

2. Though there are different subcomponents that affect which terminology applies to different designs, the term fan shell in this context refers to a blower’s scroll-shaped housing. All ± tolerance of the fan shell’s center air passage affects performance — as do variations in the fan shell length.

Another source of ± tolerances is the spacer system. This is componentry that includes a spool and provides correct spacing between working fans. Here, the spacer spool length and diameter also exhibits ± tolerance that can slightly affect blower performance. Last but not least, another component to contribute to tolerance stack-up is the fan bracket — the mounting hardware to anchor the blower in place. Here, ± tolerances in the inside diameter of fan chamber area, the discharge tube’s inside diameter, and fan chamber length all affect blower airflow output.

So many slight blower variations — what to do about it

We’ve outlined why blowers manufactured in the same facility on the same day and installed side-by-side in a given setting will still be different — exhibiting slightly different performance profiles. So how might a design engineer compensate for this fact if installation-to-installation consistency is a design objective?

Well, if a blower design uses a brushed motor, there’s little a design engineer can do to compensate for inter-design blower-performance divergences.

In contrast, a brushless-motor blower does allow for compensation of even slight inter-design blower-performance deviations. That’s because brushless motors accept software-driven controller commands that monitor rpm and make fine adjustments to input voltage via the drive for finely modulated blower output.

Keep in mind that blower-design performance is far more dependent on environmental conditions than that of most other electromechanical designs. This manifests as a load on the blower that’s essentially always variable. Because blowers handle air, that load and that vent speed is almost always changing anyway. For example, weather in a given area changes might shift and send a high-pressure front through a manufacturing facility. All of a sudden, the air in that facility becomes much denser and change the performance of the blowers in that location.

Contrast this blower-output parameter with those of a leadscrew on a machine moving a set load some distance — exhibiting roughly the same coefficients of friction through the stoke every move. The latter is a far more closed system.