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issue: October 2004 APPLIANCE Magazine Part 2: Motors & Air-Moving Devices

Part 2: Motors and Air-Moving Devices
Small Adjustments, Large Improvements


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When asked to improve the cleaning performance of a vacuum, Johnson Electric started by analyzing the fundamental physics behind a successfully working product.

A vacuum cleaner works by creating airflow through the machine. The air enters through a nozzle held close to the floor and passes through filters to extract the dust and dirt that is drawn in by the airflow. Clean air is then exhausted back into the room.


Figure 1. Guidance blades were added to the upper side of a motor diffuser plate to help re-direct airflow, passing it past the diffuser plate (left). Johnson Electric says by doing so, the airflow became more fluid and losses were diminished.

Generally, the greater the airflow, the greater the vacuum created at the nozzle (depending on nozzle size) and, consequently, the greater the prospect of removing more dirt and dust. The company says it would have been easy to use a more powerful motor with a bigger fan, but this would have led to increases in product size, weight, power consumption, noise, and cost—product aspects consumers do not want in their vacuum cleaners.

The Key Aspect

According to Johnson Electric, air, like any other fluid material, can be made to flow from one point to another by creating a pressure differential between those points and by being channeled to follow a particular route. The motor drives the fan at a specific power output and converts the mechanical energy from the moving rotor to the mechanical energy of the moving air.

Unfortunately, as air is pulled into the fan and expelled from its perimeter, losses occur. Every time the air passes through a channel or is made to change direction, there is resistance to the air movement and power losses occur. In search of a solution, Johnson Electric undertook a study focused on reducing the losses, thereby increasing the air output power without increasing the input power to the motor.

Simulation Experimentation

The company utilized a computer simulation software program to review the product’s air performance by developing a program for inputting all of the known parameters of dimensions, material, and airflow chamber shapes.


Figure 2. Modifications were made to the blade angles and dimensions of the impeller to help improve airflow and force. The design changes helped to increase the performance of a customer’s vacuum cleaner.

The company says the greatest attention was paid to the motor/fan itself, comprising the impeller (fan) and the diffuser plate, which captures the air thrown out by the impeller and re-directs that airflow to where it is needed. According to Johnson Electric, this particular system is known as a “flow-through” system because the air is directed through the motor to cool down the heat in the motor generated by internal motor losses. The simulation demonstrated that there was excessive turbulence around the perimeter of the impeller from which the air was forced outwards by the centrifugal forces of rotation. The company found that this turbulence was diminishing the useful flow of air.

To reduce the turbulence, guidance blades were added to the upper side of the diffuser plate. The blades were designed to re-direct airflow downward from the tangential direction given to it by the impeller, permitting the air to pass below the diffuser plate. By having a multiplicity of blades, the exiting air is now re-directed almost immediately, avoiding collision and turbulence with other channels of air exiting from neighboring impeller sections. According to the company, airflow now becomes more fluid and losses are diminished (see Figure 1).


Figure 3. Using a software program, Johnson Electric analyzed the motor’s air pressure at the inlet of the impeller. After design changes were made, an increase in the amount of negative pressure and airflow was evident.

The performance of the impeller itself was also reviewed. Since the air enters into the eye of the impeller and passes through the impeller to be centrifuged out from the periphery, the impeller is part of the total airflow path while also providing the pressure differential to create the airflow. This type of centrifugal impeller has been in use for many decades, and it was agreed that the number of blades selected and the shape of the cover that shrouds the impeller needed to be optimized. The company said it also felt there was room to optimize the angles of the blades and the orifice sizes.

Figure 2 shows how the modifications to the blade angles and dimensions were made. Using the software, the company predicted a change in the air pressure at the inlet of the impeller. As shown in Figures 2 and 3, there was a substantial increase in negative pressure, or vacuum, and a valuable increase in airflow volume.

Confirming the Results

Johnson Electric made tooling changes to produce a new impeller and diffuser plate according to the suggested design, and a series of tests were performed to validate both the new impeller and diffuser plate. Tables 1 and 2 illustrate the improvement in performance for both the new diffuser and the new impeller. An inlet orifice diameter of 0.75 in is used in both cases. Both tests confirmed the airflow rate improved due to the design modifications to the diffuser and impeller.

 

 

Model
Vacuum Performance
Old
297.9 mm H2O Initial
Modified
329.6 mm H2O Plus 10.6 percent

Table 1. Diffuser Performance

 

Model
Vacuum Performance
Old
293 mm H2O Initial
Modified
322.4 mm H2O Plus 9.8 percent

Table 2. Impeller Performance

Conclusion

Vacuum cleaner motors have to be designed with their application in mind and with thought given to how the vacuum cleaner manufacturer will mount and employ the motor within its cleaner. This requirement does not allow the motor designer to make drastic modifications to the motor but, nevertheless, it is possible and advantageous to make a series of incremental improvements in vacuum cleaner airflow efficiency by fine-tuning the impeller and diffuser designs within the motor.

This information is provided by Roger Baines for Johnson Electric.

 

issue: October 2004 APPLIANCE Magazine
Part 2: Motors and Air-Moving Devices

Click for the online Part 2 Table of Contents.

 

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