Vacuum Motor Selection - DeeEmm/DIY-Flow-Bench GitHub Wiki

Overview

The most important part of any Flow bench is the Vacuum source. This can range from a simple shop vac or a stand-alone 'vac box' to banks of high powered vacuum motors integrated within the flow bench itself. The choice of what to use for your vacuum source will depend on many factors. At the most basic level the style of bench you are building will drive vacuum source selection. For a basic floating depression bench using a MAF sensor, the shop vac might be a good choice, but if you are building something to compete with those big blue professional benches, then you might want to look a bit further into vacuum motor selection.

For a shop-vac powered bench, the actual depression that you are measuring at is usually not a consideration. But if you want to make measurements at a specific depression, for example the industry standard 28"/h2o, then you will need to ensure that your vacuum source has enough oomph to not only flow above the maximum flow capacity that your bench might reasonably see, but that it can do this whilst pulling the required depression.

So the two basic driving factors are...

• The maximum flow that you would ever need to measure (cfm)
• The maximum depression that you want to measure at (in/h2o)

Other considerations

As well as the physical ability of the vacuum motors, you also need to consider cost and power. To achieve the magic figure of 28"/h2o will will most likely need to run several vacuum motors simultaneously. This means that there will be an electrical power requirement that needs to be considered. For example, in the US and Australia, the general power outlets have a current carrying limit of 10 amps (in the UK this is 13 amps). So if you need 5 motors to achieve 300cfm @ 28"/h2o, then they will need to draw 2 amps or less. Or you will need to consider running a dedicated power feed to your bench.

Cost is always a factor, especially when you need to run multiple vacuum motors. The cost can vary massively, from guaranteed specification brand new motors down at the top end of the scale down to unknown provenance and performance recycled vacuum motors at the other end of the scale. Then of course there is the potential additional cost of a dedicated electrical feed.

So how do you determine what vacuum motor you will need, and how many?

Vacuum motor selection

Given that we know what depression that we want to target and have a flow figure in mind, we can utilise vacuum motor datasheets to determine what motor and how many we will need.

Vacuum motor data sheets are available on some manufacturers websites. Lighthouse vacuum motors is a great source. They even have a dedicated flow bench motor page (they are the recommended motors for the PTS flow bench) https://centralvacuummotor.com/flowbench.htm

The page lists two basic types of motor, each are available in a variety of voltages.

• LLH9563
• LH2093

You will note that each motor has a link to what is described as a 'Product Bulletin', this is essentially an excel spreadsheet that includes the flow characteristics of each motor. We will use these data sheets to determine our motor selection. A technical drawing of each motor is also provided which will aid in your design.

Determining Static pressure requirements

To be able to use the data sheet, we need to determine the static pressure requirements. The static pressure is the Test Depression + Differential Pressure. So for example if you are testing at 28"/h2o and have a differential pressure of 16"/h2o, you will have a static pressure of 28 + 16 = 44"/h2o.

You can then look at the 44" line on the airflow chart to determine what airflow the motor can produce.

On motor selection you want to look at the total static pressure your flow bench will see and compare that to the flowchart for the motor at that pressure, this will give you a good idea of what your motors will actually flow.

For the 119563 motor, testing at twenty eight inches (28+16=44 inches required) would be about 85 CFM per motor.

Number of vacuum motors required

So for the 119563 motor at a total static pressure of 44"/h2o we would get approximately 85cfm of flow. This means that if we wanted to target an overall maximum flow value of 350cfm, we would need 5 vacuum motors. (350 / 85 = 4.11 motors)

Using 5 motors gives a margin of approximately 20%. This means that the maximum duty cycle is 80%, which is good as you do not want to size your system for 100% duty cycle. i.e. motors running flat out to achieve the target performance.

You can also allow for this in your initial calculations by using 80% of the motors flow rate in your calculations.

i.e. 85cfm x 80% = 68cfm

So 350cfm / 68 = 5.14 motors

Using pre-loved unknown motors.

If new motors do not fit within your budget, you might want to consider sourcing motors from vacuum cleaners. The issue there is that the motors are of unknown provenance and performance.

In this case is might be wise to consider using additional motors, or at the very minimum, making allowance within your design for the provision of fitting additional motors but blanking off those mounting locations until they are needed. This philosophy is good for all benches as you never know when you might need more flow.