Motor CFM @28"

[Tony]

Interestingly the guy I know running 4 motors from 1 controller is using a single 13A fuse, he says if the motor rpm's are increased gradually all works fine but if he starts the motors with the pot set too high the fuse needs replacing

This can be a real problem if your bench is powered up with enough motors to use all of the available mains current.

You may need to accelerate the motors gently up to speed, or you may blow a fuse or trip a circuit breaker.
Fitting an amp meter to your bench and knowing from experience what current the main power board trips out will prevent you being constantly plunged into darkness and silence.

With simple manually controlled power modules like the Kemo, you need to take care.
One of many advantages of Bruce's power controllers is they have a soft start.
The motors automatically accelerate up to speed over a few seconds up to whatever the speed is set to.
You can leave the speed knob set pretty high, and it gently returns to that speed without doing anything.

You don't have to remember to "ride the throttle" when switching the bench on and off between runs.
It also means that if there is a brief unexpected power interruption, the motors automatically glide back up to speed.

Without that, a brief mains interruption and recovery can easily blow a fuse.
These current inrush surges are not good for either the motors or the controller.

[SSR]

Thanks for your explanations Tony.

[rwdford]

That is another plus for the PTS speed controller, soft start is a great idea

I am lucky that I have a 60A 240V power source, I will be using a small breaker rocker switch on the bench rated for 4 to 5A per motor, the main breaker will be rated a little higher than this so that it will be less likely to trip, lights etc will be on a separate breaker

Will do some more testing measuring the amperage at 44" and different vacuum levels with full mains power to the motors


The best way of rating motor efficiency would be to test them at full power with a sharp edge orifice that gives 44" water and measure the voltage + amperage at this point, take the orifice ID and convert it to CFM @28" per motor, finally divide this by the KW per motor to get "CFM per KW"

CFM per KW is the most logical rating to use since it works with whatever voltage you are using


When looking at vacuum motor manufacturer ratings you would have to look for the CFM @44" and then multiply this by 0.798 to get what CFM @28" the motors will flow with a 16" Delta P, take the voltage and amps at this test range, divide the CFM by KW and you have an answer

As an estimate looking at the results so far these motors will flow around 62CFM/KW (@28" on the bench and 44" at the motor), Ametek is probably more like 66.5CFM/KW with a single stage motor (78.2CFM @28" / 240V x 4.9A)

Will measure the amps next and get an accurate CFM/KW figure but it is looking good so far!

[Tony]

62CFM per Kw works out to 4.167 amps at 240 volts
That is 14.87 CFM per amp which is excellent.

The Amtek:
78.2 CFM at 4.9 amps.
That works out to 15.95 CFM per amp, only slightly better.
That is right up there with the very best.

You are right, CFM per Kw is technically a better figure of merit.
For most of us, amps are much more convenient and easy to directly measure.

[rwdford]

Thank you Tony, I will get some more precise readings with a good amp meter and get the CFM per A and also the CFM per KW, I agree per A is quicker to measure and get a good indication of what the motor efficiency is like

Happy with the testing so far, I now know the motors are more than capable of what I am looking for and the speed controller is also up to the task

Measure twice build once!

[rwdford]

I have got some interesting findings and also found that the power supply line I was using was giving quite a large voltage drop when running 3 or 4 motors

The max amps are certainly around wide open flow and get much lower at high test pressures

I wonder if running lots of motors 6 to 8+ if the reduction in CFM per motor is actually mains voltage related rather than airflow robbing or some other effect, have you tried measuring the voltage with the motors off and also with the motors are full speed with a large internal orifice and just a wide open 5" test hole? I imagine there would be a significant voltage drop in the line feeding the bench unless it is sized plenty large enough for the bench

The Kemo controller is giving Zero voltage drop at full speed vs power feeding into the controller

[Tony]

The max amps are certainly around wide open flow and get much lower at high test pressures

Certainly possible to add an extra motor or two to take some advantage of that, but you then need to take extra care during motor acceleration not to overload the power source and keep blowing the supply fuse back at the main power board.
Also the age and condition of the wiring needs a bit of consideration.
If you are dropping a lot of voltage in the supply wiring, there is a lot of heat being generated somewhere, so be careful...

[rwdford]

Exactly Tony, there is resistance in the line and it must be generating heat, will upgrade the line first before testing again

Something I found quite interesting/surprising is that a 13A fuse will deliver over 20A of power! and the D20 breaker trips if starting 4 motors straight to full power, 3 motors will start straight to full power with a 20A breaker, fuses must be rated for way over what they are labelled as

I am going to use variable speed ramping up the motors gradually, and use a small neat rocker trip switch on the bench as a safety trip rated lower than the main breaker I will be using

What I am wondering now is what breaker switches are commonly rated for in terms of max surge amps for motor start up, I found a video that says the peak start up amps for a 1400W vacuum are 33A and then the peak current reduced to 6.5A that would be a peak current of about 5 times running current, I will find out what the breaker switches are rated for, ideally I would like it to run perfect at full speed but quickly trip if the motors were ever started straight to half speed or higher

[Tony]

Fuses are just a deliberate weak link in the system intended to protrect something just slightly more robust than the fuse.

The problem is, you want the fuse to provide a sufficient level of protection without nuisance blowing, and many things can have a fairly large and prolonged inrush current when first switched on.
So you specify a fuse that can handle the inrush, (if that is what you want it to do) or specify a fast blow fuse to protect something more fragile.

There is a vast range of fuse types available, and the continuous current rating is only one aspect.
Some special fuses can be frighteningly expensive.
Yes, a 13 amp fuse may well carry 20 amps or even more, maybe for quite a while, or it may fuse in milliseconds at 20 amps.
Circuit breakers are also specially designed for specific purposes.
They have two very different methods of operation, thermal or magnetic, or sometimes both.

Magnetic breakers trip instantly at threshold current, thermal breakers have a heating element that trips the breaker after a delay that can be really short or extremely long.
You could spend your whole life studying this, it is a vast subject.

But the idea is to fit some motor protection that will safely disconnect a motor in the process of violently self destructing, without disconnecting motors due to normal inrush surges.

The simple rough and ready way is to just fit the lowest rated "normal" fairly fast fuses that are proven to not frequently fail in typical use, and that just involves a bit of trial and error in fuse selection.

[rwdford]

Thank you Tony, you are completely right

Fuses come in many varieties, very fast blow, very slow blow and anything in between

About the breakers yes the electromagnetic breakers are designed to immediately trip when there is high inrush current such as when a motor starts, there is a coil and piston inside just like a solenoid and they are rated for different peak current levels as a percent of the rated running current before they will trip

The other main type are thermal breakers, they do not protect against inrush current but if the running current is too high a bimetallic strip heats up and trips the switch

MCB's as in houses and workshops have both thermal and electromagnetic trip mechanisms inside

In the EU MCB's have 3 main different ratings for inrush current, labelled B, C and D, B is meant to withstand 3 to 5 times the rated current before it will trip, C = 5 to 10 times and D = 10 to 20 times the rated current for Inrush protection, C and D are most suitable for starting motors with their high peak starting amps

With all of that said I tested a new Schneider B6 MCB running 2 motors to see when the thermal cut would kick in, the motors ran with 9.3A for 3.5 minutes before it tripped! and at 7.6A they ran for 7 minutes with no trip and some heat being generated in the MCB

The magnetic cut will trip anywhere from 2A to 7.6A but only intermittently!

I can only conclude that MCB's are unsuitable and will only protect a circuit from very high starting amperage or sustained amperage higher than the rated running current but with a very "slow blow" operation

Fuses are looking like the Only option at this point, I will get some different fuses and test them to find a decent solution