Paxton blower, speed, hp, tq required - Will i have enough with 10 hp

[Tony]

Yes Shawn,

Brush type motors as found in vacuum cleaners, electric drills and other similar small household appliances will operate from either ac or dc. The speed can be controlled over a very wide range by simply varying the operating VOLTAGE. A variac is a very robust, reliable and simple way to do that, and is ideal for a flow bench that uses ordinary vacuum cleaner motors.

Induction motors work on a completely different principle. A rotating magnetic field is created by the internal windings, and this drags the solid metal rotor around at a constant operating speed. The running speed is really fixed by the mains frequency (50 or 60 Hz). To change the running speed, the FREQUENCY needs to be changed as well as the voltage.

An electronic variable frequency drive is exactly what is needed with a three phase motor. these VFDs can be found reasonably cheaply on e-bay. If you need several (or very high) horsepower, and a continuously variable speed right down to zero, a three phase motor and a VFD is by far the best solution.

[DAveK]

I usually just lurk & learn but Tony's obvious expertise raises a question or two.

1: Is the 3 phase a critical component to using the VFD or can 220 single phase be used?
2: If yes on the 3 phase, would a phase convertor be a problem?
3: If phase convertor is OK, then is there a particular brand or type you prefer?

Thanks
Dave Koehler
www.koehlerinjection.com

[Shawn]

thanks Tony,
very good info. I really excited to get this new bench up and running. Is there anything i should look for when buying a VFD for my application? I see there is a huge difference in pricing. I would assume that a lot of it has to do with load and time used. Since this is going to be short duration usage, could i get by with a less expensive one? Will it effect the ability to control speed in finer incriments?
thanks again,
shawn

[Tony]

The way these VFDs work is that the ac mains voltage is first rectified to generate a high voltage constant dc supply. The lower horsepower units just require ordinary single phase incoming mains power (230/110). But as the horsepower rating goes up, the larger VFD's always need three phase incoming power. Finding a single phase unit above about 3Hp may be difficult.

This constant dc voltage is then converted electronically to a three phase ac output. The electronics can vary the output voltage and the output frequency independently to operate the induction motor at any speed. This can be even higher than the normal speed on the rating plate. Motors can often be run up to 80Hz, 100Hz, or even sometimes at 400 Hz ! And the motor Rpm goes up accordingly.

The characteristics of the VFD mean that you will get full rated motor torque at full rated operating current, at any speed from zero up to the motors normal operating speed (at 50 or 60 HZ).

So basically the available motor torque is constant, and output horsepower increases with Rpm up to it's full normal rating.

You can run a 60Hz motor at 120Hz, and the Rpm will double, but only half the torque will then be available. In other words you can increase the Rpm, and still get full rated horsepower. Maximum speed is only really limited by the bearings, and the rotor flying apart.

VFDs have been around for many years, but the technology is improving, and the cost is coming down rapidly. The older ones are very simple and offer few features beyond just being able to vary the motor speed with a knob or incoming control voltage. All the later ones now use microprocessors with a VAST number of features, and the ability to do some very sophisticated things. For example ramping up speed slowly, to avoid belt slippage, or even possibly using flow bench test pressure to control blower speed directly with a feedback system. That may not be possible with all units, but the latest VFDs often have an inbuilt PID controller that could do it. Some have a serial data port so that the motor can be fully controlled or monitored by an external computer.

VFDs are universally used with three phase motors, and as these are cheap and readily available in a wide variety of sizes, that is not a problem. Single phase induction motors are not suitable, mainly because the start/run capacitor can create abnormal and excessive loading on the VFD. The low speed performance of a single phase motor would be very poor.

A mechanical analogy would be like saying a single cylinder engine might run roughly at very low Rpm, where a three cylinder engine might be better. Three phase induction motors (like the three cylinder engine) are much smoother and much more efficient at reduced Rpm.

When you buy a secondhand VFD make very sure you get the correct operating manual. Even with that, it will probably take several hours to figure out how to program the thing with the hand controller to get it to do what you want. It is like programming an unfamiliar video recorder (VCR), only worse.

I purchased a new 10Hp Chinese VFD which cost $1,200 Aussie dollars ( $880 US). E-bay may be another alternative, but I preferred to get something new with all the latest features, a really good instruction manual, and a warranty.

[riga team]

I have found a pipe with an input 4" rotor 12 inch on 3 phase engine 1.1kw i2800 RPM by 180 dollars, which gives out 1500m3/h or 377 l/s. Also there is a motor on 0,7 kw 4.3 amp 3000 RPM. If I shall replace with their place (for adjustment of turns by a rheostat) that I shall receive on an output? Who can comment? Yours faithfully Anatoly

[Tony]

Anatoly, 377L/sec is 799 CFM, and your Rpm, rotor diameter, and motor power, suggests it might only be capable of developing about 7" of water pressure. That is really too low a pressure to be useful for a flow bench.

Speeding up the blower Rpm sufficiently may not be possible because the horsepower required to drive it would then become excessive. See if you can find something with a larger diameter rotor. That would develop far more pressure at similar Rpm.

[riga team]

senk tony !0,7 kw it is not enough . to lift rpm 9000?

[Tony]

If you increased the Rpm by three times, you might expect:

Flow would triple to about 2,400 CFM

Pressure would go up around nine times to 63"

Horsepower would increase 27 times !!! to 30Kw

It might also fly apart and explode at 9,000 Rpm, it is asking rather a lot.

[Tony]

It might be better to start off with something that has an 18" rotor, and perhaps is rated at 300 CFM with 1Hp motor. That should produce about 17" pressure at 2800 Rpm as it is.

Now if you doubled the speed:

Flow doubles to 600 CFM

Pressure goes up four times to 68"

Horsepower goes up eight times to 8 Hp

A larger diameter rotor and less Rpm would be more practical. A 24" or 30" rotor would be even better.

[Thomas Vaught]

Tony, you think someone could make a blower like you are talking about? 24" diameter aluminum wheel with welded vane structure. CNCed scroll for the deal.

Probably cost more than the Vortron deal.

What would the application be for a 24" scroll blower like you describe?

Tom V.

[Tony]

Yes, I think making one from scratch would be quite feasible, the construction is really rather simple. I have played around with these now for a couple of years and have a fair idea of what sizes/dimensions and Rpm will create in terms of flow and pressure.

They are used for all sorts of odd industrial purposes, furnace and forge blowing, moving product through pipework pneumatically, inflating things, anywhere that a LOT of air at fairly high pressures are required.

I absolutely must buy myself a digital camera and post some pictures here !!!!

My most recent thinking is to use a spare 18" rotor blower that I already have here, that is already fitted with a 1.5Hp 2,800 Rpm motor. I could easily fit a 15 Hp 2,800 Rpm flange mount motor directly onto the back of it.

The idea is to then operate my variable speed drive up to perhaps 100 Hz or 120 Hz and at least double the maximum motor Rpm. That eliminates all the problems with pulleys and belt slip, and it would be much more compact.

My VSD is only rated to 10hp continuous, but these things are built rugged enough for starting up under very heavy load with starting current well in excess of the 10 Hp full load rating. The motor would only be running near full load at maximum Rpm anyway. At anything less than absolute maximum flow and maximum pressure, it will be running at far lower power than 15 Hp. I estimate it should be good for 900 CFM and 60" absolutely flat out, at perhaps around 6,000 Rpm.

It would consume less power than equivalent vacuum cleaner motors, and be infinitely quieter, especially at reduced speed, as well as being potentially more long term reliable.

An automotive centrifugal supercharger would probably be even better, but they do not come cheap.

[riga team]

Whether who that tried the Lord air ejektors princips? it is necessary to go such by ?
anatolij