Newb with 8/71 based bench needs some help!

[TK]

Tony - thanks for that, motor is 1450rpm & geared 2:1 to drive the blower at 2900rpm.
Probably a bit fast, I'll get a 1:1 pulley for it & start from there to see what it does, I can't see me needing any more than 2500-3000rpm on the blower, but you never know.

I'll post up pics of the VFD when it gets here.

[GordonE]

Hi!
Nice project you got there! It will be very interesing if the VFD can control the speed to achive the correct pressure!

The only downside I see is the size and weight of the motor compared to a bench with vacuum motors.
Isnt there any high speed motors that is a little bit smaller?

I have a friend at work that collects many things, and he got a Roots blower from an Rolls Royce two stroke tank diesel. I hope that it is as the 6-53 in size. It should be enough for a sturdy flow bench! Im just have to get him to sell it to me for a good price

Keep us informed about the progress!

[Tony]

[hdwgfx]

TK, I was just thinking out loud concerning the extra VAC. motors. I was just thinking some extra fine tuning if needed.

I had run my blower/ele. motor combo with a Meriam LFE (long time ago, wish this place was here then) I only have some vague data from that project left around , but spinning the blower at @3400 rpm 28" TP 4 1/2 DP (1000 cfm LFE) got @570 cfms /503.83 scfm through a 1 3/16 1/8" thick crude test orifice on top of that bench, the ele. motor was pulling 30 amps at that point. After reading all the info here about wood leaking air through the panels I do not doubt that I had air leaks also.

[jsa]

[jsa]

double post

[TK]

Danfoss VLT series have a PID loop, so it could do pressure control.[quote]

Sweet, VLT rings a bell, I may be on a winner!

Motor is 7.5A per phase - 22.5A total, rated at 5HP.
But when I had it rewound the guy seemed to think it was a 7.5HP motor that had been under-rated for some reason.
If I stick to 22A current draw it'll be fine, but in theory I should be able to run it up to 30A - I won't do that unless I need to, but it's nice to know the motor will most likely go there if I need it to.

hdwgfx - if you were getting 500+ cfm with 30A current draw & 28" I guess it's a safe bet that at 22A I *should* be able to pull 360+ cfm @ 28" - enough for almost everything I do.

#### I'm itching to get into finishing this off, stupid work backlog.

[hdwgfx]

Tony said" Basically, the back pressure acts directly upon the blower lobes, and tries to turn them backwards. How much torque you need to sustain (say 1psi, or 28 inches) working pressure, will depend on rotor diameter and length, exposed to that back pressure. AND Note that motor or blower rpm does not come into this. We are talking only about blower drive torque and motor current, and required blower operating back pressure. Optimally matching the motor and blower for the required test pressure."

TK this IS what's important period!

The example I gave you of my test was in the early 90's and that piece of info was all I could find and as I said I am vague on that testing. After I wrote that I read what Tony said and it made me remember that the problem I had was when I loaded the bench (restricted as a real test, that back pressure Tony is talking about) there was just not enough motor to push against the restriction and it would pull enough amps to shut down the 40 amp breaker which was max for my motor and I figured I was gonna need at least 15 hp maybe 20 hp! which was a real bummer

So I Hope that I had did something wrong way back then and could still use my set up??? Hence me also making the statement about using/adding a couple of VAC motors. I hope this makes sense and I didn't mislead you?

Tony you really summed it up for me with those statements, thanks man.... from the early 90's to now there's lots of cobwebs lololol

[TK]

Ahh, cool, so your drive ratio may not have been ideal - meaning you needed to load the motor up more to get the flow.
Now it makes sense why you wanted to boost it with some vac motors.

I'll let you know how it goes & what drive ratio I end up with, hoping the VFD can help me make the most of the motor I have.

[hdwgfx]

I tried numerous drive ratios and it just didn't have enough motor torque to take the load... I think you are going to need the VFD IMHO but I hope not... anxiously looking forward to your results!

[jsa]

[TK]

It does have the wire size for 7.5hp, that's why the rewinder was doubtfull about the power rating - as far as he could tell it is a 7.5hp motor.

I have much to learn about elec motors it seems - ahh well, I'll blunder along & try not to blow anything up.

Danfoss drive is rated at 15kw.

[jsa]