Wiring,speed contol ideas

[Shawn]

Hi
I touched on my bench i'm building on another post about motor contol, but here it is again. I'm using 12 of the surplus center 16-1193-J motors. I'll tell you what i have planned and please correct any mis info. that i have been given. The motors draw 12 amps each @ 110v. I plan on wiring them in series or parallel @ 220v (can't remember which one) to reduce the load by half. Even doing that it would still put my requirements at 72 amps. To keep from having that very high draw and to help extend the life of the motors (not to mention the diffcult start-up procedure) i have come to the conclusion that i need a variac of some sort. I have found some that are pretty resonably priced that will handle the volt/amp requirements, if i use two of them. The ones i located will handle 40 amps each @ 240v. While the "sweep" on these appears to pretty large from 0-240, I plan on "gearing" it down by using one handle and a belt linking them together to try and gain more refined depression control.I have a friend that is an electrian that is going to help with the wiring and elecrical requirements, but he doesn't have a clue as to what a flow bench is. I guess what i'm lookin for is any opinions on my ideas to see if someone has something that has worked better or would work better than what i have planned. Oh, by the way, this is a pitot style bench.
thanks
shawn

[86rocco]

I'm going to use a variac too, my motor are Ametek 115923's, they draw about 11 amps each, I'm starting off with only 4 motors but I'm designing things with the intention of using 8 should I need to. My strategy is a bit different then your, I'm only going to run 1 of the motors off of the variac. The remaining motors will be switched individually or in pairs by relays. The variac and relays will be mounted at the back on the bench and I'll extend the shaft of the variac through to the front panel, the relays will be controlled by an 8 position rotary switch also mounted on the front panel, along with a master shut-off switch.

Controlling only one motor with the variac will not only allow me to use a much smaller and probably cheaper variac, it also gives me a much fine degree of control than would running all of the motor off of the variac.

[Tony]

Sounds like a pretty good plan Shawn.

Gearing down the variac control shaft will give much finer control, many turns of a small knob are one possible way to do it.

Another way might be to use a suitably large control wheel mounted on the side of the bench. That would be much faster for large changes, and the rim of the wheel could have numbers or marks so you can quickly return to the same exact speed setting.

I use a car steering wheel mounted on the side of my bench for indexing the orifice turret, and it is very effective, but something even larger would make a very simple and convenient speed control for some large variacs.

Some sort of tubular steel hoop fitted with spokes, perhaps ?

[blown928]

Running the motors on 220v vs. 110v will halve the current requirements, but still have the same power requirements. There is no free lunch. 12 amps @ 220 volts is the same power as 24 amps at 110 volts. Using 220 volts just splits the current over 2 wires in stead of one, which can help but it's still the same amount power.

You really don't need to run all the motors on a variac. Rocco's plan, which is the same as mine, to have a switch for each motor and one variable control on one motor, is a simple way to handle the power.

You still need to deal with all that power through the house (garage) wiring and the outlet/plug you choose. I'll probably just wire it directly into the breaker panel in the garage and eliminate the plug.

12 motors at 12 amps is 144 amps @ 110v or 72 amps @ 220v. That's a lot of power, you'll be hard pressed to find an outlet and a plug that can handle that without turning to slag. Electric stove and dryer outlets are rated at 30 amps, 220v. Getting more capacity than that gets into some specialized, expensive hardware.

There are a bunch of flow benches running out there with 6, 8, 10 or more motors. What are you guys doing to get power to the bench?

[Tony]

Golly that is a lot of power.

Seriously though, vacuum motors are a great way to get going with a small to medium sized bench, but beyond a certain airflow requirement they are just too power hungry and inefficient.

A single large blower that is speed controlled will be quieter and much more power efficient, and probably more reliable long term.

My current bench can flow around 500 CFM flat out, and if I ever needed more than that, I would probably use an old Vortech centrifugal supercharger driven by a three phase motor and electronic drive. Sure it would be expensive, but I really cannot see a more efficient way to do it, and at very high power levels, efficiency becomes much more important.

[86rocco]

True, a variac is not required but according to Ametek product literature, varying the voltage is IS the preferred method of varying motor speed, other methods may reduce the motor's longevity. Besides, I wanted one for some other stuff I've been working on anyways so, now seemed like the time to put out the extra cash.

[blown928]

[Powerman]

It would seem to me that it would better to use frequecy control of the motors?

I have a pressure transmitter coming to hook to a mini controller, the 4-20 ma signal drives a the bypass valve to regulate the depression.

[Tony]

Powerman, there are two fundamentally different types of electric motor.

The motors in small appliances are usually ac/dc brush type motors that run at fairly high Rpm. These motors only know about voltage, and will run at whatever speed the available supply voltage will allow. 50Hz, 60Hz, or pure dc voltage, the frequency does not matter for these motors.

Then there are induction motors that have a solid steel rotor and no brushes. These use a rotating magnetic field, and the operating speed will change directly with the mains supply frequency. There will be a definite exact operating Rpm on the rating plate. Proper wide range speed control requires BOTH the voltage and frequency to be changed. You really need an electronic variable frequency drive to do it properly.

[Tony]

[Thomas Vaught]

I deal with the Vortech people daily and have taken apart many of their units. Not much to them really to repair. The bearing preload on the front input bearing seems to be the only critical spec.

Tony, Vortech makes smaller units for 4 cylinder and 6 cylinder applications that have less parasitic losses (belt drive) that would still
move a lot of air with a few inches of HG depression.

The old famous C.J. Batten head porter company at one time had three flow bench stands that worked very well.

Each stand used a stationary Paxton supercharger (like the Vortech stationary deal I posted the link to a while back).

They controlled the flow with a butterfly valve on the discharge side of the supercharger and with pulley ratios.

The units could move about 500 cfm of air at 28" H2O.

They had three orifice tubes for flow measurement per bench:
100 cfm, 200 cfm, and 400 cfm.

They used a 55 gallon steel drum for the settling chamber below the bench surface. From the drum the air went to the orifice tube and then on to the inlet of the supercharger. The discharge of the air was routed outside the building. The supercharger was mounted above the specific bench in a loft area.

You have a good idea there.

Tom V.

[Tony]

My current bench uses a single variable speed centrifugal blower, but the thing is physically rather large. The blower is two feet in outside diameter, has an eighteen inch diameter rotor running at around 6,000 maximum Rpm off a speed controlled 10Hp three phase motor. It is absolutely wonderful to use and gives very fine control independent of mains voltage fluctuations.

The monster could easily be replaced with a much more compact centrifugal supercharger. A three inch rotor running at 36,000 Rpm would generate roughly the same pressure (55" water 500 CFM).

If flow bench design number four ever eventuates, that would be how I would do it. The beauty of a supercharger is that flow maps and horsepower/Rpm curves are usually readily available, and the whole thing can be planned and engineered to give the desired final result. Vacuum motors are always a bit of an unknown quantity, and the results in the bench can sometimes be quite disappointing for the electrical power consumed.