Depression control via motor speed?

[86rocco]

How effectively can one control depression by controlling the speed of the vacuum motors? I have a variac that I'm thinking of using to control the motors, in place to control valves, is viable? What are the pros and cons?

[gofaster]

I am currently using SuperFlow's depression control in conjunction with their FlowCom data acquisition and Performance Trends Software. It holds depression at within 0.07" or less under, never over. It's more accurate than manually adjusting the depression. On my new project I am using Audie Technology's depression control, with their data acquisition and software. I have been told that it works better than the SF unit, but I won't know until the bench is done.

[84-1074663779]

A variac to control motor speed is a GIANT step in the right direction.

It will cut down both noise and power consumption, as well as giving you a very wide and stable adjustment of airflow.

The variac will be almost indestructible, whereas the clever electronic speed control boxes are renowned for sometimes spontaneously blowing up.

Suitably huge variacs can be found on e-bay.

[cboggs]

I have 9 motors in my bench, .. what would be needed for
a proper variac ?????????

Curtis

[86rocco]

I was thinking of only running one or two motors off of the variac and switching the remaining motors individually. Turning on the motors one at a time will give very coarse depression control and the variac will let me fine tune it, hopefully with considerable precision.

[DaveMcLain]

On my flow bench I'm using the Superflow Flow Com and motor control with the the Perf Trends software on a very inexpensive computer that I put together out of spare parts. The whole system works excellent and is very easy to use and very repeatable.

I would try the variac idea and see how it works. I think you might have a hard time finding one that can handle the load of the motors however. I have one and I think it's only rated to handle about 10 amps. Mine is quite heavy and it was pretty expensive, I think about $60 from MCM electronics. I bought it about 10 years ago and I use it when I work on tube guitar amplifiers.

One thing I like about using the motor control is the fact that the motors are no longer screaming along making lots of heat and noise when you are at low lifts of flowing something small. Now they are crusing along and they are much quieter, suposedly they will also last much longer, I wouldn't doubt it..

[86rocco]

My variac is a 20 amp 0-140 VAC output so, it's easily capable of handling at least one motor and two if I'm careful not to go past about 85%.

[Shawn]

[Terry_Zakis]

I don't think you could afford one to run all of your motors from. I had search tools set up on eBay for over a year, and finally found a 220 volt, 30-amp variac for several hundred $$. Big and heavy, the shipping alone was over $80. It had been used for lighting controls, and is made by STACO.

I was also able to find a smaller 110 volt, 20-amp model, that has a motorized operator, in the hopes of working out a depression control. This one was made by Superior Electric.

Superior Electric and STACO are two reputable manufacturers of these larger variacs, so you may want to set up some search tools by their names, and for variac, and variable autotransformer.

As someone stated previously, I think the best bet is to use switches to turn off motors which aren't needed, and then use the variac to control test pressure on only a few motors.

Terry Terezakis

[84-1074663779]

Don't overlook the possibility of buying a three phase variac, which is really three quite independent variacs mounted one behind the other on a common control shaft. The three input windings could all be all run off the same phase, and the motors grouped to run off each of the three outputs.

I have a three phase 15Amp variac here, but I don't use it to run my flow bench. It would be quite practical to parallel up the three inputs and run them all from one phase, and use it to control three separate loads with a combined current of up to 45 Amps.

Another trick is to immerse the whole variac in a tank of transformer oil, and the greatly improved cooling provided would enable the "air rated" current to be almost doubled quite safely.

Another way would be to use one variac per motor (or pair of motors) and run some sprocket chain around the whole lot. Lot's of ways to use variacs. True they are big and heavy, and not exactly cheap, but they will work reliably forever.

[86rocco]

[Terry_Zakis]

Good points Tony! Yes, I've seen some of the 3-phase variacs and they too get heavy pretty quickly, but you are absolutely correct that they can be used for single phase. Out of the box idea on the transformer oil. I hadn't thought of that one, but that's probably a bit more advanced than most would venture into.

Excellent point 86rocco on the better ability to control test pressure, through only controlling one motor, and you save money that way as well.

One comment not mentioned in this go-around is that the brush life of the motors are maximized with the use of the variacs, as oppposed to solid state control. In speaking to the Application Engineering Dept. at Ametek, I was told that the solid state controls tend to eat up brushes due to the sine waves being clipped. Any insight on this Tony, as I'd guess you're an Electrical Engineer?

On controlling test pressure with that motorized variac I picked up, my plan was to use an industrial process controller, with a pressure transducer, to try and regulate the test pressure. Has anyone done this already?

I figured that if my drive motor was set up run the slowest speed, that this would reduce the change of getting the loop in a swing. Controlling only one motor as 86rocco suggested, should also help to reach steady state sooner.

I don't think the controller I purchased has PID tuning, but may be on the lines of a dead-band controller. So as long as I can put up with some error in the test pressure, it should stable out quicker.

Thoughts?

Thank you in advance.

Terry Terezakis

[Nick]

At the risk of sounding dumb I'll post. I know nothing about variacs. Could an arc welder be used for this purpose? I have an old Lindy (spelling?) It has a crank handle on the top to vary amps, from 0%-100% or anything in between. Is variac just a transformer?

Nick

[84-1074663779]

Nick, no welding control is a completely different thing altogether.

A variac is an ordinary transformer built in a round shape, with a movable wiper arm that can tap off a continuously adjustable output voltage.

Terry, I had not thought of brush erosion, and that is a most interesting point. Running the motors flat out all the time, especially with a restricted flow of cooling air is not going to be good for the life of the bearings, brushes, or windings.

Reducing motor power and Rpm is definitely going to reduce the wear and tear on the motors, as well as the heat and noise. I don't know what the design life of a vacuum motor is, but like most things these days it would not be built to last thirty years. Half an hour per day for three years maybe ??? That could be 500 hours planned life. I have no idea, but if you are really planning to punish your flowbench, make sure the motors can be accessed fairly easily.

The beauty of a variac is that it will withstand short term overloads and stray voltage spikes that often appear on the mains supply. Electronic controllers are far more electrically fragile. They can work reliably for many years without trouble, or they may suddenly fail without warning.

A variac will reduce the motor operating voltage to something less than the full mains supply voltage. The current and voltage switched by the rotating commutator will also be reduced along with the commutator Rpm. Everything runs at much lower stress levels and should last much longer.

An electronic control always works by switching the full mains voltage on and off at a very fast rate, in such a way that the average "on" voltage is reduced. While it will certainly reduce the motor Rpm, it does not reduce the peak commutator voltage and current in quite the same way as the variac does. Sparking from the brushes would be higher, and so would erosion and wear of both brushes and commutator. Good point Terry, I had not thought of it before.

I am a retired electronics design engineer, one of my specialties was power electronics.

[Jesse Lackman]

What I don't like about a variac is they are so big and clunky, how does one build them in a bench so they look good?

I'm using 7 5923 motors they are listed as having a 10.8 max amp draw. (I think the 10.8 amp draw is at max airflow/min pressure. There is also a max air watts rating of 450 per motor. That would be a total of 3150 watts which doesn't really jive with 75.6 amps.) But no matter the variac or variacs would have to be monsters wouldn't they?