Two Tech Questions - Blower Speed and Capacitor Leaks

[K1JJ]

Hola,

Question 1:   I have a 5kv @ 55 uf  "photoflash" filter cap in my 3KV supply, about the size of a loaf of bread.  I looked at it today and saw a small puddle of oil, maybe 2 teaspoons worth, on the top surrounding the HV posts.  I was able to soak it up without spill. Why would the oil get pushed UP and out like that against gravity?  Is it maybe cuz the cap has a small internal problem with leakage/ heat and is boiling it out?


Question 2:  I have three squirrel-cage type blowers on the outside of the house for various amplifier cooling jobs. They are all controlled by Variacs using  120VAC.   Using the Variacs, the blowers will slow down with less AC voltage, but hit a certain minimum where they will either draw too mch current and heat up or they will slow down to a crawl/stall.

Will they work at lower speeds using a PWM AC signal? I think a standard light dimmer uses PWM and I wonder if that is a better solution than a Variac?  One of the blowers uses 8A, so may be too much for a smaller dimmer unit. How big are they rated?  Anyone ever run an AC blower on a dimmer with slow speed success?

Thanks.

T

[N4LTA]

The AC motors are made to run on 60 Hz . Their speed is controlled by frequency. The rated output speed is 1800 (synchonous speed) for a 4 pole motor minus the slip (which is controlled by output torque being demanded by the fan)

Squirrel  cage motors run at slips of 2-10 percent or so. If they slow down any more (due to torque ) - they quit working

Controlling an AC motor with voltage will overheat it.

You can control a motor with PWM but you must keep the equivalent volts/hz constant. If the motor has internal fan cooling - It may not cool itself properly at low speed. A light dimmer will not work. Most are Phase controlled Thyristor circuits.

The volts/hz on a 120 volt 60 hz motor is 2.0  If you drop the motor to 30 hz - you need to drop the terminal voltage to 60 volts.

That is why most 380 volt -  50 hz three phase European motors work just fine on 460 volts in the US

The volts /hz is  7.6  in both cases.


If the capacitor is leaking - I'd change it  as it is most likely failing.


Pat
N4LTA

[KA1ZGC]

Question 2:  I have three squirrel-cage type blowers on the outside of the house for various amplifier cooling jobs. They are all controlled by Variacs using  120VAC.   Using the Variacs, the blowers will slow down with less AC voltage, but hit a certain minimum where they will either draw too mch current and heat up or they will slow down to a crawl/stall.

Your current is rising because the motor's load is diminishing with the decreased voltage (less voltage = less air flow = less back pressure). You might try restricting the exhaust aperture (using an output damper of some kind). Added mechanical resistance will translate to added electrical resistance in the motor, decreasing the current drawn.

As with everything, there's a point of diminishing returns, but if you're almost at the point you want to be now, that may be sufficient bias to get you the rest of the way.

With regards to question 1, it seems logical that the oil is expanding when heated. Unfortunately, that means it's also contracting when cooled, and could be pulling who-knows-what into the case with it. I would start hunting for a suitable replacement. Just like airplane fuel, it only takes a small amount of moisture to ruin your day.

My US$0.02.

[N2DTS]

Odd about the oil.
What is a photoflash cap?
Surely nothing that big would be involved in a camera?

It must be getting warm, from the inside or from the outside...

Good question on the dimmers, they make some bigger ones, and ones designed for motors.
At work we have an exhaust fan on the roof, looks like a 1/2 hp motor at least, its on a dimmer...
Been running for 20 years...

I have variacs on the blower on the 813 rig, and in the 4x150a mod deck.
Odd, but when the rig warms up, the speed increases....so I never did anything about it.
It starts off barely moving, and as things warm up, it speeds up and moves a good amount of air.

A dimmer might be much better, but might generate rfi noise in the shack....

Brett

[N4LTA]

Your motor is stalling because you are dropping the voltage on the motor to the point that is hasn't got enough torque to run. When an AC motor stops running and falls below slip speed (for and induction motor )  or synchronous speed for a synchronous motor - it is essentially a low resistance "short".

AC motors  (induction and related types) can only be speed controlled with frequency - not voltage.

Most AC industrial motors are speed controlled with PWM inverters where speed control is required.

Or - Maybe all the stuff they teach in engineering school is wrong?

[WQ9E]

Your photoflash cap is in the class of energy storage caps and they have to be de-rated to live in filter service.   It seems the rule of thumb is around 50% of rated voltage but in the case of high continuous current, as you would have in a power supply for a class C final, further de-rating may be needed.  If the cap is noticeably heating up then it isn't going to survive long and you should probably start looking for a replacement more suitable to the operation.

Photoflash caps are used with large photo flashes that use xenon discharge tubes.  In design operation, the cap is charged from a filtered DC supply and then discharged through the tube to create the flash.  Filter usage is much more stressful than the original use because it is being subjected to a high amount of AC ripple.

 

[KA1ZGC]

AC motors  (induction and related types) can only be speed controlled with frequency - not voltage.

I guess that 240V blower we're running at 120V in a Harris FM-25K we restored last year didn't get that memo, because it's running at half speed.

Or - Maybe all the stuff they teach in engineering school is wrong?

Either that, or our blowers went to different schools.

[WQ9E]

No one told the Rotron fan motors I am using on my Desk KW pedestal.  These are nominal 120 volt motors running in series across 120 volts to lower the speed and provide more air and less noise than the original.

AC motors are definitely not my specialty but I believe that most of the heavier duty split phase motors (including those with capacitor start; cap run and start; and external starter are the ones that will cause problems with lower voltage as they will draw more current as they attempt to reach design speed (tied to the power line frequency).  Most of the smaller motors are of the shaded pole variety. 

I would not advise the light dimmer solution as it create an ugly waveform which will result in a very "buzzy" fan.

If you subscribed to ER look for the article on the Heathkit KL-1 Chippewa because the author experimented with various types of controllers to reduce the speed and noise and you can learn from his experiments and avoid reinventing the wheel.  I decided to live with the noise from my Chippewa and with it located on a desk with shelving above I don't find the noise to be too bad.  For my homebrew amp, the blower is located in its own box and connected to the amp with air hose.  The box has air filters which both control the noise and provide clean air to the blower and amp.  With the price of tubes, too much air is far better than too little.

[K1JJ]

Thanks for the info, guys.

1) I just replaced the leaking cap with another - all set.


2)  Yes, I've been running all my blower at various voltages, Variaced down for years now.  The reason is simply to lower noise.  I can get away with about 1/2 voltage before the motors start to heat up too much. There is often a sudden "drop-off" voltage point where the motor dies.  I've not lost a motor yet from heat damage, though once had one shut off from its temp safety circuit.

The problem now is I am using outside air to cool the amplifiers and sometimes it's very cold, like 10F. So I can get away with less air to cool. But the air exits the tube at like, 45F degrees or less, so it's getting the room cold...    If I could find a way to run these at 1/4 speed, I'd be FB in the coldest days. Otherwise I'm OK in the spring, summer and fall.

OK on the PWM circuit to control the speed. I'll have to think of the easiest solution for this. At least I avoided trying the dimmer idea - tnx.

T

[KA1ZGC]

Now I'm really thinking "damper".

You're actually trying to accomplish two things, here: first is lower blower noise, second is reduced air flow when the incoming air is borderline-cryogenic.

You've already met your first objective with reduced voltage. Applying a damper between the blower and the rig will not only reduce your airflow to where you want it, but will also mitigate overcurrent caused by the reduced voltage.

No additional circuitry required, unless you want to get really hi-tech about it and drive the damper with a synchro-motor.

I'd be on it like hair on soap, but that's just me. 

[N4LTA]

I'll tell you what I learned in engineering school and in 35 years of experience and then you tell me where I am wrong and how your motor is working?

We are talking about a common squirrel cage induction motor - not a wound rotor induction motor or a high slip induction motor - correct?

The stator in this motor has windings that develop a rotating magnetic field - With a three phase motor it is a natural occurance - with a single phase motor you need a capacitor in one winding to phase shift one winding to develop a rotating field  -  

The field rotates at 1800 RPM for a four pole motor  or 3600 RPM for a 2 pole motor  - slower for higher poles.

Some lower cost single phase induction motors do not develop a rotating field. They developing a pulsing field at the frequency of the applied voltage. The rotor aligns to this pulsing field.

The rotation of the field has NOTHING to do with motor terminal voltage - How could it? - Please explain ?

The rotor (field)  is an inductive device - a transformer and current is induced into the rotor which induces another magnetic field - That is why it is called an induction motor.

The rotor magnetic field produces torque in the rotor and the rotor spins. The initial starting condition draws about 6 time the normal terminal current, as the motors "catches up" to the rotating  field, terminal current drops.

At no load - The rotor approaches the speed of the rotating field  - but can never catch it - the difference is called slip. Different induction motors can operate at different levels of slip - but unless the motor is a wound rotor motor the slip is limited to about 5% . As the motor is loaded, slip increases until it reached break down torque  - at that point  - current rises rapidly up to about 6 x rated current when it stops.

With the cheap single phase motors (not a capacitor run type) - the ones with the pulsing field develop maximum torque at 10% slip (about 1620 RPM for a 4 pole motor). Torque drops off quickly at slips above and below this range

Possible the motor you speak of is far over-rated for its required us and is running at a torque far lower than design and at a high level of slip. Fans are cubed law devices if I remember correctly - so a slight reduction in fan speed  - results in a large reduction of air flow.

Reducing motor voltage will reduce motor torque causing slip to rise. When slip rises the maximum rating - the motor will pull excessive current and overheat.

It works great if you want to reduce speed 2-5% or more if the motor is over-sized - after that you begin to overheat the motor.  In low cost single phase motors you can probably get a 10% speed reduction or a little more before current begins to rise substantially. Since almost all motors are fan cooled, then reducing speed also reduces the motor cooling by the cube.

BTW - your fan motor running at half speed is developing   1.2 x 1/2 x 1/2 = 12.5% the airflow rated of full speed.

That's another basic law of physics I am sure you won't agree with either?



Pat
N4LTA

[N2DTS]

I suppose the hot setup would be a thermal controlled fan, set to exhaust a nice warm 80F air into the shack in the winter, warm the place up while you have fun.
In the summer, exhaust the hot air outside.

In the short term, cant you just not strap the 4-1000's and have a quiet muffin fan moving some air?
There were rigs built using 4-400's with little air, the globe king 500 had a fan but it did not do anything...
The 30K1 used a 4-125 with no air and strapped it (bright orange plate), and I put a 4-400 in without air and had no problems. The 4-1000 is likely fine without air if the plate shows no color...
So some gentle air to warm the shack should be ok, no?

Brett

[K1JJ]

Thom,

Yes, I like the damper idea. In fact I was just about to add one to each ducting system to shut the inlets off completely when not in use. Cold air still seeps into the shack otherwise...   So adding one to reduce the airflow when running reduced speed and provide more back pressure sounds logical to me.  Tnx for the idea.

Brett,

I was talking with Chuck / K1KW about the risk of running cold air to cool the finals of both my 4X1 AM rig and the 8877 linear. (Fabio and Dr. Love)  Chuck is an ex-Varian (Eimac) engr.   It seems that as long as I bring up the filaments SLOWLY, starting with a few minutes with 1/4 reduced voltage, I should be OK, more or less.  I set it up this way for both rigs and ran some tests. I am able to ramp up the fil pins / seals slowly and maintain them at 125F even with cold air blowing by, so this is good. There is some temp gradient from the bottom to top of the glass envelope, but reasonable.  The temp delta (range) of full operating seems about the same whether the inlet air starts at 10F or 70F.  (It will rise about 30-40 degrees F rom inlet to outlet)  Of course, all of this will disappear in spring, summer and fall with higher temps.  But for now, I will try the damper idea to reduce both air flow and give the motor a better load when running slower.

T

[WQ9E]

JJ,

See the following for a speed controller and the type of motor it can control:  http://www.electricmotorwarehouse.com/PDF/kb_electronics.pdf

As to CFM, for a standard fan somebody better sue Grainger because their specifications present a far different story on fan speed versus CFM reduction and for their multi speed large fans the reduction in CFM is shown as being almost perfectly proportional to the reduction in RPM.

Also JJ if you want to play with some "what if" scenarios there is a handy online blower calculator that will help you:

http://www.forestryforum.com/members/donp/Blowercalc.htm  It makes some simplifying assumptions but will get you in the ballpark. 

[K1JJ]

Outstanding Rodger, thanks!

I will give them a call tmw and try one out.

Pat - what do you think of these for my specific situation? (A diac drivng a triac)
They say there's a minimum speed setting adjustment. Based on the circuit they show, do you think it will run the motor at 1/4 speed or less, or is there still a limitation?

T

[W7SOE]

How about a damper, actuated by a servo motor, controlled by a microcontroller.  You wouldn't have to do any fancy PID control stuff, just set the damper to one of, say, 10 positions based on the exit temperature of the air (you pick what you want it to be).

I like to try to fix everything with a micro and a bit of code.  Except my r-390a, art-13, sx-42, oh, nevermind.  ;-)

Rich

[KA1ZGC]

I'll tell you what I learned in engineering school and in 35 years of experience and then you tell me where I am wrong and how your motor is working?

...and I'll ignore you, because it is working. No amount of education or experience trumps a solid, physical fact.

We are talking about a common squirrel cage induction motor - not a wound rotor induction motor or a high slip induction motor - correct?

Well, gee... you didn't ask that question before issuing the broad statement that blowers cannot be slowed down by changing the input voltage, then following it up with a smart-ass comment about everything taught in engineering schools being wrong, did you?

Next time, you should probably get all the facts before dismissing any of them.

That's another basic law of physics I am sure you won't agree with either?

Don't look at me, I'm not the claiming that something that's working in real life is physically impossible.

[WQ9E]

I believe maybe some of the earlier confusion is between blower power requirement and CFM.  If you cut the blower wheel speed in half then the theoretical horse power requirement is now 12.5 percent of the previous but the air flow is certainly far greater than 12.5 percent of the original.

In the real world efficiency comes into play so the actual power draw will not be an 8 times difference.  And since the blower is less than 100 percent efficient and is working into back pressure the exact reduction in air flow would be far easier to measure than to calculate.

It has been a long time since I spent some time in some introductory engineering classes (before switching to business)! 

[N4LTA]

Fan Law 3

The third fan law relates the total or static air power (and the impeller power), W, to the fan rotational speed: Power, W, is proportional to the cube of the fan rotational speed, N.

(W1/W2) = (D1/D2)5 (N1/N2)3 (ρ1/ρ2)

Looks pretty clear to me       Power  is proportional to the cube  of the rotation - the fan laws are not intuitive.

 

BTW - I answered correctly that his squirrel cage motor was slowing down and drawing high current because it was below the breakdown torque.

You came up with the hocus-pocus BS about the motor drawing more cureent because it was less loaded.

The question was about a squirrel cage motor - not a Rotron Fan - not a PSC motor which I suspect that you have.

"Question 2:  I have three squirrel-cage type blowers"


You essentially said I didn't know what I was talking about - I answered back.


Something that "works" trumps physics and enginnering only in in the world of the ignorant.

The things that might appear to do so are always because the data is bad or the problem is not accurately described.



You drop the speed on your fan by 10% by lowering the terminal voltage. You drop the power input and airflow by 27%. Things quiet down and the motor power input drops by 27% so even if it is operating in high slip , nothing bad happens.

So someone says "working" trumps engineering.

Then the same dumbass tries it on a loaded 10HP pump - smoke and fire

A real good search through the Rotron Data does not mention controlling their fans with voltage for whatever reason. The suggest that you use a brushless controller if you need speed control.

Enough of this BS

Pat
N4LTA

[KA1ZGC]

You came up with the hocus-pocus BS about the motor drawing more cureent because it was less loaded.

Unloaded blowers (or motors of any kind) draw more current than loaded ones do. That's a fact.

You essentially said I didn't know what I was talking about - I answered back.

No, I said your all-encompassing statement about blowers not slowing down when the input voltage is dropped was inaccurate. You got all emotional and took it as a personal attack, then started responding with personal attacks. Your problem, not mine.

The things that might appear to do so are always because the data is bad or the problem is not accurately described.

...or the problem wasn't accurately understood by the person claiming that at least three different people on this thread couldn't possibly be experiencing what we obviously are. Again, your problem, not mine. Don't blame everyone else because you went off half-cocked before you had all the data.

Then the same dumbass tries it on a loaded 10HP pump - smoke and fire

Calling me a dumbass doesn't add validity to your position, and is completely uncalled for.

Enough of this BS

Agreed. Now stop harassing me already.

[W1RKW]

You could run it full blast then dump the extra air with a waste gate and divert un-needed airflow outside, sort of like an automotive turbo charger yet maintain motor speed and load.  You could control it thermostatically and by duct pressure so you don't have to adjust it all the time depending on outside temps.

[W7SOE]

I would take a different approach than those involved in this lovely thread.  I would try to understand nothing.  I would set the blower speed to some comfortable level and then adjusted a simple damper (metal plate over the opening) until the exit air is 70F or whatever.

You could do this with a servo and micro or do it the way a reasonable person would, and have a little lever on the wall that you probably had to adjust four times a year.

ORRRR, you could study loaded motor thermodynamics blah blah blah.

Rich

PS  You should also send me pictures of blowers mounted to your house which I will use to prove to my wife that am really quite reasonable.  ;-)

[K1JJ]

PS  You should also send me pictures of blowers mounted to your house which I will use to prove to my wife that am really quite reasonable.  ;-)

Hi Rich,

Sure thing  - here's a link to more pics on page 2 of the thread:  (not XYL approved, before white painting)
http://amfone.net/Amforum/index.php?topic=22073.25
The blower outside, even at full speed on 240AC, is very quiet in the shack, enough to run my 6-band audio processor on AM. When it was in the shack, I couldn't even run unprocessed audio without a racket. It's a great way to go.



Pat,

Let's see if we can move on from here... do you feel these diac driven triacs (link below) would do the job for my squirrel-cage type AC motors?

BTW, I do notice a huge drop off in air flow when the Variac is throlled back just a little. A cube function for power vs: air flow wouldn't surprise me at all.

http://www.electricmotorwarehouse.com/PDF/kb_electronics.pdf



Bob - Keeping the blower at full speed is very noisy, even when dumping air. A certain amount of noise still comes thru the ducting even though the motor is acoustically insulated from the wall and the ducting is fiberglass. The best  compromise is running the motor slower.

T

[KA1ZGC]

The blower outside, even at full speed on 240AC, is very quiet in the shack, enough to run my 6-band audio processor on AM.

A full-speed 240VAC blower mounted outside is just enough to run your 6-band audio processor? You're my hero! 

[K1JJ]

Hey, I wiz out banging beaver while your'e was in Engrish crass studying centence struckchure - wada ya want from my?