Two Tech Questions - Blower Speed and Capacitor Leaks

[WQ9E]

JJ,

Do be sure and check your blowers to make sure that the motors are either shaded pole or PSC type because those controllers will NOT work with capacitor start motors.  The rating plate will normally identify the type; otherwise the internet is your friend to run down the specs.

[K5UJ]

1.  I don't know vy much at all about electric motors but intuitively for AC motors, it makes sense to me that slowing the AC frequency is the way to slow the motor speed.  Lowering the peak voltage may produce a desired result but I have a hunch it is one of those types of situations where everything seems okay but there are hidden consequences.

2.  Tom, I would forget all the slowing, damper blah blah business and simply run a second blower in a baffled, filtered, box across the room indoors in winter and switch the duct over to the outside unit some time in early spring.  Now your fears about breaking glass, bringing the fils up slow and so on are all moot points (although my method may not be as interesting).  Sometimes the simple way turns out to work the best.
I plan to make a big plywood box and line the interior with a lot of foam rubber and such with one side holding air filters, mount the squirrel cage inside it and get it about 20 feet away from the ham shack in a far corner of the basement.


3.  Yes such a cap would indeed be used in a flash pack for a pro photo shoot.  This isn't for a little Instamatic flash cube.  This is for touching off a bunch of big flashes in diffusion tents to light up a pro studio shoot.  Igniting those lights takes a lot of voltage and thick low resistance cables.

Rob

[K1JJ]

JJ,
Do be sure and check your blowers to make sure that the motors are either shaded pole or PSC type because those controllers will NOT work with capacitor start motors.  The rating plate will normally identify the type; otherwise the internet is your friend to run down the specs.

Thanks, Rodger -

Well, two do NOT use capacitors and the other one does. Unfortunately, the biggest one that I wish to slow down the most is the capacitor start motor, darn.

So, what wud be the effect with using the controller with the cap motor - wud it simply act the same way as when using a Variac and stall at slower speeds and heat up?


BTW, for the non-cap motors, do you think those will run very slowly without stall or excessive heat using the controllers? If so, at least 2/3's of the problems here are licked...

T

[K1JJ]

Rob,

You will be surprised how much forward pressure is eaten up from turbulance when using a long 20' run like that. The problem is unless you use fiberglass or other soft material for the ducting,  the ducting will amplify the motor sound like a trumpet. The noise comes out of the tube outlet.  Using plastic or fiberglass flexible ducting has ribs inside that will cause some air turbulance, but is quiet outside.  Solid PVC or metal ducting is very loud. My fiberglass 4.5" flexible ducting is 24" long for the 4X1 AM rig and about 36" long for the 8877 linear. Very efficient and quiet.

I'll have to think over all this info and sleep on it.

TNX, Guys!
T

[N4LTA]

My concern all along is that the motors in question are squirrel cage motors  and I assume since these are mounted outside, that they are not a Rotron fan with a shaded pole motor.

These can be slowed down about 5-15% or more by lowering the voltage - but at some point the current will rise and the motor goes into the same mode as during starting - And it looks like he is at that point. It appears that this is happening.

Some small motors are marked "impedance protected and this means they have a high enough internal inpedance that they can be stalled all day and not burn up.

A variac is probably the best way to reduce the voltage - better than using a triac system because they generate nasty RF noise in some cases.


I believe that a damper is the best solution - if a variac is not working then the motor does not like reducing voltage and a VF drive is way too expensive

With a Rotron that probably has a shaded pole motor or for a PSC motor a triac controller should work OK.

The thing that I was trying to state earlier - is that any motor running a fan has a cube law relationship - a change in speed makes a cubed difference in air flow - so you don't need to change the speed very much to make a huge change in air flow.

Dropping the motor speed just 10% make a lot of difference (27%) in noise and flow.

Pat
N4LTA

[WQ9E]

Thanks, Rodger -

Well, two do NOT use capacitors and the other one does. Unfortunately, the biggest one that I wish to slow down the most is the capacitor start motor, darn.

So, what wud be the effect with using the controller with the cap motor - wud it simply act the same way as when using a Variac and stall at slower speeds and heat up?


BTW, for the non-cap motors, do you think those will run very slowly without stall or excessive heat using the controllers? If so, at least 2/3's of the problems here are licked...

T

JJ,

As long as the other two are either shaded pole or PSC (permanent split capacitor) the controller should work just don't slow them down so much that there is insufficient air flow to cool the motor and make sure they do not stall.  I wouldn't slow them more than 40% or so.

Do not try to use a controller like this with a capacitor start or cap start/run motor.  You really don't want to slow down a motor like that by starving it for voltage either since that abuse is going to heat it up.  It is the same bad thing that happens when central AC units are subjected to a brownout and the current draw will increase as the compressor is slowed and will begin approached locked rotor current.  Your only protection is the overload protector will hopefully prevent burnout.

Again, there is confusion between power and air flow in this thread.  See the following website from Ametek which clearly delineates the relationship between fan rotational speed and the resulting flow, pressure, and power  required:  

http://www.ametekaerodefense.com/technical-corner/fan-laws.cfm

[N4LTA]

Yes - I stand corrected on the airflow.

If you double the speed, the flow doubles, the static pressure goes up times 4 and the power goes up by x 8.

Not being a mechanical engineer, the only thing I usually worry about is the power going up by X 8. I have seen a lot of burned up motors because of the 3rd law.

Pat
N4LTA

[WQ9E]

Pat,

And the third law is what results in so many burned up motors in home shop equipment.  In addition to radio I do wood working and I always cringe when someone tries to feed a tablesaw or shaper too quickly causing the motor to bog down which results in poor quality cuts followed by a much shortened motor life.  Drill presses often suffer from the same abuse.

 I was at a distant relatives house years ago and he was ripping wood on one of the "less low end" Sears table saws and he was feeding it so fast that several times the speed dropped enough to re-engage the centrifugal starter switch.  I tried to gently educate on the danger to himself and the saw but he informed me that everything was fine since he increased the breaker size in the feed line.   If  a bigger hammer doesn't fix it a bigger fuse will.  A few months later he called to ask some questions about my Delta unisaw because his 2 year old tablesaw had just burnt up the second motor so he wanted something better.  He is basically one of those clowns who could somehow break an anvil with a foam mallet but somebody has to keep Baldor in business.

[K1JJ]

"If you double the speed, the flow doubles, the static pressure goes up times 4 and the power goes up by x 8."

A good relationship to remember.

Yep, static pressure - it's the same thing with sailing.  Double the wind speed and the pressure on the sail goes up four times.  If the wind goes up four times, the pressure on the sail goes up sixteen times!

I learned the hard way. When the wind is at 5 mph, the pull on the sail rope is about 5 pounds. Then the wind gusts to 20mph - the pull is now 80 pounds... yikes!  Could break an arm if the rope gets snagged around your wrist.


Also it's the main reason it takes so much more power (non-linear) to increase speed in a dragster or airplane due to air drag.

Interesting stuff.

T

[KL7OF]

Tom...consider this in regard to your fans...You are using a small,  hi pressure-low volume type fan(s)  ......How about a low pressure-hi volume setup with larger ducting and much slower fan speeds...This system is also more damper control friendly..lots of slow moving air as opposed to a small volume at high speeds...certainly less noise..... good luck...Steve

[KL7OF]

A slow speed belt driven furnace blower in a plenum box outside with 4 inch dryer ducts plugged into the plenum.....a volume control damper in each duct ....dampers controlled on the shack side of the wall.

[WA1GFZ]

Gee tom,
I think I still have the motor off my old furnace burner. It is 1800 RPM but has a 1.2 shaft. I used it as a circulation blower for my passive solar before I went to a bigger unit to move more air.  You can have it if that helps

[N4LTA]

Roger,

I know what you mean. I was the engineering and maintenance manager at a very large printing plant until I opened an engineering firm 17 years ago. You wouldn't believe some of the abuse that I witnessed.

An induction motor is made to run in the slip zone. If you slow it down below that - it goes into the starting mode - it gets extremely inefficient and all that unused energy gets turned into heat inside the motor. Insulation gets overheated and the motor fails early.

After an induction motors gets running at speed - it is essentially a transformer and the resistance seen at the terminals is the amount of power it is delivering. The more load the lower the resistance.

Pat
n4lta

[K1JJ]

Thanks for the offer, Frank, and the suggestions, Steve.


It takes a special blower to produce the pressure needed to put a big airflow thru the tube. The standard "ventilation" blower falls apart with a big back pressure. The rotor needs to be close to the housing. You can test it by putting your hand over the outlet while running and see how it acts.  What I have now has good pressure and is a good sized blower, maybe 10 pounds. 

I already use 4.5" flexible fiberglass ducting and the final tube box is a small sub-chassis  - small for best efficiency. I've spent a lot of time with configurations and feel I have the best compromise. It works great with lots of quiet air at 1/2 to full speed, but I just need that extra slower speed (less than 1/2 speed) when the air outside is below freezing.  The room gets cold too quickly and the tube is overcooled.  I'm still thinking a damper or speed control is what I need.  I'm not about to change the overall air system config at this point.

Still thinking...

T

[WQ9E]

JJ,

In addition to the clearance all other things being equal the higher wheel speed provides more ability to overcome back pressure.  For home HVAC quietness and thus lower speed is a concern. 

I use a furnace blower motor in my homebrew air cleaner/sanding table dust collector for the wood shop and it sucks air in through a pair of large deep pleated filters with pre-filters ahead and it does a good job of removing particles from the air.  But it wouldn't do well with high back pressure or intake suction.  My central dust collection system does a good job of pulling air through the tool and connecting tubing while making plenty of noise to let you know that it is working!

[Mike/W8BAC]

Tom,

Would it be possible to find a DC motor to replace the AC fan motor? I'm sure you gave it some thought. Here is a link to just one. http://www.grainger.com/Grainger/items/3HMH7. good luck

Mike

On second thought, how about reversing the flow? Would it be possible to draw air from top to bottom over the 4X1's and pull the hot air out, instead of in, during the winter? DC motors are often reversible by changing the polarity or just reverse the flow on your AC motor.

[w8khk]

Tom, instead of slowing the motor, or adding a damper to limit the amount of cold outside air you bring into the house, have you considered adding a duct from the house to feed warm air into the blower in the winter?  Perhaps a simple hinged flapper valve could be fashioned, that would either allow all outside air, or all inside air to be ducted into the blower.  A filter on the inside vent could limit the noise.  With this solution, you would be aiding, instead of competing with the central heating system.  Just some random thoughts, hope they are helpful.

[K1JJ]

Hi Mike,

Yes, the DC motor wuda been a better solution for speed control. Though, I wanted to stay with commonly available AC motors (flea mkts) cuz they have a limited life out in the weather. One has been out there about 8 years now.  I'll take a look at the motors listed anyway...

Adding a second outlet into the house to grab pre-warmed air is an idea, though it wud require another 4" hole in the house and the added noise of incoming air - along with the motor noise. The worst air noise comes from the intake, I've found.  Right now I'm at the limit of noise for AM - it's still just a hiss and low enough for audio processing.

The cold air problem in the winter is a bug-a-boo and I will give your idea some more thought.


Rick -  Yes I did have the air reversed, sucking out before. It cost me an expensive tube. You see, the bottom seals need the air blown directly on them for cooling. When air is pulled from top to bottom, the air basically misses the seals (hidden) and they overheat. In addition, the air is now hot from the anode cooling when it reaches the bottom seals.

I reversed the air flow with air blown directly on the seals on its way to the anode. The seals now stay luke-warm, even at room temp.

While the max rated exit air temp is probably about 250F,   I see about 90F during winter and maybe  130F max in summer - so the tubes shud last a long time, seals-wise..

T

[WA1GFZ]

Tom,
A DC motor will make plenty of brush noise.

[W1RKW]

This is probably overboard and maybe not possible but I wonder if it is possible to drive an AC induction motor with a high powered audio amp and signal generator then vary the frequency of the sig gen to adjust motor speed. Sort of like a homebrew motor drive.

[k4kyv]

Whether or not an a.c. motor can safely be slowed down by reducing voltage depends on the type of motor.  This works with the shaded pole motor. These motors do not need a capacitor to start or run. An example is the old fashioned ceiling fan (pre- about 1985, with cast iron motor case and coils visible through the ventilation holes). These were slowed down with a "speed coil", an inductance wired in series with the motor and a.c. mains voltage.  The inductive reactance reduced  the voltage to the motor without generating the heat that would result from resistors, and it was less bulky than a variable transformer.  A series capacitor will also slow down these motors.

My Gates BC1-T uses a pair of motors to drive the cooling fans.  These motors are 120v, but two in series are fed with 240 volts.  I chaged the tap to feed these motors with 120 volts (60 volts per motor) and they slow down beautifully.  Neglible noise, but they still move enough air to cool the interior of the transmitter and the motors do not overheat.

The other type of simple a.c.  motor is the Permanent Split Capacitor motor.  These motors run with a capacitor. They are more efficient than shaded plole motors, but do not lend themselves so easily to speed reduction by reducing voltage. Variable speed motors usually have separate windings for each speed.

An a.c. motor may be  compared to an aircraft.  Reduce the voltage or the speed beyond a certain point, and each stalls out.  The plane drops like a lead weight, while the motor comes to a halt and the windings burn up.

Also, there is the synchronous shaded pole motor.  These use a permanent magnet rotor, and rotate in synchronism with the a.c., but are capable of very low torque.  They are used in such applications as electric clocks and turntable motors.   



Permanent split capacitor motor

Shaded pole motor

[WBear2GCR]

Brushless DC motor?

But anyhow, with a DC motor you could use a thermistor or similar to actually sense the temperature at the toobe, and thereby control directly the speed and airflow via a simple Vreg circuit (yes it needs a lower limit to keep it from stalling) back out to the fan motor.

The smart-guy's way would be a digitally controlled big size servomotor - of course. But I have no clue how to make dem dere fan-c thangs woik.

                          _-_-bear

The other thing that is out there are commercial speed controllers, including big uns for 3 phase industrial gear... but they make 'em for smaller motors and you can dial up any speed, and control them off an external source (usually a voltage) check ebay... we're not talkin those things Sears sells with a knob and an AC outlet now... I forget the precise name they are called by... <brain cramp>

Which brings up yet another idea, steal the motor off a new fangled washing machine (from the dumpster of a big appliance store, of course) and fire it up with an ebay 3 phase controller - they are 3 phase, and powerful and run from dead slow up through rather quick...

[N4LTA]

VFDs (variable frequency drives) or Inverters. Expensive.

For precision use - they run permanent magnet synchronous motors.

Pat
N4LTA

[W1RKW]

A slow speed belt driven furnace blower in a plenum box outside with 4 inch dryer ducts plugged into the plenum.....a volume control damper in each duct ....dampers controlled on the shack side of the wall.

Yeah, something like this but on a smaller scale.  How about a setup with stepped pulleys like those on a drill press?

[WA1GFZ]

I worked for a company years ago that used a lot of allen bradley controls on variable speed motors. They were controlled with a pot that I suspect could be modified with a thermistor(s) They were DC but I bet they make them for 3 phase motors. The 3 phase controller I have has a pot to adjust the speed. Bob N9NEO designed motor controllers. 3 phase has a wide range of control.
Bob's idea with the pully system sounds cool. You could set up a bike shifter to slip the belt up and down the stack.