temperature-controlled variable speed ac motor controller

Thread Starter

steamngn

Joined Aug 26, 2012
14
Ok gang (This means you LowQCab );)

Got the transformer and t-stats, and got this all wired up. All three speeds worked, and the fan was doing an awesome job cooling the enclosure.
Problem is, after about an hour of running the Hammond transformer got so hot it shifted on the mount and caused a short.
Wiring all looks fine and everything worked as expected - is this much heat normal? If so, what is the proper cooling used for this type of transformer?
It appears that the blower was on low most of the time it was running, so this would be the largest load on the transformer I would think....

Advice? ideas?
 

LowQCab

Joined Nov 6, 2012
4,023
Are You familiar with how to hook-up a Transformer in an "Auto-Transformer" configuration ?
This is where the Secondaries are wired in Series with the Primaries,
and depending upon the Polarities of the Windings, the Voltage will be boosted, or reduced,
by the Nominal-Voltage of each Secondary-Winding.
This is also called a "Buck-Boost" Transformer configuration.
This will allow You to subtract a total of ~44-Volts from the nominal 120-Volt supply, resulting in 76-Volts.
This arrangement will run substantially cooler, but the lower speeds probably won't be quite as low.

You can also insert a ~100-Ohm, ~10-Watt Power resistor in series with each of the Secondary-Windings,
This will tend to "move" the Heat generated from the Transformer-Windings to the Resistors.
You may need to experiment with different values to get the Fan-speeds that work best for You.

"" got so hot it shifted on the mount and caused a short ""
What was the nature of the Short ? visible, or internal ?
The Transformer should do just fine below about ~80C.
Any type of small Fan blowing on the Transformer should reduce the Heat quite a bit.
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strantor

Joined Oct 3, 2010
6,782
What is the reason for not running 100% speed all the time? Is it to decrease noise? Save electricity? If so then, unless I'm missing something, the transformer with shunted windings idea (as well as the one with power resistors) is going to consume the same or more power than just running full blast all the time.
 

Thread Starter

steamngn

Joined Aug 26, 2012
14
@LowQCab I've heard of (and have seen) buck-boost transformers but I'm not familiar with the wiring - Can you provide an example for me to understand better how each winding would be wired for this?
@strantor the need for the speed control is not to save electricity and not necessarily to reduce noise (although that is a plus) - The room this cabinet we are cooling is in has ventilation equipment, and when the blower is on high speed we are having an issue with reversion - where we are pulling air backwards into the room through the ventilation equipment. This is causing issues elsewhere as duct-flow switches are cycling off
o_O- And just to make it more of a challenge there are times when the heat load is high enough in this cabinet to require full speed...
if only for a very short period of time.
Now, the initial wiring of this transformer produced three possible speeds - and it appears that the blower would stay constantly on the lowest speed and then cycle to high as the cabinet heat load ramped up.... so it wouldn't be the worst thing in the world if we had medium and high only if that makes this easier.

The other thought I had was to move the transformer directly into the exhaust air stream of the blower. This would provide constant cooling (I may do this anyway) with a typical air temp of ~85F. Highest temp before alarm and shutdown of equipment is ~135F at the exhaust so this shouldn't be detrimental in any way.

I guess the most important thing here is maintaining the longevity of the transformer - this design really has my interest as it is simple and (should be) far more durable.

Oh, and thanks for everyone's input on this! Really do appreciate the guidance and friendly conversation.
Andy
 

LowQCab

Joined Nov 6, 2012
4,023
PDF attached ..........

The Yellow-Wire connects to the Orange-Wire,
Gray connects to Blue,
Red is the left-over "Line" Wire,
that used to be connected to Orange,
( it can be Neutral or Hot, it makes no difference ).
Wire Colors .PNG

Motor Auto-Transformer 2 FLAT .png
Temp Rating .PNG
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Attachments

Thread Starter

steamngn

Joined Aug 26, 2012
14
Ok gang,
sorry for the delay updating, been a crazy couple of weeks......
At any rate, after I posted on 7/2 about the overheating issue I went back and re-examined my transformer enclosure design. I concluded (rather quickly) that I had my head firmly planted in rear when I designed that.... just not good.
SO! I made some brackets up quick-like as I was going back and forth with you all about buck-boost (thanks for the training- learned a bunch!) and the such... things have been running nicely ever since:
toroidtran.jpg
That is mounted directly in the exhaust stream of the blower, and is running at a cool 90-95 degrees fahrenheit... very good indeed. As a plus this allowed keeping the low-medium-high speed options.
comments/critiques welcome!
 

LowQCab

Joined Nov 6, 2012
4,023
What Wiring-scheme did You eventually use that worked the best ?,
Shorting the secondaries, or, an Auto-Transformer style Buck-Boost scheme ?
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Thread Starter

steamngn

Joined Aug 26, 2012
14
@LowQCab I went with the original secondary-shorting method. I was curious to see if the temp would be ok (and thus proving my original design was crapola) or not...
As an aside, the low-speed setting (one secondary shorted) could even be lower. I haven't had time to look into what it would take to drop the speed a bit more but I can't imagine it would be too hard. This would really be more about noise reduction than function as the current slow speed isn't hurting anything.... it just never needs to ramp up to medium.
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
I f you are in N.A., Home Depot have all kinds of Triac style fan controllers, from variable to 3 set speeds etc.
Amazon even have a $15.00 one!

1658157885663.png
 

LowQCab

Joined Nov 6, 2012
4,023
I was going to suggest buying 3 or 4 different sized Power-Resistors,
and then see which one gives You the performance you're looking for,
but Max beat me to it and made a great point,
even if it turns-out to not work very well at such a low Voltage ( 22-Volts ),
a Light-Dimmer from a Hardware Store might be just the ticket, and they're cheap.
Just open the shorting-splice for the Low-Speed Secondary-Winding and insert Dimmer.
Let us know how well it works ............
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MisterBill2

Joined Jan 23, 2018
18,167
Keep in mind that for blower service the load varies with the square of the speed. So when the speed is dropped to 80% the load drops a lot. so a transformer to supply voltage for 80% speed should work, and then a thermostat and contactor to switch over to high speed at some higher temperature, and back to the lower speed when it reaches some lower temperature. It will cycle more, but are small excursions in temperature a problem?
I did one test stand that kept the recirculating oil at 250 degrees C. and with the on/off control it held within +/- 2 degrees all day and all week. After five years of constant running it wore out. So they ordered a second identical stand and then sent the original back for repairs. On/Off proportional control can be very good if the control loop is tight and the feedback is faster than the process..
 

Thread Starter

steamngn

Joined Aug 26, 2012
14
@MisterBill2 very good advice. In this particular scenario the temperature fluctuation can be minimal (1-2 degrees F) and then extreme (50+ degrees F) depending on the loads being placed on the equipment and also the ambient temperature in the room itself. This is very nearly but not quite a candidate for mechanical cooling, but we cannot even entertain that idea until we are further along in the construction of the building itself.
I will say this current setup is behaving very nicely!
 

MisterBill2

Joined Jan 23, 2018
18,167
Speed shifting blower control with fairly close limits and fast sensor action will be the solution. Especially the fast sensing.
If there would be some way for the cooling fan controls to know that suddenly the heat load was rising that could be a benefit toward holding the temperature closer to the target. Certainly it is a radical concept, but worthy of some thinking.
 
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