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HVAC Electrical stepdown transformer questions


paulbates

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I'm automating a damper. I have an iolinc to control the 24 volt feed to the actuator, what I'm struggling with is the right transformer to feed actuator.

 

Requirements from the documentation and notes:

  • 24v, ac or dc,
  • 7VA and 5 Watts.
  • A Class 2 Transformer
  • There will be no more than 15 wire feet between the transformer and that actuator

I'm having difficulty finding a class 2 transformer to match to those specs in terms of 7VA and 5W.

 

Here is the questions:

I found general HVAC transformers that deliver 20VA and 40VA.  Does the HVAC crowd here know, will either on of these do, given that they oversupply?

 

There is a wiring document from Bilmo, the manufacturer. I've read it, Other than class 2, and how to pick wire gauge for the distance, it doesn't say.

 

post-5496-0-75212400-1430613877_thumb.jpg

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I'm automating a damper. I have an iolinc to control the 24 volt feed to the actuator, what I'm struggling with is the right transformer to feed actuator.

 

Requirements from the documentation and notes:

  • 24v, ac or dc,
  • 7VA and 5 Watts.
  • A Class 2 Transformer
  • There will be no more than 15 wire feet between the transformer and that actuator

I'm having difficulty finding a class 2 transformer to match to those specs in terms of 7VA and 5W.

 

Here is the questions:

I found general HVAC transformers that deliver 20VA and 40VA.  Does the HVAC crowd here know, will either on of these do, given that they oversupply?

 

There is a wiring document from Bilmo, the manufacturer. I've read it, Other than class 2, and how to pick wire gauge for the distance, it doesn't say.

 

attachicon.gifIMG_0116-2.JPG

The va capacity should not affect anything as long as it is capable of delivering the voltage at the load attached.

 

If you have a 7va load a transformer 7va or larger can feed the load properly and may be a boon for later units you want to connect to the same transformer.

 

Voltage is determined by the transformer and the current (and thus va) is determined by the load (not the supply) at that voltage. Multiply the applied voltage times the load current or the supply current and you have the volt-ampere of the load or the supply (the same).

 

The 5 Watts is the usage the load makes out of the 7va it takes to make that load (watts).

Power factor is the watt / voltamperes, so 5/7  = 71% p.f.  Less than 100% means the current drawn by the load is not exactly in phase with the voltage. Wound coils like to be slow at changes and lag.

 

IOW get the most common (and usually cheapest) Class 2 transformer and use it. Lighter loading than it's rating will probably make it last longer with it's larger heat dissipation capability.

 

 

I doubt the wire size will be a problem as long as it is bigger then a human hair for those distances and 7 va load. Just use thermostat cable or even Cat 3.

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The rating for the transformer is the minimum requirement. You'll be OK with a transformer that has a higher ampacity. In fact, you can use the existing HVAC 24VAC power supply.You should also be fine with 18AWG thermostat wire.

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The va capacity should not affect anything as long as it is capable of delivering the voltage at the load attached.

 

If you have a 7va load a transformer 7va or larger can feed the load properly and may be a boon for later units you want to connect to the same transformer.

 

Voltage is determined by the transformer and the current (and thus va) is determined by the load (not the supply) at that voltage. Multiply the applied voltage times the load current or the supply current and you have the volt-ampere of the load or the supply (the same).

 

The 5 Watts is the usage the load makes out of the 7va it takes to make that load (watts).

Power factor is the watt / voltamperes, so 5/7  = 71% p.f.  Less than 100% means the current drawn by the load is not exactly in phase with the voltage. Wound coils like to be slow at changes and lag.

 

IOW get the most common (and usually cheapest) Class 2 transformer and use it. Lighter loading than it's rating will probably make it last longer with it's larger heat dissipation capability.

 

 

The rating for the transformer is the minimum requirement. You'll be OK with a transformer that has a higher ampacity. In fact, you can use the existing HVAC 24VAC power supply.You should also be fine with 18AWG thermostat wire.

 

 

Thank you Larry and Stu!

 

Stu, this is not a traditional damper application, and is not located near the furnace, but using the supplies on my new HVAC units is great suggestion. 

 

Paul

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Thank you Larry and Stu!

 

Stu, this is not a traditional damper application, and is not located near the furnace, but using the supplies on my new HVAC units is great suggestion. 

 

Paul

If you have a 20 va transformer on your furnace and it also runs the A/C contactor you may want to consider not connecting further loads to it.

 

Since the load for the damper is short term I would hook a voltmeter across the transformer and turn on the A/C, running the furnace fan also and then activate the damper. If the voltage dips below about 22-23 vac I would upsize it or use a separate unit.

 

Low voltage on your A/C contactor coil inside the outside compressor unit can make the contacts chatter or not fully close, damaging your contacts with burning or worse damage the compressor with intermittent power. For the purpose of economy many of these furnace transformers are sized very minimal.

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Thanks Larry-

The board for these new furnaces (Carrier 59TP5A) has a lot of 24VAC capacity, the concern is noted.

 

However, I won't be able to connect there, my application is elsewhere in the house and I will buy a transformer, electrical box, etc, dedicated to this project. 

 

Paul

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It'd have to be a pretty cheap contractor to install any HVAC control panel that couldn't handle several dampers B)

HVAC control panel?

 

Me thinks the home heating contractor puts in the builder's spec furnace with the smallest transformer that can turn on it's own fan contactor and run one  thermostat to activate it.  Many A/C installations have to add another transformer to run the A/C contactor to the furnace or upgrade the existing one, voiding the furnace warranty.

 

On a side note: I had my mixer panel transformer running one input to my multispeed fan motor but the seed ramp didn't work. The factory tech help on the phone identified the voltage was different and squawked about a different feed would not work. Turns out the motor input was phase sensitive and I had to move my panel breaker to the opposite phase of my electrical panel to make it match the phase the furnace transformer was on. Then my fan would do it's designed ramping. They needed more logic inputs to the motor so they programmed some inputs to detect phase polarity. Who would have thunk it?

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HVAC control panel?

 

Me thinks the home heating contractor puts in the builder's spec furnace with the smallest transformer that can turn on it's own fan contactor and run one  thermostat to activate it.  Many A/C installations have to add another transformer to run the A/C contactor to the furnace or upgrade the existing one, voiding the furnace warranty.

 

On a side note: I had my mixer panel transformer running one input to my multispeed fan motor but the seed ramp didn't work. The factory tech help on the phone identified the voltage was different and squawked about a different feed would not work. Turns out the motor input was phase sensitive and I had to move my panel breaker to the opposite phase of my electrical panel to make it match the phase the furnace transformer was on. Then my fan would do it's designed ramping. They needed more logic inputs to the motor so they programmed some inputs to detect phase polarity. Who would have thunk it?

 

I wouldn'tve tunck it as the electric supply to residences in North America is single-phase (except Philadelphia and a few rural areas). Industrial, commercial and many apartment buildings have a three-phase electric supply.

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I going with this Elk plug in transformer. It has extremely good reviews, circuit protection.. and given the project's remote location, its one less enclosure to build / deal with. Considering that the alternative involves parts and time to deal with an enclosure and do wiring, its cheaper.

 

Thanks again for the advice

 

Paul

 

trg2440.jpg

 

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That ELK transformer is indeed an excellent product that will give you years of service life. I should note that the security screw that replaces the wall plate screw should not be used.

 

There are very specific NEC / CEC rules that govern the use of this screw in a residential application. Should the device be faulty or light on fire having the screw in place will hinder your ability to remove the transformer from the outlet.

 

Thus causing more damage and risk of house fire . . .

 

Ensure you wire the strain relief loop per the users manual.

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Thanks Teken.

 

In looking at Amazon reviews, that was the only complaint I found, and I appreciate the confirmation. The install location is in unoccupied and almost inaccessible space. I have an iolinc in the same unfinished location and I've lightly tie wrapped around the box and it. It makes sure it stays in place, but it can be slid off.

 

The alternatives to the elk are scary. Variable reviews, no certainty of the source. Elk would not "bet the security farm" on a power supply that might fail. And compared to viable alternatives, its not much more, less than $20

 

Paul

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That ELK transformer is indeed an excellent product that will give you years of service life. I should note that the security screw that replaces the wall plate screw should not be used.

 

There are very specific NEC / CEC rules that govern the use of this screw in a residential application. Should the device be faulty or light on fire having the screw in place will hinder your ability to remove the transformer from the outlet.

 

Thus causing more damage and risk of house fire . . .

 

Ensure you wire the strain relief loop per the users manual.

Sounds ridiculous! Who is going to pull out a transformer from a receptacle while it is on fire? Interesting rule using the receptacle as an electrical disconnect during a fault. :shock:

 

The fire may be caused by a bad connection as the transformer vibrates loose from the receptacle because somebody didn't install it, according to manufacturer's instructions using the screw provided! :)

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The ELK and similar cUL / UL rated security transformers with screws are intended so a person can not unplug the unit without some effort and tools.

 

Back in the day transformers from various makers had many fire related issues which caused untold financial damages.

 

Thus came in more stringent certification of these devices and the level of reliability and safety. Unfortunately do to the massive amount of failures NEC / CEC instituted rules that forbid the use of the security screw for good reason.

 

Even though fire related failures have since dropped off the map.

 

The ELK transformer is one of a small hand full that has a true lifetime warranty on the product.

 

We've used them and similar for more than 15 years without a single failure.

 

 

Ideals are peaceful - History is violent

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