[flyingjwm007]

Trane 2 ton HP. Indoor unit contactor started buzzing and fluttering. Fan continued to run but outdoor unit and elcetric heat would not . Suspected 24 v T-former but voltage checked out. Replaced time delay and relay. With T-stat in heat mode and on emerg heat fan would run but no heater contactor engagement. 24 v going to electric heat contactor. Can hold contactor in and will heat. If T-stat is placed in normal mode, outdoor unit will cyle on for about one second and off and 24v t_former starts overheating as though a short is being applied. Question. 1. why would electric heat contactor not engage with 24v applied? 2. could t-stat be shorting. 3. Outdoor unit will run if its contactor is manually held in. 4. I diconnected all t-stat wiring to outdoor hoping to just have fan and electric heat. Fan runs but electric heat contactor will not engage. Can I assume that contactor is bad? The unit is a TWV025B104A mfgrd in 1996. All ran fine until the indoor relay started rattling and sparking. I suspect a short either in the t-stat or in the outdoor unit. I would be okay with just indoor fan and back up electric heat for now. Any suggestions.
Jerry

[icehouse]

Check your Transformers VA

Volt amps (VA) is the effective power rating for a transformer. In the perfect transformer, the voltage and current (combined) applied at the primary will exactly equal the voltage and current (combined) on the secondary(s).



Instructions


Things You'll Need:

• Digital Multimeter (DMM)
• Paper
• Pencil

1. VA Calculation
2. 
   Read the transformer's secondary winding rating (printed or tagged on the transformer case). This should give a rating such as 5 V , 500 mA.
   
   Multiply the voltage times the current to receive the VA.
   Ex: 5 V times 500 mA (or 0.5 A) equals 2.5 VA
3. 
   Convert the secondary winding VA value to the primary winding VA rating as the equation VA (primary) equals VA (secondary) or VA (p) = VA (s).
   
   For the example of a 2.5 VA secondary, if the primary voltage is 120 V the current will be A (p) = VA (s)/V(p). Giving A(p) = 2.5 / 120 = 0.020833 (roughly 21mA).
   This is the calculation for a single input winding/single output winding transformer.
   Verify using DMM to read input voltage and current versus voltage at a given output current.
4. 
   Add the separate VA ratings for a transformer with multiple secondary windings.
   For secondary winding 1 = 5 V at 500 mA
   Secondary winding 2 = 24 V at 100 mA
   Total secondary VA = ( 5 x 0.5 ) + ( 24 x 0.1) = 4.9 VA
   Primary VA will still equal the total secondary VA.