Air Conditioner Low Voltage Question

[Mgraw]

Usually A/C transformers are multi voltage 120,208,240 and some 460 primary and 24v secondary. Just guessing but I think the OP is using a 120v circuit for his transformer. Unusual but I have seen it done that way before. The transformer may be mounted outside the unit.

 

[gadfly56]

Intermittant Ground?

Intermittant Ground?

It sounds like you may have an intermittant ground. Every once in a while something gets grounded and pops the relay. Can you monitor the current draw on the circuit to check for spikes?

 

[mrcanby]

try just shutting down the fan conductor tap maybe the overall load from the 24v is overloading the 3 amp relays . 10 ton unit probly has dampers and other devices that use 24 volt, not just the thermostats.. either that or just supply your own heavy duty 24v relay. their cheap and can handle more load. But deffently check loads and wiring for faults.

 

[GlennH]

Put a 3 amp circuit breaker on your trans. If it dont trip tell your FA guy to get you a good relay.

 

[R Bob]

acguy

acguy

1. Make sure your control system is working within specified parameters.(you probably already know this from start-up)

2. Check the voltage and amperage(I doubt you'll read anything remotely close to 3amps) on the secondary of the control xfmr under full load.(stage 1 cool+stage 2 cool+evap fan+cond fan+etc.)

3. Provide the electrical characteristics of the control circuit(75va @24V) and your readings to the FA contractor. Unless their is some other contractual arrangement or your control circuit is actually malfunctioning, you have done due diligence and the ball is now in the FA contractors court. They need to adapt to the prevailing conditions. Unless you choose to do so, don't put any more of your time and material into this. I would probably get involved out of curiosity or the distinct possibility that I may be wrong.

Some other thoughts:

If there is, in fact, some type of intermittent ground fault/spike, I would suspect that it would be made apparent by a blown control fuse(if there is one), a fried circuit/logic board, or tstat. It's hard to comprehend that the contacts on a relay would fail and leave the electronic components of the system intact unless there is some type of substantial current limiting function engineered into the electronic components. I doubt it based on the number of circuit boards I've seen fried.

As GlennH suggested, you can always insert a 3amp non-time delay control fuse in the secondary and see what happens.

Also, mrcanby suggested installing a relay to isolate the HVAC control circuit from the FA switch loop, not a bad idea and quite common in many control applications. In fact, large, top of the line equipment has this feature built into the control scheme.

As far as the FA relay is concerned, what is the resistance across the closed contacts, what is the voltage drop across the contacts under full load(FOP), what is the current draw on the line side vs the load side of the contacts?

Good luck and be sure to let us know how this turns out...I'm currious now.

 

[markstg]

All I know is the FA contractor mentioned they were good for 3 amps. My control voltage is running thru the contact side while the FA is the coil side. This particular transformer is 75 VA and controls (2) compressor contactors, (1) ID fan motor contactor and (1) tstat. In this case, they are not robbing 24v to powere the smoke detectors but have their own source. The failures, I am told, are on th econtact side of their relay. What I am having trouble undderstanding, is how can just those items powered by the transformer pull greater than 3 amps? In my opinion, no matter what we find, I don't thinks its my problem. They apparently need to upsize their relay to handle the load. Also, i was look thru the tstat lierature for the electrical ratings. the running current values range form max 0.6 to 1.0 amps, depending on which terminal is energizing, ie 1st stage cooling, 2nd stage cooling, etc. I have a meeting with the GC, FA contractor and the municipality on monday and just wanted some more opinions on the issue. Thanks again Guys.

Guys.....The transformer is powering through the FA contact 2 contactors, 1 Motor Starter Coil, and a T'Stat. These loads could easily draw over 3 A. An ammeter measurement when the FA contact closes (inrush) and with it holding the load is in order provided the Contact doesn't burn up when closed. Also, 75VA xrmer at 24V is rated for alittle over 3 amps. My question to ACGUY is why he thinks these loads don't draw over 3A. You may need to add interposing relay to control the contactors/motor starter.

 

[Mgraw]

My question to ACGUY is why he thinks these loads don't draw over 3A.

I have not run across a 10 ton A/C system with a starter on the blower motor. More than likely just a 3 pole contactor. The coils are probably in the 250mA range. That is why I think these loads don't draw over 3A.

 

[Electric-Light]

The OP is right there is no way in hell 3A is flowing through programmed thermostat and the current in the current loop is the same regardless of where it is. If the alarm is using N/O relay in series between red and thermostat with the coil always on(as it should be for better safety, so it fails open). The relay should be a continuous duty since the 24v coil is ALWAYS energized.

If they're using non-continuous rated, that might explain the coil-burn out and any respectable relay should be able to handle +/- 10%.

As far as contacts, if you're seriously exceeding 3A on a 75VA transformer, your transformer will burn out way before the relay contacts that remains closed under normal use. 48v or 24v, it won't affect the contacts that remains closed.

 

[markstg]

I have not run across a 10 ton A/C system with a starter on the blower motor. More than likely just a 3 pole contactor. The coils are probably in the 250mA range. That is why I think these loads don't draw over 3A.

Well, I wish ACGUY would let us know what it was.