weak transformer?

[little john]

it's a small water source heat pump. it worked great for years with the mercury bulb tstat. I upgraded to the digital programmable today, but whenever the compressor contactor is energized, I get a voltage sag, the display drops for a second, and I lose the call for cooling. Compressor is only on for a second.

found the primary tap of the control transformer on the factory setting (230 v) but my mains are 208. switched that. same problem, only stays on maybe a second longer.

when 'jumped out' at stat location unit works fine, and in the control box as well.

no unexpectedly high resistance through my control circuits (but I would expect the transformer to overcurrent and fail if I did anyways)

I'm suspecting that years of running at the wrong voltage (and hence higher currents then rated for) have caused this transformer to 'weaken' and it can't handle the inrush current of energizing the compressor contactor coil. Does this line up with anybody's experience?

[freonguy]

I have seen a small transformer ' weaken ' on a bunch of old York Sunline units several years ago when hooked up to the wrong tap, and cause havoc on digital stats.

Check your va ( volt / amp ) rating of the present transformer and ensure you are not drawing excessive current for some other reason. A 40 va transformer should not draw more than about 1.5 amps on the control side.

Easy enough to hook up a auxilliary transformer and see what happens - also, what does the voltage drop to upon startup of the compressor? Is there a bad connection somewhere that is letting the voltage drop below 190 or so on initial inrush?

Keep us posted.

[griz]

If it works by jumping from stat and unit, sounds like a problem with the stat. It sounds like the problem arrived when stat was put in circuit

[little john]

thanks.
actually (and sorry I didn't mention this), I ruled out the stat as the problem by trying other stats (I keep several in truck stock) and had identical problem with all of them. I think it's either the transformer or a damaged common wire. I have other conductors available so will rule that possibility out tomorrow before changing the transformer.

I'll measure my volt drop at compressor startup as well to see what's going on there.

[griz]

Check for a faulty contactor coil, I have had that happen before.

[little john]

would that not cause the transformer to simply overcurrent and fail, if it was a faulty contactor coil? I'll test it as well though. thanks

[atmosphere]

Any kind of problem with the contactor mechanism such as binding or a foreign object in the plunger mechanism can cause a high amp draw that will not always be fatal to the LV transformer.

I think if the new t-stat was the cause there would be a slight ala Barbecue odor waft in the bargain.

Maybe something got into the contactor when you removed a panel.

[mikelcs]

make certian that your common is really common at the xformer. if the stat is acting as "power robbing" it could be the issue.

[freonguy]

I was in a rush last night when I posted my reply but should add a couple of points -

- when I say the transformers were weakened, I actually measured a difference in the winding resistance on both primary and secondary sides compared to a '' good '' identical transformer, and upon closer inspection under the winding wrap, discoloration was present - they still put out over 23 volts when wired up to line however. I believe the stats the owners were trying to use were Enerstats, which, especially in the early years, had their share of problems.

- the incoming line voltage to this facility was a paltry 198 volts - 3 phase, but other than the transformers not being wired up on commissioning, we had few if any voltage related problems ( ie: compressor or fan motor failures ) except for shortened compressor contact life and control relays burning out.

- the other posts to this thread have been right on!

[Trublshter]

I've had found on occasion a weak contactor coil that closed the contacts but had a voltage drop across the contacts. When I visually inspected them after replacing you'd swear they looked ok.

[Tool-Slinger]

it's a small water source heat pump. it worked great for years with the mercury bulb tstat. I upgraded to the digital programmable today, but whenever the compressor contactor is energized, I get a voltage sag, the display drops for a second, and I lose the call for cooling. Compressor is only on for a second.

found the primary tap of the control transformer on the factory setting (230 v) but my mains are 208. switched that. same problem, only stays on maybe a second longer.

when 'jumped out' at stat location unit works fine, and in the control box as well.

no unexpectedly high resistance through my control circuits (but I would expect the transformer to overcurrent and fail if I did anyways)

I'm suspecting that years of running at the wrong voltage (and hence higher currents then rated for) have caused this transformer to 'weaken' and it can't handle the inrush current of energizing the compressor contactor coil. Does this line up with anybody's experience?


Where are you mesuring this 'voltage sag'? If measured at the transformer it sounds like a VA problem. If it good at the transformer, but drops at the thermostat, then it sounds like you are dropping voltage between the two. i.e. a wiring problem or bad connection somewhere.

I assume you have common wired in at the thermostat, if not and relying on a trickle from "W" to power stat then that is another problem altogether perhaps.

[roc service]

My old partner had a problem trying to use an electronic thermostat with a McQuay Water Source Heat Pump unit that he installed in his apartment years ago. It seems that the McQuay unit uses a DC volt relay board to control the loads and it has compatibilty problems with newer electronic stats. He put back the old Honeywell electro-mechanical stat and all the conflicts went away. I don't know if this info will help you. Good Luck.

[maxster]

make sure you have a ....TRUE and read it off subbase 24v.... to the new stat...no feedbacks(old styles R only with Y,G,W other side of TR) you have to have the C side at the digital to run it.with that primary 208V and your tapped for 230V could give you a lower secondary when it draws as your seeing.do you have 24 or better coming out ...final solution get a new 208V/24V and more VA if changed out

[wormy]

Also consider adding a 'little black relay'

A three phase contactor coil is a bit BIG to be running through a digital stat.
Have the Y power the coil on a little black relay
Let the contacts on the relay close and send power from the 24V positive of the transformer directly to the contactor coil.

Think of it as the same concept they use on the bigger equipment.
Little relay to Big contactor to a LARGE starter
Wam bam thank you mam :)

I haven't seen the 'load' specifications of a digital stat (I admit... I haven't really looked either), but it would seem like they can't handle a whole lot.

[madhat]

The T-stat for a single stage McQuay WS HP with the DC/AC control boards is a Honeywell FocusPRO P/n TH6110D1021 Check your control wiring too, a lot of 22 and 20 gauge plenium rated wire ended up being used on WS units.

[Trane Tech NYC]

It seems from your posts its not the Tstat, check all the coils in the circuit, sounds like you may have a weak coil somewhere.

Good Luck.

[tr23]

What kind of digital stat did you install? Does the stat 'click' repeatedly before the contactor opens?

[little john]

these units were wired with the "canada only" option of grounding one side of the transformer. I guess because of the way the coil loads are arranged, this 'grounding' was the culprit. Once I lifted that "optional" ground wire, I had no problems. Original transformer is fine, original contactor is fine. I did change out the contactor because it had some pitting, but that's another story.

three other units that I upgraded with digital stats had the same wiring schematic. I changed them all and all ran fine.

thanks everyone for the input, and sorry I updated this so late

[tr23]

Glad you solved it. Seeing as I live in the Great White North, would someone perhaps explain what was happening here, so I can understand the impact of grounding one side of the transformer?

[little john]

generally, one could 'ground' one side of the secondary output of a transformer, and do all the loads on the other side. that way, on back end of each load, one can just ground it and cut down on the wires back to the transformer. it's pretty common.

this unit had a different wiring schematic than that. typically, running the transformer common back to ground would work fine, but there was something funky about the way the 'loads' in this unit were connected. I found that if I just lifted the ground wire, everything was fine.

I could probably have rearranged the wiring a bit to make the 'grounded' transformer work, but I prefer to just stick with the original wiring schematic if at all possible. makes it a little easier for the next guy

does that make sense?

[tr23]

Thanks for the explaination. It seems to be code up here that the neutral side of the transformer is grounded, and all loads connect to the common of the transformer, and are grounded. I'm going to ask the electrician at work some more about that. Thanks for the info.

[madhat]

I guess because of the way the coil loads are arranged, this 'grounding' was the culprit. Once I lifted that "optional" ground wire, I had no problems.

I hate guys that do this, you just masked the problem, (something is leaking to ground on the 24V hot side of the transformer. If the transformer is over 48VA, it has to be grounded per UL. If the transformer is crossing over, then hope no one gets out of the shower and decides to "adjust" the thermostat.

[little john]

it's a 40va transformer, and it was clearly written in the wiring schematic "Canada ONLY" to ground the one side. Also, three other units were identically wired. I know something was leaking to ground on the 'hot' side, but since we're not in Canada, why not just made it legal for here?

[fxb80]

I once had $8,000 worth of chocolate candy melt in a store because the heat stayed on after the thermostat ceased calling. The thermostat cable was worn where it entered the furnace, and R to W was shorted. If the transformer had been grounded the transformer would have failed rather than the heat running wild. One side of the secondary should always be grounded so that wiring faults will cause transformer or fuse failure rather than undesired operation.

[skpkey9]

fxb80 ~ If R&W shorted together it would be no different than thermostat calling. Theres no way that could cause a transformer to fail unless you overload control circuit which that would not do.

[control distributor]

The McQuay 24vdc issue is different. In this situation read the thermostats contacts ratings, most are not good for switching 24vdc. Ironically many old Enerstats could handel the load.

Some power stealing tstats loose the display, but still work. There is a good chance that you need to add the resister into the circuit that came with the stat. Whoops you probably threw it away already.

[little john]

these digital stats are the new version of the Carrier Debonair 220. I've not had a single problem using them for switching controls in any unit I've hooked them up in. This unit certainly is not anywhere near as big as some that have run perfectly behind one of these stats. I think it's just a design miscalculation from an era that wasn't counting on digital stats coming into the picture anytime soon.

[onebadgrif]

is there a solid state time delay in this unit? try taking it out or replacing it

[fxb80]

skpkey9, what I should have said was that R and W were shorted together through the furnace casing. If the other side of the transformer had been grounded there would have been a direct short from R to C, which would have been the overload that would cause the transformer to fail.

[skpkey9]

makes more sense now. but only if wires have touched the furnace casing