MEP 803a Short Circuit / Air Conditioner startup.

[impi]

I realize this may be a mute point but I'll ask anyway.

Would / Should the MEP 803a be able to start a 5ton AC unit ?

If not then you can stop reading.

If yes, the here's the situation.

My MEP runs all the well pumps, pool pumps and even the 2.5ton Air Conditioner without really going much over 60% load utilization.

I have tried to start up the 5ton Air Conditioning unit with all other loads shut down and I get a short circuit error - not an overload. The AC unit has a hard start kit installed and I recently replaced it with a new one just in case. I tried starting just the compressor and outside fan unit (not air handler) but still get a short circuit error. Based on the TM the unit short circuits when it output current in any phase exceeds 425 +/- 25% of the rated value.

The startup Amps jump to 112 for about 1 second or so before settling down around 18 or 19 with things running. Measured with a clamp style multi meter on the supply line to the compressor motor excluding fan motor.

The ratings on the unit is as follows: LRA 141 , RLA 25.3, 230Volts 1 Phase.

Based on these numbers if the load are too high I would have expected that the overload would have kicked in and not the short circuit. - Is this a correct assumption ? The fact that the short circuit kicks in could point to a failure elsewhere based on what I read here on SS. I

I tested the R10, R11, R12 and R13 resistors and obtained the following readings: R10-7.9 ohms, R11-7.8 ohms, R12-8.2 ohms, R13-8.1 ohms.(Technical manual states a range of 7.2 to 7.8ohms for these). Measurement was taken after disconnection 1, 2, 3 and for from K8 to isolate the resistors. R14 and R15 tested within range. Based on the readings it looks like R10, R12 and R13 are outside the numbers and need to be replaced or will need to be replaced in the near future - I'm not familiar with what a failing resistor looks like - no resistance or high resistance ? or depends?


Or am I hunting unicorns and should not be expecting the 803 to start a 5ton unit ?

Regards

 

[Zed254]

MEP-803A set up for 120/240 volt single phase is good for 52 amps. I doubt it will handle the 112 amp surge.

 

[csheath]

My 803 starts and runs my 3.5 ton AC with lights, tv, dishwasher, and stove burners in use.

 

[jamawieb]

The initial surge is to much for the 803a. Your LRA (locked rotor amps) is 141 amps so your initial surge far exceeds it capacity. The 141 amps only happens for a millisecond and most meters can catch the reading. Your set did exactly what it is suppose to do. They are made for sensitive electronics, not to start large compressors or items that need large amounts of energy to start.

 

[impi]

MEP-803A set up for 120/240 volt single phase is good for 52 amps. I doubt it will handle the 112 amp surge.

Yeah - I realize that, was kinda hopeful and overly optimistic that it would take the hit, not completely jump of the concrete pad it's sitting on and keep going . That answer's the first part of the question.

I'm curious about the short circuit error thought - shouldn't this create an overload error condition ? Looking past the inability to start the ac and back to the current resistor measurements - am I looking at a potential point of failure ?

Regards

 

[Light in the Dark]

To overdraw a machine by such a large amount, you are only asking for problems (safetys in place, or not). You are involving burden resistors that are sized to the actual output of the machine, so with such a massive draw, it very well might be overloading these (and there IS a small spark/grounding event). I think you should test this machine to the max of its stated range, and see if you have any additional issues. I personally would not attempt to 'recreate the condition'!

 

[impi]

Ok, I think I'll get a resistive load that's big enough to see if she'll go to 10KW without kicking out. Won't mess around with the AC anymore.

Thanks for the feedback.

 

[jamawieb]

I don't believe your unit has any problems. The short circuit fault is activated because of the large inrush of amperage from the A/C unit. The overload fault happens when the unit takes the load or gradually increased to cause the fault. But when its a sudden inrush of current it automatically trips the short circuit to save sensitive electronics the generator is hooked to. Had the same experience when trying to start larger motors on compressors and electric hydraulic motors.
Usually when you have a problem with the short circuit fault, is when it automatically shows a short circuit as soon as you hit the ac interrupter switch. If its holding loads until you try a specific item (A/C unit), then your fine.

 

[DieselAddict]

I bet if you replace your outside unit with an inverter (VFD) based one a MEP803 would start it.

 

[DieselAddict]

I know this is most commonly used for an RV but I don't see why you can't put one on your AC at home. I've seen videos of these in action and they appear to make a HUGE difference. Soft starts are very common on industrial equipment. They are becoming much less so with the gain in installations with VFDs on motor loads. Looks like the cost is reasonable for what you could get in return. According to the sheet you can start a 6 ton unit on 38 amps. That would bring it into range that a 803 can handle. Might be worth a conversation with the vendor.

http://microair.net/main/products/specsheets/EasyStart_Spec_Sheet.pdf

 

[impi]

Thanks for this - I will take a look at it at a later stage. I believe there's some issue that needs to be adresed.

I know this is most commonly used for an RV but I don't see why you can't put one on your AC at home. I've seen videos of these in action and they appear to make a HUGE difference. Soft starts are very common on industrial equipment. They are becoming much less so with the gain in installations with VFDs on motor loads. Looks like the cost is reasonable for what you could get in return. According to the sheet you can start a 6 ton unit on 38 amps. That would bring it into range that a 803 can handle. Might be worth a conversation with the vendor.

http://microair.net/main/products/specsheets/EasyStart_Spec_Sheet.pdf

 

[impi]

So she doesn't hold 75% load for too long.

Being in South Florida we don't do a lot of heating, but when we do its electric for most part. The heating unit tied to the air handler that drives the 5 ton ac is capable of producing decent load (9.6kw @ 42 Amps). So I fired it up this morning (fortunately we have a bit of a cold snap and the outside temps are sitting in the low 70's - so I just opened all the windows and doors, and set the HVAC to heat so the heat would kick in, doing my bit to warm up the neighborhood a bit).

Here's what I found.
After running the MEP for 5 minutes to warm up I applied load.
The load meter on the MEP shows about 75% load when things get going.

I have digital Watt/Amp/Volt meters on each branch coming in to the house so I can see what's happening from a load perspective as the way things are set up I do manual load distribution.

On each of the branches with nothing else but the HVAC running on heat it shows around 42 Amps @ 4800 watt load. This matches up with the label on the unit which shows 9.6kw @ 36 Amps and another 6 Amps for the fan motor. - so at least I know that side adds up (36Amps for the heat, and 6 amps for the fan = 42 Amps, and 4800 watt per branch (L1, L3) gives me 9.6kw).

It would handle the load ok and show around 75% (Yeah I know 9.6kw is not 75% - closer to 100% but more on this later) on the load meter on the MEP, run for about a minute and then the MEP would trip and the overload light would come on.
My understanding is that the MEP can handle 10kw constant load so the 9.6kw shouldn't really be an issue. I'm guessing those resistors with the shaky numbers are part of the problem.
The load meter also appears to be tempremental at this point as it would not show any load if I close the circuit even with a lesser load once the overload has tripped and been reset. I'll have to troubleshoot upstream and get to the back of S8 where the one branch comes form to make sure everything is ok back there.
The unit is from 1995 and only have around 1500 hours on it, but looking at the connector blades on the wiring harnass it seems to have a bit of oxidation so I''ll clean that up.

 

[Light in the Dark]

Yeah, you should consider getting a few cans of deoxit and taking the top off the machine, and cleaning all electrical contacts (and giving the S8 a nice clean out and dozens of cycles).

 

[devilphrog]

This post intrigued me as I'm going to be connecting my genset to my house and I've got a 4 and 2.5 ton units. I found one recommended by Copeland Compressors / Emerson Electric. http://www.emersonclimate.com/Documents/CopelandSecureStartOverviewPresentation.pdf Looks to be about 400 bucks though...

 

[impi]

On the to-do list - I've ordered a bunch of replacement fasteners as I had to cut some rusted fast fasteners when I removed the front top section just after I got the unit to address a diesel leak and do a fluid changeout. I will open up the back section over the next week or two to install the remote start kit I ordered from Inova - it will be a lot easier to do with the top off, and I might as well clean everything out good while I'm in there.

Yeah, you should consider getting a few cans of deoxit and taking the top off the machine, and cleaning all electrical contacts (and giving the S8 a nice clean out and dozens of cycles).

 

[jamawieb]

Clean S8. The problems you are reporting, resolve around S8 (reconnection switch). Take the top cover off and spray the connections on S8 as you rotate the switch, as describe by Light in the Dark. Do this about 2 dozen times and you will see your issues resolved or at the very least improve. If you do the process and you see no change, then start troubleshooting wires and resistors. Don't make the mistake of making this more complicated because everyone has been in your shoes and thinks it can't just be dirty connections but 99% it is.

 

[impi]

I learned that the hard way with a boat ignition switch that was badly corroded - I ended up replacing everything in the ignition circuit from starter motor, solenoids relays etc. Problem turned out to be the cheapest and last part I replaced - the ignition switch itself - I started replacing from the wrong end . I learned a lot from buying a used boat - no intention of repeating those lessons... I bought a couple of cans of Deoxit. I have the bolts soaking in penetrating lube before I break off another one.

I picked up some good info here on the discussion Farmitall started with a similar issue. Hopefully it's just the switch, if not I'll start looking upstream.

Clean S8. The problems you are reporting, resolve around S8 (reconnection switch). Take the top cover off and spray the connections on S8 as you rotate the switch, as describe by Light in the Dark. Do this about 2 dozen times and you will see your issues resolved or at the very least improve. If you do the process and you see no change, then start troubleshooting wires and resistors. Don't make the mistake of making this more complicated because everyone has been in your shoes and thinks it can't just be dirty connections but 99% it is.

 

[impi]

Glad to report that cleaning S8 did the trick. She now holds the large heating element without any issues and I added the secondary air handler on the smaller AC unit's heating element which allowed me to push the load up to 125% and she just kept chugging along without hitting overload.

Blasted half a desert's worth of sand and dust out of S8 with big can of contact cleaner while continuously rotating the switch. Gave all the other switches and contact points a clean out as well while in there. Also checked for any evidence of arcing - found none.

Happy camper over here - thanks for all the input.

 

[Guyfang]

This is what we would call working smart, not hard. When ever you have a problem, it pays to sit down, and think about it. Way too many folks start throwing parts at a gen set, when getting smart about the set/problem brings better results. And you can't beat brain storming with all the smart people in this forum. The adage, "two heads are ALLWAYS better then one" is a good one.

 

[impi]

Loaded it up this morning using the HVAC as the improvised load bank. The main HVAC gave me a load of around 43 amps per leg, and around 5200 watts. I ran the MEP with this load after letting it warm up for 5 minutes. I ran this load for 10 minutes and then added the 2nd HVACs heat.


The 2nd unit bumped the load to just north of 125% on the load meter, added pushed the amps up a bit as well. I maintained this load for 5 minutes then took it off for 5 before re-applying it. I ran it for an additional 10 minutes with the additional load before the MEP decided it was time to kick it off and gave me the overload trip. Voltage and frequency dip between the initial load and adding the 2nd load was not too horrific.



A good cleaning seems to make the MEP very happy. I'm a happy camper - this is more than sufficient capacity for what I need when everything else goes sideway during hurricane season. Next step is to install the remote start, and then start fudging with some of the technology around getting the 5 ton going. Some good contributions by DieselAddict and Devilphrog.