Help with hard start compressor

[HP708]

New compressor won’t start under load. It’s a 0 deg box application, R404a. It’s a pump down control. When the unit is fully equalized it starts and runs great. Box pulls down fast and system cycles off. On start up system it is locked rotor at 70 amps and continues to cycle out on internal overload. Voltage drops from 246 to 211. When I manually equalize it starts fine and repeats the above sequence. I temporarily wired the working unit next to the problem unit to the problem disconnect and all was good so I don’t suspect the electrical service.

The things that have been replaced.
TXV
Liq line solenoid and motor
Inside thermostat
Filter dryer
Compressor
Breaker
Start and run caps (with new compressor)
Potential relay

A similar unit running next to the problem one starts fine at 200/50 gauge. The problem one tries to start at 150/80 and fails.

The problem unit has these specs at near 0 deg box temp.
Superheat: 12 Deg
Liq Sub: 10 ( outdoor ambient 78deg F)
Compressor current: 9.8 with spec at 8.2

I have bumped start cap up to two sizes larger and still no start under these conditions.

This is the second replacement compressor thinking it was a fluke bad replacement but the second replacement does the same thing.

Any thoughts and/or ideas would be much appreciated.

[mbc]

Hi
with one check valve on discharge line and puting one solenoid valve between that and compressor to suction line you can run system in equalizing pressure and after compressor runs turn off solenoid valve by timer.

[HP708]

Are you saying to put a check inline on the hot gas line then tee upstream of that to a solenoid going to suction line?

[mbc]

hi
might help you

ab1.pdf

[al]

Is the compressor correct for the application, can you keep condenser fan running after compressor cycles off and see if it starts easier? Also you say voltage drops to 211 on start, that is a big drop, check line feeding unit, maybe run a temporary supply with the next size up cable from your fusebox.

[Brian_UK]

You say that it stops on pump down control, how does it start? Why isn't the suction pressure rising on startup?

We have to assume that the replacement compressor is an identical replacement as you haven't told us, was the replacement TXV identical too?

[HP708]

The compressor starts when room stat opens liquid line solenoid and pressure rises thru the txv. It is stopped at 5 psi and on at 18 which is common here for a pump down system. Low side comes up to 70 to 80 and high side drops from 225 roughly to 160 and stalls. This is when the compressor should start but it does not. Unit next to it is on the same circuit and starts easily at 200 over 50.

The TXV is supposed to be compatible with the older one of 18 years. I have been suspect of that but it doesn't make sense to me that it can run so well when the unit is started equalized and not be able to allow it to start at 160 over 70 ` 80 psi.

[HP708]

Ok thanks. I see this is a common setup for an oil separator which this little unit dosen't have. I can see what you are thinking about equalizing the system on startup. I doubt the hot gas line has enough volume to do that. Maybe from the liquid side with a fixed oriface to flash it to the low side. It kind of defeats the purpose of a pumpdown type control tho.

[HP708]

You say that it stops on pump down control, how does it start? Why isn't the suction pressure rising on startup?

We have to assume that the replacement compressor is an identical replacement as you haven't told us, was the replacement TXV identical too?

Let me try to answer again. I am new to this system so please bear with me.

The actual control that starts and stops the compressor is a dual pressure control mounted at the compressor. The box temp is controlled by an air stat that connects only to a liquid line solenoid. It is in the NC position. So when the air stat in the box is satisfied, it opens and the solenoid closes the liquid line just before the TXV. Meanwhile, the pressure control continues to evacuate the unit cooler and condense the refrigerant into the receiver. When the pressure control low side reached 5 psi it stops. To start, just the opposite happens and yes, the pressures begin to equlaize. When the low side reaches about 18 psi, the compressor tries to start. The pressure rises to 70 very rapidly and tends to stall there. The old compressor worked in the same pressure range and would start with the pressures I am seeing.

As far as it being the correct compressor, I am at the mercy of the supplier. They do the cross referencing but you make a good point. Maybe I should try to cross reference it. I plan to call Tecumseh when they open in the morning.

[monkey spanners]

Put your amp meter round the one of the start capacitor leads when the compressor is trying to start, if no current shows the capacitor is not in circuit or is faulty. Would be worth going through all the electrical connections and making sure they are good and correctly wired. I have once found a broken wire inside the insulation that caused something similar.
Many start relays are sensitive to mounting direction, some have arrows pointing up so you know how they should be mounted.

[HP708]

Put your amp meter round the one of the start capacitor leads when the compressor is trying to start, if no current shows the capacitor is not in circuit or is faulty. Would be worth going through all the electrical connections and making sure they are good and correctly wired. I have once found a broken wire inside the insulation that caused something similar.
Many start relays are sensitive to mounting direction, some have arrows pointing up so you know how they should be mounted.

Thats good advice and I have done that. I have installed larger caps as well (temporary) to see with no luck.

A broken wire under insulation...... ouch. It wouldn't be broken and it's not a miswire because it starts and runs great when equalized. It's only on start up under load. Maybe a wire that has become too thin somehow... don't know how that happens.

This call has me stumped. At 67 and 40 years under my belt, I don't want to retire with a mystery machine lingering on the brain, so please keep the ideas coming. I thank you all.

[frank]

The volt drop worries me.

Is the electrical supply the same as the adjacent unit that starts without a problem? (cable length, cable size etc.)

[martinw58]

can you fit a crankcase pressure regulator to stop low pressure rising when unit starts

[HP708]

The volt drop worries me.

Is the electrical supply the same as the adjacent unit that starts without a problem? (cable length, cable size etc.)

Yes it is but it is a smaller unit. Its the walkin cooler half. The freezer ran on that circuit for 18 years with no problem. I changed condesing unit for him 3 years ago and it ran but I started noticing this issue about a year ago. Now its a no start.
This unit is in the mountain region of New Mexico and power relability has never been exceptional but has been getting better over time. I did notice that the electricians wire comes out of the wall on a retrofit so I think what he did was use the old wire in the building which means there is no telling how far it goes or what actual gauge it is.
I just spoke with Tecumseh and he tells me it would take 86 amps to start it which is their LRA rating. I get 70 with a drop from 246 to 211. Pretty big drop and that bothers me too. What I didn't know is that it takes LRA to start. It still doesn't sound correct to me but yet here I am. So, I think I may take a ride back up with some 10 ga wire and wire it direct from the panel for a test.
That voltage drop has been knawing at me as well. Thanks for your input. Its a big help.

Also, on the latest Tecumseh listing they have this compressor listed twice. AWA2460ZXD AW6613-111. Both are listed at 12.6/86 amps yet one is listed as 1-1/2HP and the other at 2 hp. I asked him about that and he said thats just how they list them or something along those lines. So maybe, the new compressor is up a little on HP but not sure how they have the same current ratings.

[Tycho]

can you fit a crankcase pressure regulator to stop low pressure rising when unit starts

what he said, https://assets.danfoss.com/documents...325.1614713199

[Glenn Moore]

Hi
This Tecumseh compressor is a Low Back Pressure machine . It’s working Evaporating temperature range is from minus 40 deg F (5psi )up to plus 10 deg F (43 psi). It is a High Start Torque machine but if the suction pressure goes above 43 psi the compressor motor will stall.
Therefore you have 2 options to resolve the problem.
1) Fit an expansion valve with an MOP ( Maximum Operating Pressure or as some people express it as Motor Overload Protector .The valve must have an MOP point of around 10 deg F (43 psi ) . The Expansion valve will stay closed to prevent an overload condition until the suction pressure has fallen to less than 43 psi ,at that point the TEV will start to open.
Or
2) Fit a Crankcase Pressure Regulator set to somewhere below 43 psi .
Either solution should solve the problem although the low voltage needs to be investigated as that will certainly have a bearing on the compressors ability to start , but I think you have a normal range of TEV fitted rather than the required MOP TYPE

[mbc]

HI

MOP expansion valve and KVL does not help to our compressor during start time ( until our suction pressure comes down ) .

Both help us to protect our motors from over currents in running times .

If our motor Torque is not enough to run our compressor in start time we should put unloading system .
in medium and large system normally compressor has this system .

otherwise by putting one check valve and one solenoid valve between compressor and condenser and connect that to suction line ( PDF of Drowning on the top) we equal suction and discharged pressure.

Few second before starting solenoid valve comes on and after running times is finished it goes to OFF.

In this system before starting time our suction and discharge pressure equaled so we have minimum torque for running .

[RANGER1]

You could try altering pump down settings so starts sooner
Cut out 1 psi, Cut in sooner 10 psi as long as LLSV good.
Try to simulate what works.
Otherwise as others have mentioned for reliability.