Compressor No Start intermittently on Self-Contained application.

Hi guys,
We have been building this model display case for years. Copeland has retired the condensing unit we had been using and recently replaced it with a new model. Other than the new condensing unit, nothing has changed. I say this with regard to all aspects of the case including evaporator, piping and controls. We have built a few cases with this new unit without issue.

It is a 1/2 hp. 404A Copeland reciprocating condensing unit. 115Volt / 60hz. Cap start/ induction run. It is brand new out of the box from Copeland.

The problem;

At the initial start-up there are no issues. Cycling through the initial pull-down the t-stat preset temperature is satisfied and the unit cycles off. Everything about this initial cycle is as expected. The operating pressures are as we expect and even the sound of a running cycle is fine. When the t-stat calls for the second cycle and subsequent starts thereafter, the external over-load trips a few times before compressor starts. Unplug the unit for half an hour or so and the above scenario repeats.

What is known;
Ambient Temperature 68°f

Supply Volts, 116V
Running Volts, 116V
LRA 39.5A specs are 40.0A
RLA 9.8A specs are 10.0A
Discharge PSIG 210 running
Suction PSIG 55 running
Windings; run .8 start 3.8

My first thoughts are to just replace the start components, so that's exactly what I did. Cap, Relay and O/L. Same issues.
It is a TXV system and although the charge is standard for us and head wasn't high, I reduced the charge. Same result.

Thanks for letting me pick your brains.
Ken

I think put it back in the box and return it to the supplier under warranty

Is the compressor starting equipment suitable for TXV usage, ie unbalanced start.

Ken
This is obviously a high start torque machine as it has a start capacitor , and is also designed for use with a txV. But even with a start capacitor there is a start limitation as to the pressure difference between suction pressure and discharge pressure. During the off cycle this pressure difference should fall to a value that the compressor can safely start against.
Suggest fitting a system analyser or a simple valve between the suction and discharge to act as a simple manual bypass arrangement. The bypass must be closed during the run cycle ,and opened during the off cycle . Once the pressures have equalised shut the bypass and wait to see if the compressor starts when the thermostat calls for duty.
If it starts with no pressure difference across the compressor ,then the compressor is ok but maybe the start capacitor is faulty. If the compressor starts ok with no pd,then try to start it with 30 psi pd , then try 50 psi pd and so on until the compressor fails to start cleanly . It maybe that the tev is holding the suction /discharge pressure difference too high that the capacitor cannot get the motor torque to overcome the motor load that the pd is exerting on the motor. Try the next size up capacitor to see if that helps.
Sometimes the start capacitor is sized on the small size and reduces the start ability of the compressor motor.
You could contact a US company who manufacture capacitors and relays for small hermetics who can supply start capacitors that can boost the start torque from memory up to 500 % They are called Superior controls , or SUPCO

In addition you can check whether this compressor is suitable for Capacitor start and capacitor run if so try that option as well.

There could be a problem with compressor winding [it happens],
A faulty starting device,
A faulty overload protector,
A partially blocked pipe on the high side.
What is the compressor full model?
Have you checked resistance on the start and run winding?
Is it getting hotter then the others while running?

Thanks for all the input. I have checked the electrical specs and am leaning toward the pressure differential cause a higher torque condition. I have added a few minutes to the anti short cycle delay on the digital control with the hope that this does not negatively affect the product temps or the temperature recovery time. Any other down and dirty ways to get the pressures equalized.

Hi Ken
Most manufacturers manufacture expansion valves with bleed . This is a tev with a small hole bypassing the valves main orifice.The bleed capacity is only a few percent of the valves duty so the bypass has no effect on the valves superheat control but during the off cycle the bleed helps to quickly equalise the pressure around the system allowing an easy start situation for the compressor, or a small length of cap tube across the tev will also do the same effect . Try a bigger start capacitor before you start altering the system ,plus the longer time on the anti recycle timer will help.
Do a few tests watching the pd and see when the pd is low enough for the compressor to start cleanly then set the anti recycle timer accordingly

Hi to All,
Again thanks for the input and after exhausting all the great suggestions and then some, nothing seemed to solve this one. I replaced the condensing unit with another new one, with no other changes and its been running perfectly for 24 hours. I do remember a new compressor that was said to be tight as determined by the compressor manufacturer. It kinda sucks not to able to actually identify the problem. Thanks again for the ideas and have great spring.
Ken

Great to hear you found the issue,guess the testing procedure for new compressors is not as robust as we all take for granted let's hope it's the last faulty one you buy,totally frustrating.