So-Low Cascade freezer problem

[aerobat72L]

I am trying to service So-Low model# U85-18( -40to-85DegC). It starts up normally, and both cascade compressors (Copeland RF41C1E-CAA-210) run normally until reaching target temp, which I have set at -60 Deg C. They they then shut down normally at target temp. When deadband ends, and refrigeration system attempts to start, unit blows 20Amp breaker. As I am not there when it occurs, it has been a bit difficult to predict when it will fail. Unit runs on 115V,1Ph,60Hz. I am beginning to suspect either potential relays or strt capacitors, except for the fact that it runs normally when started from scratch after several hour shutdown.

[Gary]

When it reaches target temp, do both compressors shut down?... or just the low stage compressor?

[aerobat72L]

gary, It appears that they both shut down, but I think I know where you are going with this. This will get me thinking. Perhaps there is a start sequence to prevent both compressors from starting at same time? I do not know if this is so. I will check ASAP.

[Gary]

The high stage must be down to temperature before the low stage is allowed to start. Usually the low stage cycles on temperature while the high stage runs 24/7.

[aerobat72L]

Gary, The manual for this unit states the following:Plug the freezer into the proper outlet.The high stage will start immediately. An automatic timer will start the low stage system. It will start to operate and pull down to the set point. When the freezer reaches set point, both compressors will cycle off until the temperature changes and activates the compressor system. At this point, the high stage will start, and in two minutes the low stage will start and maintain the setpoint of the temperature control.

[Gary]

Seems things have changed since I worked on cascades.

Two minutes may not be enough for the high stage to get down to temp... especially if it is in a hot environment and/or the condenser gets dirty. If the high stage is not down to temp when the low stage starts, then it is trying to start against high pressure.

[norseman]

Hi from rainy Norway.

Several of the brands now use a first time start up timer +-10 minutes to get the HX down and it is mostly a waste of time, but just to be sure that the HX is really at a proper temperature.
After a unit's chamber is down at the cycling point you do not normally need a long delay between the high start up to the low stage start up. The main brand I work with has used a delay of just 20 second and that
does not make any trouble. Since those system start up with short delays, the HX is always pretty cold so you do not stress the system to much or say draw a lot of amperage to get the low stage up going.
It is more normal these days to have a temperature sensor on the HX itself so you are always shure that you never get a low stage up running before the HX is at a certain temperature. A common setting is around -40c and that protect against some wild condensing pressure and the high amperage.
The US maker Revco was early on controlling by HX temperature. It has to be 35 Years ago or like it when they used a simple mechanical thermostat to handle it and named D61 if I remember correct. The drawback was that a short power interuption made the both compressors to kick in at the same time due to the thermostat still was active. So Low is a little slow on design to not use the common HX control most makers use now? May be your delay timer just kick in without a delay after it has been on power for a certain time or the place ithe timer is situated is getting warm and by that make it to a malfunction? You never know with electronics, sometimes they do not withstand some heat caused by a weakness/malfunction.