What secondary circuit breaker should I use for my fridge compressor?

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Discussion Overview

The discussion revolves around selecting an appropriate secondary circuit breaker for a refrigerator compressor, specifically focusing on the amperage rating needed to protect the compressor and solid-state relay (SSR). Participants explore various specifications of the compressor and the implications of different breaker ratings, considering factors such as locked rotor amps and running current.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant mentions replacing a broken inverter compressor with a standard compressor and seeks advice on the appropriate amperage for a self-resetting circuit breaker, noting that 15 amps seems too high and 2 amps too low.
  • Another participant provides details from the nameplate of an AC condenser unit, suggesting that most compressors are damaged by overcurrent during the starting phase and recommends checking breaker sizes available for the compressor's start circuit.
  • A participant shares detailed specifications of the refrigerator compressor, including its locked rotor amps and cooling capacity, but notes the lack of information on the proper inline circuit breaker.
  • One participant estimates that the 1/3 HP compressor will draw around 2 to 3 amps running and suggests a thermal, slow trip breaker rated around 8 to 12 amps, emphasizing the need for a delay to allow the compressor to reach operating speed.
  • Another participant questions the suitability of a specific thermal circuit breaker for outdoor temperatures, advising to examine the trip characteristic curve.
  • There is a suggestion that the actual current rating needed depends on the compressor's starting current, with recommendations varying between 6 to 10 amps based on the installation environment's temperature conditions.
  • A participant inquires about using a varistor across the SSR terminals for voltage spike protection and shares a specific model for consideration.
  • One participant revises their earlier breaker suggestion to 5 amps after re-evaluating the compressor's locked rotor rating and expresses confidence in the selected varistor's adequacy.

Areas of Agreement / Disagreement

Participants express varying opinions on the appropriate amperage for the circuit breaker, with no consensus reached. There are multiple competing views on the best rating based on different assumptions about the compressor's operating conditions and starting current.

Contextual Notes

Participants note the importance of considering the compressor's locked rotor current and the ambient temperature conditions when selecting a circuit breaker. The discussion highlights the complexity of determining the appropriate breaker size due to the variability in compressor specifications and environmental factors.

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TL;DR
I am replacing a broken inverter compressor with a regular compressor that has the standard electromechanical relay. I plan on controlling the compressor/electro relay with an ac to ac Ssr. The Ssr will be triggered by the condenser fan that is a 110v circuit.
The compressor looks like it runs on 2 amps, but has 15 locked rotor amps. I would like to install a self resetting circuit breaker between the compressor and Ssr. What amperage breaker would I need? 15 seems too high and 2 too low.
I am replacing a broken inverter compressor with a regular compressor that has the standard electromechanical relay. I plan on controlling the compressor/electro relay with an ac to ac Ssr. The Ssr will be triggered by the condenser fan that is a 110v circuit. The compressor looks like it runs on 2 amps, but has 15 locked rotor amps. I would like to install a self resetting circuit breaker between the compressor and Ssr. What amperage breaker would I need? 15 seems too high and 2 too low.

Reference: https://www.physicsforums.com/forums/electrical-engineering.102/post-thread
 
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Please, see picture of nameplate of an AC condenser unit, which fan's label shows MCA of 1.0 A
Note how much over the recommended maximum protection (30 A) is respect to the total MCA of the condenser (17.8 A).

I would see what breaker sizes are available to install between the solid state relay and the start circuit of the compressor (assuming the run circuit is left out of the additional protection).
Most compressors are damaged by overcurrent at the starting phase.

DE338103-B9AB-47C9-86C7-21379A4FA236.jpeg
 
How do I determine the proper in line circuit breaker? This is a refrigerator compressor and it does not say much on it, other than it is 15 locked rotor ampsHere are some specs:
1/3- hp
Displacement 7.7cc
From 100W to 671W Cooling (341 to 2289BTUs) ASHRAE : 220w - 751btu
110-120VAC
50-60Hz
R134A
Low Back Pressure
PTC Starting Relay
Motor Type CSIR
Includes electricals, grommets and sleeves.

Cooling capacity ASHRAE
-35C (-31F) - 100w - 341btu
-30C(-22F) - 130w - 444btu
-25C(-13F) - 169w - 577btu
-23.3C(-10F) - 220w - 751btu - input power 165W - current 2.40 Amps - COP 1.35 - EER 4.55
-20C(-4F) - 275w - 938btu
-15C(5F) - 344w - 1174btu
-10C(10F) - 430w - 1467btu
-5C(23F) - 537w - 1832btu
0C(32F) - 671w - 2289btu
Certification : UL Underwriters Laboratories Recognized
Nationally Recognized Testing Laboratories (NRTL)
Reply
 
The 1/3 HP compressor will draw around 2 to 3A running.
With the locked rotor current of 17.8A (as @Lnewqban mentioned) I suggest a Thermal, Slow Trip (time delay), Auto Reset breaker rated around 8 to 12A. (I'm leaning towards the 12A range.)

That will yield a delay long enough for the compressor to reach operating speed but low enough to trip if the compressor gets fluid locked or the motor develops an internal short circuit.

Picking an exact breaker is not possible without details of the starting current and the delay curves of available breakers. The above values, however, should be well within the working range.

By the way, be aware that once the refigerant line is opened, the dessicator will need replacement, and the system evacuated with a vacuum pump before charging with refrigerant - otherwise the ambient moisture that enters the system will freeze in the evaporator (and also dilutes the oil in the compressor, sometimes causing it to foam).

Cheers,
Tom

p.s. Let us know what works -- or doesn't!
 
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That looks like a decent option.

The actual current rating needed depends on the compressor starting (locked rotor) current. The breaker ratings so far are based on 18Amps peak load.

Let us know if your chosen compressor is significantly different.

After looking at the data sheet, I would suggest a 10Amp rating if it is used in a conditioned (living) area. If in an area that gets cold (close to freezing) then a 8Amp or maybe 6Amp is more appropriate.

As @DaveE noted, if it is in an un-heated or un-cooled garage or utility room, then the temperature extremes need to be accounted for because the time to trip depends on the ambient temperature.

Cheers,
Tom
 
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I just re-read this thread and realized your compressor is rated at 10A Locked Rotor. Based on this I change my Breaker suggestion to 5Amp.

Now to the MOV:
:cry: Sorry, neither DigiKey nor LittleFuse search pages will talk to my old browser.

However based on the information on the page you linked to, that MOV should be quite sufficient, Good Choice!

Cheers,
Tom
 

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