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Phil, Mechanical Engineer
Category: HVAC
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Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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9 year old Rheem 5 ton unit with a scroll compressor

Customer Question

9 year old Rheem 5 ton unit with a scroll compressor experiences a series of locked rotor events (1 to as many as 11 observed) before finally getting a clean start. This is more common during the hottest part of the afternoon in cooling, and during the coldest early morning hours when heating. As far as I can tell, the unit has never failed to start and has never tripped a breaker. This appears to be occurring about 5 to 10 percent of the time, but seems to be slowly getting worse.
Submitted: 2 months ago.
Category: HVAC
Expert:  airheatman replied 2 months ago.

I would start by checking the capacitor. Do you have model on unit?

I do how ever have questions about the locked rotor. If compressor draws locked rotor, breaker should definitely trip. Locked rotor is probably around 130 amps, so breaker should be tripping

Customer: replied 2 months ago.
Unit is a RPNE060JAZ. Capacitor has been replaced even though old one still checked OK. Can't tell exactly how many amps its pulling, but I have a Hall effect current sensor on the supply. The output voltage is supposedly proportional to the current through the sensor and the false starts spikes to more than twice the normal starting current, so by eyeballing the graph, the current spike is at least on the order of 90 plus amps. The false start spikes are short, less than 2 seconds, and when multiple false starts occur, the spikes show an increasing interval between spikes.
Expert:  airheatman replied 2 months ago.

Locked rotor is actually about 148 amps on this unit.

Wiring diagram shows a hard start as optional. Do you have the hard tart kit with a black round start capacitor?

Expert:  airheatman replied 2 months ago.

Start cap and relay

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