The most common field failure we see in OEM controllers is not a bad sensor or a firmware bug — it's a burned-out relay contact. Usually the compressor relay. Usually because someone sized it by running current instead of startup current.
Why Compressor Relay Contacts Weld Shut
A refrigeration compressor is an inductive motor load. When it starts, the locked-rotor current (LRA) can be 5 to 7 times the running current. On a 1/4 HP compressor drawing 1.2A running, startup can hit 7–9A for half a second.
That initial inrush is where contacts wear. If the relay is rated close to running current, repeated startups arc the contacts, they pit, they overheat, and eventually they weld shut — the compressor runs nonstop and the cabinet freezes solid.
Sizing Rule of Thumb
For the compressor output, choose a relay whose rated current is at least 2× the running current, and confirm it can handle the LRA. In practice, this means a 16A relay for most commercial refrigeration compressors up to 1/2 HP.
| Load | Typical Current (220V) | Recommended Relay |
|---|---|---|
| Compressor (1/8–1/3 HP) | 0.5–1.5A run, 4–9A start | 16A |
| Evaporator fan | 0.05–0.15A | 5A |
| LED lighting | 0.02–0.2A | 3–5A |
| Defrost heater | 1–7A (200–1500W) | 10–16A |
| Defog / door heater | 0.05–0.5A | 3–5A |
Two More Loads People Under-Size
Defrost Heaters
An electric defrost heater is resistive, so there's no motor inrush. But a cold heater wire has lower resistance than a hot one — the initial current spike can be 1.5× steady state. A 1000W heater draws 4.5A steady, but 6–7A on first switch-on. Use a 10A or 16A relay.
Fans with Capacitors
Most evaporator fans are small shaded-pole motors, fine on a 5A relay. But if the fan uses a run capacitor, the inrush is higher than a simple shaded-pole unit. Check the fan's nameplate for LRA.
Common Mistake: One Relay for Everything
Some OEMs specify a controller with identical relay ratings on every output. That's fine for fan and light, but the compressor output needs the heavy contact. When we build custom firmware, we confirm which output drives the compressor and spec that relay separately — the others can stay at 5A to keep cost down.