Fridge compressor relay function
A refrigerator compressor start relay briefly energizes the compressor’s start winding to overcome mechanical inertia, providing the initial high torque required to get the motor spinning. Once the motor reaches approximately 75% to 80% of its operating speed, the relay disconnects power from the start winding, leaving only the run winding active during normal operation.
PTC start relay and overload protector internal components. Source: edu-search-genmedia
How It Works Step-by-Step
- Initial Startup Demand: Single-phase AC compressor motors cannot start on the run winding alone because they lack a rotating magnetic field. They require a secondary start winding to establish directional torque.
- Power Applied: When the thermostat signals cooling, full current flows into both the run winding and start winding simultaneously via the relay.
- Current Cut-Off: Within a fraction of a second to a few seconds (depending on the relay design), the start relay switches off power to the start winding. If left energized, the start winding would overheat and burn out.
Common Types of Compressor Relays
- PTC (Positive Temperature Coefficient) Relay: The modern industry standard. Uses a solid ceramic disc that rapidly heats up when current passes through it. As it heats, its resistance spikes drastically, cutting current flow to the start terminal.
- Electromechanical / Current Relay: Found in older or heavy-duty units. Uses a magnetic coil wire connected in series with the run winding. High inrush current pulls a plunger up to close the contacts for the start winding; as motor speed climbs and current drops, gravity drops the plunger back down, opening the start contacts.
Symptoms of a Failed Relay
Rattling Noise: On PTC relays, shaking the detached part often produces a rattling sound, indicating the internal ceramic disc has shattered.
Clicking Sound: The compressor tries to turn on, makes a distinct “click” sound every few minutes, but never starts (the overload protector trips due to high current).
Warm Fridge / Compressor Cold: The fan runs inside, but the compressor body remains idle or cold to the touch.
PTC start relay and overload protector internal components. Source: edu-search-genmedia
How It Works Step-by-Step
- Initial Startup Demand: Single-phase AC compressor motors cannot start on the run winding alone because they lack a rotating magnetic field. They require a secondary start winding to establish directional torque.
- Power Applied: When the thermostat signals cooling, full current flows into both the run winding and start winding simultaneously via the relay.
- Current Cut-Off: Within a fraction of a second to a few seconds (depending on the relay design), the start relay switches off power to the start winding. If left energized, the start winding would overheat and burn out.
Common Types of Compressor Relays
- PTC (Positive Temperature Coefficient) Relay: The modern industry standard. Uses a solid ceramic disc that rapidly heats up when current passes through it. As it heats, its resistance spikes drastically, cutting current flow to the start terminal.
- Electromechanical / Current Relay: Found in older or heavy-duty units. Uses a magnetic coil wire connected in series with the run winding. High inrush current pulls a plunger up to close the contacts for the start winding; as motor speed climbs and current drops, gravity drops the plunger back down, opening the start contacts.
Symptoms of a Failed Relay
- Clicking Sound: The compressor tries to turn on, makes a distinct “click” sound every few minutes, but never starts (the overload protector trips due to high current).
- Warm Fridge / Compressor Cold: The fan runs inside, but the compressor body remains idle or cold to the touch.
- Rattling Noise: On PTC relays, shaking the detached part often produces a rattling sound, indicating the internal ceramic disc has shattered.