Compressor Cycling

The repetitive start/stop operation pattern of fixed-speed refrigeration compressors, occurring 20-40 times daily in courier applications and wasting 25-35% of consumed energy through startup surge inefficiency, inability to match partial loads, and accelerated mechanical wear. Compressor cycling is the observable symptom of 1960s technology still being sold for 2020s applications.

Why Fixed-Speed Compressors Cycle

Fixed-speed compressors have one operating state: maximum output. Temperature control happens through thermostat cycling:

Temperature rises above setpoint → compressor starts
Compressor runs at full capacity
Temperature falls below setpoint → compressor stops
Temperature gradually rises → cycle repeats

This binary control cannot match variable thermal loads. A 50% thermal load still triggers 100% compressor output in shorter bursts rather than continuous 50% operation.

Why Compressor Cycling Wastes Energy

Compressors consume maximum energy during startup, creating power surges that dwarf steady-state operation. A fixed-speed compressor might cycle 30-50 times daily in courier operations compared to 6-8 times for properly matched equipment—a 400-600% increase in energy-intensive startup events. Industry research confirms this cycling pattern increases energy consumption by 20-50% compared to systems that run continuously at variable speeds.

The Energy Cost of Cycling

Each compressor start:

Draws 3-5× steady-state current (inrush/locked rotor amps)
Consumes 2-3 minutes establishing stable refrigerant flow
Requires additional energy to restart compression after pressure equalization
Generates heat from inefficient transitional operation

Estimated energy overhead per start: 5-10% of full cycle consumption

At 30 cycles daily × 5% overhead:

Daily waste: 150% of one cycle’s energy
Annual waste: 500+ hours equivalent of wasted compressor operation
Cost at R18/L diesel, 2L/hr consumption: R18,000+ annual waste per vehicle

The Multi-Stop Problem

Transport refrigeration in last-mile delivery faces wildly variable thermal loads: doors open and close, ambient temperatures change between shaded and sun-exposed locations, and cargo mass decreases throughout the route. Fixed-speed compressors respond to these variations by cycling—running at full power when any cooling is needed, then shutting off completely. This binary operation cannot match partial loads efficiently.

When a fixed-speed compressor operates under light-load conditions, it runs for short spurts providing rapid cooling, which is inherently inefficient. The compressor delivers maximum cooling capacity whether the cargo needs 100% or 20% of that capacity.

Mechanical Wear Implications

Compressor starts stress:

Motor windings (thermal expansion from inrush current)
Shaft bearings (no hydrodynamic oil film at startup)
Valve plates (pressure differential shock loading)
Electrical contactors (arcing during switching)

A compressor rated for 100,000 hours continuous operation might fail after 50,000 hours of cycling operation due to accumulated start/stop wear.

Each start-stop cycle is equivalent to 10-15 minutes of continuous running in wear accumulation. A courier compressor cycling 30 times daily accumulates wear equivalent to 5-7.5 hours additional operation—every single day.

The Variable-Speed Alternative

Variable-speed compressors modulate output continuously:

50% thermal load → 50% compressor speed
No cycling wear
No startup surge loss
Precise temperature control
25-35% energy reduction
Extended component life

The technology exists, performs proven, and pays back in under 12 months. The industry sells fixed-speed systems because they cost less at purchase and generate more maintenance revenue throughout ownership.

Compressor Cycling

SA Cold Chain Directory