The ratio of compressor running time to total elapsed time, expressed as a percentage. A 60% duty cycle means the compressor operates 36 minutes per hour. Duty cycle reveals whether refrigeration capacity matches thermal load—and exposes why fixed-speed compressors waste 25-35% of their energy through crude on/off control.
What Duty Cycle Reveals
Optimal duty cycle for transport refrigeration: 40-60%
This range indicates:
- Sufficient capacity to handle thermal loads with margin
- Compressor cycling that allows proper oil return and component cooling
- Reserve capacity for peak events (door openings, ambient temperature spikes)
High duty cycle (>80%) indicates:
- Undersized equipment struggling to maintain temperature
- System running near maximum capacity continuously
- No reserve for thermal events
- Accelerated wear from constant operation
- Impending temperature excursions
Low duty cycle (<30%) indicates:
- Oversized equipment (wasteful but not harmful)
- Or extremely light thermal load (empty vehicle, mild conditions)
The Fixed-Speed Problem
Fixed-speed compressors operate at one speed: maximum. Temperature control happens through cycling—compressor runs until setpoint reached, stops until temperature rises, runs again.
This creates problems:
- Startup surge: Each compressor start draws 3-5× running current, stressing electrical systems
- Cycling wear: Start/stop cycles wear bearings, valves, and electrical contacts faster than continuous operation
- No partial-load matching: A 50% thermal load still triggers 100% compressor operation in shorter bursts
- Temperature oscillation: Cargo temperature swings ±2-3°C around setpoint with each cycle
Typical fixed-speed courier operation: 20-40 compressor cycles daily
Each cycle includes startup surge, thermal shock, and mechanical stress. Across a year, that’s 5,000-10,000 wear events that variable-speed systems eliminate.
Variable-Speed Advantage
Variable-speed compressors modulate output to match load:
- 50% thermal load = 50% compressor speed
- Continuous operation at appropriate output
- No cycling wear
- Tight temperature control (±0.5°C)
- 25-35% energy reduction
A variable-speed system achieving 40% duty cycle at partial speed consumes far less energy than a fixed-speed system achieving 40% duty cycle through on/off cycling—and experiences dramatically less mechanical wear.
Monitoring Duty Cycle
Fleet operators should track duty cycle as a diagnostic metric:
- Rising duty cycle over time suggests developing problems (refrigerant loss, condenser fouling, door seal degradation)
- Consistently high duty cycle indicates undersized equipment
- Sudden duty cycle changes warrant immediate investigation
Professional temperature monitoring systems log compressor runtime; amateur systems log only temperature, missing the diagnostic value of duty cycle trending.
Related Terms: Variable Speed Compressor, Fixed-Speed Compressor, Compressor Cycling, Energy Efficiency (Cold Chain)
Related Articles: The Constant-Speed Curse: Why Variable-Speed DC Compressors Could Slash Your Duty Cycle by 40%