Refrigeration compressor operating at single fixed rotational speed (RPM) determined by motor design and electrical frequency, controlled exclusively by on/off cycling in response to temperature demand – the dominant but increasingly obsolete technology in transport refrigeration despite superior variable-speed alternatives being commercially available. Why It Matters: Fixed-speed compressors represent 1960s technology perpetuated by transport refrigeration industry despite modern variable-speed DC alternatives delivering 25-35% better fuel efficiency. The fundamental limitation: fixed-speed compressor must run at 100% capacity or shut off completely, unable to modulate output matching actual thermal load. When cargo space requires 40% cooling capacity, fixed-speed system delivers 100% cooling briefly, then shuts off completely, then restarts repeatedly – versus variable-speed compressor simply running continuously at 40% speed. This cycling wastes energy, creates temperature instability, and causes mechanical wear, yet industry continues installing fixed-speed systems because they cost R25,000 less to manufacture and generate more maintenance/fuel revenue throughout vehicle lifetime.
Technical Characteristics:
- RPM: Fixed by motor pole count and electrical frequency (typically 1,450 or 2,900 RPM on 50Hz supply)
- Capacity Control: On/off cycling only; no intermediate speeds
- Electrical Draw: 100% rated current when running, 5-7x surge during startup
- Efficiency: Highest at full load, zero at shutdown, averaged across cycling
- Temperature Control: ±2-4°C around setpoint due to cycling hysteresis
Operating Pattern Example:
Typical Fixed-Speed Cycle (40% Average Load):
00:00 - Compressor OFF (temperature drifting upward)
00:12 - Temperature reaches upper setpoint (+1.5°C above target)
00:12 - Compressor starts (5-7x current surge for 3-5 seconds)
00:13 - Compressor reaches steady operation (100% capacity)
00:45 - Temperature reaches lower setpoint (-1.5°C below target)
00:45 - Compressor stops (wasted cooling in last minutes)
00:46 - Temperature begins drifting upward again
...cycle repeats every ~45 minutes
Daily: ~32 on/off cycles
Energy waste: ~20% from startup surges and temperature overshoot
Mechanical stress: 32 startup surges per day = 11,680/year
Comparison to Variable-Speed:
| Characteristic | Fixed-Speed | Variable-Speed |
|---|---|---|
| Speed Range | Single speed only | 20-100% modulation |
| Capacity Control | On/off cycling | Continuous modulation |
| Daily Cycles | 20-40 cycles | 0 cycles (continuous) |
| Startup Surges | 20-40/day | 0-1/day |
| Temperature Stability | ±2-4°C | ±0.5-1°C |
| Fuel Consumption | 100% baseline | 65-75% of fixed |
| Maintenance | Standard | Reduced 30-40% |
| Purchase Price | R45,000 | R70,000 |
| 10-Year TCO | R1,300,000 | R950,000 |
Why Industry Persists:
- Lower Manufacturing Cost: R20-25K less than variable-speed
- Familiar Technology: Bodybuilders trained on fixed-speed installation
- Fuel Revenue: Diesel TRU manufacturers profit from higher consumption
- Maintenance Revenue: More cycling = more wear = more repairs
- Buyer Price Focus: Purchase price visibility vs lifecycle cost invisibility
Related Terms: Variable Speed Compressor, Compressor Cycling, Energy Efficiency (Cold Chain), Total Cost of Ownership (TCO), Compressor Displacement