VSD Application
Variable Speed Drives (VSD), also called VFD (Variable Frequency Drive), allow adjusting compressor motor speed according to demand. They are the most efficient technology for variable loads.
Operating Principle
How It Works
Power grid VFD Motor Compressor
60 Hz → [Rectif.] → [Inverter] → ══════►
480 VAC DC bus 30-60 Hz Variable RPM
│ VAC adjust.
└── Controller
│
└── Pressure signal
Affinity Laws
For compressors (turbomachines), the affinity laws apply:
| Parameter | Relationship with Speed |
|---|---|
| Flow (Q) | Q ∝ N (linear) |
| Pressure (P) | P ∝ N² (quadratic) |
| Power (W) | W ∝ N³ (cubic) |
Example: Reduce speed to 80%
Flow: 80% of nominal
Pressure: 64% of nominal (0.8²)
Power: 51% of nominal (0.8³)
Great energy savings!
Operating Range (Turndown)
VSD Limits
Speed (%)
│
100 ┤══════════════════════════════════════ Maximum
│████████████████████████████████████████
90 ┤████████████████████████████████████████
│████████████████████████████████████████
80 ┤████████████████████████████████████████
│████████████████████████████████████████
70 ┤████████████████████████████████████████
│████████████████████████████████████████
60 ┤████████████████████████████████████████
│████████████████████████████████████████
50 ┤████████████████████████████████████████ OPTIMAL RANGE
│████████████████████████████████████████
40 ┤════════════════════════════════════════ Typical minimum
│░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░
30 ┤ Risk zone (overheating)
│░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░
20 ┤══════════════════════════════════════ Prohibited
│
| Parameter | Typical Value |
|---|---|
| Maximum speed | 100-105% |
| Minimum speed | 40-50% |
| Turndown ratio | 2:1 to 2.5:1 |
Why There's a Lower Limit
- Motor cooling: At low speed, fan cools less
- Lubrication: Reduced oil flow
- Efficiency: Fixed losses become dominant
- Stability: Centrifugal compressors may surge
Electrical Considerations
Harmonics
VFDs generate harmonics that can affect the electrical grid:
Ideal waveform: With harmonics:
╭───╮ ╭─╮
╱ ╲ ╱ ╲ ╲
╱ ╲ ╱ ╲ ╲
╱ ╲ ╱ ╭╮ ╲ ╲
╱ ╲ ╱ ╱ ╲ ╲ ╲
Pure sine wave Distorted
| Harmonic | Frequency | Effect |
|---|---|---|
| 5th | 300 Hz | Heating |
| 7th | 420 Hz | Heating |
| 11th | 660 Hz | Interference |
| 13th | 780 Hz | Interference |
Harmonic Mitigation
| Solution | Resulting THD | Cost |
|---|---|---|
| No filter | 30-40% | - |
| Line reactor | 20-25% | Low |
| Passive filter | 8-12% | Medium |
| Active filter | less than 5% | High |
| AFE (Active Front End) | less than 3% | Very high |
Bearing Currents
VFDs can cause parasitic currents in motor bearings:
High frequency
current
│
VFD ══> Motor ═══════╪═══════> Rotor
│
┌────┴────┐
│ Bearing │ ← Arc damage
└─────────┘ (fluting)
Symptom: Arc marks on bearing races (fluting)
Solutions:
| Solution | Effectiveness | Cost |
|---|---|---|
| Grounding ring | 80-90% | Low |
| Insulated bearings | 95% | Medium |
| Ceramic bearings | 99% | High |
| Shielded cables | 70-80% | Medium |
VSD Configuration
Main Parameters
| Parameter | Description | Typical Value |
|---|---|---|
| Minimum frequency | Lower limit | 25-30 Hz |
| Maximum frequency | Upper limit | 60-63 Hz |
| Acceleration ramp | Time 0-100% | 10-30 sec |
| Deceleration ramp | Time 100-0% | 10-30 sec |
| Current limit | Motor protection | 100-110% |
| Carrier frequency | PWM frequency | 2-8 kHz |
Integrated PID
Most VFDs include PID control:
Setpoint (target pressure)
│
▼
┌────────┐
Actual pressure ───→│ PID │──────→ Output
│Control │ frequency
└────────┘
│
┌─────────┴─────────┐
│ │
Kp Ki
Proportional Integral
| PID Parameter | Effect |
|---|---|
| High Kp | Fast response, possible oscillation |
| Low Kp | Slow response, stable |
| High Ki | Eliminates error quickly, possible overshoot |
| Low Ki | Residual error, very stable |
VSD vs Fixed Speed
Economic Comparison
Scenario: 100 HP Compressor, 8,000 h/year, $0.10/kWh
| Average Load | FS Annual Cost | VSD Annual Cost | Savings |
|---|---|---|---|
| 100% | $60,000 | $60,000 | 0% |
| 80% | $54,000 | $42,000 | 22% |
| 60% | $48,000 | $28,000 | 42% |
| 40% | $42,000 | $18,000 | 57% |
When to Use VSD
| Condition | Recommendation |
|---|---|
| Load greater than 80% constant | Fixed speed |
| Load 50-80% variable | VSD recommended |
| Load less than 50% frequent | VSD highly recommended |
| Single compressor system | VSD recommended |
| Multiple compressors | One VSD as "trim" |
Installation
Requirements
| Aspect | Requirement |
|---|---|
| Motor cables | Shielded, limited length |
| Ventilation | 10-15% VFD capacity in BTU/hr |
| Ambient temperature | 0-40°C typical |
| Altitude | Derating above 1000m |
| EMI filter | Required in most installations |
Typical Diagram
┌─────────────────────────────────────────────────────────┐
│ VSD INSTALLATION │
│ │
│ ┌────────┐ ┌────────┐ ┌────────┐ ┌────────┐ │
│ │ Main │ │Harmonic│ │ VFD │ │ Motor │ │
│ │Breaker │═══│ Filter │═══│ │═══│Compressor│ │
│ └────────┘ └────────┘ └────┬───┘ └────────┘ │
│ │ │
│ ┌────┴────┐ │
│ │Pressure │ │
│ │Transducer│ │
│ └──────────┘ │
└─────────────────────────────────────────────────────────┘
Common Problems
| Problem | Cause | Solution |
|---|---|---|
| Motor overheating | Low speed prolonged | External ventilation |
| Overcurrent fault | Ramp too fast | Increase ramp time |
| Pressure oscillation | Poorly tuned PID | Adjust Kp, Ki |
| Excessive noise | Low carrier frequency | Increase PWM frequency |
| Bearing damage | Parasitic currents | Install grounding ring |
| EMI interference | No filter | Install input filter |
Only One VSD as Trim
Never operate two VSDs in parallel trying to control the same pressure. They will "fight" oscillating between them. Use one VSD as trim and others at fixed speed.