Power Measurement

Accurate electrical power measurement is essential for calculating system efficiency and the true cost of compressed air.

Why Measure kW (Not Just Amps)

The Problem with Measuring Only Current

    100 HP Motor:

    Case A:              Case B:
    I = 120 A            I = 120 A
    V = 460 V            V = 440 V
    PF = 0.85            PF = 0.75

    kW = 81.2            kW = 68.6
         ↑                    ↑
         │                    │
    Same current, different real power

Three-Phase Power Formula

$$kW = \frac{V \times I \times \sqrt{3} \times PF}{1000}$$

Where:

• $V$ = Line-to-line voltage (V)
• $I$ = Line current (A)
• $PF$ = Power factor (0-1)
• $\sqrt{3} \approx 1.732$

Parameter	Affects kW	Notes
Voltage	Yes	Typically varies ±5%
Current	Yes	Indicates load
Power Factor	Yes	Varies 0.7-0.95

Power Factor

What It Is

                 Apparent Power (kVA)
                        ╱│
                      ╱  │
                    ╱    │ Reactive Power (kVAR)
                  ╱      │
                ╱────────┘
              Real Power (kW)

    PF = kW / kVA = cos(θ)

Typical Values

Motor Condition	Power Factor
100% load	0.85-0.90
75% load	0.80-0.85
50% load	0.70-0.80
25% load	0.55-0.70
VSD operating	0.95+ (with filters)

Impact on Measurement

Assumed PF	Actual PF	Error in kW
0.85	0.75	+13%
0.85	0.90	-6%
0.85	0.80	+6%

Types of Meters

Basic Clamp Meter

    ┌─────────┐
    │  120 A  │ ← Only measures current
    │   ⊙     │
    └────┬────┘
         │
    ═════╪═════
       Cable

Measures	Does Not Measure
Current (A)	Voltage
	Power factor
	Real kW

Use: Quick reference only, not for analysis

Power Analyzer

    ┌─────────────────────┐
    │  kW: 75.2           │
    │  kVA: 88.5          │
    │  PF: 0.85           │
    │  V: 458 V           │
    │  A: 111 A           │
    └─────────────────────┘
         │ │ │
         V I V (3 phases)

Parameter	Typical Accuracy
Voltage	±0.5%
Current	±1%
Power	±1-2%
Power factor	±0.01
Harmonics	Up to 50th

Energy Logger

    Logging period: 7 days

    kW
    │    ╭───╮     ╭───╮
    80├───╯   ╰─────╯   ╰───
    │
    60├─────────────────────
    │
    40├──
    │  Night
    └────────────────────────── t
        Mon   Tue   Wed   Thu

Records continuous data for load profile analysis.

Meter Installation

Typical Connection

    Breaker
        │
    ┌───┴───┐
    │       │         Current
    │   M   │         transformers (CT)
    │       │              │
    └───┬───┘         ╭───┴───╮
        │             │ ○ ○ ○ │
    ════╪═════════════╪═══════╪════> Motor
    L1  L2  L3        │       │
                      │       │
                 ┌────┴───────┴────┐
                 │     Meter       │
                 │      kWh        │
                 └─────────────────┘

CT Ratios

Motor Current	CT Ratio	Secondary
0-100 A	100:5	5 A
100-200 A	200:5	5 A
200-400 A	400:5	5 A
400-800 A	800:5	5 A

Efficiency Calculations

Motor Efficiency

Motor Efficiency = Mechanical power output / Electrical power input

                 = (HP × 0.746) / measured kW

Example:
  Nameplate: 100 HP
  Measured kW: 78 kW

  Efficiency = (100 × 0.746) / 78 = 95.6%

System Specific Power

Specific Power = Total kW / (Total Flow / 100)

Example:
  Compressors: 150 kW total
  Total flow: 750 CFM

  SP = 150 / 7.5 = 20 kW/100 CFM

Measurement with VSD

Special Considerations

    ┌─────────────────────────────────────────┐
    │                                          │
    │   Grid ══> [VSD] ══> Motor              │
    │              │                           │
    │         Harmonics                       │
    │         Distortion                      │
    │                                          │
    │   Meter must be:                        │
    │   • True RMS                            │
    │   • Capable of measuring harmonics      │
    │   • Variable frequency                  │
    │                                          │
    └─────────────────────────────────────────┘

Measurement Location	Measures	Accuracy
VSD input	Total system consumption	High
VSD output	Motor only (excludes VSD losses)	Complex

VSD Losses

VSD Load	Typical Losses
100%	3-5%
75%	4-6%
50%	5-8%
25%	8-12%

Common Errors

Error	Impact	Solution
Measuring only current	10-30% error	Use power analyzer
Ignoring power factor	10-20% error	Measure actual PF
Incorrect CT ratio	Proportional error	Verify CT nameplate
Not considering harmonics	5-15% error	Use True RMS meter
Instantaneous measurement	Doesn't represent actual operation	Log over extended period

Data Verification

Plausibility Check

Maximum theoretical power:
  kW_max = HP × 0.746 / Motor efficiency

Example for 100 HP motor, 0.93 efficiency:
  kW_max = 100 × 0.746 / 0.93 = 80.2 kW

If you measure more than 80 kW, there's an error.

Comparison with Nameplate Data

Data	Source	Compare
Rated voltage	Motor nameplate	Measured voltage (±10%)
Full load current	Motor nameplate	Measured current
Service factor	Motor nameplate	Actual load
Efficiency	Motor nameplate	Calculated efficiency

Practical Rule

For audits, always measure real kW over a representative period (minimum 24 hours, ideal 7 days). Instantaneous measurements can be misleading.