Motor Input and Electrical Measurement Worksheet

It supports screening calculations from voltage, current and power factor, but verified true package kW from a suitable power analyzer remains the preferred input for specific-power, energy and lifecycle calculations.

1. Purpose and Engineering Decision

Electrical input is the numerator behind specific power, load/unload energy, fleet dispatch, heat recovery and lifecycle cost. If the power boundary is wrong, every downstream energy calculation can be wrong even when the arithmetic is correct.

It records what was measured, where it was measured, whether the compressor was loaded or unloaded, whether auxiliaries were included, and which values were measured directly versus calculated.

2. Measurement Boundary - Motor, Compressor Package or System

The worksheet must preserve the measurement point so a motor-only kW value is never silently compared against another machine's package kW.

3. True Power, Apparent Power and Power Factor

Power Factor = Real Power (kW) / Apparent Power (kVA)

kVA = kW / PF

Current by itself does not determine true electrical power unless voltage, phase configuration and power factor are also known. For VFD-driven motors and nonlinear loads, a suitable true-power analyzer at the drive input or package supply is preferred to a simple current-based estimate.

4. Balanced Three-Phase Screening Equation

P(kW) = sqrt(3) x V_LL x I_L x PF / 1000

This equation is appropriate as a screening calculation when the three-phase system is sufficiently balanced and the entered current and power factor represent the same operating point. For unbalanced or distorted systems, use direct multi-channel power measurement rather than an average-current shortcut.

5. Single-Phase Screening Equation

P(kW) = V x I x PF / 1000

The same warning applies: this is real-power estimation only when the voltage, current and PF belong to the same load state and waveform conditions are suitable for the instrument and method.

6. Energy over Time

Energy (kWh) = Average Real Power (kW) x Operating Time (h)

If power varies materially during the period, the worksheet should use logged interval energy or the time-weighted sum of operating states rather than multiplying one instantaneous kW reading by total hours.

7. Electrical Measurement Register

8. Three-Phase Per-Phase Worksheet

Per-phase values allow the worksheet to detect imbalance and to avoid hiding one abnormal phase behind a simple average.

9. Voltage Unbalance Screen

Voltage Unbalance (%) = Maximum Deviation from Average Voltage / Average Voltage x 100

The appropriate limit depends on motor, drive, equipment and site requirements.

10. Current Unbalance Screen

Current Unbalance (%) = Maximum Deviation from Average Current / Average Current x 100

Current imbalance can reflect supply imbalance, motor condition, connection issues or process loading. It is a diagnostic flag, not proof of a specific fault.

11. VFD-Driven Compressor Boundary

For a VFD compressor, the most useful energy measurement point is normally the VFD/package input because it captures drive losses and represents purchased electrical power. Measurements at the VFD output require instrumentation suitable for PWM waveforms and are not directly interchangeable with utility-side input kW.

12. Auxiliary Loads

Fans, oil pumps, cooling-water pumps, dryer loads, heaters and controls can create a meaningful difference between main-motor kW and total package/system kW.

Package kW = Main Motor Input kW + Sum(Auxiliary kW)

13. Reference Test Case A - Balanced Three-Phase Estimate

A compressor feeder is measured at 415 V line-line, 150 A average line current and 0.92 power factor during a stable loaded period.

P = sqrt(3) x 415 x 150 x 0.92 / 1000 = 99.2 kW

This remains a screening result.

14. Reference Test Case B - Why Current Alone Is Insufficient

Two operating points both draw 150 A at 415 V, but one has PF 0.92 and the other PF 0.72.

The same current corresponds to a difference of about 21.6 kW in this example.

15. Reference Test Case C - Voltage and Current Unbalance

The worksheet reports the numerical imbalance for review. It does not diagnose the cause or apply a universal pass/fail limit.

16. Reference Test Case D - Annual Energy from Logged States

A compressor operates 3,000 h/y at 105 kW loaded, 2,000 h/y at 38 kW unloaded and 1,000 h/y stopped at 1.5 kW standby/control power.

Annual kWh = (105 x 3,000) + (38 x 2,000) + (1.5 x 1,000)

This state-weighted energy result is more defensible than multiplying one instantaneous loaded kW value by the full annual hours.

17. Synchronized Flow, Pressure and Power

For specific-power analysis, electrical power must be synchronized with delivered flow and operating pressure. A flow reading from one hour and a power reading from another load state should not be divided together.

18. Input Model

19. Confidence Model

20. Validation and Guardrails

21. Output Specification