Motor input and electrical measurement worksheet

Why electrical input is the number everything depends on

Package kW is the numerator in specific power (kW per m³/min), the basis of load/unload energy, fleet dispatch decisions, heat-recovery estimates and lifecycle cost comparisons. If the power boundary is wrong, every downstream calculation is wrong, however careful the arithmetic.

The worksheet therefore records not just numbers but context: what was measured, where it was measured, the operating state of the compressor, which auxiliaries were included and which values were measured directly versus calculated.

Choosing the measurement boundary

The most common error in compressor energy work is comparing figures measured at different boundaries. Decide the boundary first and state it on every result.

Manufacturer data sheets usually quote package input power at stated test conditions. If you measure motor input only, your result will look better than the true package figure. If you include a dryer, it will look worse. Neither is wrong, but they cannot be compared directly.

The formulas behind the worksheet

For a balanced three-phase supply, using line-to-line voltage V_L and line current I:

• Apparent power: S (kVA) = √3 × V_L × I ÷ 1,000
• Real power: P (kW) = √3 × V_L × I × PF ÷ 1,000
• Power factor: PF = P ÷ S
• Reactive power: Q (kVAr) = √(S² − P²)
• Energy: kWh = average kW × hours

For single phase, drop the √3 and use phase voltage. Motor shaft output is input kW × motor efficiency; for a fixed-speed motor near full load the efficiency is usually high, but it falls at light load.

The three-phase screen only works when voltages and currents are reasonably balanced and the power factor is known for the actual operating state. Power factor on an induction motor changes strongly with load: it is reasonably high when fully loaded and drops sharply when the compressor is unloaded. Using a full-load power factor for unloaded readings is a classic mistake.

Worked example: loaded and unloaded

A 415 V fixed-speed screw compressor is checked with a clamp meter and then with a power analyser. The readings are illustrative.

Loaded: √3 × 415 × 150 × 0.86 ÷ 1,000 = 92.7 kW. Unloaded with the measured power factor: √3 × 415 × 70 × 0.45 ÷ 1,000 = 22.6 kW. Had the loaded power factor of 0.86 been assumed, unloaded power would be calculated as 43.3 kW – nearly double the real figure. Any unload-energy or VSD-saving estimate built on that number would be badly overstated. This is why a true power meter, or at least a measured power factor for each state, is essential. Our load/unload cycle energy calculator uses these state-by-state kW values.

How to take the measurements

1. Plan for safety. Work on live panels only by qualified, authorised people under the site’s electrical permit system, with correct PPE and insulated, rated instruments. Use clamp-on current transformers; never open a live circuit to insert a meter.
2. Pick the point. Measure at the incoming supply to the package for the package boundary, or at the motor terminals or starter output for motor-only work.
3. Connect all three phases. Use a three-phase analyser with voltage on each phase and a current clamp on each line. Check clamp direction and phase rotation; a reversed clamp gives a nonsense power factor.
4. Check balance. Note voltage and current on each phase. Significant voltage imbalance raises motor losses and heating and is worth reporting on its own.
5. Log through full cycles. Record loaded, unloaded, blowdown and stopped states across representative shifts, with a sampling interval short enough to capture state changes.
6. Synchronise with pressure and flow. Align logger clocks so kW can be matched with discharge pressure and flow for specific-power calculations.
7. Record conditions. Ambient temperature, discharge pressure, running hours and service condition all affect kW.

VSD compressors and harmonics

Variable-speed drives draw non-sinusoidal current. Simple clamp meters and the V × I × PF method can be misleading on the supply side of a drive. Use a power analyser that measures true RMS values and true real power, and measure on the supply side of the drive, not between drive and motor, for the package boundary. The analyser will also report harmonic distortion, which matters for the plant’s power quality and for any DG set supplying the compressor.

The displacement power factor of a drive can look good while the total power factor, including harmonics, is lower. Record both if the analyser offers them.

Indian site conditions that affect readings

• Supply voltage variation. Grid voltage in many industrial areas varies through the day. Low voltage raises current for the same power; record voltage alongside current.
• High ambient heat. In summer, compressor rooms in Telangana and Andhra Pradesh can be very hot. Discharge temperatures rise, coolers work harder and fan loads increase, so summer and winter kW differ.
• DG set operation. When compressors run on standby generators, voltage and frequency can differ from grid supply, changing motor speed and power. Note the supply source on the worksheet.
• Capacitor banks. Plant power-factor correction improves the bill but does not change the compressor’s own kW. Measure downstream of any shared correction.

What the worksheet records

• Asset, make, model, serial number, rated motor kW and supply voltage.
• Measurement boundary and exact connection point.
• Instrument make, model, range and calibration date.
• Per-phase voltage, current, kW, kVA and power factor for each operating state.
• Logging start, end, interval and the shift pattern covered.
• Which values were measured and which were calculated.
• Discharge pressure, ambient temperature and supply source during the test.

Using the same instrument at the same points before and after a change removes much of the systematic error from the comparison. The measurement uncertainty worksheet shows how to quantify the remaining doubt.

How Reddy Compressors helps

Reddy Compressors is an independent multi-brand compressor specialist at Tarnaka, Secunderabad, serving industry since 2002. We sell, rent and service ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and other makes – we are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. Our service team of over 20 service engineers and technicians covers all of Telangana and Andhra Pradesh, and we supply equipment across India.

• Carry out power and flow logging at a clearly stated boundary, synchronised with pressure and operating state.
• Calculate specific power and energy on a consistent basis for every compressor in the house.
• Investigate high readings: fouled coolers, restricted inlets, separator pressure drop, electrical imbalance.
• Recommend and supply new or used/refurbished compressors and VSD trim machines where the data supports a change, and rent machines for trials or cover.

What to send us

• Compressor make, model, serial number and nameplate photos of compressor and motor.
• Single-line diagram or a photo of the panel and supply arrangement.
• Any existing meter readings, logs or electricity bill data for the compressor feeder.
• Number of compressors and dryers, and how they are controlled.
• Shift pattern and whether a DG set ever supplies the compressors.

Frequently asked questions

How do I calculate the kW of a three-phase compressor motor?

Multiply 1.732 (the square root of three) by the line voltage, the line current and the power factor, then divide by 1,000. For example, 415 V, 150 A and a power factor of 0.86 gives about 92.7 kW. This only holds for a reasonably balanced supply and a power factor measured at the same operating state.

Why are my compressor amps high when it is unloaded?

An unloaded induction motor still draws magnetising current, so current stays relatively high even though real power falls a lot. The power factor drops sharply, which means much of that current is reactive rather than doing work. Measure true kW with a power analyser rather than judging unloaded consumption from amps alone.

What is the difference between kW and kVA for a compressor?

kW is real power, the energy actually converted to work and heat, and it is what you pay for in kWh. kVA is apparent power, the product of voltage and current, which sizes cables, transformers and generators. The ratio of kW to kVA is the power factor. A DG set for a compressor must be sized on kVA and starting current, not kW alone.

Should I measure motor input or package input for specific power?

Use package input – main motor plus cooling fan, controls and any package pumps – when comparing compressors, because that is the basis manufacturers normally quote. Motor-only figures look better than the true package value. Whatever boundary you choose, state it on every result so that figures are compared like for like.

Can I use a clamp meter on a VSD compressor?

A basic clamp meter can mislead on a variable-speed drive because the input current is distorted by harmonics. Use a true RMS power analyser on the supply side of the drive, which measures real power directly and can also report harmonic distortion. Readings taken between the drive and motor do not represent package input.

How long should I log compressor power?

Long enough to capture every operating pattern that matters: complete load and unload cycles, each shift, and ideally weekdays and weekends. For a stable single-shift plant a few days may do; for variable demand a week or more is better. Seasonal differences, especially summer heat, may justify repeat logging later in the year.

Related pages

• Specific-power assessment
• Specific power and station KPI engineering
• Energy baseline adjustment worksheet
• Part load fleet dispatch calculator
• Compressed-air energy and efficiency engineering
• Compressed air knowledge and calculation centre