No-load subtraction is not full segregation
A no-load test includes friction, windage, core, and some stator copper effects at different current and slip. Direct subtraction cannot uniquely allocate loaded losses.
Electrical Engineering
Motor Loss Calculator
Reconcile measured motor input and shaft output, estimate hot stator I-squared-R loss, and expose residual loss.
MOTOR LOSS RECONCILIATION
Split measured motor loss into stator copper and an auditable residual
This field-oriented model compares measured three-phase input with measured torque-speed output, then estimates stator copper loss from per-phase hot resistance and connection. The remaining loss is deliberately labeled residual rather than falsely allocated among rotor, core, friction, windage, and stray components.
Measured total loss (kW)—
Measured efficiency—
Stator copper estimate (kW)—
Residual loss (kW)—
Residual above no-load input (kW)—
Measured shaft output (kW)—
MOTOR LOSS RECONCILIATION
Measured motor loss and residual ledger
Use the residual as a diagnostic quantity, not a named failure mode. Separating rotor, core, mechanical, and stray losses requires a standard segregated-loss test or additional measurements.
| Power component | Measurement A | Measurement B | Multiplier/context | Calculated value | Unit or boundary |
|---|
CURRENT CALCULATION PROCESS
Formula, current substitution, intermediate values, and reconciliation
P_total loss = P_in − P_shaft; P_stator cu = 3I_phase²R_phase; P_residual = P_total loss − P_stator cu
Three-phase real input and torque-speed output establish measured total loss. Wye phase current equals line current; delta phase current equals line current divided by √3. Hot per-phase resistance gives a stator copper estimate; everything else remains residual.
| Symbol | Engineering meaning | Unit | Default |
|---|---|---|---|
| V_LL | Measured line-to-line voltage | V | 460 |
| I_line | Measured line current | A | 40 |
| PF | True power factor | ratio | 0.88 |
| T_shaft | Measured shaft torque | N*m | 90 |
| n | Measured shaft speed | rpm | 1760 |
| R_phase | Hot per-phase winding resistance | ohm | 0.12 |
| P_0 | Measured no-load input | kW | 1.2 |
Intermediate values remain unrounded until display formatting.
HOW TO USE THIS MODEL
Close the measurement boundary before naming losses
- Synchronize three-phase electrical input, shaft torque, and speed at a stable state.
- Identify wye or delta connection and use a per-phase winding resistance corrected to operating temperature.
- Compute total measured loss before estimating any component.
- Subtract only the supported stator copper estimate and keep the balance labeled residual.
- Use no-load input as context, then apply a standard segregated-loss method if component allocation affects a decision.
MOTOR LOSS RECONCILIATION FUNDAMENTALS
Loss accounting without false precision
- Total measured loss
- Difference between real electrical input and mechanical shaft output at the same boundary and time.
- Stator copper loss
- Three-phase winding I²R loss using phase current and hot per-phase resistance.
- Connection effect
- Delta phase current is line current/√3; wye phase current equals line current.
- Residual loss
- Unallocated balance containing rotor copper, core, friction, windage, stray-load, and measurement error.
- No-load input
- Combined losses at unloaded operation; it is not identical to core loss.
- Temperature correction
- Winding resistance rises with temperature, so cold resistance understates operating copper loss.
MODEL AND FORMULA
Measured balance plus one supported loss component
P_total loss = P_in − P_shaft; P_stator cu = 3I_phase²R_phase; P_residual = P_total loss − P_stator cu
Three-phase real input and torque-speed output establish measured total loss. Wye phase current equals line current; delta phase current equals line current divided by √3. Hot per-phase resistance gives a stator copper estimate; everything else remains residual.
DEEPER ENGINEERING ANALYSIS
Why the residual should remain residual
Resistance must match connection and temperature
Using line-to-line resistance as per-phase resistance or ignoring delta phase current can create a copper estimate larger than total measured loss.
Uncertainty belongs in the balance
Input power, torque, speed, resistance, temperature, and timing uncertainty can be comparable to a small residual. Repeat tests and quantify uncertainty before diagnosing degradation.
WORKED DECISION CASES
Diagnostic uses of the loss balance
Rewind quality investigation
After a rewind, measured total loss increases. Hot-resistance I²R explains only part of the change, so the remaining residual directs a standardized test instead of an unsupported core-loss claim.
Bearing or ventilation concern
A motor shows elevated residual loss with stable copper estimate. Vibration, temperature, no-load test, lubrication, and airflow evidence are gathered before assigning mechanical loss.
TECHNICAL LANGUAGE
Motor loss-analysis terms
- Per-phase resistance
- Resistance of one stator phase winding at a stated temperature.
- Stator copper loss
- Electrical heating in stator winding resistance.
- Rotor copper loss
- Electrical loss in the rotor conductors associated with slip.
- Core loss
- Magnetic hysteresis and eddy-current loss in iron.
- Mechanical loss
- Friction and windage associated with rotation and cooling.
- Segregated-loss test
- Standardized procedure that separates efficiency losses using multiple test observations.
EVIDENCE AND DATA LINEAGE
Loss-test records for later diagnosis
Retain raw three-phase power channels, torque and speed series, motor and drive identification, connection diagram, hot-resistance measurement and temperature correction, no-load test conditions, calibration certificates, sample timing, ambient, load state, vibration and temperature observations, and unrounded balance values.
LIMITS AND EXCLUSIONS
What the field balance cannot allocate
- The page assumes balanced three-phase measurements and a known wye/delta connection.
- It does not perform a standardized segregated-loss test or individually calculate rotor, core, friction, windage, stray-load, harmonic, or VFD losses.
- The no-load value is shown only for context; it is not used to claim a specific component split.
RELIABLE SOURCES
References for this page's method and boundaries
FREQUENTLY ASKED QUESTIONS
Motor loss reconciliation questions
Why not label the residual as core loss?
Because it also contains rotor copper, mechanical, stray-load losses, and measurement uncertainty. Additional tests are needed to separate them.
Should I use cold winding resistance?
No. Correct resistance to operating winding temperature or measure it appropriately after the run; cold resistance understates I²R loss.
How does delta connection change copper loss?
For a balanced delta, phase current is line current divided by √3. The page converts line current before applying 3I²R.
Can no-load input be subtracted directly as fixed loss?
It can be a comparison, but no-load conditions differ in current, slip, temperature, and airflow. It does not uniquely identify core and mechanical components.
Why can copper loss exceed measured total loss?
That inconsistency points to wrong connection, resistance basis, temperature, current, timestamps, torque, speed, or instrument data. The calculator rejects a material mismatch.
Does the page work at VFD output?
Only with waveform-capable power measurement and a clearly defined boundary. PWM, harmonic losses, drive output methods, and low-speed cooling require special care.
RELATED CALCULATORS
Continue the electrical engineering review
Use the next model to test a separate operating boundary without hiding it inside this result.
IMPORTANT ENGINEERING NOTE
Do not diagnose a failure mode from the residual alone
Use calibrated instruments, correct winding-resistance temperature, synchronized boundaries, uncertainty analysis, applicable standardized test methods, and qualified motor expertise before efficiency guarantees, repair acceptance, or fault diagnosis.