Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC 61986: Rotating Electrical Machines — Equivalent Loading and Superimposed Single-Phase Method for Temperature Rise Testing
✅ Standard at a Glance
IEC 61986, published in 2002 by IEC Technical Committee 2 (Rotating machinery), specifies equivalent loading methods for temperature rise testing of rotating electrical machines without the need for a mechanical load connected to the shaft. The standard is particularly valuable when testing large machines (above 500 kW) where supplying a rated-load mechanical load is impractical, or where no suitable load machine exists on site. The primary method is the superimposed single-phase (SSP) method, which creates rated-load thermal conditions by injecting a single-phase current into the machine windings while the machine operates at synchronous or near-synchronous speed with no mechanical load.
IEC 61986, published in 2002 by IEC Technical Committee 2 (Rotating machinery), specifies equivalent loading methods for temperature rise testing of rotating electrical machines without the need for a mechanical load connected to the shaft. The standard is particularly valuable when testing large machines (above 500 kW) where supplying a rated-load mechanical load is impractical, or where no suitable load machine exists on site. The primary method is the superimposed single-phase (SSP) method, which creates rated-load thermal conditions by injecting a single-phase current into the machine windings while the machine operates at synchronous or near-synchronous speed with no mechanical load.
🔌 1. The Superimposed Single-Phase (SSP) Method
1.1 Operating Principle
The SSP method works on the principle that the temperature rise of a rotating electrical machine depends primarily on the total losses (I²R losses, core losses, stray losses) rather than on the mechanical output power. By applying a single-phase excitation to the machine windings while the rotor is driven at rated speed (by the machine itself or an auxiliary motor), the combined effect of forward and backward rotating magnetic fields creates a current distribution in the windings that closely approximates the thermal conditions of rated-load operation.
For a three-phase induction motor, the SSP method is applied as follows:
ISSP = IN × √(1 + sN)
Where ISSP is the single-phase current to be injected between two phases of the motor (with the third phase open-circuited or connected through a specific impedance), IN is the rated line current, and sN is the rated slip. The formula accounts for the fact that under SSP excitation, the effective current in the windings must be slightly higher than the rated current because the mechanical power developed in the backward-rotating field (which opposes the forward rotation) produces additional losses that are not present in normal three-phase operation.
| Machine Type | SSP Connection | Test Current Formula | Rotor Condition |
|---|---|---|---|
| Induction motor (squirrel-cage) | Single-phase supply between two phases; third phase open |
ISSP = IN × √(1 + sN) | Free rotation at rated speed (self-driven or auxiliary motor) |
| Induction motor (wound rotor) | Single-phase supply to stator; rotor short-circuited or through external impedance |
ISSP determined from equivalent circuit calculation |
Driven at synchronous speed by auxiliary motor |
| Synchronous machine | Single-phase supply to armature; field winding separately excited |
ISSP = IN (direct axis) or adjusted for zero power factor |
Driven at synchronous speed by auxiliary motor |
💡 Engineering Insight
The key engineering insight behind the SSP method is the understanding of forward and backward rotating fields. When a single-phase current is applied to a three-phase stator winding, the resulting magnetomotive force (MMF) can be decomposed into two equal-amplitude components: a forward-rotating field (at synchronous speed, in the direction of rotor rotation) and a backward-rotating field (at synchronous speed, opposite to rotor rotation). The forward-rotating field induces rotor currents at slip frequency (s × f), producing a torque that attempts to accelerate the rotor. The backward-rotating field induces rotor currents at (2-s) × f, producing a braking torque. The net torque is near zero when the rotor is driven at synchronous speed, but the I²R losses in both the stator and rotor windings approximate those under full-load conditions. This elegant electromagnetic equivalence is what makes the SSP method such a powerful tool for large-machine testing.
The key engineering insight behind the SSP method is the understanding of forward and backward rotating fields. When a single-phase current is applied to a three-phase stator winding, the resulting magnetomotive force (MMF) can be decomposed into two equal-amplitude components: a forward-rotating field (at synchronous speed, in the direction of rotor rotation) and a backward-rotating field (at synchronous speed, opposite to rotor rotation). The forward-rotating field induces rotor currents at slip frequency (s × f), producing a torque that attempts to accelerate the rotor. The backward-rotating field induces rotor currents at (2-s) × f, producing a braking torque. The net torque is near zero when the rotor is driven at synchronous speed, but the I²R losses in both the stator and rotor windings approximate those under full-load conditions. This elegant electromagnetic equivalence is what makes the SSP method such a powerful tool for large-machine testing.
1.2 Test Setup and Procedure
IEC 61986 specifies the detailed test setup for the SSP method. The test arrangement requires:
Power supply: A single-phase AC supply with sufficient capacity to deliver the SSP current (typically 0.8-1.2 times the rated phase current of the machine). The supply voltage must be adjustable and stable to within ±2% during the test. For large machines, a single-phase transformer or a back-to-back converter arrangement is typically used.
Auxiliary drive: When the machine under test cannot maintain synchronous speed under SSP excitation (as is the case for induction motors), an auxiliary prime mover is required to drive the rotor at rated speed. The auxiliary machine must have sufficient power to overcome the friction, windage, and SSP-induced braking torque. Typically, the auxiliary drive power requirement is 10-20% of the rated power of the machine under test. For synchronous machines, the machine itself can be operated as a synchronous motor at no load, with the SSP supply superimposed on the armature current.
Instrumentation: The standard requires measurement of: (1) SSP current (true RMS with ±0.5% accuracy), (2) SSP voltage, (3) input power (wattmeter method), (4) rotor speed (±0.1% accuracy), (5) winding temperatures (resistance method or embedded thermocouples), (6) stator winding resistance (at ambient and during cooldown), and (7) ambient temperature.
💡 2. Validation and Accuracy Considerations
2.1 Equivalence Verification
IEC 61986 requires that the validity of the equivalent loading test be verified by demonstrating that the loss distribution under SSP excitation closely matches that under rated-load conditions. The standard defines an equivalence criterion based on the comparison of calculated loss components:
| Loss Component | Rated Load (Three-Phase) | SSP Method (Equivalent) | Required Match |
|---|---|---|---|
| Stator I²R loss (total) | 3 × IN² × Rs | 2 × ISSP² × Rs (two phases) + Izero² × Rs (third phase if applicable) |
≤ ±5% deviation |
| Rotor I²R loss | (Pag × sN) / (1 – sN) | Calculated from equivalent circuit using measured electrical quantities | ≤ ±10% deviation |
| Core loss | From no-load test at rated voltage | Varies with SSP voltage setting; must be matched | ≤ ±5% deviation |
| Stray load loss | 0.5-2% of input power | Inherently reproduced by the SSP field pattern | No separate adjustment required |
| Total losses | Sum of all components | Sum of all components | ≤ ±5% deviation |
If the loss agreement is not within these tolerances, the test current or the SSP voltage may be adjusted iteratively, or the test may need to be conducted using an alternative equivalent loading method described in the standard.
⚠️ Design Warning
A critical practical limitation of the SSP method that IEC 61986 explicitly warns about is the increased torque pulsation at twice the supply frequency (100 Hz or 120 Hz) caused by the interaction of the forward and backward rotating fields. These torque pulsations have an amplitude of 10-30% of the rated torque and can excite torsional resonances in the shaft system if the shaft’s natural torsional frequency coincides with 100/120 Hz or a sub-harmonic thereof. Before conducting an SSP test on a large machine (especially above 5 MW), the standard recommends a torsional vibration analysis of the shaft train to ensure that the 2f torque pulsation does not coincide with any torsional natural frequency within ±10%. If a resonance is detected, the test speed should be adjusted (within ±5% of rated speed) to avoid the resonance, or a different equivalent loading method should be selected.
A critical practical limitation of the SSP method that IEC 61986 explicitly warns about is the increased torque pulsation at twice the supply frequency (100 Hz or 120 Hz) caused by the interaction of the forward and backward rotating fields. These torque pulsations have an amplitude of 10-30% of the rated torque and can excite torsional resonances in the shaft system if the shaft’s natural torsional frequency coincides with 100/120 Hz or a sub-harmonic thereof. Before conducting an SSP test on a large machine (especially above 5 MW), the standard recommends a torsional vibration analysis of the shaft train to ensure that the 2f torque pulsation does not coincide with any torsional natural frequency within ±10%. If a resonance is detected, the test speed should be adjusted (within ±5% of rated speed) to avoid the resonance, or a different equivalent loading method should be selected.
2.2 Temperature Measurement and Correction
IEC 61986 specifies that winding temperatures during the equivalent loading test be measured using the resistance method (measurement of winding resistance change between cold and hot conditions) as the primary method. The temperature rise Δθ is calculated as:
Δθ = (Rh - Rc) / Rc × (235 + θc) + (θc - θa)
Where Rh and Rc are the hot and cold resistances, θc is the cold winding temperature, and θa is the ambient temperature at the time of measurement. The standard specifies that for copper windings the temperature coefficient 1/α = 235 (for aluminium: 225) be used, consistent with IEC 60034-1.
The standard also provides guidance on correcting the measured temperature rise for deviations between the test conditions and the rated operating conditions. If the total losses measured during the SSP test deviate from the calculated total losses at rated load by more than ±5%, the temperature rise must be corrected using the following relationship:
Δθcorrected = Δθmeasured × (Prated losses / Ptest losses)n
Where n is an exponent between 0.8 and 1.0 (typically 0.9 for induction machines, based on empirical data).
💻 3. Alternative Equivalent Loading Methods and Applications
3.1 Comparison of Methods
IEC 61986 describes several equivalent loading methods in addition to the SSP method, each with specific advantages and limitations:
| Method | Principle | Power Requirement | Best Suited For | Limitations |
|---|---|---|---|---|
| Superimposed single-phase (SSP) | Single-phase excitation superimposed on three-phase windings | 10-20% of rated power | Large induction motors and synchronous machines | Torque pulsation at 2f; not suitable for machines <100 kW |
| Phase opposition method | Two identical machines coupled together, one acting as motor and the other as generator | ≈ 10% of rated power (only losses supplied) | Medium and large machines where two identical units are available | Requires two identical machines; complex setup |
| Loading-back (back-to-back) | Two identical machines mechanically coupled and electrically connected | ≈ 10% of rated power (only losses) | Production testing of standardized machines | Requires two identical machines |
| Reduced-frequency method | Machine supplied at reduced frequency while rotor is driven at rated speed | 20-40% of rated power | Large synchronous machines during commissioning | Requires variable-frequency supply; limited to synchronous machines |
| Zero-power-factor excitation | Synchronous machine over-excited to draw lagging current at rated value | Low (reactive power only) | Large synchronous generators in power plants | Only reproduces armature losses; requires special excitation |
✅ Best Practice Recommendation
For commissioning tests of large induction motors in industrial installations (pumps, compressors, fans) where the driven load cannot be operated for extended periods, the SSP method per IEC 61986 is the most practical approach. The standard recommends the following procedure: (1) Perform a no-load test to establish core loss and friction/windage baselines; (2) Measure the stator DC resistance and calculate the rated I²R losses at the reference temperature; (3) Set up the SSP test circuit with an auxiliary motor (if needed) and verify that the machine reaches rated speed; (4) Apply the SSP current calculated from the formula and adjust the auxiliary power to maintain rated speed; (5) Monitor temperatures until thermal stabilization (change < 1 K/hour); (6) Record hot resistance immediately after shutdown (within 30 seconds); (7) Apply the correction formula if the measured losses deviate from the rated-load target by more than ±5%. This method has been successfully applied to motors up to 15 MW in petrochemical installations, saving weeks of commissioning time compared with full-load testing with mechanical loading.
For commissioning tests of large induction motors in industrial installations (pumps, compressors, fans) where the driven load cannot be operated for extended periods, the SSP method per IEC 61986 is the most practical approach. The standard recommends the following procedure: (1) Perform a no-load test to establish core loss and friction/windage baselines; (2) Measure the stator DC resistance and calculate the rated I²R losses at the reference temperature; (3) Set up the SSP test circuit with an auxiliary motor (if needed) and verify that the machine reaches rated speed; (4) Apply the SSP current calculated from the formula and adjust the auxiliary power to maintain rated speed; (5) Monitor temperatures until thermal stabilization (change < 1 K/hour); (6) Record hot resistance immediately after shutdown (within 30 seconds); (7) Apply the correction formula if the measured losses deviate from the rated-load target by more than ±5%. This method has been successfully applied to motors up to 15 MW in petrochemical installations, saving weeks of commissioning time compared with full-load testing with mechanical loading.
3.2 Application Scope and Limitations
IEC 61986 applies to both induction and synchronous machines of all voltage ratings. However, the standard explicitly states that the equivalent loading methods are not intended to replace full-load testing when the driven load is available and can be applied safely. The equivalent loading methods are intended for:
(1) Commissioning tests where the driven load is not yet available or cannot be operated (e.g., pumps in a pipeline not yet commissioned); (2) Type tests of large machines where supplying a rated mechanical load would require special facilities (e.g., a dynamometer exceeding 10 MW capacity); (3) Routine maintenance tests where removing the machine from the driven load is impractical; (4) Verification tests after rewinding or repair, to confirm that the thermal performance meets the original design specification.
❓ Frequently Asked Questions
❔ How does the SSP method account for the differences in stray load loss distribution between SSP and full-load conditions?
The SSP method inherently reproduces the stray load loss pattern because the harmonic fields generated by the backward-rotating MMF component create similar leakage flux patterns and induced eddy currents as those under full-load three-phase operation. However, the stray loss distribution is not identical — the SSP method tends to produce slightly higher stator stray losses and slightly lower rotor stray losses compared with three-phase operation. IEC 61986 addresses this through the equivalence verification procedure, which requires that the total losses under SSP be within ±5% of the three-phase rated-load total losses. If the deviation exceeds this tolerance, correction factors are applied to the temperature rise measurement. For machines where stray losses are a significant portion of total losses (e.g., large 2-pole machines with deep rotor bars), a correction factor of 1.0-1.05 is typically applied to the measured temperature rise.
❔ Can the SSP method be used for variable-frequency drive (VFD) supplied machines?
The IEC 61986 standard was developed primarily for machines operating on sinusoidal supplies. For VFD-supplied machines, the additional harmonic losses (from the inverter switching waveform) are not reproduced by the SSP method, which uses a pure sinusoidal supply at line frequency. However, the SSP method can be used to determine the temperature rise due to the fundamental frequency component of the VFD supply, and the additional harmonic losses can be estimated separately using the procedures in IEC 60034-2-3 (specific test methods for converter-fed machines). The total temperature rise is then calculated by superposition. Engineers should note that for VFD applications, the temperature rise at the machine’s fundamental (rated) frequency may be only 60-70% of the total temperature rise under converter operation, depending on the cable length, switching frequency, and modulation technique.
❔ What auxiliary motor power is needed for an SSP test on an induction motor?
The auxiliary motor must supply the total mechanical losses (friction and windage) plus the braking torque developed by the backward-rotating field in the SSP configuration. For a standard induction motor, the auxiliary power requirement is typically 10-20% of the motor’s rated power. A 5 MW induction motor may require an auxiliary drive of 500-1000 kW. The auxiliary motor should have a variable-speed drive to allow fine adjustment of the rotor speed to the exact rated value. In cases where an auxiliary motor of sufficient power is not available, the phase opposition method or back-to-back method may be considered instead, as these methods require lower auxiliary power (typically 5-10% of rated power, covering only the losses).
❔ What is the typical uncertainty of temperature rise measurement using the SSP method?
The expanded uncertainty (k=2, 95% confidence) of the winding temperature rise measured by the SSP method is typically ±2-5 K, compared with ±1-3 K for a direct full-load test. The additional uncertainty in the SSP method arises from: (1) the loss equivalence approximation (±1-2 K), (2) the correction factor for loss deviation (±1 K), (3) the temperature measurement during cooldown (resistance method uncertainty is approximately ±0.5-1 K depending on the speed of cooldown and the accuracy of the resistance bridge), and (4) the temperature correction for ambient variations (±0.5 K). Despite the higher uncertainty, the SSP method is considered acceptable for all standard acceptance testing per IEC 60034-1, which specifies a tolerance of +10 K on the guaranteed temperature rise for machine acceptance.
