Generator No-Load Characteristic Test: Principles, Procedures, and Technical Applications

The generator no-load characteristic test is a fundamental commissioning and diagnostic procedure that establishes the relationship between field excitation current and stator terminal voltage when a synchronous generator runs at rated speed with an open-circuited armature. This test reveals the magnetization and saturation behavior of the machine's magnetic circuit, provides essential baseline data for excitation system configuration, and serves as a powerful tool for detecting winding faults and air-gap anomalies. This article explains the test principles, applicable standards, measurement procedures, and acceptance criteria in accordance with IEC 60034, IEEE 115, and GB/T 1029.

Technical Overview

The no-load characteristic test, also known as the open-circuit characteristic (OCC) test, is performed by driving a synchronous generator at rated rotational speed with the main output breaker open and the armature carrying no load current. The field (rotor) excitation current is progressively increased from zero, and the corresponding stator terminal voltage is recorded at each step. The resulting curve of terminal voltage versus field current effectively represents the magnetization curve of the complete magnetic circuit, including the stator core, rotor body, pole pieces, and air gap.

This characteristic is one of the most important performance signatures of a synchronous machine. In the unsaturated region, the voltage rises almost linearly with field current; as the iron begins to saturate, the curve bends away from the initial tangent, known as the air-gap line. The degree of departure from the air-gap line defines the saturation coefficient, which directly influences excitation demand, voltage regulation, transient behavior, and overall machine performance.

The no-load characteristic test is governed by internationally recognized standards and recommended practices, including:

• IEC 60034-1 – Rotating electrical machines, Part 1: Rating and performance
• IEC 60034-4 – Methods for determining synchronous machine quantities from tests
• IEEE 115 and IEEE 115A – Test procedures for synchronous machines
• GB/T 1029 – Test procedures for rotating electrical machines (three-phase synchronous machines)
• GB 755 – Basic technical requirements for rotating electrical machines

Because the generated electromotive force is proportional to speed and flux, the test must be conducted at rated frequency. Any deviation in speed is corrected by scaling the measured voltage in direct proportion to the ratio of rated speed to actual speed.

Testing Samples

Turbo Generators (Cylindrical Rotor Machines)

Two-pole and four-pole turbo generators installed in thermal, gas-fired, and nuclear power plants are the most common subjects of no-load characteristic testing. These large hydrogen- or air-cooled units require the test during commissioning, after major overhauls, and following rotor rewinding or field winding repairs to verify that the magnetic circuit retains its design saturation behavior.

Hydro Generators (Salient Pole Machines)

Salient-pole hydro generators, typically operating at low speed with a large number of poles, are tested to establish the no-load saturation curve during installation and after stator rewedging, core repairs, or rotor pole refurbishment. The characteristic is also used in conjunction with the short-circuit characteristic to compute the short-circuit ratio (SCR), a key parameter for hydro units connected to weak grids.

Synchronous Condensers and Industrial Generators

Synchronous condensers used for dynamic reactive power support, as well as standby and industrial generators in plants, data centers, and marine applications, undergo no-load characteristic testing to verify excitation system settings and to record baseline curves for future condition assessment.

Generator-Excitation System Assemblies

When a generator is tested together with its static excitation system or brushless exciter, the no-load characteristic confirms the coordinated output range of the automatic voltage regulator (AVR), the ceiling voltage capability, and the residual magnetism level required for reliable voltage buildup.

Testing Items

A complete no-load characteristic test program typically includes the following measurement and evaluation items:

• Ascending no-load saturation curve: stator terminal voltage recorded at 10 to 15 field current steps from residual level up to approximately 125–130% of rated voltage, subject to manufacturer limits
• Descending branch: field current reduced stepwise to zero to capture hysteresis effects; the average of ascending and descending curves is adopted as the practical no-load characteristic
• Residual (remanence) voltage: the stator voltage measured with zero field current, typically a few percent of rated voltage
• Rated field current at no-load: the excitation current required to produce exactly rated stator voltage at rated speed, a cornerstone datum for excitation system commissioning
• Three-phase voltage balance: verification that phase voltages and phase angles are symmetrical, indicating healthy winding distribution and uniform air gap
• Saturation characteristics: knee point identification and saturation factor calculation relative to the air-gap line
• Excitation ceiling verification: maximum field current and voltage achievable within the permitted overvoltage window
• Voltage waveform quality: measurement of harmonic distortion and telephone influence factor at rated no-load voltage where specified
• Comparison with factory reference curve: evaluation of curve shift, slope change, or asymmetry that may indicate inter-turn short circuits in the field winding or rotor-stator eccentricity

Testing Methods

Test Preparation and Safety Measures

The generator is brought to rated speed and frequency, verified with a calibrated tachometer or frequency transducer. The main breaker remains open, and the armature terminals are either isolated or connected only through calibrated voltage transformers (class 0.2 or better). Field current is measured with a precision DC shunt or Hall-effect transducer, and all signals are logged by a data acquisition system with synchronized sampling. The excitation system is placed in manual (field current) control mode so the test operator can raise and lower excitation independently of the AVR. Protective limits for overvoltage, overfluxing (V/Hz), and overspeed must be active, and personnel must be protected against induced voltages on open terminals.

Step-by-Step Test Procedure

• Run the machine at rated speed and record the residual stator voltage with field current at zero
• Increase the field current in uniform steps, pausing at each point until readings stabilize, and record phase voltages, field current, and frequency
• Continue raising excitation until the voltage reaches the maximum value permitted by the manufacturer or the applicable standard, commonly 125–130% of rated voltage
• Reduce the field current through corresponding descending steps back to zero, again recording voltage at each plateau
• Correct all voltage readings to rated speed where speed differs from nominal, and plot the ascending and descending branches
• Construct the average curve, draw the air-gap line through the linear origin region, and determine the saturation factor and the field current at rated voltage

Data Interpretation and Acceptance Criteria

A healthy machine exhibits a smooth, repeatable saturation curve that matches the factory reference within the tolerance agreed between manufacturer and purchaser, typically a few percent in field current at rated voltage. A significant shift of the curve to the right, requiring more field current for the same voltage, suggests shorted turns in the field winding, increased air gap, or core degradation. Unbalanced phase voltages during the test point toward stator winding faults, rotor inter-turn shorts, or magnetic asymmetry caused by rotor eccentricity. The measured rated field current at no-load, combined with the field current from the three-phase short-circuit test, yields the short-circuit ratio used in grid stability studies. All deviations beyond acceptance limits should trigger supplemental diagnostics such as pole voltage distribution measurement, air gap monitoring, or flux probe analysis before the unit is returned to commercial service.

Applications

The no-load characteristic test supports a wide range of engineering and operational objectives across the life cycle of a generator:

• Commissioning of new generating units, where the no-load curve validates the design magnetic circuit and provides the reference datum for all future tests
• Excitation system and AVR commissioning, including automatic voltage regulator gain setting, ceiling voltage confirmation, and voltage buildup verification
• Post-maintenance verification after rotor rewinding, stator replacement, core repairs, or major overhaul, ensuring the magnetic circuit has not changed materially
• Fault diagnostics, where comparison of the current curve against the baseline reveals field winding inter-turn shorts, air-gap eccentricity, and core saturation anomalies
• Performance parameter derivation, including saturation factors, short-circuit ratio, Potier reactance (together with zero power factor tests), and excitation demand at various loading conditions
• Power system model validation, supplying manufacturers and grid operators with accurate saturation data for stability, protection coordination, and dynamic simulation studies
• Baseline archiving for predictive maintenance programs operated by utilities and accredited third-party laboratories performing periodic machine condition assessments

FAQ

Why must the no-load characteristic test be performed at rated speed?

The generated voltage of a synchronous machine is proportional to speed and magnetic flux. If the machine runs below or above rated speed, the voltage readings will be scaled accordingly and the curve will not represent the true saturation characteristic. Small speed deviations can be corrected arithmetically, but the test standard requires operation at, or corrected to, rated speed for valid results.

What is the difference between the no-load characteristic and the short-circuit characteristic?

The no-load characteristic relates field current to open-circuit terminal voltage and reflects the magnetization curve of the iron and air gap, including saturation. The short-circuit characteristic relates field current to armature current with the terminals three-phase shorted, in which the armature reaction demagnetizes the machine and the curve remains essentially linear. Together, the two curves define the short-circuit ratio and the excitation requirements across the full operating range.

Why are both ascending and descending branches recorded?

Magnetic hysteresis causes the iron to follow slightly different magnetization paths depending on whether excitation is increasing or decreasing. By averaging the ascending and descending branches, the test yields a reproducible mean characteristic that is independent of prior magnetization history and comparable with factory reference data.

How high can the voltage be raised during the test?

The voltage is typically raised to 125–130% of rated value to fully define the saturated region, always within the limits stated by the machine manufacturer and the applicable standard. The test duration at elevated voltage is minimized, and V/Hz protection must remain active to prevent overfluxing and core overheating.

What does a significant deviation from the factory curve indicate?

A curve requiring noticeably higher field current for the same voltage usually indicates shorted turns in the field winding, an enlarged air gap, or degradation of the stator core. Such findings justify follow-up diagnostics before continued operation. All comparisons should be performed by qualified engineers or accredited third-party laboratories using calibrated instrumentation traceable to national standards.

Is the no-load characteristic test mandatory?

The test is a standard type test and commissioning requirement under IEC 60034-4, IEEE 115, and GB/T 1029, and it is universally specified by turbine and hydro generator manufacturers for new units. It is also contractually required after most major electrical overhauls, because it provides the most direct and sensitive indication of the condition of the machine magnetic circuit.