Permanent Magnet Generator Testing Guidelines: Key Operating Standards, Load Procedures and Safety Essentials

Permanent magnet synchronous generators (PMSG) use high-performance rare earth permanent magnet materials to produce a constant magnetic field, requiring no external excitation current, thus offering extremely high electrical energy conversion efficiency (typically up to 92%–96%) and excellent power density. However, their unique physical structure determines significant characteristics in electrical output:

During generator operation, the no-load voltage is usually significantly higher than the full-load voltage. This is mainly because there is internal impedance (stator winding resistance and leakage reactance) inside the generator. When the generator is unloaded, the output current is extremely small, the internal voltage drop is nearly zero, and the terminal voltage is close to the ideal back electromotive force; when a full load is connected, the large current flowing through the internal impedance produces a significant voltage drop, causing the output terminal voltage to drop.

For example, for a permanent magnet generator with a rated voltage of 690V, its no-load voltage may be around 756V~780V, which is a completely normal physical phenomenon and not a fault.

The output voltage of a permanent magnet generator is highly sensitive to load characteristics. Even if the speed of the prime mover (such as a wind turbine, water turbine, or diesel engine) remains absolutely stable, when the magnitude or nature of the load power changes, the output voltage will also fluctuate accordingly.

• When a high-power load is connected, the stator current increases, the internal impedance voltage drop increases, and the terminal voltage decreases.
• When a low-power load is connected, the terminal voltage recovers.
• When non-linear loads (such as LED lighting, switching power supplies, etc.) are connected, the harmonic currents generated by the load cause additional reverse rotating magnetic fields and voltage distortion inside the generator, leading to voltage fluctuations.

Because permanent magnets cannot compensate for voltage drops by adjusting the excitation current like electrically excited generators, the voltage regulation rate is relatively weak. This is also why permanent magnet generators, even if they output 380V/50Hz and the front-end drive is stable, are not recommended to directly start with loads — the load level directly causes changes in the output voltage.

Based on the above characteristics, it is absolutely forbidden to start a permanent magnet generator directly with the output switch closed (i.e., with load connected). The correct operating procedure is:

1. First set the output AC switch to the OFF position.
2. Start the generator and allow it to run unloaded for preheating for 1~2 minutes (extend to 3~5 minutes in cold environments).
3. After the unit speed, frequency, and no-load voltage are completely stable, gradually apply the load.

If starting directly with load, the huge starting inrush current and severe voltage fluctuations can easily cause insulation breakdown of the generator winding, protective tripping, or even engine stall and mechanical component damage.

To ensure the accuracy of test results and equipment safety of the permanent magnet generator, it is strongly recommended to use a motor (or prime mover) with drive power not less than 1.1 times the rated power of the generator for driving. That is:

Drive Power ≥ Generator Rated Power * 1.1

For example, for a permanent magnet generator with a rated power of 30kW, a drive motor with power not less than 33kW should be selected.

If the drive power is much smaller than the rated power of the generator (for example, using a 5kW motor to drive a 30kW generator), the following serious consequences will result:

1. (1) Inability to reach rated speed

The output voltage of the generator is proportional to the speed. When the drive power is insufficient, the motor cannot accelerate the generator to the rated speed, resulting in output voltage and frequency both lower than the nominal values. A 30kW generator requires more than 30kW of mechanical input power to output rated electrical energy; the 5kW driving torque is far from sufficient, and the speed will be seriously low.

1. (2) Output voltage and frequency do not meet specifications

• - Low voltage: cannot reach the rated voltage (e.g., 380V/400V/690V, etc.), making it impossible to conduct rated condition tests properly.
• - Low frequency: for a 50Hz system, insufficient speed causes the frequency to deviate from 50Hz, making test data invalid.
• - Inability to perform full-load tests: only no-load or light-load tests can be conducted, and the generator's load capacity cannot be verified.

1. (3) Drive motor overload damage

Long-term operation of the drive motor under overload conditions will cause:

• - Winding overheating, accelerated insulation aging, shortening motor life.
• - Possible triggering of overload protection tripping, interrupting tests.
• - In severe cases, burning out the drive motor.

1. (4) Invalid test results

Since the speed cannot be stabilized at the rated value, all test data (voltage regulation rate, efficiency, temperature rise, etc.) have no reference value, and accurate test conclusions cannot be obtained.

In addition to power matching, the speed stability of the drive equipment is equally crucial:

• Speed fluctuation should be controlled within ±1% of rated speed.
• Excessive speed pulsation will cause output voltage frequency fluctuation and waveform distortion.
• It is recommended to use a variable frequency motor or diesel engine with speed control as the drive source to ensure precise and controllable speed.

• Confirm that the drive motor power meets ≥ 1.1 times the rated power of the generator.
• Check the alignment of the coupling between the drive motor and the generator.
• Confirm that all generator terminal connections are firm and reliable, and the phase sequence is correct.
• Check insulation resistance (stator winding to ground and between phases); cold insulation resistance should be not less than 500MΩ.
• Confirm that all measuring instruments (voltmeter, ammeter, power meter, frequency meter, thermometer, etc.) have been calibrated and are within the validity period.
• Confirm that the rated voltage of the load box or load equipment matches the output voltage of the generator.

• - Set up safety barriers in the test area; non-test personnel are prohibited from entering.
• - Operators must wear insulating gloves, insulating shoes, and other personal protective equipment.
• - Confirm that all protective devices (overcurrent, overvoltage, overtemperature) are operational before testing.
• - During high-voltage tests, a dedicated person must be assigned for supervision, and strictly follow the principle of "non-destructive tests before destructive tests, low voltage before high voltage".
• - After the test, be sure to fully discharge the equipment.

1. Ensure that the generator output switch is in the open (no-load) position.
2. Start the drive motor and slowly increase the speed to the rated speed of the generator.
3. After the speed and frequency are stable, record the no-load line voltage and phase voltage.
4. Gradually adjust the speed, record the no-load voltage at different speeds, and plot the no-load characteristic curve.
5. Measure the no-load current (should be close to zero).
6. Record the temperature rise during no-load operation (after 30 minutes of operation).

Note: The no-load voltage of a permanent magnet generator is usually higher than the full-load voltage; this is normal. For example, for a generator rated at 690V, the no-load voltage may be around 756V.

1. At rated speed, confirm that the no-load voltage is stable.
2. Adopt a stepwise loading method, increasing the load gradually at 25%, 50%, 75%, and 100% of rated power.
3. After each load level runs stably for 10~15 minutes, record the following data:

• Line voltage, phase voltage
• Line current, phase current
• Active power, reactive power, power factor
• Frequency
• Winding temperature, bearing temperature, housing temperature

1. 4. Calculate voltage regulation rate: ΔU% = (U₀ - U) / U * 100% (U₀ is no-load voltage, U is rated load voltage)

1. Operate continuously at rated load until the temperature of each part reaches stability (temperature rise change does not exceed 1K/h).
2. Use an infrared thermal imager or temperature recorder to monitor:

• - Stator winding temperature (by embedded resistance method or thermocouple method)
• - Bearing temperature
• - Permanent magnet surface temperature (strictly do not exceed the maximum temperature limit allowed for the permanent magnet material, typically 80~180°C, depending on the magnet grade)
• - Housing surface temperature

1. 3. Record the temperature rise values of each part (relative to ambient temperature)

Rated speed is the reference speed for generator design operation. At this speed:

• Output frequency is the rated frequency (e.g., 50Hz or 60Hz)
• Output voltage is the rated voltage (e.g., 380V/400V/690V)
• All performance parameters (efficiency, power factor, temperature rise, etc.) are based on test values at rated speed

Speed calculation formula: n = 60f / p

Where: f is rated frequency (Hz), p is number of pole pairs.

For example: for a 50Hz, 12-pole (p=6) generator, rated speed n = 60*50/6 = 500rpm.

• Speed too high: output voltage and frequency increase, possibly causing insulation overstress, and test data will be too high.
• Speed too low: output voltage and frequency decrease, failing to reach rated conditions, and test data will be too low and invalid.
• Speed fluctuation: causes voltage frequency fluctuation and waveform distortion, and test data will have large dispersion.

• Rated speed fluctuation: ≤ ±1%
• Speed step response recovery time: ≤ 5 seconds (after load sudden change)
• It is recommended to use a variable frequency drive motor with speed feedback closed-loop control

The output voltage of the generator must strictly match the rated voltage of the test equipment. Before testing, be sure to use a multimeter or power quality analyzer to confirm the no-load output voltage of the generator, ensuring that it is within the allowable voltage range of the test equipment.

If the generator output voltage is too high (exceeding the rated voltage of the test equipment by more than 10%), it will cause:

• Insulation breakdown: excessive voltage exceeds the withstand voltage limits of the internal electronic components and insulating materials of the test equipment.
• Capacitor bursting: electrolytic capacitors are damaged due to overvoltage.
• Semiconductor burnout: power semiconductor devices fail due to overvoltage breakdown.
• Magnetic circuit saturation: for transformer or motor type test equipment, excessive voltage causes core magnetic circuit saturation, magnetizing current increases sharply, and the equipment rapidly heats up and burns out.

• Before testing, reduce the voltage to the allowable rated range of the equipment by adjusting the drive motor speed or supporting converter.
• For permanent magnet generators, since excitation cannot be adjusted, the output voltage is mainly controlled by adjusting the drive speed.
• It is recommended to install a converter, a voltage-stabilizing frequency-stabilizing power supply, or a controller/inverter at the generator output terminals to match test equipment with different rated voltages and to meet grid connection and energy storage requirements.

Ordinary electrically excited generators can maintain output voltage stability by adjusting the excitation current, whereas the magnetic field of a permanent magnet generator is provided by permanent magnets and is constant, not adjustable. Therefore, load changes are directly reflected in the output voltage. This is why permanent magnet generators must first run stably at no load before gradually applying loads.

1. Immediately unload gradually; do not directly open the high-current switch.
2. Check whether the drive speed is stable at the rated value.
3. Check whether the load is too large or there is a short circuit.
4. Check whether the wiring is correct, with no looseness or poor contact.
5. If the voltage remains abnormal, stop the machine immediately for inspection.

1. Gradually unload to no load, run for 2~3 minutes for cooling.
2. Open the load switch.
3. Slowly reduce the drive motor speed to zero.
4. Disconnect the power supply.
5. Discharge the generator output terminals.
6. Record test data and equipment status.

The core points of permanent magnet generator testing can be summarized as the following four:

1. 1. Drive power must be sufficient: drive motor power ≥ generator rated power * 1.1, strictly prohibit "small horse pulling a large cart".
2. 2. Speed must be stable: rated speed fluctuation ≤ ±1%, variable frequency drive is recommended.
3. 3. Start no-load then apply load: starting with load is strictly prohibited; load must be applied stepwise, and each level must be stabilized before adding the next.
4. 4. Voltage must be matched: ensure that the generator output voltage matches the rated voltage of the test equipment to prevent overvoltage from burning out equipment.

Strict compliance with the above testing guidelines ensures the safety of permanent magnet generator testing and the accuracy of data.