A Wind Site That Underperformed
A small wind installer spec'd a generator rated at 12 kW at 400 rpm, then found the unit delivered well under that on a site where the rotor spent most hours between 250 and 350 rpm. The nameplate was true, but the Permanent Magnet Generator rarely ran at the point where that number applied. Reframing the purchase around the speed band, not the peak, lifted annual energy by roughly a sixth without changing the turbine.
The Low-Wind Months
Through the low-wind season the rotor sat near 280 rpm, where the Permanent Magnet Generator made less voltage and the controller clipped output to protect the bus. The operator read the 12 kW label and assumed a fault, when the machine was simply outside its best efficiency window. Logging rpm against output showed the real working range, which the original spec had never described.
The Curve That Did Not Match
The generator curve peaked at 400 rpm, but the site's wind distribution centered near 300 rpm, so the average sat far from the sweet spot. A Permanent Magnet Generator matched to the actual speed histogram would have been wound for that band instead of the marketing point. After re-matching the generator to the rotor, the same tower produced more energy across the year with no extra hardware.
How Speed Shapes Output
A Permanent Magnet Generator ties voltage and frequency to shaft speed, so moving away from the design point changes both the power and the losses. The effect is unavoidable, but it is predictable, and a buyer who reads the curve avoids paying for a peak that the duty never reaches.
Voltage and Frequency Link
Because the field comes from permanent magnets, terminal voltage rises with speed, and the linked inverter must manage a widening range. A Permanent Magnet Generator at low rpm makes less voltage, so the controller may clip or slip regulation, cutting delivered power. At high rpm the voltage climbs and stresses the devices, which is why the speed band, not a single number, defines the working machine. A Permanent Magnet Generator therefore needs an inverter rated for the whole voltage span, because clipping at the top end throws away exactly the energy the site earned from a gust. The controller, not the rotor, sets how much of the speed range stays usable.
Losses Across the Speed Range
Iron loss grows with frequency, so a Permanent Magnet Generator running fast sheds more to the core, while copper loss follows the current the load draws. The balance shifts with speed, so efficiency peaks at a specific point and falls either side of it. Reading the loss split across the band shows why a unit can be 94 percent efficient at 400 rpm and nearer 88 percent at 250 rpm. A Permanent Magnet Generator matched to the site histogram keeps both loss terms small through the hours it actually runs, so the annual yield beats a higher-rated machine that rarely reaches its peak. The efficiency curve, not the label, should drive the purchase.
Why Permanent Magnet Generator Behaves So
The behavior follows from the fixed field and the machine constants, not from a control quirk. A Permanent Magnet Generator with a known flux and winding reveals its speed dependence through back-EMF and loss terms that any competent datasheet should state, and those terms decide the real annual yield.
Flux and Back-EMF
The magnets set a fixed flux, so back-EMF scales directly with speed, and the inverter must buck or boost to hold the bus. A Permanent Magnet Generator with high flux makes strong voltage at speed but leaves little margin at the top end, while low flux eases the inverter yet weakens low-speed output. The flux choice is the core trade that sets where the efficiency peak sits.
Magnetic and Winding Losses
Eddy and hysteresis losses in the core rise with the electrical frequency, so a Permanent Magnet Generator running above its band heats the iron without adding output. Winding loss follows the load current, which the turbine power commands at each speed. Matching the magnetic design to the site histogram keeps both loss terms small across the hours the machine actually runs.
Matching and Protecting
The payback lies in matching the generator to the speed distribution, then protecting it at the extremes. A Permanent Magnet Generator selected for the real working range, not the brochure peak, delivers more energy per dollar and a longer, calmer life in the field.
Sizing to the Speed Band
Build the selection from the rotor's rpm histogram, not the rated point, and ask for the efficiency curve across that band. A Permanent Magnet Generator wound for the site's central speed yields more annual energy than one tuned to a peak the wind rarely provides. Confirm the inverter rating covers the voltage range from low to overspeed so the controller never clips the useful output.
Control and Overspeed Care
At overspeed the voltage and frequency climb, so the inverter and brake must cap the bus and protect the winding. A Permanent Magnet Generator needs a controlled stop path and a pitch or brake system sized for the gust, because the fixed field cannot be switched off like an excited rotor. Log the speed and temperature trend, and service the braking path on the interval the turbine standard sets. A Permanent Magnet Generator that is logged and braked on schedule avoids the dramatic failures that give the technology a bad name after a storm.
Frequently Asked Questions
Question: Why does Permanent Magnet Generator efficiency change with speed?
A permanent-magnet field fixes the flux, so terminal voltage and frequency scale with shaft speed, and the loss balance shifts with them. A Permanent Magnet Generator runs near its efficiency peak at the design rpm but falls away on either side as core and copper losses change. Reading the curve across the real speed band shows the true average, not the single-point nameplate figure.
Question: Is a higher rated speed always better?
No, because the site may rarely reach that speed, leaving the machine working below its peak most hours. A Permanent Magnet Generator rated at 400 rpm but running at 300 most of the time delivers less annual energy than one matched to 300. Select from the rotor speed histogram, and ask for efficiency across the band rather than trusting the peak rating alone.
Question: Which standards apply to the generator?
IEC 60034-1 covers rotating machine performance and testing, while IEC 61400 addresses wind turbine generators and their grid behavior. A Permanent Magnet Generator for wind should meet both, and IEEE 1547 governs grid interconnection where the unit feeds the network. Cite these so the supplier delivers a tested machine, not just a plausible label.
Question: How does low speed hurt output?
At low rpm the Permanent Magnet Generator makes less back-EMF, so the inverter may clip or slip regulation to protect the bus, cutting delivered power. The core also loses less, but the drop in useful voltage outweighs that gain for most sites. Matching the winding to the low end of the band keeps the controller in range through the calm months that dominate the year.
Question: What protects the generator at overspeed?
The fixed field cannot be switched off, so a pitch system or brake must cap rotor speed and the inverter must clamp the bus. A Permanent Magnet Generator needs a controlled stop path sized for the site gust, because voltage and frequency rise with speed and stress the devices. Service the braking path on the interval the turbine standard sets, and log speed and temperature trends.
Question: Can one generator suit a variable-speed site?
Yes, if selected across the working band rather than at a single point, because a Permanent Magnet Generator performs well over a range when the winding and flux are tuned to it. Ask for the efficiency curve across the histogram and confirm the inverter covers the voltage span. A machine matched to the real distribution outperforms a higher-rated one that rarely reaches its peak.
EN
