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I've seen plenty of questions wondering what happens when voltage is applied that would exceed the RPM rating, but what happens on the voltage/current side when something external - the wind spinning blades attached to the motor - spins the motor faster than its rated RPM? Does it overheat? Does the voltage eventually saturate? I'm planning on using a permanent magnet motor, but this question applies to any motor.

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    \$\begingroup\$ "but this question applies to any motor" Some motors will give a very different answer. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 22 '18 at 20:12
  • \$\begingroup\$ Are you referring to a motor under power that has over-spin due to an external force? A treadmill motor would be an example. Your answers will be limited to common motors/generators, else we could fill a book with answers. \$\endgroup\$ – Sparky256 Jun 22 '18 at 20:35
  • \$\begingroup\$ Wind turbines have one of two basic systems to prevent overspeeding : brakes or on smaller machines they are rotated out of the wind... \$\endgroup\$ – Solar Mike Jun 22 '18 at 20:57
  • \$\begingroup\$ I was not entirely clear in my original post. The intent is to have the wind spin the motor with no driving voltage, and use the generated voltage to power the system. \$\endgroup\$ – pbandjazz Jun 22 '18 at 23:44
  • \$\begingroup\$ It is just a terminology thing. In order for the load to extract power from the generator (motor in regeneration) there must be a correct voltage at the generator output terminal. If the voltage is too low, large currents will flow, giving rise to large torque at the shaft, and likely causing problems for the driving force (whether wind or a gasoline motor or whatever). Some form of matching is ESSENTIAL for the scheme to work in a satisfactory fashion, unless the motor speed is constant. \$\endgroup\$ – mkeith Jun 23 '18 at 0:14
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PMDC

All PMDC motors generate V proportional to RPM [kRPM/V] . The rated maximum power will be at approximately 82% of its maximum speed but will continue to rise with overheating above if load continues.

While the max RPM depends on the mechanical and eddy current losses in the magnets at some frequency. This maximum powerpoint (MPP), of course depends on the load current, I .

All static structures have a resonant frequency including moving parts like bearings and the risk of imbalance or approaching those resonant frequencies with stored inertial energy rising means a high risk of fatigue and catastrophic failure . Therefore all structural resonances must be much greater f than the excitation frequency is its harmonics.

Thus shunting the generator to act as a speed brake into wasted energy and inertial flap speed brakes must be designed into the system to handle worst case winds expected in the next 100 years to prevent these failures.

The motor loads will increase with conduction current losses I^2*DCR and eddy currents will rise with f^2 above max and thus liquid cooling may be needed or simply rely on increasing the drag on the blades with speed flaps and possibly an inertial clutch brake . The mechanical solutions were how we did a 20m tall egg beater type wind power Gen in 1975.

If the mechanical brakes failed and the structure held together somehow with guy wires AND the electrical system to brake failed then a 3rd protection system is needed for safety. Otherwise the voltage could increase possibly enough to breakdown and arc , if there was not at least a 300% safety margin on insulation.

PMAC

Large synchronous wind turbines however use a transmission to speed up the RPM and the PMAC generator requires a drive specifically designed for PM motors, similar to flux vector drives for ac induction motors, in that the drive uses current-switching techniques to control motor torque — and simultaneously controls both torque and flux current via mathematically intensive transformations between one coordinate system and another in order to keep in phase with the grid frequency and phase while allowing prop angle to harness more power. Therefore they are only designed to run at constant speed unless starting or stopping or shifting slowly with the grid. These are generally Betz type steerable turbines, so they can control speed by direction error with wind but still must be able to survive a near miss of tornado.

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  • \$\begingroup\$ Hypothetically, if it was a 500 rpm rated motor with no load connected, and it was spinning at 800 rpm, would the eddy currents be high enough to cause some sort of thermal failure? If I am understanding your answer, specifically about the resonant frequency, the greater above the rated RPM I go the more likely the motor will fail due to mechanical as opposed to heat? \$\endgroup\$ – pbandjazz Jun 22 '18 at 23:50
  • \$\begingroup\$ Eddy currents affect efficency but never as much power as rated. The load must see the high voltage or high RPM and take corrective action. If RPM approaches resonance plus imbalance vibrations and lack of strength, it flies apart. So these are design considerations. The load can protect the generator from overpower but mechanical brakes are essential or steeraway or change flaps \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 22 '18 at 23:54
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    \$\begingroup\$ Assuming you positively disconnect the electrical load from the generator (for example with a multi-pole contactor) the blades will be free to spin as fast as the wind wants them to. In theory, if they were to spin very fast, you might exceed the insulation voltage rating and have arcing in the stator winding. My feeling is that failure mode is less likely than mechanical self-destruction due to over-speed. Commodity motors usually have a "maximum safe speed" rating which is based on mechanical concerns such as rotor balance. \$\endgroup\$ – mkeith Jun 23 '18 at 0:04
  • \$\begingroup\$ If it is a brushed DC motor, then maybe the brushes could have problems due to over-rpm operation. I am not sure. \$\endgroup\$ – mkeith Jun 23 '18 at 0:08
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    \$\begingroup\$ Yes they could flashover so not wise to use it or less reliable \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 23 '18 at 1:10
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This question cannot be answered without making assumptions about motor type and the behavior of the electrical source which is driving.

As a general rule, most motors can potentially act as generators when overdriven by an external force. But there are a lot of details.

Let us assume you have a permanent magnet synchronous motor (PMSM) driven by an AC source with fixed frequency and voltage. When a mechanical force tries to make the rotor spin faster than the synchronous frequency, current will flow backwards. Instead of acting as a motor, the motor will act as a generator. This mode is called "regeneration." Torque will still be present, but it will be act to slow the rotor. If the torque becomes too high, the current may become very large, and the rotor may even spin faster than synchronous speed (which would likely be catastrophic in large system).

An AC induction motor driven by an AC source with fixed frequency and voltage will behave fairly similarly to the PMSM, except that the induction motor does not operate strictly at the synchronous speed. When mechanically over-driven, it will speed up a bit, and when its speed exceeds the synchronous speed of the AC source, it will also operate in regenerative mode. But, again, there are limits to how much torque can be applied by the external force. At some point, current will become very high, and some form of failure will occur. The stator may overheat if nothing else breaks first.

Brushed DC motors also can be over-driven and also go into regeneration.

A BLDC motor may be essentially identical in construction to a PMSM. But when it is driven as a BLDC, then how it responds to being over-driven is really a matter of how it is controlled. It can be controlled in such a way that it goes into regeneration, but not all controllers are designed to do this.

If you wish to harvest energy from the wind, you really need to match a lot of things. In order to harvest energy from the wind, the blades need to spin at just the right speed. If they are too fast, they will be adding to the wind and wasting electrical power. If they are too slow, they will be stalled, and not converting power efficiently. This variable speed gives rise to a need for a variable voltage also. The voltage applied to the motor (no matter which type) must be effectively varied as the speed changes. But if you want the power to charge a battery or run an appliance, it probably needs to be converted into a fixed voltage prior to use. So there has to be some form of conversion.

Another option would be to use an AC induction motor with variable pitch fan blades. The pitch of the blades can be adjusted so that over some range of wind-speed, the motor is always in regeneration. There would have to be a lot of other control, and a reduction gear so the fan blades can spin slowly, etc. But it might be the easiest way if the goal is to put power into the electrical grid.

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