ac Generators

[rbyrne15]

I've been looking at Kohler generators, to get a feel for what they are like. I noticed that they have:

permanent magnet (PM)-excited alternators

and

brushless, rotating-field alternator.

Please confirm (or else deny) that I understand these two terms.

PM-excited alternator means that a permanent magnet (no magnetising current required to generate its magnetic field) arrangement is connected concentrically to the generator rotor which, when the rotor is driven by the prime mover, induces a current in the stationary pilot exciter winding. This current is passed through a rectifier to drive the (main) exciter winding of the alternator, to produce yet another field in the (main) exciter armature. This feild rotates at the speed of rotor rotation, and finally induces the armature output current for use by supplied loads.

Brushless, rotating-field implies that no brushes (or slip rings) are used to transfer externally-produced current to the alternator field winding. Instead, solid state rectifier diodes are used to rectify the current generated by the PM generator mentioned above, to serve as field current for the main exciter winding.

Rotating field means that the main field winding is rotor-mounted. This field rotates due to the driving force of the prime mover, to produce the rotating magnetic field required for voltage generation in the stator winding.

I was also wondering whether someone could point me in the direction of a typical wiring diagram - an actual real-life, in use one, that would help me understand the setup of a brushless excitation system used in generators today. The spec sheets I have seen supply no such diagram.

Thanks for your patience.

Richard Byrne, EET.

 

[davidbeach]

Close. There are three machines on a common shaft, but not concentrically arranged. Typically, the PM generator will be in the middle, with a PM field on the shaft producing an uncontrolled AC voltage. That AC voltage is rectified and used by the AVR (automatic voltage regulator) to control the voltage into the field of the rotating exciter. The rotating exciter will typically be located on the end of the shaft, away from the prime mover, and is constructed inside-out, with a stationary field and a rotating armature. The output of the rotating exciter is run through the rotating rectifiers, mounted on the end of the shaft, and the output of the rectifiers routed back along the shaft to the rotating field of the main generator. That arrangement is necessary if one is going to avoid the need for brushes.

 

[waross]

A lot of machines are constructed with a brushless exciter on the same shaft and inside the same frame as the main alternator. The voltage regulator supplies current for the field of the exciter. The exciter generates AC in the rotor which is rectified and fed to the rotating field of the main alternator. The voltage regulator controls the output voltage by varying the current supplied to the field of the brushless excitor, which in turn varies the field strength of the main rotating field.
The basic machine will use an Automatic Voltage Regulator which is fed from the output of the main generator.
If better regulation under heavy loading such as motor starting is required, a PMG or Permanent Magnet Generator may be ordered as a factory installed accessory. This is a small generator installed outboard on the back of the generator. It has a rotating permanent magnetic field and supplies about 220 volts 3 phase AC to the voltage regulator. It requires an AVR which is designed for PMG use. In the event of failure of the PMG or the PMG regulator the machine can be used with a standard AVR until repair parts can be obtained. I can always get a standard AVR on short notice but I usually have to bring PMG parts in from out of the country.
I have been discouraged by the factory the few times I inquired about retrofitting a PMG to an existing generator.

Also, about twenty years ago I was seeing 1000 watt and 2000 watt, 120 volt generators with a rotating permanent magnet and fixed coils. I haven't seen one for many years. Is this what you were asking about?
respectfully

 

[rbyrne15]

Thanks to you both for you clear replies - now it's all straight in my mind. I'll be printing out this thread for sure for use in the future.

waross, thanks for the insight into the "just in case" design, with regard to the PM generator/ AVR and the standard AVR. I'll bear that in mind for the future too.

Richard.