3 phase power generation question

rgray

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If a 3 phase motor is over spead , it generates power back into the line.
I saw this years ago and it was for a windmill generator that used a 3 phase motor.
A simplistic and inexpensive set up as no $3000.00 grid tie was needed.
The logic was if the motor can be run (powered) then when driven to a speed above what it runs it genertes power back and all is happy as far as the a/c is concerned (in sync?)

Any body have an idea if this works with rpc or vfd driven motors? If the motor is running and I speed it up with a windmill or gas motor will it generate back and will the rpc or vfd be able to handle this?
 
I don't think the speed has anything to do with it. Motors don't need to go "a certain speed" to generate electricity, they just need to be driven.

As far as the voltage coming out, that does have to do with the speed they are being driven. At least that is my understanding.
 
You are also creating alternating current, not direct current. To get the desired frequency of the current generated (if that is needed) requires either having mains current phase it with the correct frequency or have the driving shaft controlled at the correct speed if is is not attached to existing alternating current at the correct cycles per second.
 
You have to have dc current to excite the field and motion of course.
 
I'm not an expert, But a few things I know. 1, the tie in equipment does more then just look pretty. If the grid goes down, it prevents your system from dumping power into the system, and hurting someone.

AC power generation is a lot more involved then saying more RPM's equal more power. The generator, and power grid need to have the same frequency, and be in phase with each other. The first means the generator needs to be turning at the right RPM to give the right frequency, so as mentioned, the idea of overspeed will not work. The second means they need to be in sync.
 
The generator only needs to be at the same frequency when it gets synched (which depends on how many poles it has). After that the grid controls the frequency.
 
http://www.alternative-energy-tutorials.com/wind-energy/induction-generator.html
Read "Induction Generator" part and " Asynchronous Machines"

So if I had 3 phase power and a motor hooked to it and I drove it faster it generates. This we know.

My question is since I'm not hooked to 3 phase power and am using rpc's and vfd's does over speeding a 3 phase motor generate power back through those units. Of course it would generate power back but more specifically is it ok to do so through the rpc or vfd?
 
i'm not sure if i understand the question correctly,
but if you are trying to push power the opposite direction through a vfd from a windmill or other power generating device -
i don't think it's not going to work out the way you are envisioning, you'll most likely damage the VFD.
VFD's can take slightly lower line voltages for their input and compensate and deliver full output of their rating, but are not bidirectional in regards to input and output swaps

Now, if you are asking if you use a RPC to supply power to a VFD which is controlling a motor,
the vfd will not send a surge back into the input circuit if the motor it controls is run at a different frequency.
the VFD's internal rectifiers will not allow current flow in the opposing direction
 
Simple answer: No
It is true that you can use a conventional 3 phase motor as an asynchronous generator. This works fine when the generator is small and connected to a large grid.

This totally does not work on a VFD. Everything would be backwards. A VFD has a front end rectified that converts the AC to DC, then a power section creating pseudo AC from that DC buss - usually followed by some fix up/smoothing. You cannot push 3 phase AC into what would have been the outlet - totally does not work.

Similar idea for an RPC - strictly speaking the motor in the RPC only has power on 2 legs, so you may be able to push a little power in, but the voltage / current relationship is going to be wonky.
 
i'm not sure if i understand the question correctly,
but if you are trying to push power the opposite direction through a vfd from a windmill or other power generating device -

That is what I was wondering.
What about the rpc.....pushing power back to it?
 
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