Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Micro Hydro Design Help

  1. Mar 23, 2010 #1
    Looking into “reverse engineering” a pump and running it like a turbine and using an induction motor of some sort generate power. Basically design/build my own micro-hydro power system. The system if I do go ahead with it, will be connected to the grid.

    I am very capable of handling the mechanical side of things, however I have a lot of reading to do when it comes to the electrical side. From what I have read so far I need an asynchronous generator (or induction motor).

    I will list my questions, it makes it much easier to read:

    1. From what I have read, I understand if the induction motor spins above the rated motor speed it becomes a generator, anything below the motors rated speed and it becomes a motor. Is this correct?

    2. Suppose the motors rated speed is 1500rpm, and I design my turbine to spin at 1600rpm and connected it to the motor, will it begin to generate power and automatically synchronise to the gird? What happens if there is a sudden increase in flow rate and the turbine spins up to 1950rpm what would happen to the generator, would it continue to produce power and synchronise to the grid? Is there a maximum speed the generator can handle? I know for a synchronous generator I can use that formula to calculate synchronous speed (depends on number of poles and grid frequency), is there a certain percentage of this that should not be exceeded by an asynchronous generator for example the turbine speed should not go higher than 10% of the synchronous speed?

    3. What happens if the generator is happily generating and is suddenly cut off from the grid? For example it is turning at 1600rpm and then suddenly it is disconnected from the grid, it is turning too fast to be a motor. Or if there is a spike or instability in the grid?

    4. I was told to look into a doubly-fed induction generator, (DFIG). Would a DFIG system suite my application? I have tried searching for DFIG manufacturers and couldn’t find anything, are there any companies that sell complete DFIG units? Are prices substantially more expensive than that of an asynchronous generator?

    Thanks for any help in advance, I understand I have a lot of reading to do!
  2. jcsd
  3. Mar 24, 2010 #2
    Apologies, just realised I posted this in the wrong section, was ment to go in electrical.
  4. Mar 24, 2010 #3


    User Avatar
    Science Advisor

    The power angle will determine if the electric machine operates as a motor or generator. Essentially if you have a motor and its shaft spins faster than its rated speed it will change the power angle and cause it to back-feed the source (grid in this case). However, if no electrical power is being applied to the motor and you rotate the shaft, the motor becomes a generator and will provide an electrical power output.

    If you are completely powering the electric machine via a mechanical source then you have a generator. To connect it to the grid the voltage, phase, and frequency must be within a certain tolerance or you'll have a big problem. If it is out of phase enough then it will damage your prime mover. If the machine is already connected to the grid and acting as a motor and you speed up the shaft so that the power angle changes (I forget off the top of my head which way) then it can start to provide power to the grid. However, you'll still have a big problem since your little generator won't load share properly with the grid (essentially an infinite bus).

    If you somehow managed to load share with the grid and then disconnected your generator, nothing would happen other than the generator would be running at no load. The point you need to be aware of is that an electric machine will operate as a motor or generator depending on the power angle (which depends on the electrical load and mechanical load). If the electrical power input is higher the machine will act as a motor, if the mechanical input power is higher, it will act as generator.

    Research "motor to generator transition" to find out more information. Most books on electric machines will have a section on this.

    I'm not familiar with DFIG's.

    Hope this helps.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook