Hi Guys,

In case of an Induction Generator (IG) connected to grid, the rotor runs at super synchronous (above synchronous) speed exhibiting -ve slip. This is possible since the grid frequency is fixed at say, 50Hz. However, in case of a Self Excited Induction Generator (SEIG), it starts off as a synchronous generator at low speeds due to residual magnetism. As the rotor speed eventually increases, the external capacitor injects sufficient reactive power into the stator to maintain air-gap excitation flux.

But the stator current frequency of SEIG is determined by rotor speed and hence the excitation air-gap flux would rotate at the same speed as that of the spinning rotor (if a 3 phase SEIG is considered). In this case, how will the SEIG develop output power if slip between rotor and air-gap flux is zero?.. as at every step the increment in rotor speed will cause a proportional increment in stator frequency and at each step, the air-gap excitation flux will be in synchronism with the rotor speed resultiing in zero slip. Zero slip would mean zero generated output power!

Can someone pls. clear the above concept for me? Also can this concept be applied to a single phase SEIG? Pls. help.

Pls. Note:- I've considered a 2 pole m/c. for the study.

Thanks & Kind regards,
Shahvir

 

Hi Guys,

In case of an Induction Generator (IG) connected to grid, the rotor runs at super synchronous (above synchronous) speed exhibiting -ve slip. This is possible since the grid frequency is fixed at say, 50Hz. However, in case of a Self Excited Induction Generator (SEIG), it starts off as a synchronous generator at low speeds due to residual magnetism. As the rotor speed eventually increases, the external capacitor injects sufficient reactive power into the stator to maintain air-gap excitation flux.

But the stator current frequency of SEIG is determined by rotor speed and hence the excitation air-gap flux would rotate at the same speed as that of the spinning rotor (if a 3 phase SEIG is considered). In this case, how will the SEIG develop output power if slip between rotor and air-gap flux is zero?.. as at every step the increment in rotor speed will cause a proportional increment in stator frequency and at each step, the air-gap excitation flux will be in synchronism with the rotor speed resultiing in zero slip. Zero slip would mean zero generated output power!

Can someone pls. clear the above concept for me? Also can this concept be applied to a single phase SEIG? Pls. help.

Pls. Note:- I've considered a 2 pole m/c. for the study.

Thanks & Kind regards,
Shahvir

Interesting question. I have some experience with induction motors/generators, but not really with self excited generators. So, let me start by asking a question. Are you sure that the effect of the capacitor generates a balanced 3 phase excitation in the 3 phase stator? My guess is that the capacitor would be set up in an unbalanced arrangement in order to get the process started. There would initially be power pulsations, just as there are torque pulsations when you first plug an induction motor into the wall. However, these would eventually settle to a steady state 3-phase output, as you described.

 

My interest in Induction Generators started off out of curiosity of Seimens make electric traction system used in my city which employ inverter fed 3 ph induction motors also used for regenerative braking. This curiosity eventually led me to an interest in SEIGs as they are independent source of power supply.

However, slip speed is something easily exhibited by grid connected IGs (as grid frequency is fixed and hence speed of rotating stator magnetic field remains constant), but SEIGs use capacitors which themselves depend on rotor speed for sourcing excitation amps. Hence the frequency at which the capacitors operate itself depends on rotor speed so the concept of slip between rotor and stator rotating magnetic field becomes a bit sketchy to understand as stated in my earlier post. I wish some clarity could be provided in this regard.

Thanks & Kind regards,
Shahvir

 

In case of an Induction Generator (IG) connected to grid, the rotor runs at super synchronous (above synchronous) speed exhibiting -ve slip

Interesting thought train, one that I can't comment on.

Alternators generate thier voltage in relationship to their pole configuration, in synchronous fashion. Slip is not a component. Rotational speed, and Eg is goverened to line tolerances. Bringing parallel alternators onto the line, require close matching of parameters prior to engagement. Once engaged, anyone alternator can be made to share load components. Frequency variation affects load balancing amongst paralleled alternators, which can lead to line oscillations and current tripping of individual units and/or large scale irregularities, such as the US north eastern disruption a number of years ago.

 

Thanks, but my query has been mis-understood. How is 'slip' created between stator magnetic field and rotor by capacitors in case of SEIG... unlike naturally in case of grid frequencies which remain fixed at 50 or 60Hz.