Concept of slip speed in Self Excited Induction Generator (SEIG)

[b.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

Kind regards,
Shahvir

 

[uart]

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).

I don't think that's correct shahvir. That would apply if the rotor currects were DC but of course they're not. With 3-phase AC rotor current the excitation air-gap flux will rotate at the frequency of the rotor current (divide pole pairs) plus that of the spinning rotor.

Since the slip must be negative in order to produce net electrical power then the rotor currents will actually have 3-pase currents which produce a rotating field (relative to the rotor) in the opposite direction to that of the rotors physical motion. So the air-gap flux will rotate at a frequency which is less than that of the rotors physical motion.

Hope that helps.

 

[b.shahvir]

Since the slip must be negative in order to produce net electrical power then the rotor currents will actually have 3-pase currents which produce a rotating field (relative to the rotor) in the opposite direction to that of the rotors physical motion. So the air-gap flux will rotate at a frequency which is less than that of the rotors physical motion.

Hope that helps.

Thanks Uart, although I've somewhat got the idea of what you are trying to say, but the above part is a bit unclear to me...especially the the rotor field in opposition to stator field. So, will it be possible for you to explain the physics of -ve slip in a SEIG in a bit more detail? The explanation might include the stages which a SEIG goes thru. from zero speed to, say, any desired speed. Also, since SEIG starts off as a synchronous generator due to residual magnetism, I consider rotor to be 2 pole.

How does one compute output frequency of a SEIG?

Thanks very much & Kind regards,
Shahvir

 

[dlgoff]

Hey b.shahvir,

This "www.naun.org/journals/circuitssystemssignal/cssp-62.pdf"[/URL] paper might be of some help.

Regards

 

[b.shahvir]

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