Squirrel Cage Induction generator has variable speed?

Thread Starter

Adam Brave

Joined Mar 26, 2014
13
Hi,

I've one doubt about the Induction generator with squirrel cage rotor. Is the speed of this kind of machine variable if it does not have a power converter system attached to stator windings? If not, what is the speed? Synchronism?

Regards,
 

MaxHeadRoom

Joined Jul 18, 2013
23,802
An induction motor cannot run at synchronism.
The very common way currently used is a VFD, Variable Frequency Drive unit, these will also provide 3 phase output from a 1ph source.
There once was a common method of detecting the slip frequency and once the slip difference was within 3-4 cyles, a DC current would be injected via slip rings and the rotor come up to synchronism.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
23,802
Not that I know of or come across, There is a common way of producing 3ph from single phase using a 3 ph induction motor, but that entails connecting 1ph to 2 windings and start it rotating somehow for start up, after which the motor runs off of the 1ph and generates the 3rd phase.
Max.
 

crutschow

Joined Mar 14, 2008
27,945
I missed that you were talking about an induction generator (obviously). A induction motor will operate as a generator if over-driven while connected to a power line. It has to be driven above synchronous speed, again for the slip to generate the rotor magnetizing current, to put power back into the grid. The slip frequency above synchronous is likely similar to an induction motor of 1 to 5%.

When driven as a generator the induction machine requires no power converter, it's just connected directly to the mains. But note that if the machine is not getting enough shaft input to keep the speed above synchronous (such as from a wind propeller) then it will become a motor and try to drive the prime mover. Thus there must be a way to sense when this happens and disconnect it from the grid .
 

Thread Starter

Adam Brave

Joined Mar 26, 2014
13
I missed that you were talking about an induction generator (obviously). A induction motor will operate as a generator if over-driven while connected to a power line. It has to be driven above synchronous speed, again for the slip to generate the rotor magnetizing current, to put power back into the grid. The slip frequency above synchronous is likely similar to an induction motor of 1 to 5%.

When driven as a generator the induction machine requires no power converter, it's just connected directly to the mains. But note that if the machine is not getting enough shaft input to keep the speed above synchronous (such as from a wind propeller) then it will become a motor and try to drive the prime mover. Thus there must be a way to sense when this happens and disconnect it from the grid .
So, from what you are saying, the fact that the induction machine rotor, is short circuited (squirrel cage), doesn't prevent it to work as a generator since he has enough torque provide by another machine in the shaft righ?
 
Last edited:

Kermit2

Joined Feb 5, 2010
4,162
So, from what you are saying, that fact that the induction machine rotor, is short circuited (squirrel cage), doesn't prevent it to work as a generator since he has enough torque provide by another machine in the shaft righ?
Yes, the generator must be driven. Power MUST be input to generate a power output. This is a basic physical fact of motors and generators. You never get something for free, one must always 'pay' for the reward. :)
The power input must be greater than the power being utilized by the induction machine in its motor mode. The rotor speed must be greater than when the machine operates in motor mode. If you meet those two requirements you will generate AC power that is in phase with the power grid
 

crutschow

Joined Mar 14, 2008
27,945
....................................
The power input must be greater than the power being utilized by the induction machine in its motor mode. .....................
Not sure what you mean by that (?). The input power only has to be sufficient to overcome the motor's friction and windage losses and drive it to synchronous speed. Any additional input power drives it above synchronous speed and is converted to output power.
 

crutschow

Joined Mar 14, 2008
27,945
So, from what you are saying, the fact that the induction machine rotor, is short circuited (squirrel cage), doesn't prevent it to work as a generator since he has enough torque provide by another machine in the shaft righ?
Yes, the rotor acts the same whether the machine is be used as a motor or generator. In either case it's the slip that generates the induced current in the rotor to generate the rotating rotor field. But, of course, the induction generator must be connected to the grid so their is a field current to generate the induced rotor current to get things started.
 

Kermit2

Joined Feb 5, 2010
4,162
Not sure what you mean by that (?). The input power only has to be sufficient to overcome the motor's friction and windage losses and drive it to synchronous speed. Any additional input power drives it above synchronous speed and is converted to output power.
The power input must also be greater than what it is driving, such as the large blades of a wind turbine. You are correct that once you are moving the rotor close to synchronous speed the only power the motor is using is for internal friction and electrical resistance, but I was talking about the entire setup including the power required to rotate the blades(if it is a wind turbine). For example; wind speeds under some limit, like 10 mph, would not have sufficient power to achieve above synchronous speed. The motor would be consuming power to rotate the blades and for this reason, be using more power than the friction and resistance consume.
We are on the same page here but talking past each other. :)
 

crutschow

Joined Mar 14, 2008
27,945
The power input must also be greater than what it is driving, such as the large blades of a wind turbine. You are correct that once you are moving the rotor close to synchronous speed the only power the motor is using is for internal friction and electrical resistance, but I was talking about the entire setup including the power required to rotate the blades(if it is a wind turbine). For example; wind speeds under some limit, like 10 mph, would not have sufficient power to achieve above synchronous speed. The motor would be consuming power to rotate the blades and for this reason, be using more power than the friction and resistance consume.
..............................
Well normally you don't use the generator to drive the wind turbine since that's just a power waste. If the wind isn't strong enough to drive the generator above synchronous, then you need a circuit to disconnect the generator from the mains until such speed is reached.
 

Thread Starter

Adam Brave

Joined Mar 26, 2014
13
The power input must be greater than the power being utilized by the induction machine in its motor mode. The rotor speed must be greater than when the machine operates in motor mode. If you meet those two requirements you will generate AC power that is in phase with the power grid
One more question.

In the case of the DFIG, even when the rotor speed is bellow the synchronous speed, the machine works as a generator right? In this case (Wr<Ws) it delivers power by that stator and consume power by the rotor (less than the power stator) right?
 

alfacliff

Joined Dec 13, 2013
2,458
no, the machine dosnt generate till it reaches the slip speed. for instance a 1750 rpm motor would have a synchronous speed of 1800 rpm. it will generate when it goes abouve 1750 rpm and will unlock or stop generating a little above 1800 rpm.
 

crutschow

Joined Mar 14, 2008
27,945
According to Wikipedia on doubly fed machines (which are not induction machines) "For operation as a generator a similar situation exists. At subsynchronous speeds the stator is generating the power but part of it has to be fed back to rotor. At supersynchronous speeds both the rotor and stator are producing power to the grid." so your statement appears correct.
 

Thread Starter

Adam Brave

Joined Mar 26, 2014
13
According to Wikipedia on doubly fed machines (which are not induction machines) .
I didn't invent the DFIG term. It stands for Doubly Fed Induction Machine. So it is in fact an induction machine.

My doubt is, for these type of machines they allways operate as a generator, right?
 
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