mbrooke
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How would you go about solving this? Are they talking about re-winding the motor? To me this question doesn't even seem legit yet alone having any real world merit.
Resistance and Motor Slip
- Thread starter mbrooke
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winnie
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To a _very_ rough approximation you want the rotor current in the second case to equal the rotor current in the first case. But (again _very_ roughly) the voltage induced in the rotor circuit is proportional to the slip, so you need to change the rotor resistance by the same proportion in order to get back to the original rotor current.
So as an 'in your head' answer you need a total rotor resistance of 0.136/0.04 * original rotor resistance.
_However_ this is only a very rough approximation that ignores the effect of rotor inductance as rotor frequency changes.
For a better calculation, you solve the equivalent circuit of the motor, and then re-solve it with different rotor resistance. Writing a closed form equation for how the resistance needs to change to get a particular speed change is beyond me; I'd probably simply iterate with different trial resistance values.
-Jon
So as an 'in your head' answer you need a total rotor resistance of 0.136/0.04 * original rotor resistance.
_However_ this is only a very rough approximation that ignores the effect of rotor inductance as rotor frequency changes.
For a better calculation, you solve the equivalent circuit of the motor, and then re-solve it with different rotor resistance. Writing a closed form equation for how the resistance needs to change to get a particular speed change is beyond me; I'd probably simply iterate with different trial resistance values.
-Jon
mbrooke
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To me they have to be talking about 25*C winding resistance... in that a running motor would not be based on resistance but rather impedance.
So... do I change the material of the windings or add more windings...? Doesn't make sense at all.
winnie
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When they say 'add rotor resistance' I assume they are talking about an old school wound rotor induction motor.
An old method of motor speed control was to have a wound rotor connected to slip rings. The slip rings connected the motor to an external variable resistance. By changing the resistance of the rotor circuit you could change the speed of the motor.
-Jon
An old method of motor speed control was to have a wound rotor connected to slip rings. The slip rings connected the motor to an external variable resistance. By changing the resistance of the rotor circuit you could change the speed of the motor.
-Jon
Don't see that.
mbrooke
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But why would the rotor resistance itself matter? Doesn't it vary when measured with a meter vs AC power?
winnie
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The rotor has both resistance and inductance, both of which influence the amount of current flowing in the rotor when exposed to the rotating magnetic field of the stator at different amounts of slip.
The higher the rotor resistance, the lower the rotor current flow at a given amount of slip, and the less torque produced. So with more rotor resistance you need more slip to get the same torque. Adding resistance doesn't really slow the motor down, rather it changes the shape of the torque speed curve so that the motor needs to have more slip to produce the same torque. Then the load slows the motor down.
This is best seen if you look at a 'motor equivalent circuit' model. Of course as a model it is only an approximation, but you can take the model, use standard circuit calculation techniques, and see what the effect is on the system.
I rather like the explanation given by this site: https://people.ucalgary.ca/~aknigh/electrical_machines/induction/im_circuit.html
In any case, in motor design, the rotor cage resistance is only of the design parameters; you can use Cu vs Al, or fat bars vs skinny bars, etc. When the rotor has more resistance, the motor operates with greater slip. And as I mentioned, if the rotor circuit includes a connection to external resistors, you can change speed by changing the external resistance.
-Jon
The higher the rotor resistance, the lower the rotor current flow at a given amount of slip, and the less torque produced. So with more rotor resistance you need more slip to get the same torque. Adding resistance doesn't really slow the motor down, rather it changes the shape of the torque speed curve so that the motor needs to have more slip to produce the same torque. Then the load slows the motor down.
This is best seen if you look at a 'motor equivalent circuit' model. Of course as a model it is only an approximation, but you can take the model, use standard circuit calculation techniques, and see what the effect is on the system.
I rather like the explanation given by this site: https://people.ucalgary.ca/~aknigh/electrical_machines/induction/im_circuit.html
In any case, in motor design, the rotor cage resistance is only of the design parameters; you can use Cu vs Al, or fat bars vs skinny bars, etc. When the rotor has more resistance, the motor operates with greater slip. And as I mentioned, if the rotor circuit includes a connection to external resistors, you can change speed by changing the external resistance.
-Jon
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They say Induction motor... is there such a thing as wound rotors but no electrical windings in the stator?
winnie
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Nope. At least not with any common induction motor.
In a wound rotor induction motor you have normal stator windings, and then you have windings on the rotor which are connected via slip rings to an external circuit.
I suppose you could build an induction motor with a 'squirrel cage' stator and a wound rotor, and then apply power to the rotor. But that would just be crazy
Also a common type of motor has a permanent magnet stator and wound rotor, but now we are in an entirely different class of motor.
-Jon
In a wound rotor induction motor you have normal stator windings, and then you have windings on the rotor which are connected via slip rings to an external circuit.
I suppose you could build an induction motor with a 'squirrel cage' stator and a wound rotor, and then apply power to the rotor. But that would just be crazy
Also a common type of motor has a permanent magnet stator and wound rotor, but now we are in an entirely different class of motor.
-Jon
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How could it be an induction motor with both the rotor and stator have windings?
That's exactly right.
There are specials which are stepped rotors, usually quite large. Often 2MW upwards.
It can. Switched step resistance. Usually the resistance steps up until gets until about 2/3 speed.
winnie
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All induction motors have both rotor and stator windings.
In a common induction motor the stator has what you think of as a winding; magnet wire coiled up, coils joined together, and connected to your supply. The rotor is a 'squirrel cage', solid end rings connected by thick bars. But it is still a coil; current can go down one set of bars, through the end rings, and then up another set of bars. Current gets induced in these 'rotor coils' for the current flow in the rotor circuit.
Take the squirrel cage out and replace with coils of wire similar to that of the stator, and now you have a 'wound rotor induction motor'. The function is the same as the squirrel cage motor, current gets induced in the rotor coils just as it gets induced in the squirrel cage motor, but now you have the option of adding additional resistors as desired to the rotor circuit.
-Jon
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Dumb question... does this rotor require external power?
winnie
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A wound rotor induction motor and a conventional squirrel cage induction motor both couple power to the rotor by magnetic induction. No external power is needed. For the motor being discussed in the problem at the start of the thread, no power other than that coupled magnetically from the stator. The resistors do get hot and that power has to come from somewhere.
However there is a device known as a doubly fed induction motor, which is a wound rotor IM that has a VSD connected to the rotor circuit. Yet another beast in the menagerie of motors. And I suppose if you fed DC to the rotor circuit you could treat the device as a synchronous motor.
-Jon
However there is a device known as a doubly fed induction motor, which is a wound rotor IM that has a VSD connected to the rotor circuit. Yet another beast in the menagerie of motors. And I suppose if you fed DC to the rotor circuit you could treat the device as a synchronous motor.
-Jon
mbrooke
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Wow! I had no idea! I heard of sync motors, but for the rest I learned several new things.
I just had to give you a 'like' for referencing a DFIG (doubly fed induction generator/motor). Possibly the first I've seen on this site. A whole lot of windmills out there with DFIGs...
Kramers if you like.
mbrooke
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Does anyone have a picture and 3 line diagram of such a motor?
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