Resistance and Motor Slip

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Besoeker3

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All 50Hz. Slip is 0.136. Not sure what you need for the winding the motor?
 

winnie

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Springfield, MA, USA
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Electric motor research
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
 

mbrooke

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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


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

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
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.
Don't see that.
 

mbrooke

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Location
United States
Occupation
Technician
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


But why would the rotor resistance itself matter? Doesn't it vary when measured with a meter vs AC power?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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
 

mbrooke

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Technician
They say Induction motor... is there such a thing as wound rotors but no electrical windings in the stator?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
How could it be an induction motor with both the rotor and stator have windings?
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
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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
 

bwat

EE
Location
NC
Occupation
EE
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.

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...
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
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.
 
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