Diff between Slip Ring and Squirrel Cage Induction Motor

[avinsinanan]

Hello,

Does anyone know the main difference between a Slip Ring Induction Motor and a Squirrel Cage Induction Motor?

Also does the Stator of the induction Motor need slip rigns so that the AC current source can be applied to it?

I though the Stator of the induction motor was stationary and only the rotor moved? If this is the case why does an induction motor need Slip Rings are only necessary when motion is needed?

Also I read somwhere that the Rotor has slip rings? Is this true and why would the Rotor need Slip Rings?


Thanks for your time and patience.

Yours Respectfully
Avin Sinanan

 

[Styx]

Induction machines are AC-AC machines
ie the stator and the rotor must have AC sources supplied to them. Unlike a syncronous machine that is AC-DC or DC-AC (ie "AC stator DC rotor" or DC stator AC rotor"

The stator is stationary (unless you get special insideout machines). Due to the stator being stationary you can just plug in the stator windings into the utility supply (or via an inverter).

The rotor however is "rotation" thus there exist the problem of "energising" the rotors magnetic field. If you just provided wires to the rotor they would get twisted up.

By providing "slip-ring" you now have a means that electrical power can be suppled to a rotating connector. Thus teh slip-rings are on the rotor. There are machines out there that use mercury for the brushes but they are specialist machines

As to squirrel-cage if you look at the rotor it looks like a cage. Again a squirrel-cage machine needs to establish a magnetic field on the rotor (so that it can be dragged round by the field on the stator)

How a squirrel-cage developes its rotor field is by the induced voltage that is generated by the stator field. The squirrel cage is made from two rings at the front and the rear of the rotor. A different voltage potential is developed in each ring (due to the stator). The squirrel-cage also has shorting bars linking the two rings (these bars are what makes the rotor look like a squirrel cage). These shorting bars "short" the induced voltage and current flows generating the rotors field.

Think of induction machines like transformers -essentially that is what it is, its secondary winding is just rotating

Hope this helps.

 

[avinsinanan]

Thanks a lot

Hey thanks a lot for the reply. It was the best explantion I ever got with regards to motors. Thanks.

 

[Styx]

ha
no prob. Induction machine are the hardest of the machine types to get your head round (esp the squirrel cage variety).

 

[avinsinanan]

Confused as to why the rotor would need magnetic field?

Hello,

About the induction motor. I am confuded as to why the rotor would need a magnetic field that needs to be energised.

It would seem once the Stator has its magnetic field it would create induce a maganetic field across the rotor and due to Flemmings Left Hand rule the rotor would begin to rotate.

No to my understanding the rotor does not need to be energised in the Squirrel Cage Induction Motor but in the Slip Ring Induction Motor it does. I just cannot see why one has to energise the rotor field seperatly in the Slip Ring Induction Motor and why would it be necessary?

Thanks for your time again.. Its just hard to find a text book that actually explain induction motor well.. Its very frustrating.

Yours Respectfully
Avin Sinanan

 

[Styx]

Preciely. Because of Lenz's law a field is induced on the rotor. I think you were gettting in a muddle. I was describing the physics to why a field is established on the rotor - in effect Lenz's law.

Flemmings law will show you where the eveything is pointing

Now to why the slip ring need external field and the squirrel-cage doesnt look at the piccy below. You can see where the "induced" current will flow (thus establishing it own field on a squirrel-cage machine).

A similar thing will happen on a slip ring. However because of the slip rings you can apply external power to the rotor thus establishing a higher rotor flux density that could possibly be induced from the stator alone - thus a higher power rated machine (torque and speed). This comes at the price of brushes that wear down.

**broken link removed**

you now follow how the field is established on the rotor and I am sure you can appriciate that magnetics want the path of least reluctance thus the peak of the establish field on the rotor will try to line up with the peak of the stator field and since the stator field is rotating teh rotor will rotate.

The hard part is getting your head around "slip"

 

[avinsinanan]

Hey thanks a lot again

I think I am gettintg the hang of it. I did even take into consideration magnetic reluctance. It seems reluctance has more to do with it than Flemmings Left Hand Rule. I guess Flemming Left had rule is more for DC motors.


Also is it safe to say a squirrel cage induction motor can also be a slip ring at the same time? Casue all we would have to do it just add slip rings to the side of the cage.


I assume that most slip rings induction motors have a huge coil inside the rotor.


Hey thanks a million again and is there any book you would recommed I buy from Amazon.com for university level information on motors?

Am planning to do my masters in Power and Motors and the University of the West Indies.

Thanks

Yours Respectfully
Avin Sinanan

 

[Styx]

Welcome to the world of power electronics and motor drives.
I will not lie to you it is hard and your biggest enemy is noise and I am talking about serious electrical noise.

As to a book the best book you can get is:
"Power Electronics: Converters, Applications and Design"
by: N Mohan

Unfortuanatly it is not for sale in N.America. I dunno if you can get it for the "Windies" but if you can it will be your "Bible" Mine is always on my desk at work. It covers everything about power electronics,converters and motors

It has a good section abt machines to give an everview that motor drive engineers need.

A good machine book is:
"Electric Machines"
by: Mulukutla S.Sarma

It is very deep going into machine theory but if you can cope with it it is also very good.


As to Flemmings hand rules they are not used for direct application to motors. They are mainly used for straight wires. and where tHE wound move if you passed current through it in the present of a magnetic field.
Between Flemings and Lenz's laws they give you an instantanious state of a mag system. However, machine move and that state changes. Flamings rule is good to determine which way flux is being generated within a winding. From that just look at teh rotor and picture which way it would turn. - not as hard as it sounds

As to adding slip rings to a squirrel cage 1) you would end up having to put winding onto teh rotor and this goes against using squirrel cage also the induced field in the cage will interact with the field from the rotor winding and at work not turn at best very bad torque ripple.

Yep all slip-ring Induction machine have coils on the rotor so the the electrical power fed to the rotor can produce the rotor field needed for operation

 

[Klaus]

[/quote]
As to adding slip rings to a squirrel cage 1) you would end up having to put winding onto teh rotor and this goes against using squirrel cage also the induced field in the cage will interact with the field from the rotor winding and at work not turn at best very bad torque ripple.

Yep all slip-ring Induction machine have coils on the rotor so the the electrical power fed to the rotor can produce the rotor field needed for operation[/quote]

Yet, the main reason for slip rings in induction motors has nothing to do with the ability to supply an external exitation.
With a squirrel cage motor, the start up current and torque is fixed by the size of the squirrel cage conductors.
With large Hp motors, that current becomes unmanageable at startup. Remember, the rotor is initially at standstill and there is maximum current induction in the squirrel cage. It is very much like a large transformer working into a short circuited secondary winding. This current reduces as the rotor speeds up and hence the frequency of the magnetic flux changes.

So, the slip rings simply provide an easy means to connect an external resitor bank for a single step or multi step starting sequence. The resistors limit the maximum rotor current and hence the stator startup current.
Once the rotor is up at full speed the slip rings are shorted out. The rotor then works similarily than a squirrel cage one, with the circulating currents being dependent on the load and, hence, the speed of the magnetic flux changes due to 'slip' ( the difference in rotor RPM and stator field frequency).
The slip rings are only in use during start up and there is no energy being fed into these for that purpose.
Klaus

 

[Styx]

The bars are designed (with a certain depth) to increase their impedance at higher speeds (that is what reduces the rotor flux at speed). It is because of this that slip happens automatically with no control.

Where squirrel-cage comes into their own is at high speed - no friction on the brushes. However slip ring is still used for extremely high power - low speed drives where an external source for flux generation on the rotor is needed.

Couple that with the fact with VF drive synchronous drives (esp PM types) are becoming extremely competitive with induction machine that you just plugged into the utility supply and have already taken over induction machines in some application (wont be long till they overtake Induction machines fully).

The only way that Induction machine's can cope is with D-Q control and for the widest control range slip-ring needed to control the rotor field.

 

[crust]

As to a book the best book you can get is:
"Power Electronics: Converters, Applications and Design"
by: N Mohan

Unfortuanatly it is not for sale in N.America. [/url]

I have a copy of this book (and purchased in N. America). I believe there was a time when the 2nd edition became unavailable, but there is a 3rd edition out now. Amazon amongst others carry it.

BTW, I second that this is one of the best books on power electronics.