convert a slip ring motor into a variable voltage transformer? 4

[ankervik]

Someone told me it is possible to convert a slip ring motor into a variable voltage transformer? If so what would be the set-up? We often have problems running up larger AC induction motors unloaded (380v/50Hz). Our present test bench doesn't cope too well when running motors over 250kw

 

[Skogsgurra]

It is possible, yes.

Google "induction regulator" to see examples.

The first one that turns up is quite good:

http://www.geocities.com/cindulkar/lab17.pdf

You need an electrician with good knowledge to do this.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ankervik]

Thanks Skogsgurra!
Pardon my somewhat limited (and rusty) electrical knowledge. I checked out the diagram you kindly linked. To me it didn't seem far away from an auto transformer of sorts. I am assuming (depending on the voltage ratios) that either the rotor or stator could be used as a primary, would that be correct? Could a similar setup be used as a variable voltage test output for motors up to say 400kW? (providing we can find a suitable slip ring motor).

 

[Skogsgurra]

Short answers: Yes. Yes.

It does not work like an autotransformer. The action depends on the rotor voltage being phase rotated to buck or boost the line voltage. You can have any voltage in-between, depending how you rotate the rotor. For 0 - 200 % of line voltage, you need rotor voltage = line voltage.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ankervik]

Gunnar,
I think I'm with you so far (or at least I hope I am). Imagine a S/R motor having a 400v stator & 230v rotor. We could in theory hook-up the rotor as primary 230v & get a stator output 0-460v variable at the same frequency? What then happens to the stator/rotor nameplate currents? are these to be regarded as maximum input/output values or is that too much to hope for.

 

[aolalde]

In real life applications of WRIM as voltage regulator, you should take care of:

1- Torque developed by the rotor when the load and rotor current increase. How will you keep the rotor locked and how it will be moved to adjust the desired voltage?
2- If the stator voltage is fed into the rotor winding neutral. Do the rotor winding and brush/slip rings have enough insulation to handle the increased voltage. Normally the rotor voltage is designed, constructed and insulated for a fraction of the stator voltage.

 

[ankervik]

Thanks for the input. My intuition tells me that the rotor would have to be securely locked with a hefty gear reducer, motor drive, limit switches, etc. What I am not sure of is the actual polar movement required to vary from min to max secondary output. If this movement is small then why not just dispense with the rings/current collectors & mount appropriate cable connections instead? Surely if the nameplate open circuit rotor volts are 230 then it would handle the same injected value at its rated amps (or have I got something wrong). On the drawing Gunnar kindly linked it looks like the secondary side isn't fully connected internally (i.e. all phase ends of the winding are brought out). Appreciate more good input here guys!

 

[ankervik]

Just an afterthought, what exactly does WRIM stand for? I don't think I've ran across that particular abbreviation.

 

[aolalde]

The phase angle displacement is dependent on the number of poles (P).In general G° = E°/P/2
For 2 poles the geometric degrees (G°) are the same as the electrical degrees (E°). For 8 poles to displace 90°E you move 90/8/2 = 22.5 ° geometric.

WRIM = Wound Rotor Induction Motor.

 

[ankervik]

Please correct me if I misunderstood: The higher polarity a WRIM motor has, the less geometric displacement it requires to achieve the same result. Now the million dollar question...is it feasible?

 

[electricpete]

"what exactly does WRIM stand for?"
Wound rotor induction motor

"is it feasible?"
I have seen a motor repair shop that had a WRIM permanently set up to provide a variable voltage source for testing a variety of motors.

"My intuition tells me that the rotor would have to be securely locked with a hefty gear reducer, motor drive, limit switches, etc"
That sounds similar to the setup I saw. They had a gearbox connected to control rotor position.... low speed high torque side was connected to the WRIM, the other side was connected to a small high speed low torque reversible motor used for control. Don't remember about braking.

"What I am not sure of is the actual polar movement required to vary from min to max secondary output. If this movement is small then why not just dispense with the rings/current collectors & mount appropriate cable connections instead?"
The angle of movement for full change of voltage is small... I think it is the span of one pole or less. The setup I saw had hardwired lugged connections between rotor and stationary power supply as you said.

The EASA Technical Manual has a detailed writeup on converting a slip ring motor for use as a variable voltage source. A great reference if you can get your hands on it, but pretty expensive. Most motor repair shops in the US have a copy.



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

I should say: pretty expensive for non-EASA members. I think it's much cheaper for EASA members.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[ankervik]

Thank you electripete (and all other contributors) so very much. I am new to the forum & I must say that I was extremely pleased to get so much good help from so many wise people. Now I know for certain this project has great potential I will suggest our management enrol for EASA membership a.s.a.p. I am sure it will prove invaluable. By the way I checked out EASA prices, wow...you're right it is expensive. I am a Brit working in Scandinavia, sadly EASA isn't too well known about here. Over the last few days I have been in touch with other local motor repair shops in the hunt for a suitable WRIM. Most of them thought I was off the mark in trying & use a WRIM for anything else than its intended purpose. Now I know the opposite is true!! Thanks again guys!

 

[itsmoked]

Yes $600 for a book is, um, repulsive?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ankervik]

Apologies electricpete, I misspelled your handle first time around...that's what you get for having meaty mechanical fingers!

 

[Skogsgurra]

Hello ankervik! Are you, by any chance, located in Moss?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[motorspert]

ankervik,
the other thing to be aware of is overheating. The motor will be designed to cope with the full current at rated speed - at zero speed there is no cooling. You could add some blowers or fit some thermpcouples to the rotor. It depends on how long you want to test for.

These method is alos used for exciters on variable speed synchronous motors where excitation isrequroed at zero speed.

 

[ankervik]

Thanks, I hadn't given that consideration! Guess I'd better look around for a blower unit as well suitable WRIM.

 

[ankervik]

Hi Gunnar, good to hear from you again. No, not Moss but you sure got the country nailed. I am further West (mid way between Bergen & Stavanger). I would just like to say that for my part this thread has covered some extremely valuable ground & I will be eternally grateful for information & help received. Maybe we can talk again neighbour?

 

[Skogsgurra]

Sure can! Returned from Trondheim abt ten days ago. Tried to get a hotel room there when Rosenborg played FC Valencia. Ended up sleeping on a sofa in a private home...

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...