Schrage Motors (Variable Speed AC Motors) 1

[DavT]

I'm looking for some infomation on the efficiency of Schrage motors, I am looking at replacing a 16-50 hp 720 - 1500 rpm unit with a standard motor and inverter setup but need some cost justification,I have searced the likes of ABB & Ferndau Websites to no avail, any ideas of the efficiency rane of these unit anyone ?

 

[Skogsgurra]

It's a long time ago that I saw a schrage motor! I think back in the seventies. I remember that I was very proud when I finally worked out how it works. Sorry, forgot it again.

I have some data on a 16-55 kW 420-1440 RPM 50 Hz machine and I think that it is close enough to your machine.

At highest speed, you have these data:
Pf goes from 0.85 to 0.97 when torque goes from 50 percent to 100 percent.
Efficiency is almost constant at 0.85 between half load and full load.

At synchronous speed, you have these data:
Pf goes from 0.75 to 0.87 when torque goes from 50 percent to 100 percent.
Efficiency goes from 0.82 to 0.87 when torque goes from 50 percent to 100 percent.

Hope this helps. I think that you should have modern drives. The maintainance for these machines with both slip rings and a commutator - and three sets of brushes! - is very high. It will also be next to impossible to have such a motor repaired if you have a failure in it.

 

[RalphChristie]

Skogs

I think I am far younger than the most forum members. What the heck is a schrage motor?
Where do you use it?

Thanks
Ralph

 

[Skogsgurra]

Ralph,

As I said before; I once did understand how it works - but I have forgotten. It is an electrical motor with a three-phase input and slip rings. It also has a commutator with two sets of brushes that move in opposite directions from the neutral (synchronous position). The two set of brushes are connected (IIRR) to the slip rings. With brushes in neutral, the voltage is zero and the slip rings are more or less shorted. So the motor runs like a normal AC induction motor.

When you move the brushes from the neutral position, a voltage is induced and fed to the slip rings. And this is where I start to forget how and why it works. Anyhow, by adjusting the voltage you adjust the speed. If the voltage bucks (opposite phase), the motor speed decreases and if it boosts (same phase), it increases speed.

It is possible that stator winding and slip rings have opposite roles - I think that they might have. There is something about the commutator voltage having the same frequency as the feed back voltage - and then I guess that you have to connect the slip rings to the grid.

Do a google on "Schrage Motor". I hope that you will find a better explanation there.

 

[RalphChristie]

Thx Skogs

Information on Google regarding the scrage motor is not much. The best I could find was:

http://www.lmphotonics.com/vsd/vsd_01.htm

http://www.angelfire.com/nt/ramson/schrage.html

But, I have done a search in our library, and found a book which explains the working of the schrage motor:
Control of electrical machines - Irving L. Kosow

Now, I don't want to change the orignal subject, (Sorry DavT ) but just as a piece of information for other members not familiar with this kind of motor:

The first essentials of the Leblanc system (a WRIM, a frequency converter and a 3ph variable trsf) were first combined into a single dynamo by Karl H Schrage of Sweden. The same speed-controlled motor was also produced in the USA as the BTA brush-shifting motor.
The primary excitation winding of the motor is located on the rotor (reverse of a wound-rotor motor) and is excited through slip-rings. The wound-rotor secondary is on the stator, with brushes located on the comutator 120° apart exciting the stator winding.
The auxilary winding, connected to the commutator and excited through the primary rotor winding, corresponds to the converter. When the two brushes of each phase winding of the stator secondary are in contact with the same commutator bar, i.e., shorting the phase winding or secondary, there is no foreign voltage induced in the secondary and the motor operates as a shorted wound-rotor induction motor.If the brushes are seperated, by means of a mechanical handwheel, in such a way that a foreign voltage is conductively introduced into the rotor from the converter winding which opposes the secondary emf induced from the rotor primary, the rotor current, flux and torque decrease, and speed drops. If the brushes are seperated by means of the handwheel in the opposite direction, so that the foreign voltage aids the secondary induced emf, the motor current, flux, and torque increase above synchronous speed. It is also possible to rotate the entire brush rigging structure to provide power-factor adjustment as well.
Unfortunately, the machine does not operate satisfactorily at synchronous speeds (no voltage is induced in the stator) and for this reason cannot be readily constructed in the larger horsepower ratings. The torque instability at synchronism occurs because of the inertia of heavy loads. This is its major disadvantage.
The speed is entirely variable over a range of from 3:1 to 4:1, depending on the size and the numbers of poles. (usually four or six-poles) It have the advantages of:

1. Relative good starting and max. torque (1.5 and 2.5 times rated torque, respectively) for a variable speed motor.
2. Exceeding smooth speed control above and below synchronous speed
3. Extremely high efficiency.
4. Excellent speed regulation
5. It combines all three devices of the Leblanc system in a single machine with the same advantages.

Regard
Ralph

 

[Skogsgurra]

Great research, Ralph!

See why I was hesitant about the rotor/stator and infeed part? Also surprised to see that Schrage was a fellow countryman. I always thought he was German.

 

[DavT]

Thanks for your help on this on people, it is much apreciated I also thought Schrage was a German as well !!!

 

[powerjunx]

daz,
i had handle such machine years ago..mostly i encounter was made by FARNDAU, ABB, AEM, other brand i hardly recall it.. im quite sure some motor are incorporated with HIEDELBERG printing machine package.

 

[Skogsgurra]

I had forgotten about this thread when the Schrage motor was appeared in thread 237-110269. Go there and read all about it!

 

[Skogsgurra]

OOps! How can one remove a post?

 

[ScottyUK]

Hi Skogsgurra,

Red Flag it and ask Dave to remove it as posted in error. The Red Flag just alerts him to have a look at a problem: it's not just for bad posts.



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