7 replies to this topic

[Matt303]

Does anyone know how an AC commutator motor actually works?

We have a project where existing variable speed motorshave speed control supplied via a commutator and external reactor it seems very similar to the Schrage motor described briefly on the LMphotonics main website. I have never seem these before.

The units are 400V specified as 1000/100rpm
Amps 203/97
Stator amps 190/108
50Hz

The units were originally supplied by Lawrence Scott and are 1960s vintage. I had assumed they were rotor resistance slip ring but this seems not to be the case.I have found some brief explanations of how these motors work and it seems to relate to variable turns ratio beteen the stator and rotor presumably resulting in a variable voltage / torque but I am not at all clear what is really going on and what the specific characteristics of these devices are. We are replacing them with variable speed drives. It would help considerably if we could understand how these existing devices were meant to operate.

Many thanks for any assistance you can provide

Matt

[marke]

Hi Matt

Yes, sounds rather like the Schrage motor. The speed is controlled by rotating the brush assembly around the shaft axis.
I have not found a good simple explanation of how these work, so if you find one, let me know!!
Best regards,

[jraef]

Here in the US we have something that sounds as though it is similar, we call them "Repulsion motors". They are used for applications, such as traction motors and farms, where high starting torque is necessary but low current draw is a must because of a limited supply, i.e. they are far afield. They are becoming rare though, VFDs are easier to implement and 3 phase motors easier to replace.

http://www.allaboutc...chpt_13/12.html

[marke]

"Repulsion motors"

No, a different animal completely as I understand it.
The Schrage motor has a split rotor. One half of the rotor has a delta connected winding that is brought out to slip rings and fed from the supply.
The other half of the rotor has a winding that is brought out to a commutator and the stator has three windings at 120 degree separation. These windings are connected to brushes that connected to the commutator. There are two sets of brushes displaced by 120 degrees. As you separate the two sets of brushes, the speed changes. The functionality is related to the voltage induced in the stator windings and whether this reinforces or opposes the voltage in the rotor winding.

Best regards,

[marke]

For a little bit more on the Schrage motor, look here. Schrage Motors
I will update this information as time permits.

Best regards,

[Matt303]

Hi Matt

Yes, sounds rather like the Schrage motor. The speed is controlled by rotating the brush assembly around the shaft axis.
I have not found a good simple explanation of how these work, so if you find one, let me know!!
Best regards,

Marke,

I managed to get hold of someone at Lawrence Scott who was involved in designing these units. He sent me a paper dating from the 1950's Apparently they were designed by a German Jew who fled Europe in the 1940's. This guy is the "S" of "N-S", he also designed something called a tri-slot motor (some sort of double cage induction motor with low starting torque and high efficiency) amongst other things. In short, a very clever engineer who ended up as the director of the company.
Although the operation of these units was described to me they are not simple. From what I can gather they have a commutator and something called an induction regulator. The commutator is used to correct the rotor frequency back to the stator frequency and the induction regulator is used to correct the voltage. Power can then be drawn from the rotor an injected back into the stator circuit. I'm still trying to understand the how this actually works. This gives the capability to run sub and super synchronous with higher efficiencies and the brushes can be modified to correct power factor (this was explained to me but I didn't really understand) I can send you this paper as an 18 page pdf but it is too large to attach, if you send me an email address I can forward it.
I suspect you will have more success understanding it than me.

Regards

Matt

[marke]

Hello Matt

My understanding is that the position of the brushes alters the voltage applied to the stator and that determines the torque. The frequency of the current applied to the stator is dependent on the slip.
There is a system of wound rotor motor, variable frequency variable voltage source whereby the speed of the rotor can be altered. I believe that this is known as the Leblanc system but I have very little information on this.
If you apply line voltage to the primary and a variable frequency to the "secondary" of an induction motor, you create a rotating magnetic field in the secondary, relative to the secondary. There is a rotating magnetic field in the primary which is generated by the current in the primary. The motor will spin at a speed where the two fields coincide.
For example, if we have a four pole motor on 50Hz, the stator field is rotating at 1500RPM. If we have a rotor field of 0Hz (DC) then the motor will spin at 1500RPM - Synchronous motor. If we now apply 10 Hz to the rotor, and the rotor is a three phase rotor, we will have a magnetic field rotating at 10Hz relative to the rotor. This 600RPM. The motor will spin at either 900RPM if the rotor field rotates in reverse, or at 2100RPM if the rotor field is forward.
My understanding is that to a certain extent, the Schrage motor has elements of this principle. I have seen it referred to as the integration of all the elements of the Leblanc system in one motor.

Bet regards,

[billyboro]

QUOTE(marke @ Oct 18 2007, 01:10 AM) Hi Matt

Yes, sounds rather like the Schrage motor. The speed is controlled by rotating the brush assembly around the shaft axis.
I have not found a good simple explanation of how these work, so if you find one, let me know!!
Best regards,


Marke,

I managed to get hold of someone at Lawrence Scott who was involved in designing these units. He sent me a paper dating from the 1950's Apparently they were designed by a German Jew who fled Europe in the 1940's. This guy is the "S" of "N-S", he also designed something called a tri-slot motor (some sort of double cage induction motor with low starting torque and high efficiency) amongst other things. In short, a very clever engineer who ended up as the director of the company.
Although the operation of these units was described to me they are not simple. From what I can gather they have a commutator and something called an induction regulator. The commutator is used to correct the rotor frequency back to the stator frequency and the induction regulator is used to correct the voltage. Power can then be drawn from the rotor an injected back into the stator circuit. I'm still trying to understand the how this actually works. This gives the capability to run sub and super synchronous with higher efficiencies and the brushes can be modified to correct power factor (this was explained to me but I didn't really understand) I can send you this paper as an 18 page pdf but it is too large to attach, if you send me an email address I can forward it.
I suspect you will have more success understanding it than me.

Regards

Matt

Hello Matt,
i am having problems with a laurence scott induction regulator and am trying to find as much information on these units as i can. Could you please email the pdf to me.
Many thanks
Billyboro