# Adding shunt fields



## Georgia Tech (Dec 5, 2008)

Its called a compound wound motor...The GE Aircraft Starter Generators do this. Yeah its possible. It would also help with the No Load run away effect of the Pure series part of the motor. Like I said its done but don't know REALLY how effective it is. But adding the shunt field it means you have more room to add thes coils..


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## joethemechanic (Feb 26, 2009)

Georgia Tech said:


> Its called a compound wound motor...The GE Aircraft Starter Generators do this. Yeah its possible. It would also help with the No Load run away effect of the Pure series part of the motor. Like I said its done but don't know REALLY how effective it is. But adding the shunt field it means you have more room to add thes coils..



I am very familiar with compound wound DC generators. and a generator is a motor is a generator,,,,,,,

I just am wondering if adding them to the outside of the field housing will be very effective. I was concerned with the amount of "breaks" in the path of the magnetic flux caused by mounting the coils and shoes to the outside of the field housing.


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## Georgia Tech (Dec 5, 2008)

joethemechanic said:


> I am very familiar with compound wound DC generators. and a generator is a motor is a generator,,,,,,,
> 
> I just am wondering if adding them to the outside of the field housing will be very effective. I was concerned with the amount of "breaks" in the path of the magnetic flux caused by mounting the coils and shoes to the outside of the field housing.


 Ooooo.....yeah.... putting them on the outside the Houseing of the motor is not going to help that much BECAUSE, if the motor is worth a toot then the housing its self would be good enough so that all the Flux from the main poles would let all the flux pass through it....In other words the houseing serves a main purpose for the flux path in most motors. If by putting fields out side the houseing made a differance then houseing is not good to start with...I hope I made sense..


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## joethemechanic (Feb 26, 2009)

Yeah and most motors I see do not have enough room to sneak an effective set of fields into. Although I could try getting rid of two of the series fields and winding myself a pair of shunt fields. In other words I would have 2 of each type of field.

I am starting to think DC may not be the way to go. I might want to start looking for a "wound rotor" 3 phase induction motor. I know all sorts of tricks to get a huge speed range out of them.

I better start sniffing around the scrap yards and see if I can find one from the bridge drive of an overhead crane.


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## Georgia Tech (Dec 5, 2008)

joethemechanic said:


> Yeah and most motors I see do not have enough room to sneak an effective set of fields into. Although I could try getting rid of two of the series fields and winding myself a pair of shunt fields. In other words I would have 2 of each type of field.
> 
> I am starting to think DC may not be the way to go. I might want to start looking for a "wound rotor" 3 phase induction motor. I know all sorts of tricks to get a huge speed range out of them.
> 
> I better start sniffing around the scrap yards and see if I can find one from the bridge drive of an overhead crane.


WEll I......hmmmmm...I'm not sure if you could just loose two of the poles unless You mean loose two adjecnt poles because two poles accross from each other are the same polarity.....

Not familure with a wound rotor induction motor. I assume it brushes....right? if there some sort of convser effect of controling the current in the rotor the same as you would control the field current...???


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## joethemechanic (Feb 26, 2009)

Yes the wound rotor induction motor has its rotor wire wound instead of being a squerril cage. It is wound 3 phase and connected in a wye with 3 slip rings and brushes.

You can go from 0 RPM to almost sycroness speed just by changing the resistance of the rotor. If you introduce a 3 phase current to the rotor that bucks the rotating field created by the stator you can increase speed above syncroness.

Then if you apply dc to two of the three rotor brushes, it becomes an AC alternator and can be used for dynamic braking of the mechanical load.

They are very smooth as far as low speed control goes. They also produce high torque from stall up because of the ability to increase rotor resistance.


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## major (Apr 4, 2008)

Georgia Tech said:


> I'm not sure if you could just loose two of the poles unless You mean loose two adjecnt poles because two poles accross from each other are the same polarity.....


Hey GT,

There are plenty of 4 pole compound wound motors where there are 2 series coils and 2 shunt coils. You must put the 2 series coils on opposite poles (which would be like both N) and the 2 shunt coils on the other 2 opposite coils (like the 2 S poles). The 4 pole motor actually has 4 parallel magnetic paths. Each path shares the excitation (mmf) from the coils on adjacent poles. So, in this case, would be half series coil and half shunt. If you did your way, these parallel magnetic paths would be unequal in flux and cause real havoc in the armature.

Regards,

major


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## Georgia Tech (Dec 5, 2008)

major said:


> Hey GT,
> 
> There are plenty of 4 pole compound wound motors where there are 2 series coils and 2 shunt coils. You must put the 2 series coils on opposite poles (which would be like both N) and the 2 shunt coils on the other 2 opposite coils (like the 2 S poles). The 4 pole motor actually has 4 parallel magnetic paths. Each path shares the excitation (mmf) from the coils on adjacent poles. So, in this case, would be half series coil and half shunt. If you did your way, these parallel magnetic paths would be unequal in flux and cause real havoc in the armature.
> 
> ...


Hmmmmm well how do you resolve they Electric circuit being coupled with the magnetic circuit? What I mean is The series pole "North" will have a certain Ni (ampturns) and its adjecent return flux will have a different ampturns Ni form the south pole which will be the Shunt pole...How do the "Magnetics" resolve the differances in mmf??


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## major (Apr 4, 2008)

Georgia Tech said:


> Hmmmmm well how do you resolve they Electric circuit being coupled with the magnetic circuit? What I mean is The series pole "North" will have a certain Ni (ampturns) and its adjecent return flux will have a different ampturns Ni form the south pole which will be the Shunt pole...How do the "Magnetics" resolve the differances in mmf??


The magnetic circuit behaves like an electric circuit. Except you have mmf instead of emf, flux instead of current and reluctance instead of resistance. So, in an electric circuit, you can have multiple sources of emf in series, they will add. In the magnetic circuit, the mmf sources will add. They do not need to be equal. And in fact can be zero. In the 2 plus 2 compound motor I described, if the 2 shunt coils went to zero amps, it would run perfectly fine as a series motor, 2 coils excited and two consequent poles. But those poles need to be opposite each other.

Regards,

major


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## Georgia Tech (Dec 5, 2008)

major said:


> The magnetic circuit behaves like an electric circuit. Except you have mmf instead of emf, flux instead of current and reluctance instead of resistance. So, in an electric circuit, you can have multiple sources of emf in series, they will add. In the magnetic circuit, the mmf sources will add. They do not need to be equal. And in fact can be zero. In the 2 plus 2 compound motor I described, if the 2 shunt coils went to zero amps, it would run perfectly fine as a series motor, 2 coils excited and two consequent poles. But those poles need to be opposite each other.
> 
> Regards,
> 
> major


Yeah I follow you now.. thanks bud...


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