Author Topic: Using Slip Ring Motor to Generate Power?  (Read 4594 times)

Rick Rowlands

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Using Slip Ring Motor to Generate Power?
« on: March 22, 2007, 11:39:40 AM »
I just picked up a 5 hp slip ring 3 phase motor and it amazingly looks like my 10KW generator head.  Is it possible to convert such a motor to generate 3 phase AC power?    Are there any internal differences that would prevent me from doing that?

listeroidsusa1

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Re: Using Slip Ring Motor to Generate Power?
« Reply #1 on: March 22, 2007, 01:44:33 PM »
Yes you can generate 3 phase from this. You'll need to find out the slip ring voltage and provide a suitable source of DC for the field. One thing to look for, especially on the smaller slip ring morors, is whether they are stationary field with rotating collector rings, or rotating field with stationary stator. If you have 2 slip rings you've probably got a good one to convert. (rotating field) However either can be "made" to work

Motors such as these are commonly used to correct the power factor by overexciting the field.

Mike

adhall

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Re: Using Slip Ring Motor to Generate Power?
« Reply #2 on: March 22, 2007, 03:34:10 PM »
Rick:

There is another type of slip ring motor called a "wound rotor motor" which operates as an induction motor rather than as a synchronous motor (which is the type of motor that Mike is talking about below). I don't know if a wound rotor motor can be configured as a generator or not.

Maybe Doug can answer that question.

Best regards,
Andy Hall
JKSon 6/1, 5 kW ST Head, 1992 Dodge RAM Cummins 5.9L Turbodiesel, 2001 VW TDI 1.9L Turbodiesel, 2006 Jeep CRD Turbodiesel, Yanmar FX22D Diesel Tractor

Doug

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Re: Using Slip Ring Motor to Generate Power?
« Reply #3 on: March 23, 2007, 01:09:38 AM »
If its a wound rotor motor you can excite two legs of the three and make some three phase power but it won't be as efficnet as one would like....

There are so many cool things one can do with a wound rotor motor if you do a little research....

Here is one of those places where my "old think shows" ( by that I mean I still think and work in 1950 even though I ma not that old, just a casue of being trained by very old men ).

A frequency changer:
By increasing or decreasing the speed of rotation of the rotor and drawing power from the slip rings you generate any frequency you like with one of these, as low as 25 and as high as 400 hz ( some restrictions apply based on rotor diameter and rim speed fo the rotor)

A variable speed drive with slip:
By adding resistance to the rotor circut with a resisitor bank you can slow the shaft speed, but at an efficiency loss.

Torque control:
You can change the torque curve of the motor so it produces its max shaft snaping torque at a stand still or gently rolls a load up to speed.

Two step speed control:
In some cases depending on the design of the rotor and itr limmitations you can connect it directly to shared load with an induction motor and by clever connections between the motors make the two behave as one with two speeds, with much better efficiency than the straight resisitor banks in the torque control method.

Transformer:
A little clever connections between the rotor and stator can turn a wound rotor motor into a transformer who's voltage increases or decreases with the possition of the rotor with respect to the stator poles.

Selsyn control:
Bu connecting two of these together in the right combination when you energise them turning the shaft on one by hand makes the other one's shaft turn and move in the exact same motion with the exact same amount of torque aplied

Schrague system:
I get excited over this, by combining a syncronous converter and a DC motor you can create a combination of machines that will run on AC but give you smooth stepless variable speed like a modern electronic drive just by adjusting field current on the DC motor. This is so cool 1930 technology that I've only seen done once on a decommisioned grinding mill.
Mind blowing stuff I could spend hours talking about the system....

In college to the horror of my instructor I wired three Labvolt trainers into a Shrague system. Took me hours to configuere and got me an A in machine theory....

I was once told lazy people rely on electronics clever people adapt machines. This isn't quite true but in the hands of smart man you can do a lot with a wound rotor machine.

Doug   
It's a Good Life, If You Don't Weaken

adhall

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Re: Using Slip Ring Motor to Generate Power?
« Reply #4 on: March 23, 2007, 09:13:40 PM »
Doug:

Thanks for all of that interesting information. Clearly there is much more to round rotor motors than I had suspected. All I knew about was the variable speed and torque--an old electrician who was trained as a motor winder told me about a large pump motor on a dredge that worked this way.

The research I have done so far turned up diagrams with the rotor haivng three windings like the stator of a 3-phase motor and with 3 slip rings. I also found a reference to a wound rotor motor with a "2-phase" rotor, but no wiring diagram. Can you elaborate on this? Would that have four slip rings?

What I am trying to get at here is finding a way to help identify the type of motor. If I have it right, these would be the possibilities:
 -- 2 slip rings: Synchronous motor with field in rotor.
 -- 3 slip rings and a single winding on the rotor: Synchronous motor with the field in the stator.
 -- 3 slip rings and 3 windings on stator: Wound rotor motor.

Does that make sense?

Best regards,
Andy Hall
JKSon 6/1, 5 kW ST Head, 1992 Dodge RAM Cummins 5.9L Turbodiesel, 2001 VW TDI 1.9L Turbodiesel, 2006 Jeep CRD Turbodiesel, Yanmar FX22D Diesel Tractor

Doug

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Re: Using Slip Ring Motor to Generate Power?
« Reply #5 on: March 23, 2007, 11:23:57 PM »
Two phase is realy odd stuff only still used in some aircraft aplications as far as I know.

Actual two phase power goes back to the very begining of AC power infrastructure like 25 Hz power.

There were two common types the 3 wire and 4 wire ( 4 wire was two issolated phases 3 wire had a common or neutral ). As you already know when you see a drawing of a 3 phase motor it usualy shows a Y connection with 120 deg angle between the phases ( electricaly the voiltages are also 120 out of phase with each other ). Two phase had a 90 deg shift between the two phase groups just like a modern single phase motor and in operation a capacitor run motor is actulay running as a two phase machine by taking advantage of the phase shift between the lag of the main winding and the lead of the AUX winding.

Back to the point at hand:

3 slip rings its a wound rotor machine ( two or three phase3 ).
2 its a syncronous machine

The only way to actualy tell if a poly phase machine is 2, 3 or 6 ( six phase is was done for some aplications ) is to look at the poles in the stator and try and count the number of phase groups in the pole and guess at the electrical deg difference between the two.

Eg:

A 2 pole three phase machine will have 3 coil groups per phase ( ABC ) and these will belayed out 120 deg 1/3 of the pole ( actulay less than 1/3 because of something called Cord factor done to reduce harmincs but now i'm getting too deap into theory ). So in short 2 pole 3600 rpm machine will have 6 coil groups.

A 2 pole two phase machine will have two coils per pole and look a lot like a single phase motor winding but with the same size wire for each coil group.

To complicate matters most large machines will use a lap winding in wich all coils are the same size and number of turns where small machines use a concentric coil method where the phase groups have 3 or 4 coils in decreasing size in a group.

The line to phase voltage of 2 and three phase is also different in three phase the line voltage is 1.73 times the phase where in two phase power the line is 1.5 the phase ( don't quote me on that I might be wrong its beena very long time since I did a two phase calculations ) .

Last Points:

Two phase can be converterd to three by use of a SCOTT T connected transformer. Some electricla rail ways did this convert 3 phase utility power to 2 phase for trains. Sometimes in roder to save money instead of a complete rewind 2 phase motors had their coil groups opened up atthe connection nuckel and had an internal Scott T done to allow them to run on three phase power.
Some other aplications of two and three phase are converting single to three phase power by way of a rotating two phase machine ( ususaly an ordinary induction motor, but sometimes with a few extra turns of coil in the induced phase for a voltage boost ) and a scott T transformer, as an improved way to make small amounts of three phase in areas where it isn't available.

I think this is a new record for the number of time the word phase has been used in a post....

Doug

" 3 slip rings and a single winding on the rotor: Synchronous motor with the field in the stator."

Andrew I think you mean a stationary field and 3 phase off the armature. Yes this was done and can be identified this way. An Indian 3 phase generator, the small kind on a portable genset would probably be built this way in order to save tooling....
It's a Good Life, If You Don't Weaken