Operating Wound Rotor Motor As A Synchronous Motor 2

[OhioAviator]

Hello all...
Here's a question for the group, and an opportunity for someone to tell me I'm crazy. Here's the situation: We are operating a 4,000 HP wound rotor motor that's connected to a dynamic load with quite significant load torque variations. The rotor at present is connected to a liquid rheostat that is used for 'soft' starting and for limiting stator current during heavy overload. Here's the question: Can we remove the liquid rheostat and in its place install a PWM drive to inject current into the wound rotor, thereby causing the wound rotor motor to operate as a synchronous motor? Can I use the PWM drive to control the wound rotor current and cause the wound rotor motor's stator to produce leading VAR's, just like a regular synchronous motor?

If not, why not? Thanks in advance for your thoughts!

 

[electricpete]

The rotor of a sync motor/generator is fed dc.

So you wouldn’t need a pwm, you would just need dc (figure out suitable arrangement for dc on 3-phase winding... there are several options).

The big question will be the torque capability. Sync machine is subject to pole slip when torque capability is exceeded. You’ll want to evaluate that very carefully.

Also there is a question of starting...how are you going to start this thing (no armortisseurr windings).

What kind of protection will you provide for rotor. I'm not sure there will be readily available guidelines.

The whole thing makes me uncomfortable. All in all it needs a thorough review of course from someone you have confidence in,like a competent motor oem engineer.


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[electricpete]

The above comments applied to the concept of trying to turn this into a sync motor.

There are other options, which may include pwm on rotor. That may resemble a doubly-fed induction machine...I'm not sure. At any rate it wouldn't be a sync machine.

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[OhioAviator]

Thanks electricpete, for your comments. As for starting, I know of two ways to do that: (1) start the motor normally on rotor resistance and then switch the rotor over to the PWM drive, or (2) short out the stator and use the PWM drive to inject current into the rotor.

If I inject a low frequency current from the PWM drive into the rotor at a NEGATIVE slip frequency, I should be able to actually run the motor above synchronous speed (at least by my thinking anyway). The speed range might be limited to only a few percent above synchronous speed, but that's not what I'm really after.

What I'm really looking to achieve is for the motor to produce leading VARs in order to correct power factor and possibly reduce voltage flicker. THAT's the $64,000 question. In theory, I think it will work. But theory is a long way from practical application. Am I crazy?

 

[iop995]

Theoreticaly may feed DC to rotor windings.
Will be a problem with magnetic poles separation (N vs S) because rotor distributed turns windings of induction motor. So, "new" motor may have problems, rotor speed oscilations and poor load response. Regarding start-up, may start like IM after change to SM by feeding 2 rotor phase with DC. Care must take for DC power supply that will be connected to an AC source (emf in 2 linked rotor phase), so from datasheet of IM must estimate emf and prepare DC power supply to overcame resulting AC voltage and impose a DC current.

 

[pwrengrds]

It is my understanding that is the way most large wind mills generators work, they have a wound rotor motor that is direct connected to the line via the stator and use a form of frequency drive to drive the rotor to achieve synchronous frequency. That said, yes it is able to be done, but I think that you will find it difficult to achieve.

 

[jraef]

Do a search on "Doubly Fed Induction Generator", that's essentially what you are describing. It is used a lot on large wind generators, not sure how it would work as a motor though.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[purplepete]

I am NO motor wiz but is it feasible to get a rotor built for DC excitation? Then control of the DC to the rotor may porovide th econtrol you need (meeting torgue and PF control.
I'd recruit an expert.

 

[OhioAviator]

Thanks guys. I'm reasonably sure I can use some sort of a VVVF drive, such as a PWM drive, to supply a very low frequency current to the rotor to achieve near synchronous speed. What I'm not really sure about is if I can supply enough current to the rotor to achieve generation of leading vars out of the stator. And to those who suggest I consult a motor expert... great idea! I think I will do just that.

 

[waross]

You don't get the VARs for free. They may cost you a reduction in motor capacity. Whatever the power factor of the motor is, you can go to the same power factor leading. Any further VARs will cost you a reduction in motor ca[acity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[OhioAviator]

Thanks waross! Exactly my thoughts also. The motor is nameplated 0.78 PF, so I'm figuring I can go that much leading without overheating the stator. Measuring the stator temperature through the winding RTDs isn't a problem. What I'm not sure of at all is how do I predict (and protect against) rotor over-temperature. Collector ring brush wear should improve since the rotor current will be (theoretically at least) more consistent than it has been when operating the motor conventionally. Our process application for this motor forces the load factor to be extremely poor, on the order of less than 20%, with brief overload excursions to 200% full load.

---John

 

[LionelHutz]

I'm still confused on how you expect to get leading VARs or create a synchronous motor by feeding low frequency AC into the rotor. Feeding a "negative" frequency into the rotor will just increase the speed of the motor, not turn it into a synchronous motor.

I believe you could source all the required magnetizing current via the rotor and get the stator close to unity power factor, but I don't believe you could get over unity power factor.

 

[LionelHutz]

Not proof reading very well today... I should have wrote I don't believe you can get a positive power factor and not over unity.

 

[electricpete]

I think for wound rotor machine with fixed frequency supply to stator and variable speed supply to rotor:

the frequency of supply to rotor will control the speed of the machine at a speed corresponding to either sum or difference of rotor and stator frequencies as you said.

The machine will resemble sync machine at that speed. i.e. it won't slip from the calculated speed which is sum or difference of rotor and stator frequencies.

The vars can be controlled by varying the magnitude of voltage supplied to the rotor.

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[electricpete]

Hmmm, I'm not so sure any more.
We'd better wait for more authoritative comments.

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[electricpete]

The DFIM is controlled by the magnitude and frequency of
the rotor voltage. As in the synchronous machine, the rotor
terminal voltage magnitude controls var flow when the stator
is connected to a stiff ac system. In many operating systems, it
will be perferable to control the rotor voltage magnitude to
maintain unity power factor on the rotor, and in the rotor
power converter. The machine is then excited with vars drawn
into the stator, directly from the power system, or from a nearby
var source [32].
If the rotor frequency is held constant, the shaft speed will
also be constant, and the DFIM will behave similarly to a
synchronous machine at that speed. In the case of generator
operation, an increase in driving torque will be countered by
an increase in electrical output power, and the steady-state
speed will be unchanged. Alternatively, if the rotor frequency
is allowed to follow the shaft speed, the machine will behave
in a torque control manner, similar to a dc drive [33].

http://books.google.com/books?id=4-...ac system. In many operating systems,&f=false

This suggests that voltage control on rotor side of DFIM can be used to control vars on the stator side.

My general impression: the DFIM is a well-studied, versatile machine. There are many different operating modes and control scehmes (more than I understand).



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[waross]

First issue. If the frequency results in a synchronous speed above grid frequency the machine becomes an induction generator.
Issue two. An induction generator with variable excitation control should be able to export VARs.
If this project goes ahead, please keep us posted.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Electricpete - the text suggests that the VARs required to excite the motor can be shifted between the rotor or stator. It also suggests maintaining unity power factor on the rotor and sourcing the magnetizing current on the stator side. I would assume this is done to lower the current rating requirements of the converter.

Bill - I don't follow your first issue comment. Are you referring to rotor frequency? If so, you can run a WRIM over the rated speed via controlling the frequency on the rotor and it does not become a generator.

Actually, I would expect that if you used a PWM controller that ran at more than 60Hz to -60Hz on the rotor you could either go in reverse or go over 2X the rated speed.

 

[electricpete]

I would expect that if you used a PWM controller that ran at more than 60Hz to -60Hz on the rotor you could either go in reverse or go over 2X the rated speed

Figure 2.32 of above link shows this. Interesting to note that when machine speed is controlled above stator field speed, the rotor power adds to the stator power. When machine speed is controlled below stator speed, rotor power subtracts from stator power. Seems quite inefficient when reducing speed below stator speed.

As clarification, I was by no means passing judgement of this machine for this application, just acknowledging it is wisely used, widely studied (nothing I have ever used or studied), and can be controlled in many different modes for different purposes. Certainly the advice to study carefully still holds. The pole slip protection sticks in my mind since op said this load tends to overload.

If the only problem is voltage control / reactive power... my uninformed guess would certainly be that pf corrrection caps would be much cheaper and simpler and probably more reliable. Certainly they are the more standard solution.

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[electricpete]

Actually I am a little bit unsured about the way that text uses the term "slip". I am assuming that they are defining slip to be the ratio of applied rotor frequency to applied stator frequency.... and that the rotor spins mechanically at exactly that frequency, adjusted for poles, regardless of load.

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