Operating Wound Rotor Motor As A Synchronous Motor
Operating Wound Rotor Motor As A Synchronous Motor
(OP)
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!
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!





RE: Operating Wound Rotor Motor As A Synchronous Motor
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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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous 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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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous Motor
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?
RE: Operating Wound Rotor Motor As A Synchronous Motor
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.
RE: Operating Wound Rotor Motor As A Synchronous Motor
RE: Operating Wound Rotor Motor As A Synchronous Motor
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RE: Operating Wound Rotor Motor As A Synchronous Motor
I'd recruit an expert.
RE: Operating Wound Rotor Motor As A Synchronous Motor
RE: Operating Wound Rotor Motor As A Synchronous Motor
Bill
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"Why not the best?"
Jimmy Carter
RE: Operating Wound Rotor Motor As A Synchronous Motor
---John
RE: Operating Wound Rotor Motor As 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.
RE: Operating Wound Rotor Motor As A Synchronous Motor
RE: Operating Wound Rotor Motor As A Synchronous Motor
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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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous Motor
We'd better wait for more authoritative comments.
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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous Motor
htt
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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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous Motor
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
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"Why not the best?"
Jimmy Carter
RE: Operating Wound Rotor Motor As A Synchronous Motor
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.
RE: Operating Wound Rotor Motor As A Synchronous Motor
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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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous Motor
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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous Motor
and that the rotor spins mechanically at exactly that frequency, adjusted for poles, regardless of load.
should have been:
and that the rotor spins mechanically at exactly that frequency(applied stator frequency +/- applied rotor frequency), adjusted for poles, regardless of load.
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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous Motor
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(2B)+(2B)' ?
RE: Operating Wound Rotor Motor As A Synchronous Motor
RE: Operating Wound Rotor Motor As A Synchronous Motor
Now today, I'm remembering that the VAR's will flow between the motor and the DC capacitor bank inside a standard VFD. In other words, a standard VFD will source the magnetizing current of a motor without drawing any current from the line source. This means it might actually be possible to do what you want without the exported VARs simply going back to the rotor circuit. But, the converter you choose will have to be capable of sourcing the VARs internally or else it won't work.
RE: Operating Wound Rotor Motor As A Synchronous Motor
RE: Operating Wound Rotor Motor As A Synchronous Motor