Induction motor as transformer 1

[fsmyth]

I almost posted this to the "induction motor/generator"
thread, but will start another topic. On another forum,
someone raised the question of driving a 3-phase motor
with a VFD, and using the second winding set to generate
isolated 3-phase power. I posted the following as a reply
and would like to get answers from here:

"The thing about using one set of windings as
the motor and the other set as generator sounds
interesting. What, pray tell, would be the
difference between this and a transformer
(other than the rotating core, obviously)?
I'll be the first to admit that I have a
lot to learn, but it sure seems reasonable.
Can dual-winding motors be safely operated
as a motor on one set of windings?
And if so, how much should it be de-rated?"


<als>

 

[electricpete]

wound rotor motors are commonly used as variable transformers to accomplish variable/reduced votlage start/run of motors in repair shops. Small changes in rotor positions (less than one pole pitch total movement) control the output voltage. Typically the rotor is driven through a gearbox to overcome the accomplish these small rotor movement changes with good control of rotor position (high torque). There is a chapter in the EASA Technical Manual on this.

I'm not sure if this is the application you are talking about.

=====================================
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[Skogsgurra]

I think fsmyth is referring to a normal induction motor - not one with a wound rotor. The latter, when used as a variable transformer sometimes is called an "induction regulator". Using a motor with dual stator windings is certainly possible. But I see no point in doing so - except, perhaps, if you urgently need the isolated three-phase supply and happens to have a dual winding motor at hand - but no transformer.

About available power: As long as you do not exceed the motor's nominal current(s) you should be fine. The rotor is spinning freely and doesn't contribute to the heating. That implies less losses than in normal use. So you can probably pull a little more than nominal current from the secondary. But I wouldn't undersign that ;-)

Gunnar Englund

http://www.gke.org

 

[fsmyth]

The context was single phase to 3-phase converters.
Sorry, should have added that.
And yes, the motor in question was a normal dual-
winding induction motor.

Forget the converter part:
Can the motor be run on either winding alone
(at its rated voltage), and if so, how would one
determine the output H.P.? One-half of nameplate?

What, if any, are the effects of Y vs. delta?

<als>

 

[aolalde]

Standard 3-phase motors DO NOT have two windings. Dual voltage motors have normally 9 leads that allow external connection for different line voltages. However it is a single winding and must be 100% energized in order to produce the proper magnetic poles distribution and the required full magnetic excitation that generates the designed flux per pole. Two speed dual voltage motors still could have problems to be used as a transformer since the poles in the rotating field do not match the poles in the other winding, that should result in different frequency and most probably a non symmetrical voltage.
In conclusion a standard dual voltage motor can not operate with half winding connected to a power line while the other half is used as the secondary of a transformer to feed another load.

A lot of confusion has been generated in this forum on past threads for persons recommending to leave only part of the winding energized for continuous operation. That condition is tolerated only for a few seconds in the so-called “part winding starting”. Under that condition, the motor will produce lots of noise, vibration, inadequate torque, and finally it will burnout.

 

[fsmyth]

Pretty much answers that!
Thanks, aolalde.
<als>

 

[SteveKW]

This may be of some interest...

http://easa.com/news/c0506.pdf

See page 3.

 

[Eclectic3]

Aolade, I'm the person who started this on the "other" forum.
Yes, this is a question relating to using a 9 wire dual voltage Y connected motor as a transformer, perhaps as a resonant 60hz transformer with the addition of capacitors.
It looks from the wiring diagram of my Leeson 2hp single speed dual voltage motor that there are two sets of windings cowound around the same poles and connected in series for 480v or parallel for 240v. Is the "cowound arround the same poles" an unwarranted assumption? The motor will only be used as an idler and will have no mechanical load. In that case is there any problem running the motor on just the center connected winding set and using the other set in isolation also wired Y as a transformer output? The VFD will be able to soft start the motor.
Sgogsgurra, I can find cheap used 3 phase motors almost anywhere. I don't often see used 3 phase transformers.
SteveKW, yes, that is very helpful, the question just remains if it will work as an isolation transformer.

 

[Eclectic3]

fsmythe, thanks for bringing the discussion to this forum. It's a MUCH more helpful group.
I found this thread discussing running motors on partial windings:

http://www.eng-tips.com/viewthread.cfm?qid=153756

 

[gonso]

Eclectic3,

Your plan will work just fine. Thanks SteveKW for the great link provided! Looking at page 3 fig. 1, you only need to join terminals 4, 5, and 6.

The resulting rating:
The VFD's 3-phase will keep the motor spinning, providing cooling via the axis-mounted fan. While the VFD runs at the motor's rated frequency, you will have rated cooling. Beware of lower frequencies! The cooling drops with the cube of speed.
The second winding will be able to keep the same kVA rating as before - half of the motor's total kVA. The additional heat coming from the VFD's distortion will not matter, since your rotor will remain unloaded - subtracting the rated I2R losses that the rotor would see under normal rated conditions.