wound rotors 1

[fsmyth]

I have the option to get another 20 HP Baldor -
and I really do want to learn more about these
motor types. Quoting a previous post:

" Just got a 20hp Baldor -
286TZ frame, 1425/1675 rpm, spec: 10P46W694,
stk no.: 39H314BAH, 208/240/60 and 220/380/50.
Talked to manufacturer - seems that this was
OEM'd to a mixer company, IIRC. Other than
that, no other specs.

I was hoping that someone had run across this
particular type, and could give me some pointers.
All of the documentation I can find refers to
3-phase rotor and stator windings - note that this
one has only 2 slip-rings, hence only one rotor
winding (more like a 3-phase alternator).
From what I have found, starting a wound-rotor
motor with the armature shorted has a 1000-1400%
starting current (as opposed to 500-800% on a
"standard" squirrel-cage). Is this also true
for this (single-winding) type?
What I need is practical application info, such as
how much resistance to use (near as I can tell so
far, about 3x the rotor resistance), when to switch
it out ( again, NAICT, at over 80% speed), and what
the effects of more slip have on operation.

It appears to my small mind that this could be the
equivalent of delta-Y start, without the massive
switching transient, and (because of more slip)
be more responsive to load variations when used as
an RPC idler.

I can probably interpolate values from a 3-phase
rotor type, if I could just find them. I realize
that this is a small motor for the type - most of
the literature I have found refers to 500HP+, but
any hints are welcome."

Can anyone help, or at least provide some pointers?
I do not mind experimenting, but is is difficult for
me to set up (I have access to 3-phase, but have to
make arrangements in advance, and anything I do will
be in their way until done).

<als>

 

[shagooo]

Just a quick addition to this thread, if it's still going. Years back I encountered wound rotor motors with the rotor being connected to a series of large wattage resistors. There were also from one to four contactors across banks of these resistors that when energized, would short out the resistors. This was used to control the speed of large cranes and hoists.

Just my 2 cents

 

[electricpete]

Shagoo - How many slip rings?

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[CJCPE]

I wonder if it is possible that a 3-phase rotor winding is connected to a 3-phase bridge rectifier mounted on the rotor and feeding DC to the slip rings. The DC from the slip rings could then be connected to a chopper and resistor for speed control or to a regenerative SCR bridge for speed control with slip energy recovery.

It is also possible that the motor is mounted to the housing of an eddy current clutch and the slip rings are for energizing the clutch field. The clutch assembly would be a lot larger than a regular end bell, but a lot of people don't realize that they are looking at two machines coupled together when they see an eddy current drive.

Both of these types of equipment were often used with pumps as a means of flow control before adjustable frequency drives took over that market.

 

[fsmyth]

Thanks, CJ.
From the bit that I know and have read lately about
motors, I see a couple of possibilities:
a) the rotor is fed some value of D.C. in order to
generate a constant rotor field (like a D.C. motor),
or b) a resistor is placed across the rotor winding
while starting, and shorted when up to speed, and
c) resistors are used to control slip and thus also
possibly the efficiency of the motor when running.
I lean heavier to the b) option, since there is no
information on the nameplate refering to the rotor.

If the rings go to a brake, the winding(s) are
internal to the motor housing (which I doubt, since
the housing appears to be the proper size for a
motor of this rating - a 286T).
The end bell is only large enough to contain the
brush holders (if you are referring to the small added
tin globe-like enclosure added to one end of the
shaft). Both ends of the motor housing, if that is
what you are referring to as end bells, are identical.

I am beginning to be more interested in these motors
as generators, if only I could find out a bit more on
the normal operating modes and ratings.

<als>

 

[jraef]

I think perhaps there is some confusion here between Wound Rotor Induction Motors and Sychronous Motors. Both have slip rings, but in a Sychronous motor they are used to apply DC to the rotor to change the field strength. This has nothing to do with speed control, it is used to change the power factor of the motor, and can even make the motor run at a leading power factor in order to correct a facility's PF from other induction motors on line at the same time. Synchronous motors are also more easilly capable of being used as generators since the field is separately excited, unlike induction motors, including WR induction motors.

Wound Rotor Induction Motors are a completely diferent animal. The only similarity is that the both have slip rings. This is what Pertonila was refereing to way back on Aug. 1. If I'm not mistaken, applying DC to the rotor of a WRIM makes it into a Brake!

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[edison123]

jraef,

Applying DC to the WRIM rotor (after it has reached full speed), will make it a synchronous motor. These are special motors called as synchronous-induction motor, which start as induction motors and run as synchs.

Applying DC to the stator makes it a brake - Dynamic braking.

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[davidbeach]

Applying DC to the stator makes it a brake - Dynamic braking.

I beg to disagree. Not about the brake part of it, but about the dynamic braking part. Dynamic braking is when a motor is driven by the load (becoming a generator) and the output power is dumped into a resistor (or other load) in a controlled manner, thus extracting energy from the load. If the power produced in this manner is pushed back into the power system instead of being dumped, it is regenerative braking.

There comes a point at which the shaft is turning too slowly for effective dynamic or regenerative braking, at that point the DC to the stator would work better as a brake. What you get when you apply DC to the stator of a synchronous motor is a motor with a synchronous speed of 0.

 

[edison123]

davidbeach,

regenerative braking is different from dynamic braking.

regenerative braking involves hypersynchrounous speed (negative slip) and will be ineffective when the motor reaches synch speed.

Dynamic braking involves applying DC to the stator immediately after disconnecting the 3 phase supply. With slipring motors, resistances can be introduced in the rotor winding ckt to get better braking upto standstill.

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[jraef]

Applying DC to the WRIM rotor (after it has reached full speed), will make it a synchronous motor. These are special motors called as synchronous-induction motor, which start as induction motors and run as synchs.

So what is the difference between this and a Synchronous motor which uses it's amortisseur winding (induction machine) to get it up to pull-in speed? Semantics? Being that up until reading this thread I had never heard of a WRIM with single phase rotor windings, is that what you are refering to? Maybe that is what this motor is then.

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[edison123]

No jraef, it ain't semantics.

Two types of synchronous-induction motors. One is salient pole with amortisseur/damper winding start. This starts as sq cage motor using the damper winding and salient poles take over during synch run.

The other is wrim which starts with three phase rotor winding connected to rotor resistances and then near synch speed, with DC is applied to the rotor (with two of the shorted at line ends) to make it run as synch motor.

I have rewound both types of motors.

All these types of motors and electric braking are well described in 'Performance and design of alternating machines' by MG Say.

Regarding this thread, I also have not heard of in theory or in practice 'single phase rotor' in a 3 phase stator.



* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[fsmyth]

Well, what do you think I have here? (Specs posted
previously). The motor looks physically like any
other 286T frame, with the addition of the housing
containing the sliprings at one end. I certainly
admit to not knowing enough about winding motors or
identifying rotor types, but I have read some of
what is available, and have found nothing that
resembles this motor. Plenty of 3-phase rotor stuff,
but nothing on single-winding rotors except on
synchronous motors. Would posting pitchurs help
<als>

(In case I have not posted it before, phone calls to
Baldor were not very informative, and the manufacturer
to which it was OEM'd has been absorbed by another, and
documentation apparently lost or non-existant).

 

[CJCPE]

Pictures might help.

The nameplate data seems quite inconsistant with the idea that this is any kind of synchronous motor.

I think it would be worth the effort to ohmeter across the sliprings in both directions to see if there could be a rectifier in there.

 

[davidbeach]

edison123, I've never seen dynamic braking defined the way you are defining it. Check Allen Bradley's description here:

http://www.ab.com/drives/techpapers/RegenOverview01.pdf.

They talk about pulling energy off the DC bus of a drive when the load is turning faster than the motor would would be. The energy is being absorbed from the load, converted to electrical energy by the motor acting as a generator, and being dumped as heat across a resistor. Dynamic braking does not heat the rotor any more than operating as a motor does and can be a long term operation. The DC injection braking you describe can only be used for very short periods as it dumps the energy into heating the rotor.

Dynamic braking is used by diesel-electric locomotives and they can be in dynamic braking for long periods of time bringing a train down a mountain grade. DC injection wouldn't work well in DC traction motors, but even with AC induction traction motors, there is no way that the rotor could absorb all of that energy.

 

[CJCPE]

I agree that "DC injection braking" or just "DC braking" are the more common and correct terms for applying DC to the stator. However I have ocassionally seen it called "DC dynamic braking."

 

[edison123]

davidbeach,

when you say "I've never seen dynamic braking defined the way you are defining it", do you mean the definition or the process ? If it is former, may be it is then "semantics" (as jraef says). If you mean there is no such process (of applying DC to the stator winding to brake), then I disagree.

I have read the AB paper and it seems to be "regenerative braking" i.e. load driving the motor (supersynchronous speed). You may also see the book I mentioned.

I agree that not all motors can be braked electrically (dynamic braking, regenerative breaking & plugging) since electrical braking will overheat the rotor.



* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[davidbeach]

edison123,

Two basic types of electric braking:

Internal energy absorption and external energy absorption.

The DC injection method is an internal energy absorption method, the energy given up by the rotating load is dissipated in the rotor of the motor. This method can only be used to bring a load to a stop, it can not be used as a means of speed control for overhauling loads. This type of braking is principally applicable for induction motors.

Dynamic braking and regenerative braking are examples of external energy absorption methods, the main difference between the two is that in dynamic braking the energy is wasted as heat in resistor grids while in regenerative braking the released energy is returned to the supply system. These methods can be used with DC, AC induction, and AC synchronous motors. These methods can also be used for continuous braking - speed control - of overhauling loads. Examples of overhauling loads can be elevators in one direction, mine hoist applications, locomotives, and undoubtedly many others. With currents within normal operational ranges, there is no more motor heating, rotor or stator, than there would be operating as a motor.

Maybe some of it is semantics, but names for things work better if everybody uses them the same way. DC injection is a means of braking an induction motor, but it isn't dynamic braking; to say dynamic braking while meaning DC injection doesn't convey to the listener/reader what you are talking about.

 

[edison123]

davidbeach,

I agree that we disagree. Let's leave it at that.

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[Farkel]

Maybe, just maybe the slip rings feed some other device as a secondary circuit like in a transformer.

 

[LionelHutz]

I've never seen a 1-phase wound rotor in a motor before. I don't think this is what you have there.

Generally, for a wound rotor the rotor voltage is inversely proportional to the speed and the rotor current is proportional to the percent load. So if you've got a motor running at 80% speed and 70% load the voltage is 0.2 x rated and the current is 0.7 times rated. Use this voltage and current to calculate the resistance you need. It's not really this simple since you need to include the wiring and rotor resistance but it's a close approximation.

"The other is wrim which starts with three phase rotor winding connected to rotor resistances and then near synch speed, with DC is applied to the rotor (with two of the shorted at line ends) to make it run as synch motor."

I've never seen a synchronous motor with a 3-phase rotor. We've built field controls for 100's of synchronous motors and I've never run across that before.

The only brush type synchronous machines I've ever seen use a squirrel cage type of winding in the pole faces along with adding a resistor to the field winding when starting.

 

[edison123]

lionelhertz,

AEG (Germany) supplied such synch motors with wrim windings (1000 KW rating)and I have personally rewound six of them. The theory is in the book I've mentioned before.

* Anyone who goes to see a psychiatrist ought to have his head examined *