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Replacing a wound rotor motor with a snychronous motor for pfc
2

Replacing a wound rotor motor with a snychronous motor for pfc

Replacing a wound rotor motor with a snychronous motor for pfc

(OP)
This is stricly a theoretical question.

Our plant operates at about 15MW with a power factor of .86 give or take.  To compensate for power factor there are several power factor caps at each of our 4.16kV larger motors (500hp plus) to help correct our power factor.  Over the years several of these caps have failed (most likely harmonics but a different story for a different day) and we have never replaced several of them.  Other then replacing these caps or installing a new cap bank at the incoming like of the plant I wanted to pick everyones brain here about the possibility of coverting one of our wound rotor motors to a synchronous motor to provide excess vars need for power factor correction.

Our larges motor in the plant is a 4.16kV 6500hp wound rotor motor with the motor connected to an electrolytic rheostat for starting purposes only.  The theoretical question I have been kicking around in my head for some time now was if it was at all feasable to convert this wound rotor motor to a new synchronous motor and overexcite the motor so that the motor in addition to running the load could supply the excess reactive power needed in the plant to correct the power factor.

I have heard others in the past speak about running synchronous motors unloaded to essentially act a a large power factor cap to supply necessary reactive power but I wanted to see if it was possible to do this while driving a load at the same time.  The load that is being driven is a large ball mill.  Even if this could not be accomplished with the current motor could this be done by inserting a new synchronous motor?

Some of the things I am wondering.

1) Is this at all feasable or is this merely a pipe dream
2) What type of motor and load specifics would need to be  
   considered
3) How much excess reactive power could I squeeze out of
   such an application if any to supply to the rest of the  
   plant?
4) Would this idea be at all economically feasable?

I know I have provide limited information but I'm curious to hear from others about this idea taking a broad approach.  I'll be glad to supply more specifics if necessary.
 

RE: Replacing a wound rotor motor with a snychronous motor for pfc

Sounds reasonable on the power factor correction side.   But what is the reason for original use of wound rotor motor............is the load very difficult to start?  That may complicate selection of a replacement motor

=====================================
(2B)+(2B)'  ?

RE: Replacing a wound rotor motor with a snychronous motor for pfc

1. It's not likely you could just change the rotor of the existing machine. Possible, but not likely.

2. You need to determine if the synchronous motor can start the mill.

3. This would be motor dependent. I ran across a case of a 15MW sync motor being capable of running under load at about 27MVA or 22.5MVAR.

4. How would we know? You have to collect data, make a plan, get quotes and run the numbers. But, it's very possible depending on the penalties for your poor power factor. I suggest you first look at the penalties you pay in a year due to the poor power factor and then get a budget quote on a sync motor of the appropriate size. That will at least tell you if you're in the right ballpark or not.

 

RE: Replacing a wound rotor motor with a snychronous motor for pfc

While possible, you would need a detailed study by a professional to determine its feasibility and do cost comparisons.

Motor replacement would need much more engineering to match the motor characteristics and physical dimensioning plus sophisticated controls. The end system would get much more complex and you need to run the load with motor, you cant just use the motor.
 
My "hunch" is that working on resolving the issue of static PF correction would be relatively simpler and cost effective. But is just a hunch.

You can also consider just a stand alone synchronous motor (s), a condenser, in lieu of the static caps. This gives much more control as to how and when to run it, without running the load itself.

Rafiq Bulsara
http://www.srengineersct.com

RE: Replacing a wound rotor motor with a snychronous motor for pfc

(OP)
Electricpete

The origonal use for the wound rotor motor is for use in conjunction with the rheostat to provide high toruqe and reduce current on our system during starting.  Once the motor is brought up to speed the rotor leads in the rheostat are shorted and it is run this way during operation.

For the comments above on difficulty of starting are they geared more towards the fact that synchronous motor cannot provide that much starting torque, or because of the reason that a wound rotor motor was used to reducing starting current and increase torque this may be one of the challenges in selecting any other type of motor other than a wound rotor?

RE: Replacing a wound rotor motor with a snychronous motor for pfc

That makes the use of a Synchronous motor unlikely. The Wound Rotor motor was giving you EXTRA acceleration torque without increasing current as much as a Squirrel Cage Induction Motor would. The amortissuer windings used to accelerate a Synchronous motor would be capable of relatively "normal" SCIM starting torque and may not allow starting of your load within the thermal limits of the motor.

The use of synchronous motors as a synchronous condenser (as you are proposing here) is usually applied to loads that must run continuously but are relatively easy to accelerate, such as plant air compressors that start unloaded. If you must have a WRIM to accelerate the load, chances are a synchronous motor is not a good application for it. Not absolute, but highly unlikely.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: Replacing a wound rotor motor with a snychronous motor for pfc

That is pretty much what I was thinking, but I was waiting for someone with more experience to weigh in (we have nothing but DOL induction motors at our plant). Thanks Jeff.

I see Lionel responded to your comment about "converting" as if you were considering just swapping rotor in existing stator. If that's the case I agree with him...I would say that would probably not work unless the original motor OEM does it, and even then it would be a challenge.  

=====================================
(2B)+(2B)'  ?

RE: Replacing a wound rotor motor with a snychronous motor for pfc


In general it is possible to supply reactive power from a loaded synchronous motor by variation of its excitation current. But your case is a special case.

A quick and dirty calculation gives us the following data:

15,000 kW at 0.86 p.f. gives us an apparent power of 17,450 kVA and a reactive power of 8,900 kVar (under- or over-excited). The stator of the 6,500 hp motor in question, when working as a synchronous condenser, could at first glance be capable to deliver about 50 percent of the reactive power you require. But there are two problems. First, the existing field winding of your motor would have to carry about twice the rated field current when running at 0 p.f. over-excited. Second, the stator end zones would over-heat if the unit is running at 0 p.f. under-excited as these stator regions have to be especially designed for such a service.

As rbulsara hinted, a static converter may be the best alternative to your problem.

Wolf
www.hydropower-consult.com    

RE: Replacing a wound rotor motor with a snychronous motor for pfc

I must have missed that hint. In what context is static (electronic) converter envisioned?

=====================================
(2B)+(2B)'  ?

RE: Replacing a wound rotor motor with a snychronous motor for pfc

In other words: vfd to help start up the sync motor or are you suggesting electronic var units as alternative to caps?

=====================================
(2B)+(2B)'  ?

RE: Replacing a wound rotor motor with a snychronous motor for pfc

A WRIM generally will produce a very high torque for the stator current provided. They can be a very big advantage to start a load with a lower inrush current or to start a load which requires a very high torque. Also, they can be used for speed control if you provide the appropriate rotor circuit.

A synchronous motor typically has a very poor starting torque for the stator current provided. The rotor is basically designed as for DC with a squirrel cage tacked on top of the field poles. A synchronous motor can easily draw over 200% rated current until the DC is applied.

Having posted that, I've been involved on many projects using a synchronous motor on a ball mill. I've seen cases where there was a clutch and other cases where the motor was directly coupled. So, it's possible to start a ball mill with a synchronous motor.
 

RE: Replacing a wound rotor motor with a snychronous motor for pfc

(OP)
Thanks for your input guys.

Wolf

Thanks for your repsonse.  The plant operates at 15MW rather than the 15kW you used in your post.  This may or may not change your response?

Can you explain why the stator field current would have to carry twice as much field current when running at 0pf over excited?

It seems to be a general consensus that the wound rotor motor is specifially used in this application to provide the high starting torque with low starting current.  So if this was a normal starting torque application would this be a more realistic alternative.  Most of our larger loads in the plant are process fans.  I guess I now begin to wonder that in the design stage of such a facility why not consider installing synchronous motors for larger required motors to deal with power factor and reactive power needs?  Is it too dificult to control such motors and reactive power when several of these motors are supplying reactive power?  Would it make sense at the design stage to make the largest motor (barring no high starting torque requirements) a synchronous motor to deal with power factor issues.  Or is it simply just cheaper and easier to install capacitors and attack it from that angle.

RE: Replacing a wound rotor motor with a snychronous motor for pfc

Don't forget that the motor must be sized for both the mechanical load and the VAR load. Pythagoras gives you a bit of an advantage, with kW and KVARs equal, the KVA must be root 3 of either one rather double.
If you have a machine built that will supply two or three times the VARs as kW, then you will probably be able to start the ball mill easily and then increase the excitation to pick up the KVARs.

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

RE: Replacing a wound rotor motor with a snychronous motor for pfc

A clarification:

By static I meant stationary, not necessarily electronic, although such thing may be available.

Rafiq Bulsara
http://www.srengineersct.com

RE: Replacing a wound rotor motor with a snychronous motor for pfc


rockman7892:

I learned by experience that the majority of the Eng-Tips audience is living in North America. There 15 MW is identical with 15,000 kW (a comma in the U.S. is equivalent with a dot in Europe). So, 15000 kW or 15.000 kW in Germany, for instance, is identical with 15,000 kW in North America. If you are not used to the comma, then replace it with a dot and my response must not be corrected.

6500 hp is equivalent to about 4850 kW. Assuming a power factor of 90% over-excited (I try not to confuse again with dots and commas) the rated excitation current would be 100%. If your motor is synchronized to the grid, running mechanically unloaded but excited to a reactive power of 4850 kVar, the excitation current could be about twice the rated excitation current.

Wolf
www.hydropower-consult.com     

RE: Replacing a wound rotor motor with a snychronous motor for pfc

(OP)
Wolf

I am still a bit confused with your explanation I apoligize.  Are you saying that in order to produce the excitation field in the motor there will be 4850var and then the excess Vars supplied to the grid will also be 4850V thus doubling the excitation current in the field winding?  I'm not following you.

Are you saying that this would be an issue with retrofitting the existing wonund rotor motor, or also a problem if we swapped the entire motor out for a 6500hp synchronous motor?

Thanks for the help.

RE: Replacing a wound rotor motor with a snychronous motor for pfc

I'd say the items identified by Wolf show another difficulty without swapping out the rotor (the stator is undersized no matter what), and also show that if you replace the motor with new 6500hp sync motor, there would be some unique requirements for that new motor.  But the replacement motor of this size would be special order anyway.... these added requirements would probably drive up the price some (how much I don't know).

=====================================
(2B)+(2B)'  ?

RE: Replacing a wound rotor motor with a snychronous motor for pfc

Quick calcs:
6500 HP= 4875 kW
15MW@.86PF= 8900 KVAR
4800 kW plus 8900 KVAR = 10150 KVA
10150 KVA = 13500 HP
Consider a 13500 HP motor or a 14000HP motor.
The reason Synchronous motors are able to start ball mills and provide PF correction; In this example the motor is about 200% oversized for the machanical load so even though the starting torque is low for a synchronous motor, there is twice as much as there would be if the motor was sized for the mechanical load alone.
For a motor this size, the manufacturer will be addressing high excitation levels and possible stator heating issues in the design stage.
 

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

RE: Replacing a wound rotor motor with a snychronous motor for pfc


rockman:

All what I was trying to say is that the 6500 hp motor in question at first glance could deal as a synchronous condenser (i.e. zero active power) only with about 50% of the reactive power your system requires.  For this service the stator winding is able to carry the rated current without problems. However, stator components of this motor will fail eventually when a load of 4850 kVar under-excited is applied because of excessive stray flux concentrations in the stator core end regions. The field winding of the 6500 hp motor is under-sized for 4850 kVar over-excited. The excitation current for generating 4850 kVar over-excited (again, at zero active power) can be up to 100% above rated excitation current in case the magnetic circuit of your motor is highly saturated. If the motor is magnetically moderately stressed the excitation current may be in the region of 50% above rated excitation current, but even this is still far too high for the existing field winding. All the above tells us that the existing 6500 hp motor is useless for the service you have in mind.

Bill explained it well. If 6500 hp and 8900 kVar (over- or under-excited) are required at the same time you end up with a new synchronous motor of 10150 kVA and a power factor of 4850/10150 = 0.48. The manufacturer can design this unit by considering the field winding and stator end region issues but the motor would be very expensive.  

You seem to confuse the interdepence between excitation current and reactive power. Reactive power is generated by the excitation current, not vice versa.

Wolf
www.hydropower-consult.com
  

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