2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters 1

[dubstep]

Hi Guys,

A sugar mill is planning a front end upgrade. We are taking over the project on there behlaf as they cannot deliver on a tight timeline.

My question : what (if any) implications are there in running the two MV motors on a common shaft. The idea is to directly couple the motors using both for duty, it is a Cane Knife application.

The motors are both been manufactured as we speak, and the only concern i have rasied , is that i would liek to electrically interlock the LRS so that they will share the load equally.

Is there any other concerns i should be looking into? some senior engineers have indicated concern in doing this, as they say the motors will not share the load equally.

does anyone have some litreature that can help me. I will be responsible for designing the new 11kV Board that will feed the motors.

Thanks again

 

[LionelHutz]

Is the liquid rheostat for continuous speed control or just for starting the motors?

Wound rotor motors with rotor resistance load load share very easily. You shouldn't have any trouble if you keep the rotor resistance similar. Of course, that is a big "if" with liquid rheostats if they aren't always maintained.

If this is for starting only, I have always been able to start wound rotor motors with a soft-starter and a single resistor step. It's very simple and effective.

 

[dubstep]

thanks for the response

The slip ring variable resisitor starter is for torque/current control during startup , once motor is up to speed the variable resistor drops out but maintains a slip resistor that is continously connected to the rotor.

this method of startup does not offer speed control. Yes the motors rotors are wound.

regards

 

[jraef]

If you run at a fixed resistance, it becomes more critical that the motors run at the exact same slip, otherwise the load sharing issue looms large. If you ran on the LRS, you can use the variability to maintain equal torque from both, if necessary.

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[iop95]

I think it's beter to use same rheostat (or one powerfull if current and / or heat at startup overpass specifications) for both motors. Link togheter rotor windings (same phase order / also same phase order in stator) and use a single rheostat to share same resistor values on rotor windings to assure same (almost same) slip in order to obtain same torque on both motors.

 

[volxport]

Good Luck! The Proper way to do this type of system since 2 motor's even if they were built at the same time will have slitghly different performance characteristics is to use a VFD with either encoder's on each motor or a VFD that has an electronic line shaft software that will allow you torque match each motor and ensure that both motor's are spinning at the exact same RPM.

 

[gattie]

Usually this type of drive takes advantage of kinetic energy slip
recovery to drive the cutters thru sharp load peaks.
Balancing the performance of each motor might be a bit tricky!

 

[LionelHutz]

Geeze, using VFD's is overkill unless the wound rotor motors are replaced with squirrel cage motors. Similarily, connecting both rotors together will eliminate any possibility of load matching the motors since you are forcing the same voltage on both rotors.

You are suggesting a fixed resistor. Is this a tapped resistor so you have the capability of adjusting the load sharing of both motors?

 

[iop95]

I have made same simulations with a constant torque load, 200Nm and both motors coupled to this load. I attached a file with difference between motors torque in two situations: maxim 0.5% speed difference between motors and load and no speed difference.

 

[dubstep]

thanks for the responses. As LionelHutz says, we cannot change the slip ring motors, as the client has already ordered them, we are sourcing liquid resistor starters, and the resistor banks that do not switch out after start will have 3 tapping points, +3%,+5% and + 7.5% slip. I do not have details on the motor slip ratings as yet, i assume between 1% and 3%.

Thanks for all the valuable info.

My boss has been involved with a installtion as mentioned above, involving squirell cage rotors and VFD control with enconders. maybe the preferred way to operate such a system, but at this point we have to make do with what we have.

thanks.

 

[dubstep]

thanks iop95.

very interesting in deed. We do not plan to have one "rheostat" (liquid resistor starter in my case) but 2. One for each motor, the starting will be interlocked on the MV contactors.

In the event there is an issue with one of the motors, the client would like to run the knives with just one motor in use.

Once the installtion is complete April 2012, i shall produce my own graphs and share them with you.

Thanks

 

[iop95]

It's important that both motors to "see" same rotor resitor to provide same (very close more exactly) operating point on slip curve. Only one rheostat assure this and also allow to change plot position at same time. Attached is a simulation when motors are on separate rheostat and there are 10% difference rheostats value one from other. After starting may see, will be a constant torque difference between motors.

 

[LionelHutz]

iop95 - the poster is not using the liquid rheostats for speed control and they will only be in the circuit during motor starting.

When I think about it more, I would actually say that 1 liquid rheostat could be used for starting and then it could be shorted once the motors are running. The individual fixed resistors would be left in circuit during the start and also for running. This scheme would be cheaper to purchase but it could present starting and maintenance issues if one motor is down and the machine is still operating.

 

[iop95]

LionelHutz - yes, rheostat is for startup only, but this start-up frame may be see like a variable / controlled speed regime even is a short time. To have almost same torque on both motors each one must see same rotor resistor and this scheme with a single rheostat seem to be cheaptest and simplest.
Depending of avaiability required by application, yes, must take care about service down-time. May be used more contactor to separte both rotor and stator circuits of each motor but this add more device failure risk, same control complexity and sure need more money... and now is a big crisis...

 

[LionelHutz]

iop95 - The motors producing equal and matched torque is not a big issue since the original poster has indicated this application can be started and operated with a single motor. There is no need to worry about getting rated torque from both motors when that requirement simply is not necessary.

 

[PersonalProfile]

As suggested by others you need a method of distributing the torque - the slip resistor you mentioned should be able to do this - though you need to confirm it is appropriate - i.e. sufficient to cater for expected vagaries between the motors - having them made together / tested may be a good start though is no guarantee (plus what happens when the spare or a replacement is installed?).

In your case, a model with each motor +/- 2 % of expected slip / 3 % (i.e. one motor has 1 % and the other was 5 % slip) yielded when one motor was at 100 % FLT the other motor would be at 20 % FLT..

Given the significant impact of secondary resistance on motor slip, it may be worthwhile paying attention to the location of the LRS and or installing a shorting contactor (though your trimming resistors need to be inline..) and generally keep an eye on the secondary resistance (selection, QA during installation etc).

Generally not such an issue during a start - a torque speed curve illustrates why.

Regards,
Lyle

 

[GGOSS]

It appears the original question has not been answered.

If the motors are directly coupled, they will balance themselves and therefore there is no need for tuning/load sharing resistors.
If the motors are indirectly coupled, it becomes necessary to analyse and address both the ‘start’ and ‘run’ conditions separately.

For the ‘start’ condition, the strength of the electrolyte in both tanks, as well as the length of cable between each tank and the rotor of the motor is connected to needs to be closely matched. In order to achieve this, it is necessary to introduce circuits that monitor and control the acceleration rate of each motor. See also ‘compact dual and compact triple’ starters below.

For the run condition, load sharing resistors may be used to provide load sharing. Some thought then needs to be given to the max additional slip provided by the resistors, the number of incremental steps and whether they are to be automatically or manually switched.

For applications involving 2 or 3 motors driving in to a common load, traditionally this was achieved through the use of 2 or 3 separate LR starters. To operate affectively, it becomes necessary to match electrolyte strength within each tank through additional controls, circulations pumps and external plumbing.

One LRS manufacturer has however adopted a unique approach to this and produces ‘compact dual and compact triple’ starters. These comprise a single tank with multiple electrode sets each connected to its corresponding motor. As the electrolyte is housed within a single tank, it's temperature and strength is consistent for all connected motors.

Suitability of compact dual and triple starters is dependant upon motor size and intended duty. For further details, you can download their brochure here:

http://www.nhp.com.au/lrs/common/media/LRS-C low.pdf