2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
(OP)
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:)
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:)





RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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.
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
recovery to drive the cutters thru sharp load peaks.
Balancing the performance of each motor might be a bit tricky!
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
You are suggesting a fixed resistor. Is this a tapped resistor so you have the capability of adjusting the load sharing of both motors?
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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.
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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.
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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... :)
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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
RE: 2 x 1.5MW 11kV Slip ring Motors with liquid resistor starters
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