Liquid starter

[niallnz]

I've got a 500kW, 6.6kV slip ring induction motor, which uses a liquid starter to bring it up to speed and is then switched to synchronous operation. The starter has movable plates rather than relieing on heating to change the resistance. When starting the motor the operator monitors the stator current and manually controls the starter, keeping the stator current just below the motor protection pick up point.

The operator claims that the stator current is not falling away as quickly as it used to. The motor drives a cement ball mill, and the operator is blaming the starter but I think that if it is not accelerating as it used to then the load has increased. Any thoughts?

Also the starter used soda ash (sodium carbonate) to modify the conductivity fo the water, does any one know what the chemistry of the liquid should be?

 

[GusD]

Hello niallnz

In an old motor shop we had a liquid starter that we used to load test our generators.We used a brine solusion in it and controlled the resistance by moving 3 rods in and out of the brine.We didn't have to be very concerned about the Conductivity of the solution,because our primary need was to creat a measurable load.
In your case,you have to have a constant solution or otherwise your motor starting characteristics will be changing all of the time.Temperature of the liquid is important,but your solution Conductivity is crucial.

GusD

 

[Marke]

Hello niallnz

If the start current is the same as it used to be, and the acceleration is slower, that would suggest to me that there is a mechanical problem. If the liquid characteristics have changed, that will not matter as you compensate by shifting the plates. If the current is the same, then the torque should be the same. If the torque is the same, then slower acceleration can only mean that either the inertia of the load has changed, the coupling ratio has chaged, or the work torque during acceleration has increased, reducing the accelleration torque.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[edison123]

niallnz,

If you suspect the load had increased, what is your steady state (after cutting out the rotor resistance fully) stator current ? If it is more than your previously known current, then your load has definitely increased (provided your voltage has not reduced now). This will affect your acceleration time since you apparently have a cap on the starting current. Since you mention yours cement ball mill drive, were the balls replaced recently, which may account for increased load ?

 

[jraef]

niallnz,
Go to the "Search" bar at the top of the page and enter "liquid rheostat", then click the dropdown bar to the right and select "Search Posts (Keyword)". That will give you 6 threads wherein this subject was addressed in the past. You should be able to find some good resources in them.



Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion: Visit

http://www.saetpd.it/saet_eng/elettromeccanica/reostati_liquido.htm

http://www.flomatch.com/liquid.htm

http://www.surplusrecord.com/cgi-bin/adpop.pl?303069

http://www.mdresistor.com/mdus/vectrostarRUS.htm

etc. for more info

 

[niallnz]

Thank you all for your input. A softstarter is not an option as I would still need external resistors to provide the starting torque. As much as I would like to do a VSD it is not really an option due to the expense (existing motor is 6.6kV ie MV drive, or new 400V motor and drive). We seviced the motor and brush gear, and didn't find anything unusual, and the mill started alright. Personally I think the mill is some times shut down with out being completely unloaded and once it has cooled down the starting stiction increases.

 

[Marke]

Hello niallnz

Yes, if the machine is started partly loaded, that will definitely affect the acceleration characteristics and produce the symptoms you describe.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: The liquid starter manufacturer might be contacted for the liquid starter periodic maintenance procedure.

 

[GGOSS]

Hello Niallnz,

1. The chemistry of the electrolye solution is based on a number of factors including tank volume/size, rotor volts, rotor amps, required starting current and starting torque, number of starts per hour, ambint temperature etc etc. The only people who can advise you on the correct 'chemistry' for your application are the manufacturers or suppliers of the starter. Note: Ensure the machine operator is kept in the loop, because he/she is the only person who can provide accurate information relating to starting duty.

2. Although this is a moving electrode starter, the resistance (and performance) can change from start to start. More advanced moving electrode starters incorporate a heat exchanger to ensure the electrole solution remains at a fixed temperature and therefore the only item influencing resistance is the position of the electrodes.

Hope this helps

Regards,
GGOSS

 

[jbartos]

Suggestion: If the current is not falling off as it used to, then there appears to be a higher resistance in the liquid starter circuit loop than designed by the manufacturer. Perhaps, the liquid starter overhaul is overdue.

 

[DougMSOE]

Having designed a few of these, and applied same in high torque loads, suggest that a close inspection of both the moving and fixed electrodes. They should be smooth and clean with no buildup on them. Make sure that phase to phase situation can not occur. The electrolyte resistivity is critical. If you know what the value is to be, the mixture is straight forward. Once you know what the resistivity should be, the best way to determine if the electrolyte is not correct is to use density not pH as several mfgs say. The most frequent problem with these liquid starters is the evaporiation of the water which will increase the resistance and lower the speed torque curve, slowing the accdeleration.