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Fixed Electrolytic Liquid Starter


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Hi All,


I'm looking for some information on fixed liquid starters for slip ring motors. The starter i'm dealing with is a 3 step with 2 banks of electrodes controlling the starting sequence of a 3-phase 315kw slip ring motor. First stage one bank is in series with the rotor windings, second stage two banks are in parallel and a final stage coupling contactor.


This is my first time working with such a starter and my knowledge of the operating principles of the system is limited. I'd appreciate if someone with some more knowledge than myself could explain these principles. I also have some specific questions i'd like to put out there.


* The starter hasn't been in use for some time and the electrodes have become slightly corroded, will this influence the starting characteristics of the motor?


* There are a total of 9 electrodes (all of equal area and all fully immersed in the electrolyte) per bank, 3 directly connected to each of the rotor windings and 2 parallel to each of the three rotor electrodes (1 either side each seperated aprrox 50mm from the central rotor electrode), these 6 electrodes are in turn coupled with copper busbar at the terminals above the tank. What is the purpose of the 2 electrodes either side of the individual rotor electrode?? Would 1 not suffice, i.e. a total of 6 electrodes per bank?? Does the capacitive element of the plates have any bearing??


* As mentioned the starter hasn't been in use for a few years. Will the solution which is still within the tank be usable or is it probable that it will need to be replaced. If the latter is true, what is the most commonly used electrolyte and what weighting of this vs. water should be used. Rotor current - 330amps, rotor voltage - 580 volts.


I'd be very grateful for some feedback.

Thank you.



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Hello matt84


Welcome to the forum.


This is a standard secondary resistance starter where the value of the secondary resistance (rotor resistance) is reduced as the motor speed is increased. (http://www.lmphotonics.com/m_control.htm#Slip_Ring_Motors and http://www.lmphotonics.com/slipring.htm)

The resistance is controlled by switching tanks in and out of circuit.


The value of any one resistor is a function of the surface area of the electrodes on each phase ( adding electrodes in parallel increases the area) and the distance between the electrodes, and the conductivity of the electrolyte.

Note, the conductivity of the electrolyte increases as the temperature of the electrolyte is increased.


There are a number of common electrolytes used. The volume of the tank and the type of electrodes can determine the electrolyte used. A common electrolyte used is saline solution (salt and water). Caustic soda solution is also commonly used.

I would expect that the electrolyte in the tanks could be used, but it may need additional water added to compensate for evaporation.


Best regards,

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  • 2 weeks later...

Very similar to a secondary resistance starter but very different at the same time.


First thing that happens when you apply power is bubbles form on the electrodes thereby giving a high initial resistance. Further into the start sequence the bubbles dissipate whcih means the resistance reduces. Beyond that point, the electrolyte has a negative temperature co-efficient, so as you pass current through it, it heats up and the resistance reduces accordingly. Very soft starts can therefore be achieved.


having said that I suspect there may be some confusion about the way your liquid starter is arranged. First stage will have 2 electrodes, second stage will have 1. The purpose of the second stage is to add parallel resistance to the first, thereby reducing resistance further to ensure the torque step when the shorting contactor closes is very small.


Hope that helps.




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Hi Matt84,


If I may add something to the excellent posts of Mark and Spiro. The secondary starter you are mentioning is commonly referred to as a vapormatic starter.It is based on the difference in resistivity between a liquid electrolyte and its vapour. The passage of the starting rotor current causes partial vaporisation of the electrolyte and thereby adjusting the resistance and starting torque to optimum values. During the acceleration of the motor to rated speed, the thermal interchanges which occur gradually reduce the resistance. A delayed contactor then short circuits the low residual resistance. The first chamber is usually filled with a solution of caustic soda, the concentartion is usually given by the manufacturer and the second chamber with a solution of sodium cabonate.

Hope this helps.



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