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Need help with alternate to LRS


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One of our customers has decided that they do not like the "hazardous" sodium carbonate electrolyte solution that will be used in the LRS we are supplying to start the slip ring motor for our equipment. It's already on the ship to their site.


They would like us to propose an alternate solution (no pun intended). Are there other electrolytes available?


I have also seen some posts about using grids of resistors in lieu of an LRS but have no experience with these. Does anyone know who does the design and supply thes egrids.


Our application is for 1,000 hp at 3000 V and 50 Hz.

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The reason sodium carbonate is used is because it is a known commodity for that application so the issues surrounding it's use are already common knowledge. Any electolyte, even brine, will technically work but there are always consequences to deal with such as outgassing, electolysis, corrosion etc. If your system is designed for one chemical, the consequences of switching need to be investigated thoroughly.


Most of the major resistor manufacturers offer assistance in designing a grid for motor starting, but I only have knowledge of US mfrs such as IDC and Post - Glover, so you need to find some in your country to work with.


Your existing system does not already include a method of switching the resistances in and out, so that will be a bigger issue to solve first. The paper on Mark's LM Photonics website on slip ring motor control offers a good idea of how that works, and his Electrical Calculations softare is great for determining the rsistance values you will need. http://www.lmphotonics.com/slipring.htm


Another alternative IF the mill will always run at full speed is to use one resistance and a solid state starter to accelerate it, then short the resistor at full speed. If you needed the variable resistance in the milling process for speed control, then that will not work.

"He's not dead, he's just pinin' for the fjords!"
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It's a fixed speed application with the high torque being needed only for acceleration to operating speed. Of course, that's why we use the LRS. As the speed increases, the torque demand drops, quite drastically once the ball charge cascades, which is after less than the first 90 degrees of mill rotation. That equates to about 13 motor revolutions. After that, we only need accelerate the load to full speed. The LRS, with it's adjustable resistance, allows the user to operate at minumum current throughout the starting time.


I will look into the single resistance alternative.


He's not pinin', he's passed on!

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Hi FXWeaver


Sounds like you need to have a very high torque for the machine breakaway.

This typically wil require a slip ring motor with the speed of maximum torque set close to 100% slip.

Unfortunately, this resistance wil result in a very low torque as you approach full speed and you will end up with a significant switching transient when you short it out. Therefore, you need to use a multistage resitance to start your machine. It is possible, as you need a high breakaway torque only, that you amy be able to use a two stage starter with the first stage set to around 90% slip and the second at around 10% slip.


The advantage of the liquid resistance starters is that the resistance is variable and so you have essentially a multistage starter. Aditionally, as the electrolyte heats up, the resistance typically reduces and provided sufficient electrolyte is used, the resistance change and motor acceleration can remain in step.


There are many possible electrolytes that can be used with brine (salt water) being common. If you change the electrolyte, you may need to change the tanks and probes to prevent a chemical interaction, plus the different electrolytes have different resistances and so the area and spacing of the probes may also need to be altered.


Best regards,

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