My LRS Is Experiencing trips

QuEnTiN_aDmIn — Wed, 18 May 2022 19:34:38 +0000

Question: The LRS installed on a 1MW crusher is experiencing trips. What about a soft starter solution?

According to our understanding a soft starter will not be able to supply the required starting torque for this application?

What are the preferred starting methods for MV slip ring motors sized at about 1MW?

Answer: With a moving electrode type LRS, the starting torque can be very well controlled if the LRS is tuned correctly, the water temperature is well controlled and the electrodes are in a good condition.

Therefore any start-up torque may be chosen, as hard or soft as needed. No other soft start solution is necessary.

Other liquid starters are in use around the 1MW level, eg. Vapourmatic starters, that do not have a moving electrodes but rather several fixed tanks. With these starters, the torque cannot be well controlled, in which case an upgrade to moving electrode LRS may be in order.

If you already have a moving electrode LRS (like the EPM1), we can test the LRS and tune the starter electrolyte solution to give the required start-up torque.

Crushers are typically started very often, leading to overheating of the water and subsequent tripping. If this is the case we can install our standard air-blast cooler to control temperature at a set-point.

If you need to find the root cause of the trips, we can install a portable (or permanent) recorder to monitor start-ups remotely for a period of time, in order to recommend the best solution.

Some factors that can influence a start-up of a crusher are:

The time it takes to complete the start-up cycle: if it is an EPM variable resistor starter, the cycle time might be too long. Crushers have less inertia and should start faster compared to mills and fans. Speed is also needed to give momentum for the crushing, when start with ore in the crusher.
Starting under load can cause tripping (based on the resistance/conductivity of the electrolyte solution). The type of ore can be the determining factor for tripping. If the ore is too hard (like chrome ore or magnetite), starting under load might not be possible, but with copper, gold and platinum it can be done. From experience we found that the starting torque need to be in the order of 180 – 200% of rated torque, when starting under load.

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HOW TO TEST LIQUID RESISTANCE STARTERS

QuEnTiN_aDmIn — Thu, 14 Apr 2022 09:22:54 +0000

INTRODUCTION
A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.

In wound rotor induction motors the rotor coils are connected via slip-rings on the motor shaft in series with external variable resistors. The external resistors used are liquid resistors.

Two electrodes per rotor phase are placed in a conductive liquid bath and moved closer together during start-up. This cause the rotor current to increase. When the rotor current increase the motor produce more torque and speeds up.

APPLICATIONS OF LIQUID RESISTANCE STATORS (LRSs)
Wound rotor induction motors(WRIM) are used in applications where large starting torque are required, for example in grinding mills, large fans and pumps.

IMPORTANT CHARACTERISTICS OF LIQUID RESISTANCE STARTOR. (LRS)
As the electrodes in a LRS are moved closer to each other the resistance between the electrodes is reduced. As the electrodes is moved to the minimum distance position a short is placed across the resistors to reduce the resistance to zero and supply maximum current and torque to the motor.

Important characteristics of Liquid Resistance Starters are the:
Turn-down ratio.
This is the ratio of the maximum resistance of the LRS to the minimum resistance before the short is placed in position. The short is placed in position with a shorting contractor. A good turn-down ratio will limit the starting torque of the motor to an acceptable value.
A good turn-down ratio for a 5mW LRS is a 100 or more.

The starting torque of a Wound Rotor Induction motor should not be more than 120% of the rated torque.

LRS working temperature range
The working temperature range of an LRS is the electrolyte temperature range over which the LRS can be operated without introducing too high starting and shorting spikes on the motor drive train.

The LRS working temperature range is dependent on:
i) the LRS turn-down ratio.
ii) the LRS electrolyte mixture strength.
Iii) and the condition the LRS electrodes.

TESTING THE PERFORMANCE OF AN LRS
The testing method that I would advocate is a startup torque test.
To do a startup torque test, current transformers (CTs) needs to be installed on the LRS cables to the LRS Electrodes.
The start-up LRS currents traces can the be captured with a suitable oscilloscope or recorder and analyzed.

It is important to note that the current is directly equivalent to the motor torque produced. 100% rated motor rotor current is equivalent to 100% torque.
Another important measurement is the LRS electrolyte temperature.
It is also important to be able to measure the LRS Electrolyte conductivity.
It might be necessary to dilute or increase the electrolyte strength to alter the LRS torque performance.

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The post HOW TO TEST LIQUID RESISTANCE STARTERS appeared first on SDG - Technologies.

QuEnTiN_aDmIn, Author at SDG - Technologies

My LRS Is Experiencing trips

Question: The LRS installed on a 1MW crusher is experiencing trips. What about a soft starter solution?

HOW TO TEST LIQUID RESISTANCE STARTERS

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