Hi Dear friends, Problem for Starting current of MV motor(mill motor, 2.9MW, 6.3KV,rotor current:775A, rotor voltage:2770V), in this case sometimes we recorded 775A as starting current and in other times we recorded 400 A for rotor current during start and for stator the current is variable, in some case it was 85 A, and in other was larger than 400 a. this motor starts with liquid starter.
what is your idea about differentiation ? what is cause of current reduction?
is the liquid starter resistance variable during each case of start?

The motor current will vary with the load even through the fluid coupling/clutch. There are many different setups for fluid soft starts so it is hard to say without knowing details. I am assuming you are not changing start/fill times. As a very rough rule the fluid coupling will limit starting current to torque at 100% shear/stall once full and vary with the load below that.

Unless you are having some big temperature changes causing the fluid to get thick I would guess your load, incoming power or both are changing.

Mahmoud:
The job of the liquid rheostat is to vary the external resistance attached to the rotor from a very large value (at speed = 0) to approximately a short circuit (at speed = full rated slip speed). This is done to limit the inrush current to a reasonable level; typical values are 1.5x rated current, which leaves enough torque to smoothly accelerate the connected load.

The control of the starter - and therefore the observed resistance and current values - should be extremely repeatable from one start to the next. If things are all over the map, as you suggest - AND THE LOAD IS REPEATEDLY STABLE - then there are a couple of possibilities.
1. The mechanism for driving the electrodes is damaged, causing one or more electrodes to not move in concert.
2. The mechanism for driving the electrodes is damaged, causing incomplete reset (to full resistance) prior to attempting a start.
3. The ionic potential of the liquid is changing over time, most probably as a result of chemical reaction.
4. The liquid rheostat has insufficient volume to adequately dissipate the heat buildup during normal start / run conditions, in which case the chemical composition may be varying.

One more thing - are you making your current measurements with a standard multi-meter? Or is it one that reads "True RMS" values? If the former, the readings can be all over the place as the device only takes a value for instantaneous current - which may be at any point on the sine curve. If the latter, the device should read repeatedly close values for a similar waveform, since it is averaging out the waveform to an RMS condition.

And lastly - a wound rotor induction design is a "dumb" machine. It will try to do the job, regardless of how much damage it is likely to do to itself. This means that if, for some reason, there is a higher inertial load, the inrush current will climb until one of four things occurs:
A. The motor manages to accelerate the load,
B. The motor cannot accelerate, and draws sufficient current to damage the windings.
C. The motor cannot accelerate and draws sufficient current to damage the electrodes of the rheostat.
D. The upstream protection takes the machine offline because of overloading.

Rotor current will be directly proportional to torque and stator current. In affect you could measure the stator current and calculate the approx rotor current. Measuring the rotor current directly with standard instruments will give errors as you are in affect measuring the current at what ever the slip frequency is.

For example, If the stator full load current was 250A (not sure what it is in this case) and the Stator start current was 400A (1.6x) then I would expect Rotor start current to be 1.6 x Rotor full load current = 775 x 1.6 = 1240A. Torque delivered would be around 1.6 x Full load torque, very typical for a ball mill or sag mill as Chris mentioned.

Correct measurement in the rotor current can be done with the likes of hall effect transducers or even shunts but not clamp-on ammeters unless they use hall affect or similar technology.

The liquid starter simply adds resistance in the rotor during starting to produce torque and depending on the electrode type and design, the start current and therefore torque should be similar with each new start. Generally the electrodes start wide open at start and slowly close up as the start progresses. Finaly the rotor is then shorted as the motor gets to full speed and then runs like any cage motor. Differences in start current ( eg As measured in the stator) would only be due to the electrode position at start being different each time. Liquid temperature will affect it only slightly.

The load will not vary the start current, only the resistance in the rotor and the speed of the rotor will affect that.

Hope this helps

for the following motor it could be mentioned that, the curve of stator current in some case has a ramp shape and at the end of starting sequence it reaches the 85 A. this current is the no-load current based on vendor documentation. in addition, in other case the current has different shape: during 10s ramp-up to 400 and during 10s ramp-down to 35A. Also i comment that the maximum current during ramp-up is vary between 120 to 400 for each case ,

In fact, in case of ram-up/down start, it could be supposed that the motor might be started direct online because the behavior of current in this case induce this idea to me. and I think the resistance of liquid in this case must be high, because of high resistance during starting the rotor current would be low and so the for gaining the starting torque (1.5 times) , the stator has to supply the compensation current.
please advice me about this problem

Without a lot of detailed information it is all guessing. As far as I know you could have badly imbalanced voltage and the current depends on which line you read amps from.

Is the liquid starter a resistor bank submerged in oil to reduce rotor current or is it a fluid coupling/clutch with two impellers between motor and load?

It seems the motor control has separate voltage ramp for the stator and rotor. Are these ramp time set or are they self compensating from some feedback?

based on MKS (supplier of liquid starter )time for starting sequence is 27.2 + 4 s,
the time that I mentioned is based on Monitoring system ( trend) in Central room.

the Liquid starter is contain the Na2Co3 as that solved into water. Moreover, it has two Electrode section, Moving electrode and fixed electrode.

I point out that at start the motor is no-load, because all rollers have been lifted and the motor connected to the gearbox of table through coupling.

I would look at phase voltages and currents with a recording meter with enough channels to cover the stator and rotor phases. If they are balanced and ratio of stator versus rotor would out the same for the low and high current occurence then you need to look at the load to be the cause.

With no load the motor will be up to speed really quickly iresspective of the starting sequence time setting, and your measurements are probably picking up on start transients so appear somewhat random. So at no load you will not see the true performance and affect of the Liquid Resistance Starter as it is set up to deliver 1.5x FLT which you don't need for an unloaded start.

Sodium Carbonate electrolyte solution is commonly used in these starters at a recommended dose rate to give the start resistance and you just need to ensure the interlocks prevent starting unless the electrodes are in the open postion.

By the way, the equivalent of DOL with a slip ring motor is electrodes shorted out which should not be possible with the control circuit. Also, this is not recommended due to extremely high inrush current with shorted slip rings.

This is in affect a simple device so I'm thinking you don't have any issues as long as the stator supply is solid and balanced and the slip rings of the motor are affectly connected to the LRS.

Under a loaded start I think you will see everything will probably work OK and you will have time to make the right measurements.

You could run a control sytem check without main power to ensure the electrodes are moving and the right sequence is in place

do u have special DC/AC motor carbon brush drawing?

recirculation pump in the lrs should be provided so as to equalise the concentration of na2co3 before start. also check the na2co3 should be of industrial application.