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Home » Motor control » MV motor starting current problem
MV motor starting current problem
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 the 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.
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.
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 the 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.
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.
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