HV Synchronous motor current fluctuating during starting 2

[RRaghunath]

7.2MW, 11kV Synchronous motor is coupled to a compressor through geared hydraulic coupling.
The motor starts as Induction motor. It has been in service for nearly 20-years.
Recently, when it was started the motor current fluctuated as if in a power swing and went beyond the rated starting current to trip on motor short circuit protection (set in motor protection relay set at 130% of rated starting current and 0.1s delay).
What could be the reason. Appreciate if experts in forum could save their experience / knowledge.

Rompicherla Raghunath

 

[panter]

Maybe bad amortisseur winding on rotor.
Good luck

 

[edison123]

Could you possibly post the photo of the rotor here? There are two types of synchronous induction motor rotor windings construction. I would like to see what type it is before doing my usual back seat driving.

Muthu

http://www.edison.co.in

 

[RRaghunath]

Dear Mr. Muthu, the rotor is still not withdrawn from motor. Motor manufacturer was asked to advise. Awaiting the response.
It is for sure is a Synchronous motor that is designed to start as Induction motor from the grid.

Rompicherla Raghunath

 

[electricpete]

What is the speed of the machine? That would enable an educated guess about the rotor construction.

Was the current swing before or after the field was applied?

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(2B)+(2B)' ?

 

[RRaghunath]

Electricpete, the speed is 1500rpm and the swing is observed before the excitation is switched on.
The current goes up in excess of the rated starting current during the swing.

Rompicherla Raghunath

 

[jraef]

Well when the dampening (amortisseur) winding is used as an induction motor to accelerate the load, it will behave like any induction motor and if there is excessive load, the slip will increase causing it to pull more current in order to finish accelerating. 130% of FLA is actually very very low in that regard. Locked Rotor could be as high as 600%!

On a compressor application that has been functioning correctly for 20 years and now starts doing this, I would first suspect a mechanical issue that is increasing the load on the motor during acceleration. It could be a malfunctioning compressor unloader valve (very common), clogged unloader air line (if any), bad bearing in the motor and/or the compressor etc etc etc. Don’t start tearing down the motor until you eliminate ALL other possible causes. And from experience, just because you hear the unloader valve operate does not mean it actually is. I’ve seen people fooled by that time and time again. What can happen is that the valve itself gets clogged with rust from the tank or compressor head (wherever it is) and the SOLENOID engages, so it makes the proper sound, but the valve inside is not actually opening or opening fully.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

I second jraef.
The first three things to check are;
1> Unloader system.
2> Unloader system.
3> Unloader system.
Next check the relay settings. 0.1 seconds is not much time for the starting current to drop to 130%.
It is possible that the actual operating points have changed from the original.
The actual indicated setting is not as important as a change in the operating points.
With a setting as low as 130% it is surprising that the system functioned for 20 years.
Things may not be as they seem.
The protection relay may have been out of calibration. Then or now.
The starting current seen by the relay may have been erroneous due to CT saturation.
Has anything been changed? Relay? CTs?
Oh and don't forget to check the entire unloading system.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

At that speed, I'd say you have smooth (slotwound) rotor, not a salient pole motor. So you can probably discard the idea of reluctance effects that apply to salient pole machines.

I haven't ever set the protection for this type of machine, but fwiw I agree with jraef. It's not surprising that current increases above rated during start (unless you are watching it with a meter that has heavy low pass filtering effect in which case you might not see that high current even though it's there). What changed is probably not the highest magnitude of the current but the time to get it to speed.

For typical SCIM (I know sync is different, but it's probably close), the current doesn't get below 130% until it's within a few percent of synch speed. And 0.1 seconds is not a lot of time for this big machine to accelerate within a few percent of sync speed. I don't think you had a lot of "margin" to trip to begin with, in spite of the fact it apparently started fine many times over 20 years. Some ideas what might have changed:

Increase in mechanical load torque applied during the start. I think your hydraulic coupling may play the same role as the compressor unloader valve jraef described. Maybe there are some checks that can be done on that hydraulic coupling (don't ask me what kind).

Low or unbalanced voltage - this would slow down the acceleration.

Of course, also double check your relay as-found trip points.


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(2B)+(2B)' ?

 

[waross]

Although the set point may be 130% the original system may have been designed so that CT saturation prevents the relay from seeing the full current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

electricpete - no, synchronous motors are not the same. It's typical that the current on a synchronous motor will not drop below about 150% to 250% of rated current until the field is applied. I've run into a few motors that were around 400% current.

130% current for 0.1 second is not a normal trip setting for a motor unless this trip is locked-out until the motor has started and reached steady operating conditions. It makes no sense to even attempt to start a motor with that trip in place.

So, what changed on the installation? New stator or rotor controls? New relay? Changes to the hydraulic coupling?

 

[RRaghunath]

Gentlemen,
Thanks for the posts.
Sorry, you seem to have got it wrong on the current, it is not 130% of FLA. It is 130% of starting current (which in itself is 4-times the FLA), i.e. 520% of FLA. It is there in my original post ("130% of rated starting current")
The concern is that the motor Amps is not supposed to go beyond the stipulated starting current but in this case, it is happening as the current swings slowly during initial starting period.

Rompicherla Raghunath

 

[waross]

Are your incoming phase voltages balanced? The back EMF of an induction motor (this starts as an induction motor) will be equal in all three phases.
If one incoming phase is a few percent low, it may equal the voltage of the low phase and there may be little or no incoming current.
If one incoming phase is a few percent lower, it will push current back into the low phase in an attempt to correct the low voltage.
In order to do this, energy is taken from the healthy phases.
This results in higher than normal current in the higher voltage phases.
It is worthwhile to monitor the incoming phase voltages during a start to see if one phase is going low.
Also monitor and compare the phase currents during starting. A small drop in voltage causes a large increase in current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Well, I didn't understand since I've never heard of a relay being set as a percentage of rated starting current before. On the other hand, a setting of 520% of FLA is quite common.

If it's a brushed motor then check the resistor connected to the field while the motor starts.

 

[edison123]

Assuming it's a pole face damper/amortisseur winding start, a swinging stator current and inability to pull into synchronism indicate cracked damper winding bar(s).

Muthu

http://www.edison.co.in

 

[electricpete]

Sorry, you seem to have got it wrong on the current, it is not 130% of FLA. It is 130% of starting current (which in itself is 4-times the FLA), i.e. 520% of FLA. It is there in my original post ("130% of rated starting current")

Yeah, I misread that one also. I think the word "rated" threw me (I think that typically is associated with full load current, not starting current). But I should have noticed the word starting.

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(2B)+(2B)' ?