Insane Motor Circuit 4

[ReliabilityTech]

Because I'm not an engineer, and I'm currently in heated discussions with an electrical engineer, I need some theoretical assistance with the following circuit.

13.8kV primary to 600V secondary, 600V secondary to 600V/4.16kV autotransformer to 4.16kV softstarter, softstarter to 4.16kV motor.

13.8kV-600V = 1MVA
600V-4.16kV = 330kVA (Actually a 4.16kV H-side, but reversed for whatever purpose rated at 45.8A)
4.16kV softstarter (designed for 600HP motor originally)
4.16kV motor (45.7FLA, 594RPM, 12 pole, 300HP)

Trying to convince them that this will not function, nor is it a conventional installation.
Currently they cannot get the shaft to turn due to loss of control voltage on the softstarter due to voltage drop incurred based on the inrush current.

It should be noted that, as designed, there was a 600V softstarter in the circuit in front of the autotransformer causing voltage collapse as well.

There are multiple issues with impedance as far as reversing an autotransformer in this fashion, as well as inrush conditions changing dramatically, but are there any expected values that I can present to this engineer that can backup my words rather than just saying "You can't do that"?

Even if they achieve spinning the shaft uncoupled, there's no way that this circuit will run loaded.

Any input would be great.

Thank you in advance.

 

[LionelHutz]

Voltage drop to the motor will be in the 50% range, so the motor can only produce about 25% of it's rated locked-rotor torque.

Locked rotor torque of the motor is only 110% of rated torque, so the motor has a crappy locked-rotor torque.

That's why they can't start the load.


Why not a 600V motor? Already have the switchgear and soft-starter there to support it, so just a new run of cable from starter to motor.

 

[ReliabilityTech]

UPDATE: There was monitoring equipment attached to this circuit recently and the secondary voltage of the auto transformer was (obviously) where the bottleneck was occurring. The output was ~3100V and was not generating enough torque to turn the motor's attached load. The auto transformer has since been replaced with am 880kVA delta-delta transformer which produced similar results (contractor bandaid). This is still with the softstarter on the 600V side of the circuit. It has been decided, finally, by the contractor's engineer to either swap the motor out for a 600V motor and run new cable, or purchase a 13.8kV to 4.16kV delta-wye transformer and utilize the 4.16kV softstarter. Not sure which route they are going to take, but either option will work better than what has been happening in recent months since my original post. I'll update once commissioning has taken place and verified. Thank you to everyone that has contributed to this thread.

 

[waross]

UPDATE: There was monitoring equipment attached to this circuit recently and the secondary voltage of the auto transformer was (obviously) where the bottleneck was occurring. The output was ~3100V and was not generating enough torque to turn the motor's attached load. The auto transformer has since been replaced with am 880kVA delta-delta transformer which produced similar results (contractor bandaid).

It's starting to sound as if you have a soft grid.
Has anyone checked the voltage dip on the incoming supply?
Starting a 300 HP motor on an unloaded 1 MVA transformer should work.
Starting a 300 HP motor on a loaded 1 MVA transformer on a soft grid may be problematic.
You stated that the auto-transformer output was 3100 Volts.
What was the input voltage to the auto-transformer?
The you can expect that motor to draw around 1.9 MVA when DOL starting on a firm grid.
That may be too much for a loaded 1 MVA transformer.
3100 Volts is about 75% of rated voltage.
What is the voltage into the transformer when the output is 3100 Volts?

The auto transformer has since been replaced with am 880kVA delta-delta transformer which produced similar results (contractor bandaid).

You state that the auto-transformer is obviously the bottleneck.
With years of experience running motors on auto-transformers across a geographical region twice the size of Texas, I am not so quick to blame the auto-transformer.
You have replaced the 330 KVA transformer with an 800 KVA transformer with similar results.
Maybe it's time to stop blaming the transformer and find the real problem.
You have not told us what load you are driving. That may make a difference.
What type of bearings do you have on the motor and on the load.
Some motors and some machines must be rolled on turning gear before starting.
You have not told us if you have been able to start the motor unloaded.
If so, were the currents balanced?
How much voltage drop when starting?
Do the lights dim when a starting attempt is made?
Check your connections.
An easy mistake is to swap the #6 lead with the #9 lead when connecting a 9 lead motor.
Yes, I know that this is probably a 3 lead motor but it may have an internal problem.
I am starting to suspect a broken motor.
Possibly a soft grid or an overloaded main transformer. Possibly both.




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

 

[dpc]

I'm jumping in late and haven't read every post, but with two captive transformers in series, I'm curious why a soft starter is even needed? The transformers are going to provide a lot of "reduced voltage". If it can't start full voltage due to voltage drop, a reduced voltage starter isn't going to help.

 

[ReliabilityTech]

Waross: The input voltage only drops to 563V. The 13.8kV side (primary of first transformer) doesn't budge. There is an SVC after the site's utility transformers that reacts to sags/spikes. I was following up to let the interested parties, including yourself, know that this is ongoing and that an outside consultant with no vested interest or bias has been hired to resolve the issue. The motor is quite possibly "broken" due to the amount of hail-mary starts that the contractor has attempted. There is a spare motor that I will be testing for a baseline and then comparing the guinea pig motor's results to. I cannot mention the load for other reasons I hope you can respect. When loaded, it will be a varying load that would need to have enough torque to overcome a near locked-rotor state depending on the sequence it was originally stopped while working.

dpc: I agree which is why there is a 4.16kV softstarter as well, but it has only been used once during the constructor's "troubleshooting". It should be an across-the-line starter in my opinion regardless.

Let it be known that there are zero thermal damage curves or studies done with this entire motor circuit. I'm not even sure what kind of settings have been entered into the softstarter.

 

[waross]

Doing a reality check: 300 HP, 600 Volts?
Normally we would not consider using a 4160 Volt motor less than 600 HP.
HP > Voltage is a rule of thumb for considering a higher voltage.
With only one large motor in a plant the maximum motor HP may be somewhat greater than the rule of thumb.
Is there any reason that a 4160 Volt motor was chosen?
It sounds like this may be a challenging load.

When loaded, it will be a varying load that would need to have enough torque to overcome a near locked-rotor state depending on the sequence it was originally stopped while working.

The motor may be too small for the load.

BTW NEMA lists:
CODE LETTERS for starting KVA. CODE J OR CODE F
INSULATION CLASS, TEMPERATURE RATING
DESIGN LETTER. SPEED TORQUE CHARACTERISTICS.
Do you know the design letter for the torque curve?
When testing the motor it may be more productive to test with AC.
Apply 120 Volts to each winding in turn and measure both the current draw and the voltages induced in the other windings.
You may consider a ballast load of about 1200 Watts in series to limit the current in the event of a shorted or partially shorted winding.

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