Surge test? 5

[buckeye3]

I have a 700HP 460V induction motor that I have been asked to perform a surge test on. The motor megs good with a regular hand crank megger set to 500V but I am unfamiliar with the term "surge test". Would someone describe exactly what this entails? Thanks

 

[itsmoked]

If he doesn't come back soon we'll know he should've used gloves...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[buckeye3]

I have installed a good soft start from another unit and will probably do the smoke test in the next few hours. I was going to rebuild the old soft start and use the 5 HP motor to test it. I know that even at locked rotor it cannot short the SCR.

Instead of rebuilding the old soft start I thought about installing a new soft start, one rated at 500HP across the line but 700HP inside the delta (no need to go into the pros and cons about inside the delta)to save money, time and embarrassment. I have 12 350KCMIL cables between the location of the soft start and the motor peckerhead, cable length is 30'. This will give me two 350KCMIL cables per phase, is this enough?

The shop gloves are still in their bag and have never been used. My new tool is a length of 14 gauge green wire that I use to check after I check with the meter.

 

[itsmoked]

buckeye3; Your cowboy attitude may get you someday. I think you should reconsider it. Electrocution or third-degree burns are not broncos you can get back up from.

Your 14 gauge wire may become just a source for a ball of 20,000F plasma in your hands.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[buckeye3]

Motor passed smoke test and will be coupled to the load next. Current on the B phase (where SCR short was occuring) ramped up to 2863A and then dropped extremely fast to 144A once up to speed.

itsmoked, I apologize for sounding cavalier with respect to safety. I am very safety conscious but when I got locked up in it I had been working on it for two weeks and it was locked out at the source and everybody knew that but did not think about the back feed when they did the remote wiring to the wall socket. My 14 gauge wire is just my added protection after I am sure the circuit is dead by turning off power and checking with my meter. I would not use it on anything over 460VAC.

 

[itsmoked]

Good to hear.

Wow that's 20X. Seems like a bit much especially with a SStarter.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Smoked,

A little French lesson for you: "Au corsair - corsair et demi"



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[buckeye3]

I have just been thinking the same thing. I think the thyristors are rated in the area of 2800A. I did not monitor the other two phases but only the B phase during start. After thinking about it, somewhere in the area of 1500A is what it should have peaked at during start. Would you agree that this is abnormally high warranting further testing before placing the motor under load?

 

[Skogsgurra]

Agree. Should be somewhere around 1500 A. What does the motor nameplate say about FLA?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[buckeye3]

Motor FLA is 754A. I checked the thyristor data chart and the first amps listed is "I TRMSM" and it is 2800A. The motor is also uncoupled and 2863A during start is way too high. I have already done two start and run tests but I watched B phase only during start. After start the current fell to 156A-144A-126A and those readings are good to me. I am leary about trying another but would another test monitoring either A or C phase give me any more information about the problem?

 

[Skogsgurra]

Would a longer start ramp take the current down? I think it would. Do you need the fast start?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[buckeye3]

Ramp is OEM set to 6 seconds. I will lengthen that value to say 10 seconds and do a test. I will try this and see what happens but suspect that the current will remain large because I now think there is a motor fault.

 

[buckeye3]

The ramp is controlled by a nine position switch with position 1 meaning six seconds which is factory default. I do not have information as what the different steps mean on the switch but I set it to 3 and used a stop watch. The ramp was 7 seconds and the highest current reading was 2450A. I then set the ramp pot to 5 and current peaked at 2190A but time stayed about the same. These values still seem to high to me. Would this high current cause you enough concern to do the surge test?

After the motor stops spinning I will turn the pot to step nine and see what happens.

 

[Skogsgurra]

Sorry. One of those pesky customers called.

If the motor runs OK, once started, I don't think you have a motor problem. The current may seem to be high, but not high enough to be worried about.

Especially not if the SS obeys commands. Which it seems to do.

There is always a more or less uncontrollable current peak when starting with an SCR SS.

I guess you are right, Smoked, there's one more recording. From yet another SS. The current peaks as the speed gets up to peak torque. Thar part is hard to control in most SS.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[buckeye3]

I got called away by a whining and whimpering one horse motor.

Thanks to everyone for their inputs. The voltage of the switchgear that powers the motor is 500V and this would be adding some to the current but to be on the safe side I am calling in a professional to do the surge comparison test. If the motor is declared good then I am re-pulling the soft start and installing it back where it was and make current measurements during start. From that point I will have a good footing of what a working system looks like. I should have done this before I moved it but hindsight is 20-20 and I was sure the motor was in good shape.

I am having trouble accepting that when using the factory default settings I am pulling over 2800A when the thyristors are rated 2800A. No electrical engineer, IMHO, would design a system with the current of the system having the value of the major components current rating.

 

[edison123]

Can you possibly roll the phases over (R to Y, Y to B and B to R) and check whether the high current goes with the supply lead or stays at the motor pecker head ?

* I would go green if only I were not yellow *

 

[LionelHutz]

If the motor starts smoothly and sounds good when starting then there is likely nothing wrong with it. It's very seldom a motor has an intermittent winding failure, especially when running on a grounded utility system. The fault currents during the initial failure typically cause more damage and it's permanent.

You already said you found an insulation failure in the soft-starter so why don't you think that caused the SCR problem?

One other thing that could be the problem is the snubber circuit on the soft-starter. I have seen that before. Replace the snubber and the SCR quits failing. It was an RC snubber but the R was shorted putting the capacitor directly across the SCR.

Thyristors are rugged devices. You are somewhat misreading the current rating. It's possible you could push 8000A through those SCR's for a motor start assuming good clamping, gate drive, and heatsinking. And, you could do that every motor start without degrading the devices.

One other comment - in this day and age it's a very poor softstarter if it does not protect against the imbalance cause by missing gate firing.

 

[buckeye3]

I am going comit a sinful act and install "inside the delta". All die!

 

[buckeye3]

Oh Yea? Then why is the OEM raising the current rating of the thyrisors? I am hoping to get a discount for doing the research.

 

[Skogsgurra]

It is about Iavg and I for 60 degrees. Each thyristor conducts only one 1/6th of the period - or less. So thermally a 9000 A load on the SS may look like a 1500 - 2500 A load for each thyristor.

The uncertainty is because thyristor losses are not purely resistive and not purely constant voltage either, but a mix.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Sorry 1/3rd. A 4500 A SS load may look like a 1500 - 2500 A thyristor load.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...