Surge test?
Surge test?
(OP)
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





RE: Surge test?
It is a potentially destructive test and should only be done by qualified personnel.
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RE: Surge test?
* I would go green if only I were not yellow *
RE: Surge test?
700HP motor soft start shorted a B phase SCR (cathode to anode) for no apparent reason. Checked the motor, replaced the SCR but after 5 seconds into ramp up the exact same SCR shorted again. After extensive checking I found the insulator to be electrically bad between the failed SCR and the heat sink but I could not find any visual. I have an identical soft start that I can install but was concerned about the potential damage it could receive if the motor caused the fault. I cannot picture a way that the motor can cause one particular SCR to fault. Wires look good and megs open to ground at 500V. I did a DOL the best I could but it tripped the main CB immediatly like it should have because it is sized too small for DOL. When I looked at the uncoupled motor shaft it was silently rotating a at fast clip indicating to me it is ready to run. With these conditions would it be risky to install a new soft start without a surge test done by qualified people. With what I have described it seems a good bet that the whole problem originated from the soft start. A little history, two years ago there a major phase to phase short in the motor peckerhead. The only thing done was repair the short damage in the peckerhead and it has run good since.
RE: Surge test?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Surge test?
If one SCR doesn't fire, you will get a very heavy DC component (no more AC reactance, only DC resistance) and that will certainly melt down the SCR if you have standard (not thyristor) fuses - or no fuses at all. The fact that DOL tripped the CB instead of blowing fuses points to that.
A new soft-starter wouldn't have that problem. So, I think that you should change it. Also, if there is a by-pass contactor, check it so it isn't stuck closed in one or more phases.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
RE: Surge test?
I feel rather confident that you have a problem with the soft-start. Turn-turn shorts in motors do not take out thyristors after five seconds. But missing gating pulses on one thyristor does. Can you check the gate current on all six thyristors? Easy to do with a current clamp if you can get at it. Reading shall be same on all six thyristor gates. If one deviates much, you have a problem. A DC clamp and a scope are the best tools for that, but an ordinary current clamp can also be used. Expect 10 - 100 mA readings due to the pulse character of the gate current.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
Question: Will a 700HP motor soft start run a 5HP motor correctly? It seems to me that it does not make any difference to the thyristors as to the size of the motor but I could be wrong.
RE: Surge test?
Connecting incandescent lamps to the output is also an easy test. But you need to string several lamps in series so they don't get serious overvoltage. Connect strings in wye for least voltage.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
RE: Surge test?
RE: Surge test?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
Hey Gunnar, would you just put the clamps on the gate wires to the thyristors?
I presume they are actual flying leads on something this size?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Surge test?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
When the soft starter is given the start command, all three lamps should light up together. Watch for the lamp that comes up early, or the lamp that comes up slow, or remains at reduced brightness. Triggering and SCR problems are usually very obvious.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Surge test?
Skogs stated that if one SCR fails to fire there will be a
"very heavy DC component (no more AC reactance, only DC resistance) and that will certainly melt down the SCR"
Can you elaborate? I would have thought that not firing one SCR would only result in a single phasing type situation, not damage to the SCR.
My experience is in LCI so if soft starters are not a similar breed of animal, my apologies
RE: Surge test?
A motor that big has very little resistance in its windings. As you know, it is electrically equivalent to a (rather bad) transformer.
So, the current is roughly U/X, where X is the winding's reactance (omega*L).
Now, if there is more magnetization than the system can take, i.e. saturation, the X goes to zero and the current rises beyond normal values. A lot beyond. Also, there is a lot of 0 rad/s (DC, that is) and that is only limited by the near zero R in the circuit.
The very high current heats thyristors and fuses and starts to trip breakers. If the thyristor i^2*t is less than that of the fuse - which is alewys the case if standard fuses are used - then the thyristor either melts down or evaporates before fuse cuts out. Usually the first. Breaker usually takes longer time, so it trips a little while after.
I had a similar experience a couple of years ago and the recordings from that event are interesting. See attached file.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
Clean hookups!
I'm starting to get the feeling Ginnar never met a starter/VFD he didn't record.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Surge test?
Any success?
Did you sacrifice the 5 HP to make the 700 HP run?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Surge test?
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.
RE: Surge test?
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
RE: Surge test?
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.
RE: Surge test?
Wow that's 20X. Seems like a bit much especially with a SStarter.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Surge test?
A little French lesson for you: "Au corsair - corsair et demi"
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
RE: Surge test?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
RE: Surge test?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
RE: Surge test?
After the motor stops spinning I will turn the pot to step nine and see what happens.
RE: Surge test?
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
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.
RE: Surge test?
* I would go green if only I were not yellow *
RE: Surge test?
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.
RE: Surge test?
RE: Surge test?
RE: Surge test?
The uncertainty is because thyristor losses are not purely resistive and not purely constant voltage either, but a mix.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
If the manufacturer is changing device size and giving you the runaround then it's time to look for a new soft-start manufacturer. Honestly, the unit you are working on is still set-up using pots and is likely pretty poor technology and now you are indicating the manufacturer doesn't seem to know what to supply.
A good starter for that motor size would provide a nice text display and provide a full array of motor protection features as well as a number of features to protect against the starter itself failing.
By chance is this a RAM soft-starter? I'm just trying to think of who uses 9 position switches to pick the ramp time. If it is, well, I was taught if I can't say anything good then don't say anything at all.
RE: Surge test?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Surge test?
It is not easy to determine the useful rating of a soft starter from the SCR average current rating only.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Surge test?
One thing the professionals did that I did not want done was couple the motor to the load and start it. They recorded 5200A during start but I did not get a report about run but the motor started and ran. I locked out the motor again after finding out what was done. They did this with my good soft start and it may have sustained some damage or at the least was horribly stressed. It must be a loose connection somewhere in the switchgear or peckerhead. The taped up peckerhead connections show no sign of heating and that is what I would expect with a loose connection. Does anyone have any other sugestions for the cause of high starting current? The load turns freely by hand and I do not suspect it because I think 2800A start current is too high for a uncoupled 460V 700HP induction motor. I think start current should be in the area of 1200-1300A for the uncoupled motor.
RE: Surge test?
* I would go green if only I were not yellow *
RE: Surge test?
I believe you are dealing with a low end soft-starter and in that case it is very possible that the current can reach the levels you have recorded. A number of the low-end starters only provide limited current control capability so you get what you get.
It's also possible that you have set-up the soft-starter incorrectly which can also lead to high current levels. You posted you did not know what the settings of the ramp dial means and that it was set to the minimum. Are there other pots on this starter whose settings you have not verified? Typically, in the best case the "default" settings are not the optimal settings and in the worse case they are completely wrong.
SCR's typically don't degrade. They are usually OK unless you stress them to the point of failure. Then when they fail, the puck type SCR's typically fail shorted.
RE: Surge test?
RE: Surge test?
I've got the OEM engineer answering all of my questions with, "NO!" and angered some in here but I got the nerve to start and run it under load and was successful. I was wary because I replaced the the shorted SCR in the original soft start and its unshorted twin but the same exact same SCR shorted again. I then discovered the cause of the shorts is a compromised thin sheet of plastic that acts as the insulator between between the SCRs and heat sink. I was concerned that an external component (motor) caused the insulator to fail.
As a fast remedy I installed a known good exact same soft start and found the high currents during start with the motor uncoupled and (in my opinion) extremely high currents when coupled. I was making comparrisons with another manufacturer with the same ramp times but the currents are wildly different during start.
Thanks again to everyone for their valuable inputs and giving confidence to someone who is still learning about big motors.
RE: Surge test?
The currents you have measured during starting the motor with the load indicate that the starter is basically going to full-voltage during the acceleration. So, have you verified all the settings of the starter to get the best start? To me, having the ramp time set to the minimum is not the optimal setting for a large motor that has a fair bit of inertia.
Is this a RAM starter? If so, verify the bypass contactor is not energizing before the motor has reached full-speed. That can be the cause of your high current.