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

 

[LionelHutz]

I don't really agree with you Gunner. For a rectifier or DC drive, yes your 1/3 number applies, but not for a soft-starter. The phase current in a soft-starter is divided between 2 devices so each one only sees 1/2 of the phase current. Above that, you can still over current the devices for short periods of time without damage. The capsule or puck type thyristors are amazingly rugged devices and can take a lot of abuse.

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.

 

[Skogsgurra]

Yes, you are right. One conducts during positive half-wave and the other during negative. So, you shouldn't worry at all about overstressing the thyristors.

Gunnar Englund

http://www.gke.org

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

 

[Marke]

The real issue when it comes to SCR ratings on soft starters, is the junction temperature at the end of the start. This is a function of the SCR characteristics and the heatsink characteristics. The average current rating is only an indicator of the SCRs current carrying ability on a given heatsink under continuous load conditions.
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

 

[buckeye3]

Mr. Skogsgurra, in reference to your earlier suggestion that the surge test may not be needed because the high current is only during start and not run was correct. I had the motor "surge test" tested by professionals and it passed. The OEM stated that was something they would do under similar circumstances.

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.

 

[edison123]

FLA of a 700 HP, 460 V motor would be around 850 Amps. 2800 A inrush current during start for such a motor, even uncoupled, is quite normal.


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

 

[LionelHutz]

OK, the starting current depends on the motor, the load, and the operation of the soft-starter.

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.

 

[oftenlost]

My learning is that inrush is not related to load. If this motor were started at 460 volts, had a KVA code of F, it would draw 4000 amps or so on start. If the voltage were limited to half, the current would be half. If the voltage were 500 vac,then the current would be 5% higher. I can see 5000 Amps if the instantaneous voltage goes up.

 

[buckeye3]

Time to end this thread:

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.

 

[LionelHutz]

Oftenlost has it right. The current seen during starting is basically only controlled by the voltage applied.

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.