Testing of a 380V/280KW 60Hz Motor

[BoyNaruto]

Dear Masters,

I am new to the world of motors, but more or less I can stand the principles. May I ask on how to measure and how to test this big motor that we have for our compressor? Suddenly the start up draws around 500-600A which causes our mains to trip. But when I test the compressor, I can see that the rotor is moving. I've also manually turn the shaft and is it freely moving and not stuck up. Do you have some general tips on how to check the motor resistances?

Also, could it be possible that my 400KVA Diesel Generator caused this failure on the compressor motor? Thanks peeps

 

[Zogzog]

Your basic tests will be a 1000VDC megger test for 10 minutes, calculate PI (>1.0) and temp correct your meadings to 40C. THe test results specs depend on the winding type and age but you should have at least 5M at 40C.

Winding resistance values should be meausred and comared to each other to ensure they are within .5%.

 

[jraef]

280kW at 380V would be roughly 425A FLC (Full Load Current). If your motor is only drawing 600A at start-up, you are doing EXTREMELY well. Normal Across-the-Line (DOL) starting current would be roughly 2500A. Make sure this is not just a problem in the settings of your breakers, they must be set to allow the starting current of that motor.

Sounds as though you need the services of a qualified Electrical Engineer to do a "Coordination Study" of your circuit protective devices. I highly recommend that you not try to do this unassisted if you are new to this. The ramifications of getting it wrong include fire and loss of life!


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

The minimum size Gen-Set that I would spec would be 800KVA. Normally I would spec 1000KVA to start that motor. You may have three options.
1> It is sometimes possible to start the gen-set with the motor connected. The Under Frequency Roll Off feature of the Automatic Voltage Regulator combines with the increasing speed of the genset to give Variable Frequency Drive action. You may have to supply the AVR from an Uninteruptible Power Supply. This is only suitable if there are no other loads on the gen-set, or other loads are non critical and may be turned off while the motor is being started.
2> You may have to install a VFD or Soft Start.
3> You may need to install a larger gen-set.
OR, given the starting current you are seeing, the gen set may be slowing down to the point that the UFRO action of the AVR is providing VFD action. That may open another possibility but without a lot more information and ideally on site observations I don't want to pursue possible options here.

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

 

[Compositepro]

You say this happened suddenly. So your compressor and motor were working fine at some point? Have you checked that the unloader on the compressor is working. This seems to be a common issue with questions regarding compressors. Most compressors cannot start with pressure on the discharge. There is an unloading mechanism to remove this load while the motor comes up to full speed.

 

[BoyNaruto]

Hi Bill thanks for the specs, but the problem is how to defend this one, the compressor in combo with the 400KVA has been working for a while (at least for a month). Once can easily say it was really under specs since when the compressor loads the 10Bar tank, then the generator really is being pushed hard to the limit. Do you have any formula on sizing the power requirement versus the starting current? I only have this specs 280KVA from the motor plate.

As of now I am in a dilemma which one is defective, my genset or my compressor. I am sure the unloader is in good shape since I can easily turn the motor shaft while the motor is at rest. Thanks

 

[BoyNaruto]

I just want to add this info, perhaps someone can help me pinpoint where the verdict is. The genset that we are using is just a customized genset. Somebody from the company is clever enough to advice the manager to have a genset made from junks. The V8 Mercedes-Maybach engine came from Swiss Army tanks which was junked from Switzerland, this engine is fitted with a 400KVA Engga Alternator from China. Engine Speed controls and governors are from Governors America and the Engine Controller is from DATAKOM (Turkey).

Ever since I've used this genset, it has f...'d, one of my 8KVA UPS, my new Parker Air Dryer Compressor is already stuck up (experiencing LRA). I've heard also from other branches that they had 3 UPS's which also f...'d. And now my 280KW compressor won't start.

Any idea how can a generator destroy inductive loads? From the past we used this genset only for heat treatment facility, where it's only a resistive load. Ain't got no problem with that.

Does somebody know which area to measure the load side inefficiencies? Which instruments can help me? I only have a 60Mhz oscilloscope, multimeter, clamp meter and a Saturn 100 plus multi-purpose meter (equipped with insulation and phase detection, RCD, Riso).

I just want to have a very good reason to get rid of the verdict in our system, and I am very thankful for your help guys. Thanks.

 

[waross]

As far as curves go, that's a real screw ball.
Is the Gen Set running the whole plant or just the compressor?
How stable is the speed control. Normal control is rated frequency plus 3% or 51.5 Hz or 61.8 Hz unloaded. You should be on rated frequency at full load.
Does your AVR have UFRO? Inductive loads will often overheat at below rated frequency. An AVR used on 60 Hz with the UFRO jumper set for 50 Hz may overheat inductive loads if the frequency drops much below 60 Hz.
If the compressor is taking too long to come up to speed, the unloader may be loading the compressor prematurely.
Questions:
1> Does the generator speed drop when starting the compressor?
2> Is the generator speed stable?
3> What is the approximate power of the prime mover. (The V8 Mercedes-Maybach engine from Swiss Army tanks)
Note; If the generator is overpowered, many of the accepted rules of thumb change.
Motor starting ability depends more on the prime mover power than the alternator rating. If the engine can supply the torque, the alternator may be able to supply enough current to start a large motor. The alternator may well be overloaded but the heating may be within acceptable limits if the motor starts well and quickly.
Tell us about the set; how does it perform, what do you have for an AVR, do you have an accurate indication of the speed or frequency?


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

 

[BoyNaruto]

Hi Bill,

Thanks for the quick response. Below are my answers:
1.) Yes the generator's speed drops upon compressor start up (delta to star) and also when air loading. But in between this transition, the generator is at the right speed and voltage output.
2.) Yes when at nominal load, the generator speed and voltage is stable. Only when the compressor starts then the generator attempts to compensate for the power requirement.
3.)From the nameplate of the engine, I can see a rating of 660PS? As it was written in German Leitung: 660PS, and Drehzahl: 2200 1/min (I think this is speed).

To answer further inquiries the generator is only supplying the compressor and the air dryer unit (in our branch). Yes the AVR is also made by Engga and there is a setting for UFRO. I've already increased this setting so it will ignore the changes in speed upon compressor starting. The setting for the frequency as of now is 60Hz. This is to adapt to our compressor.

I only rely on the freq reading and voltage readout coming from the DATAKOM. Everything is quite normal when running, but during start ups and loading, the engine starts to push harder and the frequency drops to about 55Hz and the voltage down to 330V for at least 5secs before the engine can recover to the right speed and voltage output.

 

[ozmosis]

660PS means 660 HorsePower ("PS" is German for Pferdestärke, meaning 'the power of a horse' or something similar).
So in kW this is about 480~490kW.
"PS" is not used in Euro land anymore, it is always kW.
I'm thinking you might need a bigger generator.

 

[ozmosis]

Or fit an electronic soft start to the compressor.

 

[BoyNaruto]

Hi Ozmosis,
Thanks for the translation, i've tried using an online translator but it gave me different meaning. I was thinking the generator is really small for my compressor. But I need to defend this by electrical formula or simulation so I can convince my boss. Also, the problem, is that it was working before and suddenly i got high starting current. Same as what happened to the air dryer compressor. Initially the starting current is only up to 16A and now up to 95A (LRA). I suspect that the generator somehow made some damages on my motors, but as to which damage? Can it be on the windings? or mechanical? I don't have an initial insulation readings so even if i make some insulation readings as of now, without a reference then it would be hard to make justification.

Thanks

 

[edison123]

Common winding tests for LV motors -

IR value measurement (phase to ground and phase to phase, if 3 phases can be separated)

Surge comparison test to check for turn shorts

DC resistance measurement tro check for loose terminations

Muthu

http://www.edison.co.in

 

[waross]

You can order Cat sizing software here;

http://www.cat.com/cda/components/fullArticle?m=37508&x=7&id=1174702

OR here;

http://www.cat.com/cda/layout?f=111183&m=329875&x=7&mode

Here are the instructions for obtaining Cummins/Onan sizing software.
Re AVR settings. This should be set at the operating frequency. This will intentionally drop the voltage if the frequency drops.
Have you checked the engine for problems?
Re the engine power,- I would take the estimate of 490 kW and adjust that for speed. 1800 RPM/2200 RPM x 490 kW = 400 kw. the engine output has dropped for any mechanical reason such as a faulty injector, or a dirty fuel or air filter, or even a lighter weight fuel it may be causing starting issues with your large motor.
As far as installing a VFD, I would try connecting the motor in delta and directly to the generator. Feed the AVR with an independent source and start the generator. As the frequency ramps up, the UFRO feature of the AVR will ramp up the voltage also. That will maintain the Volts per Hertz ratio and give the same action as a VFD but without the motor damage issues that Pulse Width Modulation may cause.

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

 

[rbulsara]

Nothing in the posts suggests there is anything wrong with the motor.

As Compositepro suggested look at the mechanical side, you are now starting against the back pressure some how. It is somehow required by process, the gen is undersized. Compressors are generally started unloaded.

As jraef suggested 5 to 6 times starting current for induction motors is not abnormal, depending upon the starting methods and load inertia/acceleration period.



Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Hi Rafiq;
As I understand the situation, the generator has been able to start the motor and now trips the main breaker when starting. If the prime mover power has dropped for any reason, the start cycle may be extended leading to an inverse time trip on the breaker. I have seen similar problems on more than one gen-set in the past.

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

 

[rbulsara]

Bill:
Yes, anything is possible and we have seen all kind of problems. We can only state opinion based on what we read. There is no substitute for an on site knowledgeable person.



Rafiq Bulsara

http://www.srengineersct.com