Utility and generator operation confussion

[mkees]

I am having a problem with a facility, that uses a generator as a back-up power source. When this facility is running on utility all is normal, when they switch to generator all the motors go to half speed. This is a 480 VAC 3 phase system. We have chacked the voltage when generator is running and it is 480, and I have all three phases, generator maintains speed, and freq. stays at 60hz.
Heres the interesting part, utility phase rotation is CAB, generator rotation is ABC, I would think that when switching to gen. from utility the motors would run backwards, but they don't change direction, they just slow down. Why? What can I do to correct this? I am afraid that if I make the gen. rotation the same as utility then the motors may go backwards, and that would be very bad.

 

[waross]

rotation:
CAB and ABC is the same rotation.
Check your frequency meter.
respectfully

 

[mkees]

I will check the freq., The firld tech assures me it is correct (60HZ), if it is correct does anyone have any other ideas?

 

[wbd]

When you say the field tech checked the frequency, did he look at the meter on the generator or did he actually measure the frequency using a scopemeter?

 

[rbulsara]

Measure the freq, voltage and currents in all three phases at couple of motors while on generator.

Do the same at the generator terminals at the same time. Post the findings.

 

[waross]

Use the same meters to do the measuremenrs on both the utility and the generator.
respectfully

 

[Marmite]

What happens to the phase/neutral volts when the generator is running? (Measured at the transfer switch or downstream, not at the generator) I presume the 480V is ph-ph. If there is a problem with the generator neutral connection and the load is inbalanced this could cause the star point to move electrically from it's central position causing inbalanced ph-n voltages which would affect any downstream single phase loads including motors. You don't mention whether the motors running at half speed are single phase or three phase, or whether they are induction motors. Induction motors generally operate slightly below synchronous speed. If they are single phase motors running at half speed due to reduced voltage they would pull very large currents.
Regards
Marmite

 

[mkees]

I want to the job site today, the motors ar 480 3 phase. again I checked the phase rotation of the utility which was CCW, the the generator which was CW. Voltage at the generator was about 485 and the freq was 60.4. We couldn't transfer, the facility wouldn't let us. I conducted a test with an unused motor, I ran the motor on the utility power, the shaft rotation was CW, then to simulate, generator I switched leg 1 and leg 3 (previously mentioned utility was CBA and generator was ABC) on the motor contactor. I could have predicted the results, the motor ran full speed reverse. Could there be something in the relationship of the phase angles?

 

[mkees]

Motors are 3 phase induction motors, phase to neutral voltage is 284 on all 3 legs, please ask more questions so I can get this fixed. this customer is getting very upset.

 

[Scramode]

You said you measured voltage at the generator. was this with motors at full load? what is the conductor size of the cables exiting the gereator going to the transfer switch ? I am thinking maybe voltage drop.

http://www.SBEI.US

 

[JBinCA]

You need to check your rotation from the same point in the system twice. Once on generator, and once on utility. If rotation at that point changes, then you definitely have a rotation mismatch between the two sources.

If you just check at the utility service entrance and then again at the generator output, you have no way to account for possible transposition of the system cabling.

I would also like to know what indication you have that your motors are running at half speed.

If you are going by things such as flow or pressure meters in mechanical systems or cfm on fans, you may be making the wrong assumption. Many pump impellers will continue to move fluid in the proper direction even when the impeller's direction is reversed. They just move a lot less fluid (50% is in the ball-park), and do so much less efficiently.

If you're going by an indication such as that, you may already be running your motors backwards.

Good Luck.

JB

 

[jghrist]

In your first post you said

utility phase rotation is CAB

but in your later post you said

utility was CBA

Which is it? These are opposite phase rotations The first is the same as ABC as noted by waross.

 

[waross]

This may seem like an offensive question, but I have seen an electrical engineer who specialized in power systems and yet could not get his phasing right with a phase meter.
To check your phasing, connect your phase meter to the output (Load Terminmals) of the transfer switch. Check the utility phasing and then check the generator phasing. The connection busbars on some transfer switches reverse the phase connections. That is, Left to right, ABC is correct for the utility and CBA is correct for the generator.
With a loaded system at 480 volts you should have 277 volts phase to neutral. With an unloaded system, magnetic hysteresis may distort the wave form and give an indicated voltage that is not exactly correct. You are close enough on voltage.
If some idiot has used the dead-rack adjustment to set the speed of the generator it may easily pull down to a lower frequency under load. The motors may be running slow, but not half speed. After 2 Hz. of frequency drop (58 Hz.) your Under Frequency Roll Off will be active and will reduce the voltage proportionally to the frequency.
It is sometimes difficult to determine the speed drop of a generator by sound. Use a frequency meter.
The most ridiculous instance that I saw was a set that would bog trying to start a motor that was about 20% of the set rating. I was getting inquiries for several months. I kept saying
"Governor, check the governor, The governor needs to be set-up by someone who knows what he is doing!"
They tore down the engine and did a complete rebuild. Put it back together with the same governor setting and guess what. Same problem. They flew in a generator expert. He couldn't find anything wrong with the generator. Of course not, it was an engine problem not electrical.
They had been avoiding me because I was too expensive. After spending 10 or 20 times or more what I would charge they bit the bullet and called me. (10 or 20 times! They rebuilt a 4 cylinder Cat engine and missed tens of thousands of dollars of lobster trapping. I spent about 2 hours. I asked fore and was paid in fresh lobster.
I was not familiar with the governor on the old set, so I carefully started to turn an adjustment. I didn't get the correct responce so I returned it to the original position and tried the next adjustment. Bingo.
You need a load bank to set the dead rack properly and I didn't have one. I cranked itt open until it started the motor and then gave it that many more turns. It's been running ever since.
That was not my only dead rack fiasco.
Dead rack symptoms;
Frequency and voltage both drop under a load within the rating of the set. This adjustment is normally made with a load bank. If you don't have one, there are a few tricks to get by with reasonable safety.
respectfully

 

[rcw retired EE]

Negative sequence currents in motor windings create reverse rotating fields in the motor. If the negative sequence gets large enough, the rotor could be thought of as developing two torques, one forward, one reverse. The result is a motor that slows down and gets hot real quick.

Unbalanced voltages create unbalanced currents that in turn "create" negative sequence currents.

Check a motor's amps and volts on utility and on generator for balanced currents.

Most likely this is not the cause, because your motors would have burned up already.

I agree with the comments about getting phase sequence wrong. I've done it and have seen it done wrong many times. Go to one spot, check it on utility, don't disconnect the indicator, and then check it while on generator.

 

[Hoxton]

I agree with waross, check the phasing carefully beteen utility and generator. When I worked closer with IEC / ANSI specifications than I do now, I saw some real mess ups over the simple facts that the spec said what the phase rotation is (A - B -C) etc BUT DO NOT SAY LEFT TO RIGHT (even though that may be logical to a lot of people). Wait until the client has put up 100 metres of bus duct between generator and switcgear, then realises this simple fact!

 

[mkees]

Thanks for the advise, I will check the the phase rotation at the same point. The utility phase rotation is CBA, I will check what I write more closely next time. Waross, saying the obvious things are not offensive, I often need reminded of the obvious.
Unfortunatly, I am the last call when the problems come along and I have to rely on what the Field Techs report, I don't have the opportunity to switch the equipment to and from the bus to check operation. I get the problem when the customer is good and PO'ed and the Field Tech is too proud to admit he messed up. Frequently I end up chasing the wrong problem.
You all have given me some things to think about, especially the governer. This was one of the orginal problems with this generator, and again I have to rely on the Field Tech (because I am not a mechanic) to ensure that it was fixed. The other problem was the generator breaker was bad, and replaced, which was why I went to the phase rotation.

Thanks again

 

[waross]

I see a lot of transfer switches with the following phase arrangement.

Utility Generator
"A" "B" "C" "C" "B" "A"
It is done this way so that the interconnection busbars may be nested.
respectfully