Power Factor Correction Caps

[ndzied1]

At the end of the day today I walked over to a panel that a co-worker was commissioning and noticed a buzzing in the panel. I traced it to the 40 HP motor starter which is used for a hydraulic pump.

The customer spec called for PF correction caps which I had never used before so we got a recommendaiton from http://www.myronzucker.com/. The caps are wired after the starter.

The motor had been started earlier in the day to check rotation but now it was not turning. There were no blown fuses although the overload blew once.

Can improprely sized capacitors (too small) cause this?

Thanks,

 

[leitmotif]

At the end of the day today I walked over to a panel that a co-worker was commissioning and noticed a buzzing in the panel. I traced it to the 40 HP motor starter which is used for a hydraulic pump.
REPLY Sounds like single phasing to me. BUT do any of the relays buzz normally??


The customer spec called for PF correction caps which I had never used before so we got a recommendaiton from http://www.myronzucker.com/. The caps are wired after the starter.
REPLY OK in parallel with the motor.

The motor had been started earlier in the day to check rotation but now it was not turning. There were no blown fuses although the overload blew once.
REPLY Check rotation implies to me start for one sec or so to get it turning then shut it down - Especially if you gotta reverse rotation.

Can improprely sized capacitors (too small) cause this?

QUESTIONS
1. When they did phase rotation did they leave it running? Does anyone know how long it ran?
2. What were running amps?
3. What were starting amps?
4. You said the overload blew -- was it still blown when you found the buzzing?
5. How are the overloads sized and are they mags or thermals or a CT relay setup?
6. How many overloads are used?
7. Did you run motor after reset overloads and what happened??
8. Are there (or were) there any mechanical problems that overloaded the motor??
9. This is across the line starting and not VFD (sorry gotta ask)
10. Are you able to measure PF when the motor is running - are you getting what you want?
11. IF an overload tripped why did (IF that is the case) the main contactor remain shut thus single phasing the motor??

My guess is that when starting the caps and the motor windings both draw current. IF the caps are undersized then they would draw less than if they were the "proper" size.

My first thoughts are
1. disconnect the caps
2. Run the motor check run amps and make sure there are no mechanical problems - run at full load and make sure the overloads do not trip.
3. Double check the control wiring to ensure the main relay opens to keep the motor from single phasing when overload(s) trips.
4. I guess because I learned it in the Navy I still like an overload for each of 3 phases.
5. If all is OK reconnect the caps and try same sequence.

Dan Bentler

 

[rsdoran]

Simple answer is YES, you are the engineer so I know you will figure it out.

 

[ndzied1]

Simple answer is YES, you are the engineer so I know you will figure it out.

Yes but I are a makanikal Injunear by edumikashun ... I just play an electrical engineer on TV ...

But you are right, I will figure it out. I have found out that bumping the motor repeatidly without allowing the caps time to discharge will cause huge amounts of current on the lines. But I would think I would have got a blown fuse if this were the case and that hasn't happened. ...maybe I have a half blown fuse...

 

[ndzied1]

1. When they did phase rotation did they leave it running? Does anyone know how long it ran?
2. What were running amps?
3. What were starting amps?

As I said it was the end of the day, Most of the tools were put away, The people who did the inital rotation check were gone. All good to know and I will document today.

4. You said the overload blew -- was it still blown when you found the buzzing?

No, I found that strange. It actually blew after I found the buzzing and noticed the motor wasn't turning.

5. How are the overloads sized and are they mags or thermals or a CT relay setup?
6. How many overloads are used?

Overloads are "W" labeled heaters (one per phase) Sized per the AB manuals. Myron Zucker said I wouldn't have to change them but I'm not sure I believe this information from what I've read last night from various searchings.

7. Did you run motor after reset overloads and what happened??

It would not turn after that or not more than jumping about 10 degrees at a time every second... And it got warm.

8. Are there (or were) there any mechanical problems that overloaded the motor??

Not obvious ones. The motor is close coupled to a pump which was supposedly tested by the power unit manufacturer. But I'm sure they didn't use pf caps.

9. This is across the line starting and not VFD (sorry gotta ask)

Yup, good point.

10. Are you able to measure PF when the motor is running - are you getting what you want?

I don't really know how to measure PF. I'll look into that.

11. IF an overload tripped why did (IF that is the case) the main contactor remain shut thus single phasing the motor??

When the o-load tripped, the contactor did NOT pull in. That worked correctly.

My guess is that when starting the caps and the motor windings both draw current. IF the caps are undersized then they would draw less than if they were the "proper" size.

My first thoughts are
1. disconnect the caps
2. Run the motor check run amps and make sure there are no mechanical problems - run at full load and make sure the overloads do not trip.
3. Double check the control wiring to ensure the main relay opens to keep the motor from single phasing when overload(s) trips.
4. I guess because I learned it in the Navy I still like an overload for each of 3 phases.
5. If all is OK reconnect the caps and try same sequence.

Dan Bentler

Dan, Thanks for your great questions and suggestions. I'll be away from e-mail most of the day but our first plan of attack was to remove the caps so we're on the same track.

I'll post again tonight.

 

[leitmotif]

Norm

Did you run motor after reset overloads and what happened??
It would not turn after that or not more than jumping about 10 degrees at a time every second... And it got warm.

REPLY Uh oh sounds mechanical. Somebody is bound maybe motor or maybe pump. BEFORE you split coupling to check does alignment look good?

Dan

 

[TRACY H]

Caps

Norm a capacitor is not going to cause your motor not to run. It`s either single phasing or mechanically bound. All a capacitor does is furnish the magnetizing current so the line doesnt have to.

 

[OkiePC]

The symptom description sounds like single phasing, and it's easy to check. My first check would be to disconnect the motor power feeding into the motor starter, disconnect the caps, pull in the coil (electrically if possible), and ohm the motor leads. You should have the same ohms between each of the three motor legs. Check ahead of the starter to eliminate it as a possible cause, especially if it's new from A/B...sorry, I love A/B PLCs, but have had lots of trouble with all their other stuff.

 

[randylud]

Really?

Norm a capacitor is not going to cause your motor not to run. It`s either single phasing or mechanically bound. All a capacitor does is furnish the magnetizing current so the line doesnt have to.

I would like to know just how this happens! Wonder if DickDV can explain this gem of knowledge?

 

[Lancie1]

Norm,

1. The capacitors could be so large that a large current was created at the start attempt. Alternatively, the capacitors are properly sized, but the system power factor is very low, and a feed-back current developed between the capacitor and the other motors in the plant.
2. This large current caused welding of two contacts in the motor starter.
3. Afterward the startup to check rotation, the motor was "buzzing" because one coil group (instead of three) was energized and a vibrator was created instead of a motor. Overload eventually got hot enough to trip, even though current was only flowing in two legs.

 

[tom_stalcup]

Randy,

I can't explain it very well myself... But I did find this page with a pretty decent explanation(with vector diagrams)...

 

[Terry Woods]

If you hear a Motor Mag buzzing... it is usually caused by a failure of the shading device of the shaded pole coil in the mag.

A DC Coil will keep a coil closed as long as enough current is applied.

An AC Coil goes through that Max-Positive-Volt to Zero-Volt to Max-Negative-Volt then back to Zero-Volt routine. It's called a Sine-Wave.

The "shading-device" on the coil of an AC mag causes the mag to maintain closure during the zero-crossing portion of the Sine-Wave.

That device is soldered into place. If it breaks it's connection then the "hold thru zero" characteristic is lost. This results in 60-cycle chatter or hum.

If this occurs, then the contacts are liable to open during zero-crossing.

It is common knowledge, among those that know, that opening and closing a contact produces spurious currents.

In the case of a motor contactor, this results in high currents applied to the motor as often as the contact is opened and closed.

In this case, the weakest phase of the motor (resistance-wise and CMF-wise)will allow the highest currents and that phase will cause the particular overload to trip.

It's my guess that the failure is the mag coil.

Change the coil and let us know how it goes.

 

[stasis]

the one solid piece of advice I haven't heard mentioned...use your meter! Grab a Fluke & ohm the motor leads. Three phase motors will show an ohm value between the phases if the motor is good. Do this before applying power (kind of a 'no-brainer', but...). All three values (T1-T2,T1-T3,T2-T3) should be relatively close, +/- 1.3 is normal on used/rewound motors.

Then check for continuity between the line-side terminals on the overload block & the load side terminals, there should be continuity.Make sure you check L1 to T1, L2 to T2, L3 to T3. I know I shouldn't have to write that, but you never know. Then check the line/load of the contactor in the same fashion...no continuity.

Then apply power & VERIFY you have 3 phase on the line terminals.

If these check good, it's not electrical, but mechanical.

 

[randylud]

Terry, thank goodness! The voice of reason and sanity!

 

[DickDV]

randylud, regarding the statement "the power factor caps supply the magnetizing current for the motor so the line doesn't have to", I'm not sure that's a 100% accurate statement.

It is true that the mag current in a motor is all reactive (it does no useful work) so it is the primary contributor to lagging p.f. in a motor. In that sense, if the caps correct the p.f. to 100%, then I suppose the above statement is correct.

I've just never heard it said quite like that and am a bit uncomfortable with it.