Fuses blowing in the wind... 2

[itsmoked]

I have a 2HP, 3Ph, 460VAC, 60Hz hydraulic pump motor.

On initial start up it was turned ON and run for ~2 minutes.
Turned OFF. Turned back ON within a minute. Turned OFF and turned back ON within 10 seconds. All three fuses blew.

Here's the fuses. (4A version)

http://www.bussmann.com/pdf/1001.pdf

Here's the motor:

[green]I think it was the repeated quick starts funneled thru time delay fuses. Do you agree? Got other ideas? I have to go deal with this Tues.[/green]


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

I would agree with your assessment...repeated starts did not let the fuses cool down and cumulative heat blew the fuses.

This can be proved (or disproved) if you repeat the sequence.

 

[edison123]

I agree.

It's the starting I^2t that did it.

*Even inside a hard-boiled egg, there is a golden heart!!!*

 

[Skogsgurra]

Hi Smoked!

Those fuses SHOULD be able to take a few starts with ten seconds interval. It should even be possible to plug brake with those fuses. I mean, the data sheet says 10 seconds time delay at 20 A (500 % of 4 A). Also, the first starts were a few minutes between. So it should only be the two ten seconds interval starts that taxed them.

How much "within" were the ten seconds? A lot less, perhaps? And even then, two starts is only half a plug braking when it comes to fuse temperature.

Was the motor working against any pressure? If started before pressure had built up, the starting can be considered easy. When starting against pressure the second time, the inrush (6.9 times rated current) may have been on longer than the fuses could take.

It is also interesting that all three fuses blew. That's not so common and usually indicates a lot more current than inrush.

Rodents behind this?

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

Thanks,
bulsara
edison123

Gunnar; I wasn't there but this was all related to me by a pretty analytical guy. He said the last start was; ON for a few seconds then OFF and they 'waited a while' before the last turn ON, Quote:"You know, not 10 seconds but long enough for the motor to probably stop."

I am not sure how fast the hydraulic pressure decays and whether or not remaining pressure would subject the motor to a loaded start or not.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

Re-reading the post..if it were a 4A fuse for a 2.75A FLA and 6.9X LRA, it probably is undersized. At low HP rated motors fuse sizing becomes tricky.

I would rather put a larger fuse (may be just fast acting-single element but 700% rated) and let motor trip on overload (over temp of winding) before the fuses blow. This again run risk of burnig the motor too.

If meeting NEC is an issue, it can be categorized as an engineered system.

 

[itsmoked]

Hmmm That motor doesn't have internal temp sensors so I'd be a little leery of 700%. hmmmm

The plant maintenance guy (different) swore that was a "9A motor" though I couldn't figure out where that assumption came from...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

Here's the hydraulic circuit.

See an obvious residual pressure problem? Keep in mind the rams go up and stay up when the system is energized. The main rams shove these rams down as needed and as the main rams rise these follow. These rams are called the 'cushion'.

Ok. So should I leave the fuses at 4A or go to 5A and caution them about rapid cycling?




Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ssn596]

Keith

Will this pump be in this frequent start stop mode?

What is current draw during start (I know hard to measure with DMM but,,,,) what is its current during steady state or better yet with pump pumping and with pump dumping to reservoir?

I think the trick here is not to worry about how it is supposed to run BUT how it is going to run and after finding that out design protection for the real world use (abuse)

If they want to be replacing motors just continue stopping and starting every few seconds. Motor shops love those guys.

Dan Bentler

 

[itsmoked]

Hi Dan.

Good points. I was only able to check the values for all the other motors in the project. This cushion pump had none of the hoses available so I was unable to do my usual measurements, confirmations, and adjustments. I will head over to this place on Tues and do a follow thru. I'll be checking everything you suggest! Nice list.

And NO! the system is supposed to be turned ON and run for hours with no valve changes and zero motor cycling.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Keith,
One thing that catches my eye on the motor specs is the fact that this looks like a high slip motor, possibly a Design D curve, although that information is missing from your data sheet. But the 690% starting current and 300% LRT point to that pretty well.

That being the case, your starting current curves are going to be worse than Design B motors, i.e. more I²t to deal with in the fuses. Looks like another job for a lockout timer!

 

[itsmoked]

Jeff,
I was starting to think the same thing...

Due to a screwup in a single letter the contactor for the cooling fan 1/2hp and this cushion pump were found to be 24VAC coils instead of the expected 120VAC, so I ran both thru the Smart Relay to isolate the 120VAC switches from the 24VAC coils. This 'error' will be bailing me out twice I think.

The temperature switch provided by the hydraulics people for controlling the 1/2hp fan (~36") has as far as I can tell Zero hysteresis! The fan was going ON and OFF 12 to 13 times a minute while deciding to actually cool. But because I'd run the this fan thru the Smart Relay I was able to just lob in a 5 minute OFF-Delay in so it always stays on at least 5 minutes if it ever gets called on.

Now I'll probably be jacking in something for this cushion pump no doubt. The button topology is all different though. Just a single twist rather than an easily lock-out-able push button set. I haven't figured out a logical way to do this. I don't really like the idea of turning the twist to ON and then maybe not having the pump come on for some period of time. Sounds like a sure way to surprise someone. (In a very bad way)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Look at the edge rising input function in that smart relay. You can use it to force someone to turn off the switch and turn it on again to re-start.

 

[itsmoked]

Ah!
A capital idea!

Thanks Jeff.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

Ok, I'm back from the trenches to report on the fuse blowing and overload tripping.

The fuses blew due to massive overloading and rapid powering up/down of the motor. Once I explained this to them they have turned the pressure down from 2250psi to 200psi and not rapidly cycled the motor and have suffered no more fuse blowing. BUT!! Now they have an overload tripping problem.

When I got there today they were complaining about the overload tripping often and quickly.

Here is the problem. The solid-state overload unit is one of these SquareD units that I have had no problems with before(Note: Download of 2 measly pages takes about a minute):

http://ecatalog.squared.com/pubs/Mo...or Logic SSOLR, Type 9065S/9065HO9601R901.pdf

The motor as previously mentioned is a 2.8A FLA motor.

The SS overload is the OOC model which is a 3-9A unit so I installed it with three wraps of each leg running thru it. This would give 2.8A x 3 = 8.4A
The adjustment dial is very generous, large, and consuming about 300 degrees.

We started the pump and I measured the current.
T1=1.4A
T2=1.3A
T3=1.2A

Just as I was finishing the measurements the overload tripped! About 20s.

We waited a few minutes and started it again putting it immediately under load @ 100psi.
T1=2.2A
T2=2.1A
T3=2.0A
It ran about 60s then tripped out.

In all cases the dial was set to max about 9.2A

<sigh>

So I pulled one wind out of every loop so I now have only two wraps.

2.8A x 2 = 5.6A
With the unit back unloaded:
T1=1.4A
T2=1.3A
T3=1.2A
Ran about 15s then tripped.

I turned the knob stop to stop then set it to 8.5A.
T1=1.4A
T2=1.3A
T3=1.2A
About 15s.

What is going on???

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

The trip is probably caused by the phase imbalance or negative phase sequence element operating. Your current imbalance is numerically small but is a large percentage of the total current. NPS currents kill rotating machines so the better O/L relays measure this. Work out why you have the imbalance - bad connection somewhere? A milliohm meter would be more useful than a multimeter for this measurement. Check the motor windings first and work back over. How well balanced are the source voltages? If your connections are good maybe someone else's aren't!


----------------------------------

Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[itsmoked]

I measured the voltage because the first time the overload went I thought the fuses had. I didn't write down the voltages. argh!

I bet you're right Scotty. That SS overload is supposed to monitor imbalance but NOWHERE in the dang spec do they even hint at what percentage it might be.

I suppose that any imbalance is multiplied by the multiplication wraps exacerbating the whole problem.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Keith,
Imbalance protection in SSOLs typically comes in two forms; Direct tripping, i.e. you have a programmable function by which you set a percentage trip level (this relay doesn't appear to have that); and biasing of the OL thermal model, in which the thermal algorithm for the Overload Trip function is biased by the detected imbalance in order to compensate for the additional heating caused by negative sequence currents. I would hazard a guess that your OL is tripping for that reason, because you have either a 10% or a 16% imbalance in those numbers, either of which is cause for concern in the motor windings.

If your line voltages are balanced (i.e. under 2%), you may have a circuit issue. An old trick learned from submersible pump manufacturers; try "rolling" the motor leads. Just swap A for B, B for C and C for A. It doesn't change the rotation but it can sometimes compensate for slight differences in motor winding and lead resistance. Check the currents after doing that and see if the imbalance changes or follows the roll. If it follows the roll you have a problem in the motor starter or leads, i.e. a bad contact, connection etc. If it changes, and it is good now, leave it alone. If it changes and it's worse, roll one more time and check again. If it changes again but is still that bad, consider getting a new motor.

 

[itsmoked]

So if I roll it and it gets better that might be because the phase voltages are different and you might align, say, the higher resistance leg with the higher voltage reducing the current imbalance. That a correct assessment?

BTW they are going to go up on top of the machine and examine the motor connections.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Yes, that's essentially correct.