Rotating Vibro-Screen Trouble 2

[PolyMike]

The trouble is on a Pennwalt rotating Vibro-screen with direct on-line 1,5Hp 380V 2,3Amps 50Hz 720rpm induction motor (no frequency converter).

http://files.engineering.com/getfil...a-5f4252355198&file=R-00814_209703____1.1.pdf

The screen was working properly, but after a preventive bearings change, the motor couldn't reach the rated speed!!!.
It seems working at slow motion; between 60rpm - 100rpm instead 720rpm!!! Obviously, the motor is overloaded (5-6 amps) but balanced in all phases as well as the coil resistance (15ohm). The earth isolation is 12Mohm (at 500V). We have measured the line voltage running and stands in 380Volts. The rotor can be moved easily by hand.

We have re-opened the motor in order to inspect bearings or a possible displacement of the rotor magnetic core... but motor and bearings seems to be perfect.

We have tried to change the angle between masses, but only after removing additional plates of the upper weight or completely removing upper and lower weights the motor reach the rated speed 720rpm at 1,2Amps.

Anybody can help me?

Thank you in advance,

Mike.

 

[jraef]

Check your motor connections.

More than likely you (or someone) reconnected the motor leads incorrectly. You say it is a 380V motor, the manual says it is a dual voltage 220/380V motor. You are supposed to connect it in Y (Star) for 380V supply, or Delta if the supply is 220V. What is your supply voltage?

If it is 220V, and you connected the motor in Y, it would only produce 1/3 the torque it is rated for. So with any kind of load, it will slow down. When you remove the load, it will still eventually spin at the full rated speed however because the frequency is correct and you no longer need all of the torque.

If your supply is 380V and you connected the motor in Delta, then the problem becomes that the motor is over excited (you have the equivalent circuit effect of applying 660V to it). The net effect is that the motor will also lose it's ability to create torque under load.



"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[waross]

Is there any magnetic dust in the area. A little grinding dust inside the motor will give those symptoms.

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

 

[roydm]

I don't know if it's applicable to your screen but when i worked in mining putting too much oil in the gearbox would give those symptoms.
Roy

 

[PolyMike]

First of all, thank you for your appreciable help.

Jraef,
there is no mismatch possible in the motor connection, because the star is internal and only three terminals are available for 380V connection. 380 Volts remains stable, even with the motor running.

Waross,
when opened the motor for inspection we couldn't see any kind of dust.

Roydm,
the motor is not connected to a gearbox and the grease quantity of the bearings have ben checked.

We keep trying changing sense of rotation with no results.

Thank you again.

 

[Skogsgurra]

These screens run in resonance mode. If you have added or removed weight or if you have changed/moved springs in any way, then your resonance speed may have moved away from the motor speed. The motor then has to supply "all the power all the time" and never gets into resonance mode where power is needed only to cover the losses in the system.

Removing things makes resonance frequency increase so that your motor always works below it. Check if there are any changes made. Replaced anything during that overhaul?

Gunnar Englund

http://www.gke.org

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

 

[jraef]

...there is no mismatch possible in the motor connection, because the star is internal and only three terminals are available for 380V connection. 380 Volts remains stable, even with the motor running.

Huh...
Well then you must have a different motor than what it came with as standard, because the instruction manual you posted is very clear on pages 1 and 2 (7 and 8 of the pdf file) about there being 6 motor connections through the motor cable. So for your installation, their standard motor was supposed to have the A1, B1 and C1 leads tied together, then A2, B2 and C2 connected to the motor starter terminals. This will connect the motor in Star. If someone tied A1 and A2 together, B1-B2 and C1-C2 so that you end up with only 3 leads, then the motor is in Delta. If you have something different, then there was no point in supplying that manual and you are on your own for motor connections. But from the description of your symptoms, I strongly believe that is where your problem lies because what you are describing is exactly what I would expect to happen if it was misconnected.


"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

 

[burnt2x]

Nice sleuthing job there, jraef!

 

[PolyMike]

The upper bearing cap pressed outer ring. It was necesary to turn on the lathe and mechanize 0,2 mm.

As said before, the rotor could be moved by hand, and I supposed the very little resistance was because of the grease retainers...

Again, the right was the most simple explanation; but even now I can hardly believe the motor ran at 100rpm instead of came to a standstill.

I'm very sorry for the inconvenience!

THANK YOU ALL FOR YOUR HELP

 

[Skogsgurra]

We all had a refreshing mental excersise there!

Gunnar Englund

http://www.gke.org

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