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Mixer Motor.

Mixer Motor.

Mixer Motor.

(OP)
Hi,
New to the forum and was seeking some help with a problem I have. We have received a Mixer Motor back from a client and on inspection the motor seems o.k. We have identified the 3 windings, checked resistance and all seems well. When you run the motor out of the water it runs o.k then as soon as it is installed in the water it ramps up and down from between 5 and 40 Amps. The motor is a 440v 50 Hz and is connected in Delta the FLC is 12 Amps. We tried all different configurations of the 6 cables but nothing seems to work. Any help would be greatly appreciated.

RE: Mixer Motor.

Sounds like a load problem to me.  bent shaft/bad bearing etc. Check the mixer gear box by rotating the shaft by hand and you might pick it up.

Can you correlate the amperage swing to the output shaft position?

 

RE: Mixer Motor.

(OP)
Thank you for the reply,

Will check,there is no gearbox all it has is a propellor, just seems strange how it works ok out of the water and even just skimming on top of the water suddenly jumps to 30 to 40 amps then flucuates.   

RE: Mixer Motor.

It sounds like the propeller is to large for the speed and horsepower of the motor. It overloads in water and vortexing or cavitation of the propeller bring the load down until the prop sees water again. You need a smaller prop or a gearbox on the motor.

RE: Mixer Motor.

I agree with the cavitation suggestion.
Is this a manufactured mixer or  has someone taken a mixer meant to be turned by a 1/2" drill motor and mounted it on a 10 HP motor?

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

RE: Mixer Motor.

(OP)
sorry guys for taking so long to get back to you, we deal with hundreds of these mixers and the prop size is not an issue. We ran the mixer in water without the prop and it runs perfect so thought that it may be a mechanical fault as pointed out, so we stripped the mixer and there is no evidence of mechanical failure ie: sound bearings and no rubbing marks on stator or rotor. We have never came across this problem before so would like to get to the bottom of it. All your assistance and suggestions are much appreciated, thank you.

P.S
Winders assure us the motor is wound o.k.  

RE: Mixer Motor.

One way to divide and conquer would be to put an alternate load on the motor and see if you get similar oscillations.  If yes, the problem in the motor, if no, the problem lies in the connected equipment or composite system.

Also if possible compare sudden application of load to gradual application of load.

One longshot that comes to mind is torsional resonance of mechanical origin or electromechanical origin.  What is the speed of the motor and what is the speed of the oscillations?  How big is the prop?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Mixer Motor.

This may be end play related drag in a bad bearing. If so, dissipating that much energy in a bearing, it will soon fail completely and the cause will become apparent.

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

RE: Mixer Motor.

Here's my theory;

You have connected the motor incorrectly. You said it "is a 440v 50 Hz and is connected in Delta the FLC is 12 Amps".

That is highly unlikely, a 50Hz motor is almost always 380-415V, 440 is usually a 60Hz motor. If it is a 440V 60Hz motor and you connected it incorrectly, you not only have 57% of rated torque, but the speed is slower as well. Uncoupled, it will spin at the speed of the frequency and poles. But as soon as you put a load on it, it is going to go into high slip and stalling. In a centrifugal propeller pump, load is speed dependent. So as you begin to stall it, it slows down, which sheds load, which allows it to speed up, which couples load, which puts it back into high slip and near stall, repeat ad nauseum.

Describe all of the motor connections and the EXACT nameplate data for additional help.


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RE: Mixer Motor.

I agree with that jraef.

Keep in mind that the wires could be mislabeled too.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Mixer Motor.

That is an interesting scenario I hadn't thought of.  Definitely doens't hurt to check.

If you'all will indulge an attempt at analysis – that scenario doesn't make sense if we anaylse it using the steady state torque speed curves:

If we plot the motor and load torque speed curve vs speed
There are two types of intersections of the torque speed curves:
1 - a stable intersection occurs whenever load curve slope exceeds motor curve slope.  Once you get to this point, you tend to remain there.

2 - an unstable intersection occurs whenever motor curve slope exceeds load curve slope.  The machine tends to accelerate away from this type of intersection in either direciton.

If you draw the two curves in almost any way imaginable, the intersections must alternate: stable, unstable, stable, unstable.    To see-saw back and forth would require two unstable intersections next to each other – difficult to imagine.

But as we have discussed before, the steady state curves don't always tell the whole story.    

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Mixer Motor.

How about this:
The prop is spinning too fast. It may be the wrong prop for the motor, or the motor may be a 3000 RPM base motor and the prop may be designed for 1500 RPM base speed. (Or 1500 RPM and 1200 RPM).
Such a mismatch may alternate between overload and cavitation.
This is not to dismiss jraef's theory which may also be investigated.  

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

RE: Mixer Motor.

Looks to me an overspeed issue. Post the full nameplate details of the motor and your shop test results including the speed measured on no-load (i.e. outside water)

Muthu
www.edison.co.in

RE: Mixer Motor.

Hi Pete;
I believe that the two intersections you speak of may exist but both are mechanically or hydraulically unstable.
The high current intersection may cause cavitation which will reduce the load. The result will be low  current. Cavitation, once started may continue, however, should conditions be such that cavitation ceases, and the prop "Grabs water", the current will again go high. Does this reconcile your suggestion with some of the other suggestions made here?
Yours
Bill
 

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

RE: Mixer Motor.

If the machine cavitates at high speed, then it gets unloaded and tends to speed up further.  At first glance that doesn't seem to feed an instability involving feedback due to motor torque-speed which was the type I was discussing and I believe jraef was discussing.  But for sure, the fluid machine/system may oscillate in other complex ways.

Caledonia - You did say the problem occured both at the customer's site and at your shop?  In that case I guess it rules out power supply problems.

What about a rotor bar problem?  That causes current oscillation.  I haven't heard of the oscillation being as severe as you described, but who knows. Try a single phase test.

=====================================
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RE: Mixer Motor.

I mentioned earlier trying a different mixer with the motor (to divide and conquer).  An alternate version of that is to try another motor with the same mixer/bucket (depending on what you have available).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Mixer Motor.

I imagine the mixer is designed to mix something other than water, right? Something highly viscous?   And the load comes mostly from viscous friction? If so I tend to agree with the others, results when putting in a completely different liquid (low viscosity water) might have unpredictable results.   

But it leads to another question - did this surging occur at the customer's plant and the shop, or just at the shop?  Was the customer running it with water?

=====================================
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RE: Mixer Motor.

(OP)
Hi Guys, Thanks again for showing an interest in our problem, I am currently away from the workshop for a few days but will try to get all relevant information for tomorrow.Surging does occur at both the shop and clients premises which is a waste water treatment plant. The motor was recently refurbished ie new bearings etc.
Regards
And thanks again for all the replies.

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