Over Speed Wound Rotor Motor

[08summit]

We have a 6 pulley conveyor system, with 2 wound rotor motors on the primary drive pulley and 1 wound rotor motor on the secondary drive pulley. The problem is the primary drive pulley sometimes gets build up increasing the diameter larger than the secondard drive pulley, therefor the secondary drive pulley is being pulled faster than the motor can drive it. The motor is calulated to be rotating 1280rpm. The motor full load speed is 1192rpm.
What happen in the rotor circuit when this happens?

 

[edison123]

What sort of material build-up in the primary pulley ?

How are the belts able to accommodate such a build-up given their length is constant ?

How did check the rotor speed of the secondary pulley motor ?

If the rotor speed is indeed above the synch speed, then the secondary motor will work as induction generator.

 

[jraef]

Are you sure this is even a problem? In some conveyor systems, especially long overland conveyors, this is a somewhat standard practice because the over-hauling motor acting as a generator will also provide some braking as a way to prevent runaway of the loaded belt going down hill.


"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

 

[waross]

Accepting that your information is correct and the belt is NOT overhauling;
The over speed motor will act as an induction generator as edison123 points out. The power to turn it will have to come from the other two motors. If the motor is regenerating 50 kW then the other motors between them must supply an additional 50 kW plus losses.
But your figures are suspect.
1192 RPM is only 8 RPM of slip,or 0.67%
1280 RPM is 6.7% over speed.
0.67% slip is so low as to be suspect.
An over speed of 10 times rated slip is so high as to be suspect.
1120 RPM instead of 1192 RPM would be more believable for a rated full load speed.
For a motor used as an induction generator we expect the % over speed to be roughly equal to the % slip at full load.
If the over speed is 10 times the rated slip we would expect a rapid burnout and a severe overload on the other drive motors.
If the 1192 RPM is the actual speed rather than the rated speed, just turn the third motor off as soon as the system is up to speed.
Please verify your information as to rated RPM, and actual RPM. Is this an overhauling conveyor application?

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

 

[08summit]

Hi Guys Great Stuff!!!
The ore we move is call Tar Sand, it is a sticky/sandy/bitumin compound thats sticks to the lagging on the pulleys. Due to lack of "mechcanical maintenance", pulley build up 2-3" or 4-6" diameter. All the pulleys are 54" only primary drive pulley tends to build up. We have a speed encoder on a non driven pulley next to the primary pulley that indicates 104% speed and a speed encoder on the secondary motor shaft this speed indicates 102% speed. The speed encoder are used for Belt slip Protection. The motors are 165flc 1250hp 4160v. Unloaded the conveyor runs at 60amps on each motor. As the conveyor becomes loaded the 2 Primary motor draw 140-165amps and the secondary pulls 80-100amps. If the pulleys are all clean the currents are balancedand ths speeds at 98% each. We had a rotor blow apart and I surspect the motor was generating or braking, But I have to prove this?

Bill, I like you way thinking! the excess current on the primary motors is because of the braking action of the secondary motor.
The same motor action of rotating magnet field in the stator trying to pull the rotor at Syn. Speed is the same rotating magnet field trying to brake the secondary motor to syn speed???????

thx
jim

 

[waross]

Without looking at the setup and verifying the calibration on the speed encoders it is impossible to calculate the actual speeds from this end.
But this information makes your first information acceptable.

Bill, I like you way thinking! the excess current on the primary motors is because of the braking action of the secondary motor.
The same motor action of rotating magnet field in the stator trying to pull the rotor at Syn. Speed is the same rotating magnet field trying to brake the secondary motor to syn speed???????
EXACTLY
To prove it, connect a Watt Meter to the over driven motor and watch the kilo Watts being pushed back into the system.
Many of the wind turbines down south of you may be using over driven induction motors as generators.
I understand the lack of maintenance issues. No manpower, even at six figures.
I may be up in your area around the end of the month. I hate Hwy 63 with a passion but Hwy 881 is bearable, just.
Can you tell us what camp your in?

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

 

[08summit]

Hey Bill
I have no reason not to trust the encoders, they are 1024ppr to a high speed counter PLC card.
I'll check the name plate data for Full Load speed tomorrow
The Wound Rotor has a slip resistor 27mohms, which should lower the motor speed even more.

No Camp the land of big trucks and Aurora Borealis has been home for 30years

 

[waross]

I was referring to the calibration. What speed corresponds to 100%, are both set identically. It doesn't matter for slip detection.
By way of example, an induction motor rated at 1760 RPM is about 2.2% below synchronous speed at full load. At about 2.2% above synchronous speed it will be regenerating close to full rated current.
Ah the memories. I was up there when Super Test hill was still a gravel road. I was on the crew that did the test and commissioning and start-upon the big B&E drag-line on display in front of the Syncrude Plant. I'll buy you a coffee the next time I'm up that way. Possibly late February or early March.


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

 

[ScottyUK]

Is there a mechanical solution to the build-up - something akin to a form tool matching the pulley groove - which could reduce it to a minimal level?


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

If we learn from our mistakes I'm getting a great education!

 

[waross]

HiScotty;
The material buildup will not be on the vee belt pulleys.
These conveyor belts are from 4 Ft. to 6 ft. wide. The conveyor belt is driven by a 54" diameter flat pulley, if I understand correctly. That is the pulley that will be subject to the material build-up.
You are correct that a mechanical scraper may work well. The problem is manpower. Some folk love the rough and rugged life style. After 30 years, 08summit must be one of the folk who love the lifestyle. Most people don't.
About 10 months ago I was casually offered a steady job in one of the tar sand plants. I declined with thanks. The supervisor mentioned that the overtime was pushing the annual gross up to about $200,000. It was a great offer but it just didn't fit my life style. Best paying job I ever didn't take.
But the point is, for most folk, the climate and the lifestyle are brutal. Some of the plants there have their own airports and staggered days off. Every day the planes fly the workers home on days off. Remember that these flights are crossing the world's second largest country. Still there is a chronic shortage of workers.
Probably the guy who fixed the scraper quit a couple of years ago and hasn't been replaced yet.



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