O.T DC Motor/Drive experts?

[Clay B.]

Be careful with extruders. The motors are sized for those cases that you mentioned. Otherwise, they usually run well under the rated load of the motor. This caused a lot of problems with brushwear where I was an engineer for several years. The brushes being used were rated for a current density that matched the nameplate of the motor. Due to running less than 50% of rated load, this caused the brushwear to greatly accelerate adding a lot of expense in brush replacement not to mention the carbon dust storms created during PM's. A DC motor can handle a hefty overload on top of its continuous rating. So I hope the 150 hp doesn't leave you in a "bind" should the plastic harden in the barrel.

Also, I guess the field was not a dual voltage field which would have helped the current capacity dilemma of the field regulator.

We had the same results here. We had an old Impco (think thats how you spell it) and the original motor went south. We had some smaller series wound motors for another aplication that were lower HP. What we ran into was excessive brush wear. I slowed down the accel curve to help reduce this some. We finally matched the motor to another brand with the same the orginal HP and this corrected the brush problem. Then we got rid of the old clunker.

I am surprised though. I would never think of starting an extruder with cold plastic in it. Replacing a screw is a nightmare. On several of our machines we would have to pull the entire barrel down to get enough room to get the screw out. You can not even purge on our machines unless temp is up.

 

[DickDV]

Excessive brush wear on DC motors is, curiously enough, more likely under light load than heavy load. On that basis, a less-than-fully loaded 200hp would be more likely to have brush/comm problems than a fully loaded 150hp.

The issue of straight shunt or compound wound fields should not be an issue as far as brush wear is concerned since, at nameplate hp, both types of motors would develop exactly the same torque. The only difference would be when overloaded. The compound field motor would increase torque faster as it gets pulled into overload than a similar straight shunt motor would.

 

[rsdoran]

My experiences coincide with what Clay and Dick are stating

All the extruders I have dealt with had lockouts that would not allow them the screw to rotate until temp was at a certain point for a specified time.

 

[swhite65]

We did a designed experiment on brushwear where the following variables were manipulated: brushtype (brand and/or different versions of the same brand), current density, brush spring pressure, and humidity. Humidity plays a large role in the quality of the film that the brush "rides" on. The results were very interesting as brush spring pressure had the most impact by far. Brushwear can be broken into frictional wear and electrical wear. The less pressure, the more arcing and sparking and the electrical wear increases exponentially as pressure decreases. With more pressure, the electrical contact is better but the frictional component increases fairly linearly with increasing pressure. When you look at it graphically, you want to be in the bottom of the "bathtub" for optimal wear. This usually is around 3 to 5 lbs of force. Many setups like the GE or Cutler Hammer springs are a coil that signigicantly weaken with age. They also are in the conduction path of the armature current which can affect the metal. Rubber hard top brushes are good to isolate the springs from current flow.


If you work in a plant with a lot of DC motors it is worth taking a look at your brush replacement budget and how often brushes are having to be replaced. In some of our cases, the life was extended by 50 times or more just by replacing the $3 springs and applying the proper brush for the application.

The smaller DC motors were eventually replaced with AC motors.

 

[allscott]

This is the first plant I have worked at that uses an extrusion process so I am not an expert by any means. I don't know if what we do here is normal or not normal as far as extrusion machines.

The product we are extruding is either PVC plastic or Cross Link polyethelene. None or our extruders are interlocked with the barrell temperature, maybe they should be?

I think the biggest problem is with the X-link. Under normal operations the Xlink action occurs after the product is extruded making it extremely hard. This product is never left to sit in the extruder but is purged out after a run by a cleaning compound.

Removing the screws here is actually quite simple. The production end of the screw does not have a bearing on it. The back of the barrell is removed and the screw is hammered out of it's bearing and the whole screw slides right out. This is done regularly during a changeover in what we call a cleanout.

There are two occasions when the cross link becomes a problem. The worst is if the power fails to the extruder and is not restored in a reasonable amount of time. In that circumstance the crosslink poly sits in the barrell and cools off very slowly. This makes it as hard as a rock and the screw usually needs to be porta - powered out of the barrell.

The second occasion is not as bad. The extruder needs to stop for some reason but power is available to run the cooling fans that cool down the barrell rapidly. In this case the crosslink action does not happen as much and the product doesn't get as hard.

The second occasion is where we are at right now, the motor failed but the operators turned on the cooling fans ASAP so the polyethelene shouldn't be as hard. They want to try and screw it out as soon as I get the motor ready.

My emergency shipment didn't go well, my motor is on a trailer in Winnipeg Canada (it's cold there right now). The trailor it is sitting on has failed brakes so they can't move the trailer to unload my motor to ship it to me. The home for this motor is on the third floor of a building. The freight elevator that goes up to this floor is currently broken so when this thing finally shows up we haven't figured out how we are going to get it up there!!

How come when things go wrong they really go wrong???

I will post with what happens when we are done. Stay tuned.

 

[allscott]

Just thought I'd follow up and let everyone know that the conversion went find. I adjusted the relevent paramaters, performed an autotune and it started up great. I found that the previouis motor had been current limited in the drive to 100% so I bumped that up to 125% on the new motor to give some additional grunt if we need it but so far it looks like we won't

Thanks for the expert advice.

 

[Leadfoot]

Glad to hear things came together.