O.T DC Motor/Drive experts?

I know this has nothing to do with PLC's and I hope Phil doesn't mind but I haven't come across a better resource than here for a variety of topics that aren't PLC related.

We are in a bit of a panic to replace a 200HP reliance DC motor that has failed catastrophically on us (commutator destroyed etc...)

I have been dealing with my trusty repair shop that I have been dealing with for years trying to get a replacement motor ASAP!!!!.

Reliance seems to have let them down. The first motor they offered to sell me was designed to be coupled rather than belt driven (which is what I need), I know enough about bearings to know that that probably wasn't a good idea.

We have determined that the original motor was way oversized and that a 150HP motor would do the job. The next motor they offered was a straight shunt 150 HP motor as opposed to the stab shunt motor that we originally had.

The motor is connected to a Siemens Simoreg drive, unfortunately I don't have the model number with me.

I know that I need to make sure that the field regulator can handle the extra current of the straight shunt motor which is about 8A as opposed to 4.2A, but other than that, should this work OK?

The application is a plastic barrell extruder where reasonably accurate speed control is necessary but the load is fairly constant once the process is up to speed.

It is now Wednesday night and I have been working with the supplier on this since Monday morning and I, as well as my boss are getting frustrated. I am not tied to Reliance, if anyone has a good source for large DC motors I would love here from them.

As always, thanks in advance

edit:
More details,
Motor is 500V DC armature, 300V DC field, 1750 RPM base speed

The drive is capable of but the process never requires operation above base speed, 1100 RPM is about average.

compound wound?

Question: Stab shunt means stabilized shunt, or compound wound, right?

And BTW, I think it's fine to ask almost any technical question here. We (mainly they, the creators and expert participants) have turned this forum into the greatest single resource for all control system questions in the world. JMHO

I got into a similar deal at Goodyear on a 3-1/2 rubber extruder and the biggest problem was an RPM mismatch. The new motor had a much different peak RPM. The PID system in control didn't care, and still cranked out the right amount of rubber, but the new motor was running too low in it's RPM range compared to the old 850 RPM reliance motor and would cause drive faults intermittently. Later, we ended up oversizing it to keep it happy at low RPM and stayed with a standard readily available motor. Just another spec to compare...RPM...

Then there's the mechanical differences that may kick your butt...we had to build a 4" tall custom base to make the darn thing fit in the old location.

OkiePC said:

Question: Stab shunt means stabilized shunt, or compound wound, right?

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As far as I know yes Stab shunt means the same as coupound, ie: there is an extra winding wound in series with the armature in order to provide more field flux as the motor is loaded. But again I'm not an expert I'm just assuming that is what the difference is.

The two motors have the same base speed but the speed regulation curves are quite a bit different (because of the compound winding). I am not sure if a modern digital drive can take care of this major difference in motor design? That is really my main question.

Stab Shunt means Stabilized Shunt. Comp Shunt means Compensated Shunt. Both are compound fields--there is a series field wound on top of the shunt field. The stab shunt motor has less series field and the comp shunt has more. This makes the torque-speed curves slightly different. The stab shunt has a nearly constant torque output as the speed increases whereas the comp shunt is a bit more like a series wound motor in that the heavier you load it and the slower it turns, the higher the torque output becomes. The comp shunt motor is normally only used where lots of slow speed lugging or heavy overloads are encountered.

A straight shunt motor's torque is going to fall off a bit as speed slows and loads get heavy. On most extruders, this is not a serious deficiency. As long as you are sure that 150hp is enough, I would use the straight shunt motor and not worry about it. It's probably a little smaller frame size as well.

I would expect that the motor has a tach and the drive is operating in closed loop tach feedback mode. Be sure that the smaller motor's field is fully excited---right up to the nameplate value. Plan on retuning the current minor loop and the speed loop. Otherwise, you should have a happy stable system.

Too bad you don't have this motor wired to a Reliance Flexpak 3000 DC drive. They are my favorite and have never let me down in a tough situation. I'm sure the Siemens drive is fine also.

Oops! In the first paragragh the fifth sentence should read "The stab shunt has a nearly constant torque output as the speed DECREASES whereas......"

DickDV, thank you for your response and clarifying the differences.

The way that I concluded that the motor was oversized was by measuring the armature current of an identical extruder while running. The 200HP motor was rated at 320A Ia and was pulling 120A. This motor was running at 1100RPM and the max it will ever run is about 1400RPM.

The major torque requirement happens in the rare occasion that plastic is allowed to harden inside the extruder. This only happens after a power failure (or motor failure for that matter). In that instance much more torque is required to push the hardened plastic out of the extruder screw. How much torque? who knows.

Considering that the max speed was less than the base speed I thought that we could increase the shieve ratio in order to give us more torque from the smaller motor and run closer or even above base speed during normal operation and still have the grunt at low speed when things go bad with the extruder.

From what I can tell this should be OK. I sure hope so because I need a motor NOW!! and I can't think of another option.

BTW, I worked at a pipe mill that had 10 flexpak 3000 drives between 75 and 125HP. We had absolutely nothing but problems with these drives and had reliance service techs out on several occasions to try and figure out what was wrong. We never did. The failure rate was about 1 per month. The failures ranged from bad field regulator cards, blown SCR's, failed comm boards (Reliance DCS), and many nuisance (SCR X misfire trips). The plant I am in now has several of these and have never had a problem????

You are correct. The change in sheave ratio could be used as a fallback if the smaller motor proves to be a little short on torque.

As to your experience with FlexPak3000's, I guess everything has its vulnerabilities and your former location must have hit on it. But, for me, I would sooner blame the troubleshooting for failing to find the cause than the drive that has, generally, a bulletproof reputation.

Just my personal preferences, I guess, and I sell ABB!!!

The Stab type motors are motors with an "S" winding, I'm guessing. The S winding is an integrated speed regulator and will buck a DC drive that has it's own speed regulator. Since you are going from a motor that has an S to one that doesn't, you may be asking more from your drive than you were before; may require some tuning on the existing speed regulator. With an extruder, I don't see it being a major issue. More likely, your problems will be the frame of the new motor and adapting your mounts to it. If Siemens is a typical DC drive, the field supply should be good to 10 amps, so you should be ok on higher current field. I work on GE drives mostly but have some exposure to ABB also.

I bet the environment where the drives were problamtic had dirty line voltage and/or atmosphere conditions.

My experience with the FlexPak 3000's are like every other brand. If you keep them clean and have stable power they act like cadilacs. Noisy power or let them get dirty and they are problamatic. I fixed several on a film line by blowing the dust out of them.

Your smaller motor non stabilized should work using the additional gearing. You might want to extend the accel time a bit for cold starts. That or let the barrel heat soak at least 2 hours at normal temp. I absolutely go nuts waiting on the heat soak, but the motor, drive and extruders fair much better.

I have been thru a couple of drive/motor replacements with material in the barrel and if they are willing to let the heat stabilize usually starting goes smoothly. Remember it is OK for the motor to pull 150% rated armature amps for up to 1 minute. During the initial starting monitor the armature amps. Extending the accel time will also let the drive accel smoothly and not in current limit.

DJM, both the Stab Shunt and Comp Shunt motors have an "s" winding with leads S1 and S2. The only difference is how much more of the Comp Shunt winding is adding to the shunt field. The Stab Shunt tends to be a rather short winding (not much series boost) and the Comp Shunt tends to be a little longer (a little more series boost for the same armature amps).

Certainly with the new motor, you will need to retune the inner and outer loops not only because of the change in field but because of the change in hp, as well.

Thanks for all of the replies, We are going with the 150HP shunt motor and I am planning on changing the motor NP data in the drive obviously as well as retuning. We are going to stick with the same sheave for now and resize if we determine it is necessary.

Turns out some of my trouble getting Reliance to source me with a motor was due to the takeover at Reliance. I didn't know Baldor bought Reliance and Dodge.

I am real curious to know how that will affect their product and customer support.

Should make my tech connect cheaper though right?

Thanks for the info Dick. This is a great forum!

DJM said:

Thanks for the info Dick. This is a great forum!

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Agreed. Dick, you are consistently one of the most interesting and informative posters on this forum and I am always keen to hear what you have to say. Good work mate!

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.

Thanks for the kind comments, guys! Actually I'm just an ordinary guy with a love for machinery and controls. At 61 years old, I find it especially satisfying to help someone else understand something new, especially younger people struggling to get a grasp on all the new stuff.

I've been lucky in several ways: lots of people in my past that took the time to help me understand, the ability to make a living while specializing in a fairly narrow field (AC and DC drives---no PLC's, no sensors, no stepper motors, no servo's), and the gift from my Creator of a brain that works fairly well and stays curious even at my age. It also seems to help to remember the struggles I went thru to understand when I watch others going thru the same struggles.

As most of you know, this is not a good field for people with a lot of pride or just a little patience---especially anything having to do with troubleshooting!

Let's just keep enjoying solving problems together! And thanks again!