Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


severely oversized motor questions, need answers, can't sleep at night!!!!

severely oversized motor questions, need answers, can't sleep at night!!!!

severely oversized motor questions, need answers, can't sleep at night!!!!

I work in a facility that just recently replaced an old 750 hp dc submarine, (yes submarine) motor.
This dual armature motor was used on a rolling mill, which sees intermittent loading.
basically thin sheets of metal coming through once every 10-15 seconds.
The failed sub motor although rated at 750 hp was being powered by a mg set providing it with approx 75 hp.

What is keeping me up at night is the fact that our management team, replaced it with a 400 hp constant torque motor. Being driven by a 450 hp vfd. Running at 3.5 hz
The no load power being used as per the ABB drives front panel is .25%
When a piece is being processed the power being used never goes above 8%
I can't find effiency curves that even go down that low.

The starting torque is very low in that I can rotate this machine by hand.
My gut tells me this machine is oversized by at least 4 times if not more.
They are convinced that they have done a good thing and are saving money....
I am so incensed by the whole ordeal that I am considering quitting and accepting being unemployed, rather that work in a place that can't seem to get it right.
Did I mention this forced ventilation motor was run for almost two weeks with no cooling. What I noticed when operating like this (with no cooling) is that when the mill became loaded the shaft runout or float seemed severe to me, and the motor growled accordingly. It literally would snap backwards after the piece went through the mill. I didn't notice ANY shaft movement when this was first installed. Now the cooling has been restored, but the float is still present, although not as severe.

I contacted ABB to try and get some clarification and they simply emailed my concerns to the vendor that put this in in the first place. Why would I want to consult with them, when in my opinion they intentionally ignored a basic design criteria for efficiency, which is to match the motor the the load....

Any and all comments are welcomed, and hopefully this forum can put me at ease.
Thanks to all in advance who choose to comment.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

The concern is about efficiency. That is a very valid concern and energy costs will probably outweigh the motor's cost in the first few years. I am not so good at effiency things but I hope that Jeff or Bill will have a look at this.

I think that you may have involved yourself a bit too much. But, as I said, it is worth looking into. Definitely.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

The concern here is energy consumption, and what I could have done
to influence their decision making to make a more prudent purchase.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

First, do not quit your job in frustration. The economy is poor so you may not find another.

Second, if you were not included in the decision making process, then there is nothing that you could have done to influence the outcome. The saying is that you should know when you can make a difference and to do so when you can but, you should also know when you cannot make a difference and accept the inevitable outcome when this happens.  

"You don't tug on superman's cape, you don't spit into the wind, you don't pull the mask off of the old long ranger, and you don't mess around with Jim...." - Jim Croce


RE: severely oversized motor questions, need answers, can't sleep at night!!!!

The problem seems to be your huge turndown ratio: 3.5Hz, assuming a 50Hz machine, is 7% of base speed. The motor can deliver rated torque, but at only 7% of rated speed it can deliver 7% of its rated power, which correlates quite well with the drive metering. The motor is arguably sized quite well for the torque requirement, but based on the small amount you have told us about the load it is crying out for a reduction gearbox so the motor can operate somewhere nearer its rated speed and rated output. I can only guess that there is some reason why a direct drive is preferable to a gearbox or cyclodrive. The motor efficiency that low down is way below the normal range quoted - the drive losses are likely to be comparable with or higher than those in a decent gearbox.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

Well said Scotty.

I would be looking at the speed of the driven load, operating at that low speed on a VFD, I would expect very poor speed regulation. If you stalled the motor, it would not draw much more than rated torque, in fact it may be lucky to even get there at 3.5Hz.
Operating continuously at 3.5Hz, there is a high possibility of overheating the motor just due to the iron loss alone with no cooling.
As you reduce the motor speed, you reduce the effective rating and I do not like seeing motors operate below 20Hz for long. The bearings will not be lubricating properly, the iron loss will not be dissipated properly and the speed regulation would be very poor.
If this is a 2 pole machine, replace it with an 8 pole machine and run the drive at 14Hz and use a 100HP. Better still use a gearbox and run the motor as close as you can to rated frequency and you can use an even smaller motor. The advantage of running at this speed, is that you will not overload the motor/VFD by stalling it to the same exttent as you would at a higher speed.

Best regards,

Mark Empson
Advanced Motor Control Ltd

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

As stated, HP scales directly with rpm or frequency. You're using Hp and you're in the US so it's fairly safe to assume 60Hz. 3.5Hz/60Hz is 5.8% which equated to 23Hp. The drive showing the load goes to 8% or 32Hp means the motor is actually overloaded as the sheet of steel runs through the mill. The motor can't be made 4 times smaller for that application or it would stall under load.

In your case, you don't want to look at efficiency. It will be terrible, but was also likely terrible for the old motor. You have a forced ventilation fan running using power all the time and this motor running idle for a fair bit of the time. What you really would want to determine is if the new system uses less energy than the old system.

Efficiency is a tough one to calculate. Installing a VFD does not mean you'll save power despite what the years of marketing have tried to tell everyone. A proper efficiency calculation is comparing the mechanical work being done to the power being used. In most of the typical marketing cases, much of the savings illustrated was achieved by removing the waste work being done. For example, slowing down a fan that is moving too much air.

Overall, it doesn't sound like a terrible situation but I agree with the others that a smaller motor and a gearbox might have been a better solution. A 40Hp or 50Hp motor running at full speed with a gearbox reducing the motor speed to the required load speed would likely save more energy especially if you can get rid of the VFD and run the motor directly off line power.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

I forgot one other thing. It's only a job unless you are an owner and something like this really is just not worth getting upset about.

Now, if the project was going south and the fingers were pointing at you it'd be a different story.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

I appreciate the input. Let me elaborate further on this install. The old motor was only able to provide 75 hp, (limited by the power the mg set provided to it), then it
should be able to be mathematically derived as to how much torque was actually being delivered to the load.
Remember this was an old DC motor that was replaced.
The install requires variable speed, the operator will vary it depending on production material being run. What I'd really like to be able to do is present the facts mathematically, and try to pinpoint the closest match in terms of torque requirements.
The motor we are using is a beast no doubt, but I believe the motor efficiency has to be close to nil. It is a baldor/reliance (modelIDDRPM404006) rated at 1780 rpm @ 40 hz.
I wanted them to stick with DC and a modern DC drive or go with AC and a gearbox.
I know it isn't worth leaving a job over, but consider that if every company that needs an upgrade were to do an install such as this, thats all the more energy being wasted.
I unfortunately am the type that cannot always just accept what is being done as being as being sound, and I know that instead of crunching the numbers, they just oversized to guarantee that it never hits stall torque and sacrificed the efficiency of the project to do it. Doesn't seem like very good business regardless of how much money you may have to throw away on wasted energy. As to work ethic, sometimes doing your job requires you use your experience to try to influence poor decisions being made by those "above you" Its not about being right but about doing the right thing. In a world that is considerably taxing its power grid, it would have been worth the extra effort to spec this unit properly.
If anyone has some equations in mind or a link to the same I would appreciate it.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

As I stated before, I suspect you'll have a hard time arguing efficiency. Can you calculate the mechanical power required to process a piece of steel? Forget about calculating efficiency if you can't do this because anything other number you come up with will be next to meaningless.

Have you put a meter on your system and measured how much power it is using? You might be surprized at how little power a VFD draws to spin a unloaded motor slowly.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

I think you should check the power being used BEFORE you make this your fight to live or die by.... as others have said I think you will find it fairly efficient after all, and not using a lot of power.....

You had 75hp avail before, others have shown your 8% peak is only 32hp, so that makes sense so far. I think you said idle is only .25% (so 1hp), and the spike comes only every 10 seconds. You did not say how long the pike is; lets assume 2 seconds.

so your rms load is [(2sec*32hp^2+8sec*1hp^2)/10sec]^.5 or 14hp (google rms load calc for background proof).

If you can believe your ABB drive, that is the rms load it is putting out. since the ABB is about 96% efficient, you are pulling 14.9hp at near 1.0 power factor so 14.9*.746kw/hp = 11.1kw

Not bad for a 400hp motor!

Although a gearbox DOES seem like a prudent idea, have you looked at the cost of a 32hp*5252/100rpm= 2,000 #-ft rated gearbox? Maybe a lot more added cost than the direct drive approach? For this load I think I would pick one rated 4-5000#-ft. I dont know cuz I didnt look but bet it is expensive - probably more than the 400hp motor itself! Ya, the motor would be spinning faster and I'll swag 1.25-1.50x more efficient, but would that small extra amount on 11kw input really be significant?

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

this also jives with picking a 400hp motor - it is rated 400*5252/1800= 1200#-ft. It is typical for a good drive to output 2x rated intermittently so it can output about 2400#-ft.... If I did this app I too would upsize the drive to a 450hp model due to the heavy peak requirement every 10 seconds.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

how about sticking a clamp on ammeter around the feed to the vfd lines and see if this is correct and let us know? Not sure about others, but I sure would like to hear back on it!

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

Appreciate all the replys, I still think it is too big for the job, wrong regardless of how the drive makes it appear cost effective, the fact is the motor is never ever even gonna get close to running at 50% or more load. thats where I am looking at effiency to start.
As i said before there is so much gearbox in front of this motor, I can spin it by hand. I guess I don't understand big business.

The incoming current is lower than the blower current. the motor no load is 100 amps,
when loaded jumps to between 300 and 400 amps. this is all just to keep the speed at setpoint. My feeble mind tells me that a motor sized to the rated speed/load/torque requirement is ultimately more efficient and running happy than one that is running at less than 10% load.

And the upfront costs would have been substantially lower as well.
I hate doing the math but I will and if I am wrong then I will put this to rest.
If I'm right then instead of quitting I will just look to have my management team replaced. hahaha
thanks again all of you!

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

Early in my career I had to go to a steel mill and measure the torque of a motor they wanted to replace. It was a big pit motor that Edison must have wired himself. This was a four hour job to hook up the telemitry and get the data, it took four days because they refused to stop production. I remember it well because this was before established lock out safety procedures. Just a horn would blow before they started it. The mill was about 75 feet from the motor in another bay and was told to look out for hot flying steel if it should jam up. The multiple drive shafts to the mill were all twisted up and would slap on the floor. The motor seemed like it would almost grind to a halt by the end of the bar. I had nothing to do with the selection of the replacement motor and never heard what they finally did. It was obvious that the mass of this motor was a big part of the equasion. Those twisted drive shafts were clear evidence of the forces needed. Bet that submarine motor had a lot of old school mass. If I was to loose sleep over anything, it would be that this motor is not big enough. Like to hear the weight comparison between the old and new motor. This was an expedient way to buy a flywheel. Hope it is big enough so it doesn't tear the motor apart.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

The motor is sized to the speed/load/torque requirements.

Measure the input power before you lose more sleep over this.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

I have, on several occasions, been admonished for looking at machines from a strictly electrical standpoint. Here is one tidbit that I learned in that way a long time ago.

On some types of rolling mills, you need inertia (mass) more than you need HP because if the machine slows down WHILE the steel is in the rollers, it increases the contact time and changes the bending outcome. The optimum performance some rolling processes is that the machine barely notices that the steel sheet went in. If you use a smaller motor based solely on the HP needed at the motor shaft, you have to use it to spin up a flywheel for the inertia and that takes time, bit on initial start-up and recovery time in between production runs. So it could very well be that the machine designer opted instead to use a large motor as the mass for the inertia, which could also give them a quicker recovery / start-up time, increasing productivity.

"Will work for salami"

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

Well spot on reply there. Inertia was huge on the old motor. Even at only 70 hp max,
that old antique dc drive and tacho would ramp her up when the piece hit the mill.
I tried to insist that we needed a gearbox in front of this install to get some speed out the motor. They declined and well this beastly 400 hp stalls when the piece hits the rolls, then it cranks up the amperage to try and catch up. Speed regulation goes to hell in a handbasket and before you know it the piece clears the mill and we became regenerative for a second. No one really commented on the shaft runout on this forum either. When new this motor would stall like it still does, but the rotor would never runout, at least not visibly to the eye. When my superior said it was ok to run without the cooling fan, I watched it run for an hour and literally watched this output shaft push toward the load at least 3/8 to 1/2 inch each time it loaded up, growlin like a MF. Then it would violently snap back to center. Two weeks they ran like this. Now the fan is on but the runout is still there although only a quarter inch or so. Was there bearing damage done?
I dunno, but it does stall, you can see it stop then on with the current surge to try and make target speed. Overshoot then become a generator and do this 7 days a week 24 hours a day.WTF

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

Honestly, I can't see either motor storing much energy rotating when running at about 100rpm. Rotational energy is inertia times rpm squared so speed is king when storing rotating energy.

Is the poor operation not a concern to anyone else? Does it mess-up the material being processed? The poor performance could be due to the speed loop in the VFD not being programmed correctly.

RE: severely oversized motor questions, need answers, can't sleep at night!!!!

Well I am sure the operators are concerned but they only can work with what they have been given.
I would have replaced this machines drive by getting a torque measurement of the input shaft with a piece in contact with the rolling mill. Then I would have purchased a gear box with a high ratio and plenty of output torque. A 100 hp motor would have been sufficient. With the motor running fast enough when the reflected torque seen at the motor output shaft shows up, the gearbox would buffer some of the shock and the motor could be regulated to speed that much easier.

Its all too late now anyways.

Its time for me to move on.
So tired of arguing all the time.
Remember this, just because you may have a degree, doesn't necessarily mean that the man on the floor has less of a handle on how things should be.

Still seeking a reply on the shaft movement issue. Bearing damage or no???

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close